Patent Publication Number: US-2020284266-A1

Title: Motor drive control device, fan and motor drive control method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of Japanese Patent Applications No. 2019-038257 and No. 2019-038258, filed Mar. 4, 2019, which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a motor drive control device, a fan, and a motor drive control method, and relates to, for example, a motor drive control device that controls the air volume of a fan rotated by a motor. 
     Background 
     Conventionally, fans (fan motors) have been widely known as devices for cooling components and the like which are provided inside home appliances, OA devices and the like. 
     In general, the performance of a fan is expressed by an air volume-static pressure characteristic (hereinafter also referred to as “P-Q curve”). The P-Q curve represents the relationship between the air volume and the loss (static pressure) caused by the pressure between a suction port and a discharge port of the fan. In the P-Q curve, when the static pressure is maximum (ventilation resistance is maximum), the air volume of the fan becomes zero, and when the static pressure is zero (ventilation resistance is zero), the air volume of the fan becomes maximum (for example, see Japanese Patent Laid-Open No. 2009-174414). 
     Note that a state where the static pressure is zero, that is, the air volume of the fan is maximum is also referred to as “free air state”. 
     SUMMARY 
     Generally, a fan is designed so that a required air volume can be obtained in a predetermined operating range (for example, a region where the static pressure is in a middle range). In other words, the fan is designed so as to realize a P-Q curve that provides a predetermined air volume in the required operating range. On the other hand, for the fan, quietness is more important than air volume in a region other than the required operating range, for example, a region where the static pressure is a predetermined value or less. 
     With respect to conventional fans, the rotational speed of a motor is controlled so as to obtain a rotational speed instructed by a host device, and the air volume changes depending on a pressure loss (static pressure). For this reason, the conventional fans have a problem that an air volume equal to or higher than an air volume in a required operating range occurs in a region where the static pressure is low like a free air state, resulting in increased noise. 
     The present disclosure is related to enhancing quietness while securing a necessary air volume in a fan. 
     A motor drive control device according to a representative embodiment of the present disclosure comprises a control circuit that generates a drive control signal for controlling a rotational speed of a motor based on a speed command signal indicating a target rotational speed of the motor, and a motor driving unit that drives the motor based on the drive control signal, wherein the control circuit performs speed feedback control for generating the drive control signal so that the rotational speed of the motor coincides with the target rotational speed when the target rotational speed is lower than a rotational speed threshold value, and generates the drive control signal based on a relationship between current flowing through the motor and a reference current value when the target rotational speed is higher than the rotational speed threshold value. 
     According to the motor drive control device of the present disclosure, it is possible to realize a fan having enhanced quietness while ensuring a necessary air volume. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a fan according to a first embodiment of the present disclosure. 
         FIG. 2  is a view showing air volume control of the fan by a motor drive control device according to the first embodiment. 
         FIG. 3  is a view showing a correspondence relationship between a rotational speed and a motor current of a motor in a maximum air volume control line of  FIG. 2 . 
         FIG. 4  is a block diagram showing a configuration of the motor drive control device according to the first embodiment. 
         FIG. 5A  is a view showing an example of a method for adjusting the rotational speed of the motor during maximum air volume control by the motor drive control device according to the first embodiment. 
         FIG. 5B  is a view showing an example of a method for adjusting an operating point of the motor during maximum air volume control by the motor drive control device according to the first embodiment. 
         FIG. 6  is a flowchart showing a flow of the air volume control of the fan by the motor drive control device according to the first embodiment. 
         FIG. 7  is a view showing a relationship between the rotational speed of the motor and a target rotational speed in the fan according to the first embodiment. 
         FIG. 8  is a block diagram showing a configuration of a fan according to a second embodiment of the present disclosure. 
         FIG. 9  is a view showing air volume control of the fan by a motor drive control device according to the second embodiment. 
         FIG. 10  is a view showing an outline of an air volume control method by the motor drive control device according to the second embodiment. 
         FIG. 11  is a block diagram showing a configuration of the motor drive control device according to the second embodiment. 
         FIG. 12  is a view showing a method of determining a current threshold value. 
         FIG. 13  is a flowchart showing a flow of processing relating to air volume control by the motor drive control device according to the second embodiment. 
         FIG. 14  is a view showing the relationship between a target rotational speed of the motor and an actual rotational speed in the fan according to the second embodiment. 
         FIG. 15  is a view showing the relationship between the motor current and the rotational speed of the motor in the fan according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     1. Outline of Embodiments 
     First, an outline of a representative embodiment of the disclosure disclosed in the present application will be described. Note that in the following description, as an example, reference signs on the drawings which correspond to components of the disclosure are shown in parentheses. 
     [1] A motor drive control device ( 1 ,  1 A) according to a representative embodiment of the present disclosure includes a control circuit ( 4 ,  4 A) for generating a drive control signal (Sd) for controlling the rotational speed of a motor ( 20 ) based on a speed command signal (Sc) for indicating a target rotational speed (Rtg) of the motor ( 20 ), and a motor driving unit ( 2 ) for driving the motor based on the drive control signal. The control circuit performs speed feedback control for generating the drive control signal so that the rotational speed of the motor coincides with the target rotational speed when the target rotational speed is lower than a rotational speed threshold value (Rth), and generates the drive control signal based on the relationship between current flowing through the motor and a reference current value (Itg, Ith) when the target rotational speed is higher than the rotational speed threshold value. 
     [2] In the motor drive control device ( 1 ) of the foregoing [1], the reference current value may be a target current value (Itg) to be calculated in association with the rotational speed of the motor, and the control circuit may perform maximum air volume control for generating the drive control signal so that the current flowing through the motor approaches the target current value when the target rotational speed is higher than the rotational speed threshold value. 
     [3] In the motor drive control device ( 1 ) of the foregoing [2], the control circuit ( 4 ) may calculate an actual rotational speed of the motor based on a position detection signal (Sh) indicating a rotational position of the motor, and calculate the target current based on prestored correspondence relationship information ( 471 ) indicating the correspondence relationship between the rotational speed of the motor and the target current value and the calculated actual rotational speed. 
     [4] In the motor drive control device ( 1 ) of the foregoing [3], the motor may rotate an impeller ( 21 ) in the fan ( 100 ), and the correspondence relationship may represent the relationship between the rotational speed of the motor and the target current value in a region where the static pressure of the fan is lower than a predetermined value (Pb). 
     [5] In the motor drive control device ( 1 ) according to the foregoing [3] or [4], under the maximum air volume control, the control circuit ( 4 ) may generate the drive control signal so that the rotational speed (Rr) of the motor increases when current (Ir) flowing through the motor is higher than the target current value (Itg), and generate the drive control signal so that the rotational speed (Rr) of the motor decreases when the current (Ir) flowing through the motor is lower than the target current value (Itg). 
     [6] In the motor drive control device ( 1 ) according to the foregoing [5], under the maximum air volume control, the control circuit ( 4 ) may generate the drive control signal so as to increase the rotational speed of the motor when the current (Ir) flowing through the motor is higher than a predetermined range (M) containing the target current value (Itg) as a reference, so as to make the rotational speed of the motor unchanged when the current (Ir) flowing through the motor is within the predetermined range (M), and so as to reduce the rotational speed of the motor when the current (Ir) flowing through the motor is lower than the predetermined range (M). 
     [7] In the motor drive control device ( 1 ) according to any one of the foregoing [3] to [6], the control circuit ( 4 ) may include a rotational speed calculator ( 42 ) for calculating the actual rotational speed (Rr) based on the position detection signal (Sh), a speed controller ( 43 ) for generating a first control signal (Sp 1 ) so that the actual rotational speed (Rr) calculated by the rotational speed calculator coincides with the target rotational speed (Rtg) indicated by the speed command signal (Sc), a target current value calculator ( 47 ) for calculating the target current value (Itg) based on the actual rotational speed (Rr) calculated by the rotational speed calculator, a current value acquisition unit ( 46 ) for acquiring a current value of current flowing through the motor, a maximum air volume controller ( 44 ) for generating a second control signal (Sp 2 ) so that the current (Ir) flowing through the motor approaches the target current value (Itg) when the target rotational speed (Rtg) is higher than the rotational speed threshold value (Rth), and a drive control signal generator ( 45 ) for generating the drive control signal (Sd) based on the first control signal and the second control signal. 
     [8] In the motor drive control device ( 1 A) of the foregoing [1], the reference current value may be a current threshold value (Ith) for estimating an external pressure, and when the target rotational speed is higher than the rotational speed threshold value, based on a comparison result between the actual current value of the current flowing through the motor and the current threshold value, the control circuit ( 4 A) may perform switching between the speed feedback control and the maximum air volume control for generating the drive control signal so that the rotational speed of the motor does not exceed the rotational speed threshold value. 
     [9] In the motor drive control device ( 1 A) of the foregoing [8], under a state where the target rotational speed is higher than the rotational speed threshold value, the control circuit ( 4 A) may perform the speed feedback control when the actual current value is larger than the current threshold value, and perform the maximum air volume control when the actual current value is smaller than the current threshold value. 
     [10] In the motor drive control device ( 1 A) of the foregoing [8], the control circuit ( 4 A) may generate the drive control signal in the maximum air volume control so that the motor rotates at a constant rotational speed corresponding to the rotational speed threshold value. 
     [11] In the motor drive control device ( 1 A) of any one of the foregoing [8] to [10], the control circuit ( 4 A) may include a rotational speed calculator ( 42 A) for calculating an actual rotational speed of the motor based on a position detection signal indicating the rotational position of the motor, a speed controller ( 43 A) for generating a first control signal (Sp 1 ) so that the actual rotational speed calculated by the rotational speed calculator coincides with the target rotational speed indicated by the speed command signal, a current value acquisition unit ( 46 A) for acquiring the actual current value (Ir), a maximum air volume controller ( 44 A) for generating a second control signal (Sp 2 ) so that the rotational speed of the motor does not exceed the rotational speed threshold value when the target rotational speed is larger than the rotational speed threshold value and the actual current value acquired by the current value acquisition unit is larger than the current threshold value, and a drive control signal generator ( 45 A) for generating the drive control signal based on the first control signal or the second control signal. 
     [12] A fan ( 100 ,  100 A) according to a representative embodiment of the present disclosure includes a motor ( 20 ), an impeller ( 21 ) configured to be rotatable by rotational force of the motor ( 20 ), and a motor drive control device ( 1 ,  1 A) for controlling driving of the motor, wherein the motor drive control device comprises a control circuit ( 4 ,  4 A) for generating a drive control signal (Sd) for controlling a rotational speed of the motor based on a speed command signal (Sc) indicating a target rotational speed (Rtg) of the motor, and a motor driving unit ( 2 ) for driving the motor based on the drive control signal, and the control circuit performs speed feedback control for generating the drive control signal so that the rotational speed of the motor coincides with the target rotational speed when the target rotational speed (Rtg) is lower than the rotational speed threshold value (Rth), and generates the drive control signal based on a relationship between current (Ir) flowing through the motor and a reference current value (Itg, Ith) when the target rotational speed is higher than the rotational speed threshold value. 
     [13] In the fan ( 100 ) of the foregoing [12], the reference current value may be a target current value (Itg) to be calculated in association with the rotational speed of the motor, and the control circuit ( 4 ) may perform maximum air volume control for generating the drive control signal so that current flowing through the motor approaches the target current value when the target rotational speed is higher than the rotational speed threshold value. 
     [14] In the fan ( 100 A) of the foregoing [12], when the target rotational speed is higher than the rotational speed threshold value, based on a comparison result between an actual current value of current flowing through the motor and a current threshold value (Ith) as the reference current value, the control circuit ( 4 A) may perform switching between the speed feedback control and the maximum air volume control for generating the drive control signal so that the rotational speed of the motor does not exceed the rotational speed threshold value. 
     [15] A motor drive control method according to a representative embodiment of the present disclosure is a method for generating a drive control signal (Sd) for controlling driving of a motor ( 20 ) based on a speed command signal (Sc) indicating a target rotational speed (Rtg) of the motor ( 20 ), and driving the motor based on the drive control signal, and includes a first step (S 3 , S 3 A) of performing speed feedback control for generating the drive control signal so that a rotational speed of the motor coincides with the target rotational speed when the target rotational speed is lower than a rotational speed threshold value (Rth), and a second step (S 4  to S 14 , S 3 A, S 5 A to S 9 A) of generating the drive control signal based on a relationship between current flowing through the motor and a reference current value when the target rotational speed is higher than the rotational speed threshold value. 
     [16] In the motor drive control method of the foregoing [15], the reference current value may be a target current value (Itg) to be calculated in association with a rotational speed of the motor, and the second step may include a third step (S 4  to S 14 ) of performing maximum air volume control for generating the drive control signal so that current flowing through the motor approaches the target current value when the target rotational speed is higher than the rotational speed threshold value. 
     [17] In the motor drive control method according to the foregoing [15], the reference current value may be a current threshold value (Ith) for estimating an external pressure, and the second step may include a third step (S 3 A, S 5 A to S 9 A) of performing, based on a comparison result between an actual current value of current flowing through the motor and the current threshold value, switching between the speed feedback control and maximum air volume control for generating the drive control signal so that a rotational speed of the motor does not exceed the rotational speed threshold value when the target rotational speed is higher than the rotational speed threshold value. 
     2. Specific Examples of Embodiments 
     Hereinafter, specific examples of the embodiments of the present disclosure will be described with reference to the drawings. In the following description, the same reference signs are given to components common to the respective embodiments, and duplicative description is omitted. 
     First Embodiment 
       FIG. 1  is a block diagram showing a configuration of a fan according to the first embodiment of the present disclosure. 
     A fan  100  according to a first embodiment is a device for generating wind by rotating an impeller (vane wheel). The fan  100  can be used as one of cooling devices for discharging heat generated inside equipment to the outside and cooling the inside of the equipment. The fan  100  is an axial fan, for example. 
     As shown in  FIG. 1 , the fan  100  includes a motor  20 , a motor drive control device  1  for driving the motor  20 , and an impeller  21  that is configured to be rotatable by the rotational force of the motor  20 . 
     In the first embodiment, the motor  20  is, for example, a three-phase brushless motor having coils Lu, Lv, and Lw. The motor drive control device  1  is a device for controlling the rotation of the motor  20 . The motor drive control device  1  rotates the motor  20  by causing a drive current to periodically flow through the three-phase coils Lu, Lv, and Lw constituting the motor  20 . 
     Specifically, the motor drive control device  1  includes a motor driving unit  2 , a control circuit  4 , and a current detector  6 . Note that components of the motor drive control device  1  shown in  FIG. 1  are parts of the whole, and the motor drive control device  1  may include other components in addition to those shown in  FIG. 1 . 
     In the first embodiment, at least a part of the motor drive control device  1  is packaged as one semiconductor device (IC: Integrated Circuit). For example, circuits such as the control circuit  4  and the motor driving unit  2  are implemented as separate semiconductor devices. 
     Note that the motor drive control device  1  may be a semiconductor device which is packaged in its entirety, or all or a part of the motor drive control device  1  and another device may be packaged together to configure a single semiconductor device. 
     The motor driving unit  2  outputs a drive signal to the motor  20  based on the drive control signal Sd output from the control circuit  4  to drive the motor  20 . The motor driving unit  2  selectively energizes the coils Lu, Lv, Lw of plural phases of the motor  20 . 
     Specifically, the motor driving unit  2  includes an inverter circuit  2   a  and a pre-drive circuit  2   b . The pre-drive circuit  2   b  generates an output signal for driving the inverter circuit  2   a  based on the drive control signal Sd output from the control circuit  4 , and outputs the output signal to the inverter circuit  2   a . The inverter circuit  2   a  energizes the coils Lu, Lv, Lw included in the motor  20  based on a signal output from the pre-drive circuit  2   b.    
     Specifically, the inverter circuit  2   a  is configured, for example, so that a pair of series circuits each including two switching elements provided at both ends of a power supply voltage (DC power supply) Vcc are arranged for each phase (U-phase, V-phase, W-phase) of the coils Lu, Lv, Lw. In each pair of two switching elements, a terminal of each phase of the motor  20  is connected to a connection point between the switching elements (not shown). The pre-drive circuit  2   b  outputs, for example, six types of signals Vuu, Vul, Vvu, Vvl, Vwu, and Vwl corresponding to the respective switching elements of the inverter circuit  2   a  as output signals. By outputting these signals Vuu, Vul, Vvu, Vvl, Vwu, and Vwl, the switching elements corresponding to the signals Vuu, Vul, Vvu, Vvl, Vwu, and Vwl respectively are turned on/off. As a result, a drive signal is output to the motor  20 , and current flows through the coil Lu, Lv, Lw of each phase of the motor  20  (not shown). 
     The current detector  6  is a functional unit for detecting current flowing through the motor  20 , that is, current flowing through the coils Lu, Lv, and Lw of the motor  20  (hereinafter also referred to as “motor current”). The current detector  6  outputs a voltage Vs corresponding to the motor current of the motor  20 . Note that the configuration of the current detector  6  will be described later 
     The control circuit  4  is configured to include, for example, a microcomputer (MCU (Micro Controller Unit)) having a processor (for example, CPU) and various memories, a digital circuit, an analog circuit, and the like. Various signals for instructing driving of the motor  20  are input to the control circuit  4 . The control circuit  4  performs driving control of the motor  20  based on these input signals. For example, a speed command signal Sc as a signal for instructing driving of the motor  20  is input to the control circuit  4  from a device provided outside the control circuit  4  such as a host device. 
     The speed command signal Sc is a signal related to the rotational speed of the motor  20 . For example, the speed command signal Sc is a pulse width modulation (PWM) signal having a duty ratio corresponding to a target rotational speed Rtg of the motor  20 . Note that a clock signal may be input as the speed command signal Sc. 
     Furthermore, a position detection signal Sh is input from a position detection element  5  to the control circuit  4 . The position detection element  5  is, for example, a Hall element disposed in the motor  20 . The position detection signal Sh is a Hall signal output from the Hall element as the position detection element  5 . The position detection signal Sh is a signal indicating the rotational position of the motor  20 , that is, a signal corresponding to rotation of a rotor (not shown) of the motor  20 . Hereinafter, the position detection element  5  is also referred to as “Hall element  5 ”. 
     The control circuit  4  obtains information on the actual rotational speed of the rotor of the motor  20  from the position detection signal Sh to control the driving of the motor  20 . 
     Note that  FIG. 1  illustrates a case where one Hall element  5  is arranged in the fan  100 , but the number of Hall elements  5  to be arranged in the fan  100  is not particularly limited. For example, three Hall elements  5  may be arranged around the rotor of the motor  20  at substantially equal intervals. 
     Note that the control circuit  4  may be configured to receive other information related to the rotation state of the motor  20  in addition to the position detection signal Sh or instead of the position detection signal Sh. For example, the control circuit  4  may be configured to receive, as an FG signal corresponding to the rotation of the rotor of the motor  20 , a signal (pattern FG) generated by using a coil pattern which is provided on a substrate on the side of the rotor. Furthermore, the control circuit  4  may be configured so that the rotation state of the motor  20  is detected based on a detection result of a rotational position detection circuit for detecting a back electromotive force induced in each phase (U, V, W phase) of the motor  20 . An encoder, a resolver, or the like may be provided so that information such as the rotational speed of the motor  20  can be detected. 
     The control circuit  4  generates a drive control signal Sd for controlling the rotational speed of the motor  20  based on the speed command signal Sc, the position detection signal Sh, the voltage Vs, and the like described above. 
     The drive control signal Sd is, for example, a pulse width modulation (PWM) signal. The control circuit  4  supplies the motor driving unit  2  with the drive control signal Sd as a PWM signal, thereby adjusting the rotational speed of the motor while switching, in a predetermined order, an energized phase among the coils Lu, Lv, Lw of plural phases to be energized by the motor driving unit  2 . As a result, the air volume of the fan  100  is controlled. 
     The motor drive control device  1  according to the first embodiment controls the rotational speed of the motor  20  so that the air volume of the fan  100  is limited in a region where the static pressure of the fan  100  is equal to or less than a predetermined value. 
       FIG. 2  is a view showing the air volume control of the fan  100  by the motor drive control device  1  according to the first embodiment. 
     In  FIG. 2 , the horizontal axis represents the air volume Q, and the vertical axis represents the static pressure P.  FIG. 2  shows a P-Q curve for each target rotational speed Rtg when the target rotational speed Rtg indicated by the speed command signal Sc is changed. 
     In  FIG. 2 , reference sign  201 _min represents a P-Q curve of the fan  100  when the target rotational speed Rtg is set to a minimum value Rmin, reference sign  201 _max represents a P-Q curve of the fan  100  when the target rotational speed Rtg is set to a maximum value Rmax, and reference sign  201 _ x  represents a P-Q curve when the target rotational speed Rtg is set to a value between the maximum value and the minimum value. 
     As described above, the fan is designed so that a desired air volume is obtained in a required operating range. For example, in  FIG. 2 , it is assumed that the operating range required for the fan is a range indicated by reference sign  200 . In this case, the required operating range  200  exists in a region below the P-Q curve  200 _max (region where the static pressure is lower) when the target rotational speed Rtg is set to the maximum value Rtg_max, so that it can be said that the fan having the P-Q curve shown in  FIG. 2  satisfies a required specification. 
     On the other hand, as described above, for the fan, quietness is more important than air volume in a region other than the required operating range, that is, in a region where the static pressure is a predetermined value or less. For example, in  FIG. 2 , quietness is more important than air volume in a region where the static pressure is equal to Pb or less. 
     However, the conventional fan controls the rotational speed of the motor so that the rotational speed of the motor has reached the target rotational speed Rtg indicated by the speed command signal Sc, and the air volume changes due to a pressure loss (static pressure). Therefore, even in a region where the static pressure is low, the air volume trends to increase, and noise trends to intensify. 
     For example, a case where a conventional fan is designed to operate on a P-Q curve indicated by reference sign  201 _max as shown in  FIG. 2  is considered. In this case, when the target rotational speed Rtg is set to “Rtg_max” in a free air state, the conventional fan generates an air volume qmax that greatly exceeds the required operating range  200  (the range from qr 1  to qr 2  in air volume). 
     However, as described above, in a region other than the operating range  200 , an air volume that greatly exceeds the required operating range  200  is not necessary. 
     Therefore, the motor drive control device  1  according to the first embodiment controls the rotational speed of the motor  20  so that the air volume of the fan  100  is limited regardless of the target rotational speed Rtg indicated by the speed command signal Sc in a region where the static pressure is lower than a predetermined value. 
     Specifically, as shown in  FIG. 2 , the motor drive control device  1  controls the motor  20  so that the air volume of the fan  100  changes along a line (hereinafter also referred to as “maximum air volume control line”) C connecting a point A of the maximum air volume when the target rotational speed Rtg is set to a predetermined value Rx (minimum value Rmin&lt;Rx&lt;maximum value Rmax) and a point B of the maximum air volume when the target rotational speed Rtg is set to a maximum value Rmax in a range where the static pressure is lower than a predetermined value Pb. 
     More specifically, the motor drive control device  1  adjusts the rotational speed of the motor  20  based on the correspondence relationship between the rotational speed and the motor current of the motor  20  on the maximum air volume control line C, thereby causing the air volume of the fan  100  to change along the maximum air volume control line C in a region where the static pressure is lower than a predetermined value Pb. 
       FIG. 3  is a view showing a correspondence relationship between the rotational speed and the motor current of the motor  20  on the maximum air volume control line C in  FIG. 2 . 
     A characteristic (graph)  500  shown in  FIG. 3  represents the relationship between the rotational speed (actual rotational speed Rr) and the motor current (actual current value Ir) of the motor  20  on the maximum air volume control line C of  FIG. 2 . A point A in the characteristic  500  in  FIG. 3  corresponds to the point A on the maximum air volume control line C in  FIG. 2 , and A point B in the characteristic  500  in  FIG. 3  corresponds to the point B on the maximum air volume control line C in  FIG. 2 . 
     The characteristic  500  can be acquired by the following method. 
     For example, the static pressure (P), the air volume (Q), the rotational speed (actual rotational speed Rr) and the motor current (actual current value Ir) of the motor  20  at each target rotational speed Rtg when the target rotational speed Rtg of the fan  100  (motor  20 ) is changed from a settable minimum value Rmin to a settable maximum value Rmax are measured in advance. Note that at this time, the maximum air volume control described later of the fan  100  is set to be disabled. 
     Next, the measurement data of the static pressure (P) and the air volume (Q) are used to draw a P-Q curve for each target rotational speed Rtg as shown in  FIG. 2 . Next, the drawn P-Q curve is used to set the maximum air volume control line C so that the air volume is limited in a region where the static pressure is lower than a desired value (for example, Pb). A method of setting the maximum air volume control line C is as described above. 
     Next, the measurement data of the actual rotational speed Rr and the actual current value Ir on the maximum air volume control line C are extracted from the measurement data of the actual rotational speed Rr and the actual current value Ir of the motor current of the motor  20  at each target rotational speed Rtg. Then, the correspondence relationship between the actual rotational speed Rr and the actual current value Ir is plotted based on the extracted measurement data. As a result, the characteristic  500  representing the correspondence relationship between the rotational speed and the motor current of the motor  20  can be obtained. 
     As described above, the characteristic  500  represents the relationship between the rotational speed and the motor current of the motor  20  when the air volume of the fan  100  changes along the maximum air volume control line C of the P-Q curve shown in  FIG. 2 . Accordingly, it is possible to cause the fan  100  to operate along the maximum air volume control line C by controlling the rotational speed and the motor current of the motor  20  so as to satisfy the characteristic  500 . 
     Therefore, the motor drive control device  1  according to the first embodiment adjusts the rotational speed of the motor  20  so that the actual current value of the motor current of the motor  20  approaches a target value of the motor current represented by the characteristic  500  (hereinafter referred to as “target current value Itg”). As a result, the air volume of the fan  100  is changed along the maximum air volume control line C, thereby limiting the maximum air volume of the fan  100 . 
     Hereinafter, a configuration of the motor drive control device  1  for realizing the function for limiting the maximum air volume of the fan  100  will be described in detail. 
       FIG. 4  is a block diagram showing the configuration of the motor drive control device  1  according to the first embodiment. 
       FIG. 4  shows functional blocks related to the function for limiting the maximum air volume of the fan  100  described above out of functional blocks constituting the motor drive control device  1 . 
     The control circuit  4  in the motor drive control device  1  performs speed feedback control when the target rotational speed Rtg of the motor  20  indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth (for example, in the case of Rtg&lt;Rth). 
     Here, the speed feedback control is control for generating the drive control signal Sd so that the rotational speed (actual rotational speed Rr) of the motor  20  coincides with the target rotational speed Rtg. 
     On the other hand, when the target rotational speed Rtg is higher than the rotational speed threshold value Rth (for example, in the case of Rtg Rth), the control circuit  4  generates the drive control signal Sd based on the relationship between the current (motor current) flowing through the motor  20  and the reference current value. 
     In the first embodiment, the reference current value is a target current value Itg to be calculated in association with the rotational speed (actual rotational speed) of the motor  20 . 
     Specifically, when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the control circuit  4  performs maximum air volume control. 
     Here, the maximum air volume control according to the first embodiment is control for generating the drive control signal Sd so that the motor current approaches the target current value Itg. 
     More specifically, the control circuit  4  includes a target rotational speed acquisition unit  41 , a rotational speed calculator  42 , a speed controller  43 , a maximum air volume controller  44 , a drive control signal generator  45 , a current value acquisition unit  46 , a target current value calculator  47 , and a comparator (CMP)  48  as functional blocks for implementing the speed feedback control and the maximum air volume control. 
     The target rotational speed acquisition unit  41  acquires information on the target rotational speed Rtg of the motor  20  from a speed command signal Sc which is output from, for example, a host device existing outside the motor drive control device  1 , and provides the information to the speed controller  43  and the maximum air volume controller  44 . 
     For example, when the speed command signal Sc is a PWM signal representing the target rotational speed Rtg by a duty ratio, the target rotational speed acquisition unit  41  analyzes the duty ratio of the PWM signal as the input speed command signal Sc to calculate the rotational speed corresponding to the duty ratio, and outputs the calculated rotational speed as the target rotational speed Rtg. For example, the target rotational speed acquisition unit  41  has a table representing the correspondence relationship between the duty ratio of the PWM signal and the target rotational speed. The target rotational speed acquisition unit  41  reads the target rotational speed Rtg corresponding to the duty ratio of the input speed command signal Sc from the table. As a result, the target rotational speed acquisition unit  41  can acquire the information on the target rotational speed Rtg from the speed command signal Sc. 
     The target rotational speed acquisition unit  41  is implemented, for example, by an external interface circuit, etc. of a microcontroller and program processing by CPU. 
     The rotational speed calculator  42  calculates the rotational speed (the number of revolutions per unit time) of the motor  20  based on the position detection signal Sh output from the position detection element  5 . The rotational speed calculator  42  calculates the actual rotational speed of the rotor of the motor  20  by using the position detection signal Sh, and provides the calculated actual rotational speed as the actual rotational speed Rr to the speed controller  43  and the target current value calculator  47 . 
     The rotational speed calculator  42  is implemented, for example, by an external interface circuit, etc. of a microcontroller and program processing of CPU like the target rotational speed acquisition unit  41 . 
     The speed controller  43  generates a PWM command signal (an example of a first control signal) Sp 1  indicating the duty ratio of the PWM signal as the drive control signal Sd based on the target rotational speed Rtg output from the target rotational speed acquisition unit  41  and the actual rotational speed Rr of the motor  20  calculated by the rotational speed calculator  42 . 
     Specifically, the speed controller  43  generates the PWM command signal Sp 1  so that the actual rotational speed Rr coincides with the target rotational speed Rtg. For example, the speed controller  43  calculates the difference between the actual rotational speed Rr and the target rotational speed Rtg, and calculates the duty ratio of the PWM signal as the drive control signal Sd so that the difference becomes zero. Then, the speed controller  43  outputs information of the calculated duty ratio as a PWM command signal Sp 1 . 
     The current value acquisition unit  46  is a functional unit for calculating the current value of the current flowing through the motor  20 . The current value acquisition unit  46  includes, for example, an A/D conversion circuit for converting an analog signal into a digital signal. For example, the current value acquisition unit  46  is a ΔΣ modulation type analog/digital conversion circuit, and is configured by a dedicated logic circuit. The current value acquisition unit  46  converts an analog signal input from the current detector  6  into a digital signal according to a ΔΣ modulation method. 
     Here, as described above, the current detector  6  is a circuit for outputting a voltage Vs corresponding to the current (motor current) flowing through the motor  20  as a control target. For example, as shown in  FIG. 4 , the current detector  6  includes a resistor Rs which is connected in series between the coils Lu, Lv, and Lw of the motor  20  and the ground potential via the motor driving unit  2 . The voltage Vs occurring between both ends of the resistor Rs is output as a detected value of the motor current of the motor  20 . 
     The current value acquisition unit  46  converts the voltage Vs, which is an analog signal output from the current detector  6 , into a digital signal, and outputs the digital signal as the actual current value Ir of the motor current of the motor  20 . 
     The target current value calculator  47  calculates the target current value Itg based on the actual rotational speed Rr calculated by the rotational speed calculator  42 . For example, the target current value calculator  47  includes a memory unit  470  for storing correspondence relationship information  471  representing the correspondence relationship between the rotational speed and the motor current of the motor  20 , and uses the correspondence relationship information  471  read from the memory unit  470  to calculate the target current value Itg from the actual rotational speed Rr. 
     As described above, the target current value Itg is a target value of the motor current for controlling the air volume of the fan  100  along the maximum air volume control line C. 
     The correspondence relationship information  471  is, for example, information containing a mathematical expression representing the characteristic  500  in  FIG. 3 . For example, regression analysis is performed in advance by using measurement data of the actual rotational speed Rr and the actual current value Ir of the motor  20  on the maximum air volume control line C to derive a relational expression (for example, a linear function) of the rotational speed and the motor current, and the derived relational expression is stored as the correspondence relationship information  471  in advance in the memory unit  470 . 
     In the maximum air volume control, the target current value calculator  47  reads out the relational expression of the rotational speed and the motor current as the correspondence relationship information  471  from the memory unit  470 , and substitutes the actual rotational speed Rr of the motor  20  calculated by the rotational speed calculator  42  into the read-out relational expression to calculate the target current value Itg of the motor  20 . 
     Note that the correspondence relationship information  471  is not limited to the above-described relational expression, and may be, for example, a table (lookup table) indicating the correspondence relationship between the motor current and the rotational speed. 
     The comparator  48  compares the target rotational speed Rtg with the rotational speed threshold value Rth, and outputs a comparison result. 
     The rotational speed threshold value Rth is a parameter serving as a reference for switching of the control mode (the speed feedback control and the maximum air volume control) of the fan  100 . For example, when the fan  100  is controlled along the maximum air volume control line C shown in  FIG. 2 , the rotational speed of the motor  20  at the point A may be set as the rotational speed threshold value Rth. 
     When the target rotational speed Rtg output from the target rotational speed acquisition unit  41  is larger than the rotational speed threshold value Rth, the comparator  48  outputs, for example, a high-level determination signal Scmp. On the other hand, when the target rotational speed Rtg output from the target rotational speed acquisition unit  41  is smaller than the rotational speed threshold value Rth, the comparator  48  outputs, for example, a low-level determination signal Scmp. 
     Based on the determination signal Scmp of the comparator  48 , the maximum air volume controller  44  generates a PWM command signal (an example of a second control signal) Sp 2  indicating the duty ratio of the PWM signal as the drive control signal Sd. Specifically, when it is determined by the comparator  48  that the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the maximum air volume controller  44  generates the PWM command signal Sp 2  so that the actual current value Ir of the motor current approaches the target current value Itg. On the other hand, when it is determined by the comparator  48  that the target rotational speed Rtg is lower than the rotational speed threshold value Rth, the maximum air volume controller  44  does not generate the PWM command signal Sp 2 . 
     The maximum air volume controller  44  generates the PWM command signal Sp 2  based on the target current value Itg calculated by the target current value calculator  47  and the actual current value Ir calculated by the current value acquisition unit  46 . 
     Specifically, the maximum air volume controller  44  generates the PWM command signal Sp 2  so that the actual rotational speed Rr of the motor  20  increases when the actual current value Ir is higher than the target current value Itg, and generates the PWM command signal Sp 2  so that the actual rotational speed Rr of the motor  20  decreases when the actual current value Ir is lower than the target current value Itg. 
     More specifically, the maximum air volume controller  44  increases the rotational speed of the motor  20  when the actual current value Ir of the motor  20  is higher than a predetermined range containing the target current value Itg as a reference. The maximum air volume controller  44  does not change the rotational speed of the motor  20  when the actual current value Ir is within the predetermined range. The maximum air volume controller  44  generates the PWM command signal Sp 2  so as to decrease the rotational speed of the motor when the actual current value Ir is lower than the predetermined range. 
       FIGS. 5A and 5B  are views showing the maximum air volume control.  FIG. 5A  shows an example of a method for adjusting the rotational speed of the motor  20  based on the actual current value Ir of the motor current.  FIG. 5B  shows an example of a method for adjusting an operating point of the motor  20  based on the target current value Itg. 
     Note that |X|&lt;|Y|, |Z| in  FIG. 5A . 
     For example, as shown in  FIG. 5A , in the case of Itg−X&lt;Ir&lt;Itg+X, that is, when the actual current value Ir of the motor current is within a range Mm, the maximum air volume controller  44  makes the rotational speed of the motor  20  unchanged. For example, when the motor  20  is operating at an operating point c in  FIG. 5B , it can be determined that the operating point c is within the range Mm and the fan  100  is operating along the maximum air volume control line C. In this case, the maximum air volume controller  44  outputs a PWM command signal Sp 2  containing information of the same duty ratio as a PWM command signal Sp 2  output immediately before to make the rotational speed of the motor  20  unchanged. 
     On the other hand, in the case of Ir&gt;Itg+Z, that is, when the actual current value Ir of the motor current is within a range H, the maximum air volume controller  44  generates the PWM command signal Sp 2  so as to increase the rotational speed of the motor  20 . For example, when the motor  20  is operating at an operating point a in  FIG. 5B , it can be determined that the operating point a is within the range H and the fan  100  is not operating along the maximum air volume control line C (the pressure resistance against the fan  100  Is large). In this case, as shown in  FIG. 5B , the maximum air volume controller  44  generates the PWM command signal Sp 2  for increasing the rotational speed of the motor  20  so as to shift the operating point of the fan  100  from “a” to “ax” on the characteristic  500 . For example, when the duty ratio indicated by the PWM command signal Sp 2  output immediately before is 50% and a unit adjustment width of the duty ratio is 0.1%, the maximum air volume controller  44  outputs a PWM command signal Sp 2  indicating the duty ratio of “(50+0.1) %” to increase the rotational speed of the motor  20 . 
     In the case of Ir&lt;Itg−Y, that is, when the actual current value Ir of the motor current is within a range L, the maximum air volume controller  44  generates the PWM command signal Sp 2  so as to reduce the rotational speed of the motor  20 . For example, when the motor  20  is operating at an operating point b in  FIG. 5B , it can be determined that the operating point b is within the range L, and the fan  100  is not operating along the maximum air volume control line C (the pressure resistance against the fan  100  is small). In this case, as shown in  FIG. 5B , the maximum air volume controller  44  generates a PWM command signal Sp 2  for reducing the rotational speed of the motor  20  so as to shift the operating point of the fan  100  from “b” to “bx” on the characteristic  500 . For example, when the duty ratio indicated by the PWM command signal Sp 2  output immediately before is 50% and the unit adjustment width of the duty ratio is 0.1%, the maximum air volume controller  44  outputs a PWM command signal Sp 2  indicating the duty ratio of “(50−0.1)%” to reduce the rotational speed of the motor  20 . 
     Note that as shown in  FIG. 5A , the maximum air volume controller  44  may perform control so as to make the rotational speed of the motor  20  unchanged in the range Mh of Itg+X&lt;Ir&lt;Itg+Z and the range MI of Itg−Y&lt;Ir&lt;Itg−X as in the case of the range Mm of Itg−X&lt;Ir&lt;Itg+X. As a result, in the maximum air volume control, the range of the operating point of the motor  20  for making the rotational speed unchanged can be expanded from the range Mm to the range M(=Mh+Mm+MI). Note that |Y|=|Z| or |Y|≠|Z| may be set. 
     The drive control signal generator  45  is a functional unit for generating a drive control signal Sd for controlling the driving of the motor  20 . The drive control signal generator  45  generates the drive control signal Sd based on the PWM command signal Sp 1  output from the speed controller  43  and the PWM command signal Sp 2  output from the maximum air volume controller  44 . 
     Specifically, when it is determined by the comparator  48  that the target rotational speed Rtg is smaller than the rotational speed threshold value Rth, the drive control signal generator  45  generates a PWM signal having a duty ratio indicated by the PWM command signal Sp 1  output from the speed controller  43 , and outputs the generated PWM signal as the drive control signal Sd. On the other hand, when it is determined by the comparator  48  that the target rotational speed Rtg is larger than the rotational speed threshold value Rth, the drive control signal generator  45  generates a PWM signal having a duty ratio indicated by the PWM command signal Sp 2  output from the maximum air volume controller  44 , and outputs the generated PWM signal as the drive control signal Sd. 
     For example, when the PWM command signal Sp 2  is not output from the maximum air volume controller  44 , the drive control signal generator  45  generates a PWM signal having a duty ratio indicated by the PWM command signal Sp 1  output from the speed controller  43 , and outputs the generated PWM signal as the drive control signal Sd. When the PWM command signal Sp 2  is output from the maximum air volume controller  44 , the drive control signal generator  45  generates a PWM signal having a duty ratio which is indicated, not by the PWM command signal Sp 1 , but by the PWM command signal Sp 2  output from the maximum air volume controller  44 , and outputs the generated PWM signal as the drive control signal Sd. 
     The speed controller  43 , the maximum air volume controller  44 , the drive control signal generator  45 , the target current value calculator  47 , and the comparator  48  described above are implemented by, for example, program processing of a microcontroller (CPU). Note that the drive control signal generator  45  may be implemented by a dedicated logic circuit. 
     Next, a flow of the air volume control method of the fan  100  will be described. 
       FIG. 6  is a flowchart showing the flow of air volume control of the fan  100  by the motor drive control device  1  according to the first embodiment. 
     First, when a speed command signal Sc is input from the host device to the control circuit  4 , the target rotational speed acquisition unit  41  of the control circuit  4  acquires information on the target rotational speed Rtg from the speed command signal Sc (step S 1 ). 
     Next, the control circuit  4  determines via the comparator  48  whether the target rotational speed Rtg acquired in step S 1  is larger than the rotational speed threshold value Rth (step S 2 ). When the target rotational speed Rtg is smaller than the rotational speed threshold value Rth (step S 2 : No), the control circuit  4  performs the speed feedback control (step S 3 ). In other words, as described above, the drive control signal generator  45  generates the drive control signal Sd based on the PWM command signal Sp 1  generated by the speed controller  43 , whereby the motor  20  operates so that the actual rotational speed Rr of the motor  20  coincides with the target rotational speed Rtg. Note that at this time, the maximum air volume controller  44  does not generate the PWM command signal Sp 2 . 
     On the other hand, when the target rotational speed Rtg is larger than the rotational speed threshold value Rth in step S 2  (step S 2 : Yes), the control circuit  4  starts the maximum air volume control (step S 4 ). 
     In the maximum air volume control, the control circuit  4  first acquires information on the actual rotational speed Rr of the motor  20  (step S 5 ). In other words, as described above, the target current value calculator  47  acquires information on the actual rotational speed Rr of the motor  20  calculated by the rotational speed calculator  42 . 
     Next, the target current value calculator  47  calculates the target current value Itg (step S 6 ). Specifically, the target current value calculator  47  calculates the target current value Itg by the above-described method based on the information on the actual rotational speed Rr acquired in step S 5  and the correspondence relationship information  471  stored in the memory unit  470 . 
     Next, the maximum air volume controller  44  acquires the actual current value Ir of the motor  20  (step S 7 ). Specifically, as described above, the maximum air volume controller  44  acquires information on the actual current value Ir of the motor current calculated by the current value acquisition unit  46 . 
     Next, the maximum air volume controller  44  determines whether Itg−X&lt;Ir&lt;Itg+X is satisfied, that is, whether the actual current value Ir acquired in step S 7  is within the range M (step S 8 ). 
     In step S 8 , when the actual current value Ir is within the range M (step S 8 : Yes), the maximum air volume controller  44  makes the rotational speed of the motor  20  unchanged (step S 13 ). For example, the maximum air volume controller  44  outputs a PWM command signal Sp 2  indicating the same duty ratio as a PWM command signal Sp 2  output immediately before. 
     On the other hand, when the actual current value Ir is not within the range M in step S 8  (step S 8 : No), the maximum air volume controller  44  determines whether Ir&lt;Itg-Y is satisfied, that is, whether the actual current value Ir is within the range L (step S 9 ). 
     In step S 9 , when the actual current value Ir is within the range L (step S 9 : Yes), the maximum air volume controller  44  reduces the rotational speed of the motor  20  (step S 11 ). For example, the maximum air volume controller  44  outputs a PWM command signal Sp 2  indicating a duty ratio which is increased by only a predetermined width (for example, 0.1%) from the duty ratio indicated by a PWM command signal Sp 2  output immediately before. 
     On the other hand, when the actual current value Ir is not within the range L in step S 9  (step S 9 : No), the maximum air volume controller  44  determines whether Itg+Z&lt;Ir is satisfied, that is, whether the actual current value Ir is within the range H (step S 10 ). 
     In step S 10 , when the actual current value Ir is within the range H (step S 10 : Yes), the maximum air volume controller  44  increases the rotational speed of the motor  20  (step S 12 ). For example, the maximum air volume controller  44  outputs a PWM command signal Sp 2  indicating a duty ratio which is reduced by only a predetermined width (for example, 0.1%) from a duty ratio indicated by a PWM command signal Sp 2  output immediately before. 
     On the other hand, when the actual current value Ir is not within the range H in step S 10  (step S 10 : No), the maximum air volume controller  44  determines that Itg+X&lt;Ir&lt;Itg+Z or Itg−X&lt;Ir&lt;Itg−Y is satisfied, that is, Itg is within the range Mh or MI, and makes the rotational speed of the motor  20  unchanged (step S 13 ). 
     After steps S 11  to S 13 , the control circuit  4  generates the drive control signal Sd based on the PWM command signal Sp 2  (step S 14 ). Specifically, the drive control signal generator  45  generates a PWM signal having a duty ratio indicated by a PWM command signal Sp 2  output from the maximum air volume controller  44  in steps S 11  to S 13 , and outputs the generated PWM signal as the drive control signal Sd. 
     After steps S 3  and S 14 , the control circuit  4  determines whether an instruction to stop the motor  20  is given (step S 15 ). In step S 15 , when no instruction to stop the motor  20  is given (step S 15 : No), the above-described processing (S 1  to S 15 ) is repeatedly executed. On the other hand, when an instruction to stop the motor  20  is received in step S 15  (step S 15 : Yes), the control circuit  4  finishes the air volume control processing. 
       FIG. 7  is a view showing the relationship between the rotational speed of the motor and the target rotational speed in the fan according to the first embodiment. 
     In  FIG. 7 , the horizontal axis represents the target rotational speed Rtg, and the vertical axis represents the actual rotational speed Rr of the motor  20 . 
     As shown in  FIG. 7 , the conventional fan controls the motor so that the rotational speed (actual rotational speed Rr) coincides with the target rotational speed Rtg as indicated by reference sign  600 . In other words, since the conventional fan controls the motor so that the air volume increases in proportion to the target rotational speed Rtg, the air volume increases in proportion to the target rotational speed Rtg even in a region where the static pressure (pressure resistance) is low, for example, like a P-Q curve indicated by reference sign  201 _max in  FIG. 2 . 
     On the other hand, the fan  100  according to the first embodiment controls the motor  20  so that the rotational speed of the fan (motor) coincides with the target rotational speed Rtg in a range where the target rotational speed Rtg is lower than the rotational speed threshold value Rth as in the case of the conventional fan, but controls the motor  20  so that the rotational speed does not exceed a set maximum rotational speed, that is, the rotational speed threshold value Rth regardless of the target rotational speed Rtg as indicated by reference sign  601  in a range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth. 
     In other words, according to the fan  100  of the first embodiment, in the range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor drive control device  1  calculates the target current value Itg corresponding to the actual rotational speed of the motor  20  from the relationship between the rotational speed and the motor current on the maximum air volume control line C shown in  FIG. 2 , and controls the rotational speed of the motor  20  so that the motor current approaches the target current value Itg as described above. As a result, the fan  100  can limit the maximum air volume in the region where the static pressure is low (a range where the static pressure in the P-Q curves in  FIG. 2  is lower than Pb). 
     As described above, when the target rotational speed Rtg indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth, the motor drive control device  1  according to the first embodiment performs the speed feedback control for generating the drive control signal Sd so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg, but when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor drive control device  1  generates the drive control signal Sd based on the relationship between the motor current and the reference current value. Specifically, when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor drive control device  1  performs the maximum air volume control for generating the drive control signal Sd so that the actual current value Ir of the motor  20  approaches the target current value Itg calculated in association with the rotational speed of the motor  20 . 
     According to this control method, as described above, in the range where the target rotational speed Rtg is lower than the rotational speed threshold value Rth, the motor  20  is driven such that the air volume of the fan  100  increases in proportion to the indicated target rotational speed Rtg, whereas in the range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor  20  is driven such that the air volume of the fan  100  changes according to the static pressure. 
     Note that the maximum air volume control by the motor drive control device  1  performs such control that when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the maximum air volume is equal to a desired value according to the relationship between the rotational speed of the motor  20  and the pressure loss, and it does not necessarily perform such control as to keep the air volume constant. Therefore, this maximum air volume control differs from conventional so-called constant air volume control in the control method and its effect. 
     Furthermore, as described above, in the range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor drive control device  1  calculates the target current value Itg corresponding to the actual rotational speed Rr of the motor  20  calculated based on the position detection signal (Hall signal) Sh of the motor  20  by using the pre-stored correspondence relationship information  471  between the rotational speed of the motor  20  and the target current value Itg, and controls the rotational speed of the motor  20  so that the actual current value Ir of the motor  20  approaches the target current value Itg. 
     Here, by setting the correspondence relationship information  471  as information indicating the relationship between the rotational speed of the motor  20  and the target current value Itg in a region where the static pressure of the fan  100  is lower than a predetermined value, the rotational speed of the motor  20  is controlled so that the relationship between the rotational speed of the motor  20  and the target current value Itg specified in the correspondence relationship information  471  is satisfied regardless of the indicated target rotational speed Rtg in the region where the static pressure of the fan  100  is lower than the predetermined value. For example, by setting the correspondence relationship information  471  so as to satisfy the characteristics on the maximum air volume control line C in the P-Q curves shown in  FIG. 2 , the air volume can be controlled so as to satisfy the maximum air volume control line C regardless of the indicated target rotational speed Rtg in the region where the static pressure of the fan  100  is lower than Pb. 
     In other words, according to the motor drive control device  1  of the first embodiment, in a region where the static pressure is higher than the predetermined value, a sufficient air volume can be secured in a required operating range, and in a region where the static pressure is lower than the predetermined value, the air volume is suppressed regardless of the indicated target rotational speed Rtg, so that occurrence of noise and increase in power consumption of the fan  100  can be suppressed. 
     Under the maximum air volume control, the motor drive control device  1  generates the drive control signal Sd so that the rotational speed of the motor  20  increases when the actual current value Ir of the motor  20  is higher than the target current value Itg, and generates the drive control signal Sd so that the rotational speed of the motor  20  decreases when the actual current value Ir of the motor  20  is lower than the target current value Itg. 
     According to this control method, when the indicated target rotational speed Rtg is higher than the rotational speed threshold value Rth, the operating points of the motor  20  can be easily controlled so that the actual current value Ir and the actual rotational speed Rr of the motor  20  satisfy the characteristic  500  as shown in  FIG. 5B . 
     Furthermore, under the maximum air volume control, the motor drive control device  1  generates the drive control signal Sd so as to increase the rotational speed of the motor  20  when the actual current value Ir of the motor  20  is higher than the range Mm containing the target current value Itg as a reference, makes the rotational speed of the motor  20  unchanged when the actual current value Ir of the motor  20  is within the range Mm, and reduce the rotational speed of the motor  20  when the actual current value Ir of the motor  20  is lower than the range Mm. 
     According to this control method, it is possible to prevent the air volume of the fan  100  from becoming unstable due to excessive change of the rotational speed in the situation where the operating point of the motor  20  approaches the maximum air volume control line C. 
     Particularly, as shown in  FIG. 5A  and  FIG. 5B , in the maximum air flow control, the range of the operating point in which the rotational speed of the motor  20  is made unchanged is expanded from the range Mm to the range M (=Mh+Mm+MI), whereby it is possible to more effectively prevent the excessive change of the rotational speed, and thus further stabilize the air volume of the fan  100 . 
     Second Embodiment 
       FIG. 8  is a block diagram showing a configuration of a fan according to a second embodiment of the disclosure. 
     Like the fan  100  according to the first embodiment, a fan  100 A according to the second embodiment includes a motor  20 , a motor drive control device  1 A for controlling driving of the motor  20 , and an impeller  21  which is configured to be rotatable by rotational force of the motor  20 . 
     The motor drive control device  1 A according to the second embodiment performs air volume control of the fan  100 A by a method different from that of the motor drive control device  1  according to the first embodiment. 
     Specifically, the motor drive control device  1 A includes a motor driving unit  2 , a control circuit  4 A, and a current detector  6 . The components of the motor drive control device  1 A shown in  FIG. 8  are a part of the whole of the motor drive control device  1 A, and the motor drive control device  1 A may include other components in addition to those shown in  FIG. 8 . 
     In the second embodiment, at least a part of the motor drive control device  1 A is packaged as one semiconductor device (IC: Integrated Circuit). For example, circuits such as the control circuit  4 A and the motor driving unit  2  are implemented as separate semiconductor devices, respectively. 
     Note that the motor drive control device  1 A may be a semiconductor device packaged in its entirety, or the whole or a part of the motor drive control device  1 A and other devices may be packaged together to configure a single semiconductor device. 
     The control circuit  4 A is configured by a microcomputer, a digital circuit, an analog circuit, and the like, for example, like the control circuit  4  according to the first embodiment. Various signals for instructing driving of the motor  20  are input to the control circuit  4 A. The control circuit  4 A performs drive control of the motor  20  based on these signals. For example, a speed command signal Sc is input as a signal for instructing driving of the motor  20  to the control circuit  4 A from a device provided outside the control circuit  4 A such as a host device. 
     The control circuit  4 A obtains information on the actual rotational speed of the rotor of the motor  20  from a position detection signal Sh, and controls the driving of the motor  20 . 
     Note that  FIG. 8  illustrates a case where one Hall element  5  is arranged in the fan  100 A, but the number of Hall elements  5  to be arranged in the fan  100 A is not particularly limited. For example, three Hall elements  5  may be arranged around the rotor of the motor  20  at substantially equal intervals. 
     The control circuit  4 A may be configured so that other information related to the rotation state of the motor  20  is input to the control circuit  4 A in addition to the position detection signal Sh or in place of the position detection signal Sh like the control circuit  4  according to the first embodiment. 
     The control circuit  4 A generates a drive control signal Sd (for example, a PWM signal) for controlling the rotational speed of the motor  20  based on the speed command signal Sc, the position detection signal Sh, the voltage Vs described above, and the like. 
     Like the control circuit  4  according to the first embodiment, the control circuit  4 A supplies the motor driving unit  2  with a drive control signal Sd which is a PWM (pulse width modulation) signal to adjust the rotational speed of the motor  20  while switching, in a predetermined order, an energized phase among the coils Lu, Lv, Lw of plural phases to be energized by the motor driving unit  2 , thereby controlling the air volume of the fan  100 A. 
     The motor drive control device  1 A according to the second embodiment controls the rotational speed of the motor  20  so that the air volume of the fan  100 A is limited in a region where the static pressure of the fan  100 A is not more than a predetermined value. 
       FIG. 9  is a view showing the air volume control of the fan  100 A by the motor drive control device  1 A according to the second embodiment. 
     In  FIG. 9 , the horizontal axis represents the air volume Q, and the vertical axis represents the static pressure P.  FIG. 9  shows a P-Q curve for each target rotational speed Rtg of the fan when the target rotational speed Rtg indicated by the speed command signal Sc is changed. 
     In  FIG. 9 , reference sign  201 _min represents a P-Q curve of the fan when the target rotational speed Rtg is set to the minimum value Rmin, and reference sign  201 _max represents a P-Q curve of the fan when the target rotational speed Rtg is set to the maximum value Rmax, and reference sign  201 _ x  represents a P-Q curve when the target rotational speed Rtg is set to a value Rx between the maximum value Rmax and the minimum value Rmin. 
     As described above, the fan is designed so that a desired air volume can be obtained in a required operating range  200 , and quietness is more important than air volume in a region other than the required operating range  200 , that is, in a region where the static pressure is not more than a predetermined value. For example, in  FIG. 9 , quietness is more important than air volume in a region where the static pressure is not more than Pb. 
     However, since the conventional fan controls the rotational speed of the motor so that the rotational speed has reached the target rotational speed indicated by the speed command signal Sc, the air volume trends increase to be more than necessary and noise trends to intensify even in a region where the static pressure is low like a free air state. 
     For example, a case where the conventional fan is designed to operate on the P-Q curve  201 _max as shown in  FIG. 9  is considered. In this case, when the target rotational speed Rtg is set to “Rtg_max” in the free air state, the conventional fan generates an air volume qmax which greatly exceeds an air volume range from qr 1  to qr 2  which is required in the operating range  200 . However, as described above, in a region other than the operating range  200 , an air volume that greatly exceeds the required air volume range from qr 1  to qr 2  is unnecessary. 
     Therefore, the motor drive control device  1 A according to the second embodiment controls the rotational speed of the motor  20  so that the air volume of the fan  100 A is limited regardless of the target rotational speed Rtg indicated by the speed command signal Sc in the region where the static pressure is lower than the predetermined value. 
     Specifically, as shown in  FIG. 9 , in the range where the static pressure is lower than the predetermined value Pb, the motor drive control device  1 A limits the actual rotational speed Rr of the motor  20  so that the actual rotational speed Rr of the motor  20  does not exceed the rotational speed threshold value Rth (Rmin&lt;Rth&lt;Rmax). For example, when the rotational speed threshold value Rth=Rx is set, the motor drive control device  1 A controls the motor  20  so that the motor  20  rotates at a constant rotational speed Rx(Rth) in a range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth and the static pressure is lower than the predetermined value Pb. Accordingly, the fan  100 A operates to draw, not the P-Q curve  201 _max, but the P-Q curve  201 _ th . As a result, in the region where the target rotational speed Rtg is higher than the rotational speed threshold value Rth and the static pressure is lower than a predetermined value, the air volume of the fan  100 A is limited regardless of the target rotational speed Rtg indicated by the speed command signal Sc. 
       FIG. 10  is a view showing an outline of an air volume control method by the motor drive control device  1 A according to the second embodiment. 
     In  FIG. 10 , the horizontal axis represents external pressure (pressure resistance, static pressure) applied to the fan, and the vertical axis represents a motor current. A characteristic (graph)  300 A shown in  FIG. 10  represents the relationship between the motor current and the external pressure when the target rotational speed Rtg of the fan is set to a predetermined value (for example, a maximum value Rmax). 
     As shown in  FIG. 10 , when the external pressure increases while the fan is rotating at a constant rotational speed, the motor current also increases. In other words, the motor current and the external pressure are in a substantially proportional relationship in the fan. Accordingly, it is possible to estimate the external pressure applied to the fan by monitoring the motor current. 
     Therefore, in the fan  100 A according to the second embodiment, a current threshold value Ith is preset as a current reference value for determining whether the external pressure is in a high state or in a low state in order to limit the air volume under a state where the external pressure (static pressure) is lower than a predetermined value. By comparing the current threshold Ith with the motor current of the motor  20 , the fan  100 A estimate whether the external pressure is in a high state or not, and switches the motor control method. 
     In the second embodiment, the reference current value is a current threshold value Ith for estimating the external pressure. 
     Specifically, when the target rotational speed Rtg indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth, the motor drive control device  1 A performs the speed feedback control for generating the drive control signal Sd so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg. On the other hand, when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor drive control device  1 A generates the drive control signal Sd based on the relationship between the current (motor current) flowing through the motor  20  and the reference current value (current threshold Ith). 
     More specifically, when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, based on a comparison result between the actual current value Ir of the motor current and the current threshold value Ith, the motor drive control device  1 A performs the switching between the speed feedback control and the maximum air volume control for generating the drive control signal Sd so that the rotational speed of the motor  20  does not exceed the rotational speed threshold value Rth as shown in  FIG. 10 . 
     Hereinafter, a configuration of the motor drive control device  1 A for implementing the function for limiting the maximum air volume of the fan  100 A will be described in detail. 
       FIG. 11  is a block diagram showing the configuration of the motor drive control device  1 A according to the second embodiment. 
       FIG. 11  shows functional blocks related to the function for limiting the maximum air volume of the fan  100 A described above among the functional blocks constituting the motor drive control device  1 A. 
     When the target rotational speed Rtg of the motor  20  indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth (for example, in the case of Rtg&lt;Rth), the control circuit  4 A in the motor drive control device  1 A performs the speed feedback control. 
     As described above, the speed feedback control is the control for generating the drive control signal Sd so that the rotational speed (actual rotational speed Rr) of the motor  20  coincides with the target rotational speed Rtg. 
     Furthermore, when the target rotational speed Rtg is higher than or equal to the rotational speed threshold value Rth (for example, in the case of Rtg&gt;or=Rth), the control circuit  4 A switches the maximum air volume control and the speed feedback control with each other based on the comparison result of the motor current (actual current value Ir) of the motor  20  and the current threshold value Ith (reference current value). 
     Here, the maximum air volume control in the second embodiment is the control for generating the drive control signal Sd so that the rotational speed of the motor  20  does not exceed the rotational speed threshold value Rth. 
     As shown in  FIG. 11 , the control circuit  4 A includes a target rotational speed acquisition unit  41 A, a rotational speed calculator  42 A, a speed controller  43 A, a maximum air volume controller  44 A, a drive control signal generator  45 A, a current value acquisition unit  46 A, a memory unit  47 A, a comparator (RCMP)  48 A, and a comparator (ICMP)  49 A as functional blocks for implementing the speed feedback control and the maximum air volume control. 
     The target rotational speed acquisition unit  41 A acquires information on the target rotational speed Rtg of the motor  20  from the speed command signal Sc which is output from, for example, a host device existing outside the motor drive control device  1 A, and provides the acquired information to the speed controller  43 A and the maximum air volume controller  44 A. 
     For example, when the speed command signal Sc is a PWM signal representing the target rotational speed Rtg by the duty ratio, the target rotational speed acquisition unit  41 A analyzes the duty ratio of the PWM signal as the input speed command signal Sc to calculate the rotational speed corresponding to the duty ratio, and outputs the calculated rotational speed as the target rotational speed Rtg. 
     For example, the target rotational speed acquisition unit  41 A has a table representing the correspondence relationship between the duty ratio of the PWM signal and the target rotational speed Rtg. The target rotational speed acquisition unit  41 A reads out the target rotational speed Rtg corresponding to the duty ratio of the input speed command signal Sc from the table to acquire information on the target rotational speed Rtg from the speed command signal Sc. 
     The target rotational speed acquisition unit  41 A is implemented, for example, by an external interface circuit, etc. of a microcontroller and program processing of CPU. 
     The rotational speed calculator  42 A calculates the rotational speed (the number of revolutions per unit time) of the motor  20  based on the position detection signal Sh output from the position detection element  5 . The rotational speed calculator  42 A calculates the actual rotational speed of the rotor of the motor  20  by using the position detection signal Sh, and provides the calculated actual rotational speed as the actual rotational speed Rr to the speed controller  43 A. 
     The rotational speed calculator  42 A is implemented, for example, by an external interface circuit, etc. of a microcontroller and program processing of CPU like the target rotational speed acquisition unit  41 A. 
     The speed controller  43 A generates a PWM command signal (an example of the first control signal) Sp 1  indicating the duty ratio of a PWM signal as the drive control signal Sd based on the target rotational speed Rtg output from the target rotational speed acquisition unit  41 A and the actual rotational speed Rr of the motor  20  calculated by the rotational speed calculator  42 A. 
     Specifically, the speed controller  43 A generates the PWM command signal Sp 1  so that the actual rotational speed Rr coincides with the target rotational speed Rtg. For example, the speed controller  43 A calculates the difference between the actual rotational speed Rr and the target rotational speed Rtg, and calculates the duty ratio of the PWM signal as the drive control signal Sd so that the difference becomes zero. Then, the speed controller  43 A outputs the information on the calculated duty ratio as the PWM command signal Sp 1 . 
     The current value acquisition unit  46 A is a functional unit for calculating an actual current value of current flowing through the motor  20 . The current value acquisition unit  46 A includes, for example, an A/D conversion circuit for converting an analog signal into a digital signal. For example, the current value acquisition unit  46 A is a ΔΣ modulation type analog/digital conversion circuit, and is configured by a dedicated logic circuit. The current value acquisition unit  46 A converts an analog signal input from the current detector  6  into a digital signal by a ΔΣ modulation method. 
     The current value acquisition unit  46 A converts a voltage Vs, which is an analog signal output from the current detector  6 , into a digital signal, and outputs the digital signal as the actual current value Ir of the motor current of the motor  20 . 
     The comparator (RCMP)  48 A compares the target rotational speed Rtg with the rotational speed threshold value Rth, and outputs a comparison result. 
     The rotational speed threshold Rth is a rotational speed for defining the maximum air volume of the fan  100 A as in the case of the first embodiment. For example, when it is desired to control the fan  100 A along the P-Q curve  201 _ th  shown in  FIG. 9 , the rotational speed Rx of the motor  20  at the point A at which the maximum air volume is obtained on the P-Q curve  201 _ th  may be set as the rotational speed threshold value Rth (&lt;Rmax). Information  471 A of the rotational speed threshold value Rth is stored in the memory unit  47 A. 
     The comparator  48 A compares the rotational speed threshold value Rth read from the memory unit  47 A with the target rotational speed Rtg output from the target rotational speed acquisition unit  41 A. When the target rotational speed Rtg output from the target rotational speed acquisition unit  41 A is larger than the rotational speed threshold value Rth, the comparator  48 A outputs, for example, a high-level determination signal Scp 1 . On the other hand, when the target rotational speed Rtg output from the target rotational speed acquisition unit  41 A is smaller than the rotational speed threshold value Rth, the comparator  48 A outputs, for example, a low-level determination signal Scp 1 . 
     The comparator (RCMP)  48 A compares the actual current value Ir of the motor current with the current threshold value Ith, and outputs a comparison result. 
     The current threshold value Ith is another parameter serving as a reference for switching the control mode (the speed feedback control and the maximum air volume control) of the fan  100 A. 
       FIG. 12  is a view showing a method of determining the current threshold value Ith. 
     In  FIG. 12 , the horizontal axis represents the rotational speed of the motor of the fan, and the vertical axis represents the motor current of the fan. Reference sign  500 A represents a characteristic representing the relationship between the rotational speed and the motor current of the motor when the speed feedback control is performed in the free air state of the fan  100 A. 
     The current threshold value Ith is determined based on the motor current when the fan  100 A is rotating at a predetermined rotational speed. For example, as shown in  FIG. 12 , a value obtained by adding an offset amount Iof to a current value Ith 0  of the motor current when the fan  100 A is operating at the rotational speed corresponding to the rotational speed threshold value Rth in the free air state may be set as the current threshold value Ith. 
     Here, the offset amount Iof may be appropriately set according to the size (weight and shape) of the impeller  21  of the fan  100 A, the number of turns of the coil (winding), the diameter of the coil conductor, and the like. 
     Information  472 A on the current threshold value Ith is stored in the memory unit  47 A. 
     The comparator (ICMP)  49 A compares the current threshold value Ith read out from the memory unit  47 A with the actual current value Ir of the motor current output from the current value acquisition unit  46 A. When the actual current value Ir is larger than or equal to the current threshold value Ith (for example, in the case of Ir&gt;or=Ith), the comparator  49 A outputs, for example, a high-level determination signal Scp 2 . On the other hand, when the actual current value Ir is smaller than the current threshold value Ith (for example, in the case of Ir&lt;Ith), the comparator  49 A outputs, for example, a low-level determination signal Scp 2 . 
     The memory unit  47 A is a functional unit for storing various parameters, calculation results, etc. for the speed feedback control and the maximum air volume control. For example, the memory unit  47 A stores the above-described information  471 A on the rotational speed threshold value Rth, information  472 A on the current threshold value Ith, and the like. The memory unit  47 A is implemented by a storage device such as RAM or ROM, a register, or the like. 
     The maximum air volume controller  44 A generates a PWM command signal Sp 2  (an example of a second control signal) indicating the duty ratio of a PWM signal as the drive control signal Sd based on the determination signal Scp 1  of the comparator  48 A, the determination signal Scp 2  of the comparator  49 A, and the rotational speed threshold value Rth. 
     Specifically, when the comparator  48 A determines that the target rotational speed Rtg is higher than a predetermined value and the comparator  49 A determines that the actual current value Ir is higher than the current threshold value Ith (Rtg&gt;Rth and Ir&gt;Ith), the maximum air volume controller  44 A generates the PWM command signal Sp 2  so that the motor  20  rotates at a constant rotational speed lower than the target rotational speed Rtg. 
     On the other hand, when the comparator  48 A determines that the target rotational speed Rtg is lower than the rotational speed threshold value Rth, or when the comparator  49 A determines that the actual current value Ir is lower than the current threshold value Ith (Rtg&lt;Rth or Ir&lt;Ith), the maximum air volume controller  44 A does not generate the PWM command signal Sp 2 . 
     As described above, in the case of Rtg&gt;Rth and Ir&gt;Ith, the maximum air volume controller  44 A generates the PWM command signal Sp 2  so that the motor  20  rotates at a constant rotational speed lower than the target rotational speed Rtg. Specifically, the maximum air volume controller  44 A generates the PWM command signal Sp 2  so that the motor  20  rotates at a constant rotational speed corresponding to the rotational speed threshold value Rth. 
     For example, the maximum air volume controller  44 A calculates the difference between the actual rotational speed Rr and the rotational speed threshold value Rth, and calculates the duty ratio of the PWM signal as the drive control signal Sd so that the difference becomes zero. Then, the maximum air volume controller  44 A outputs information on the calculated duty ratio as the PWM command signal Sp 2 . 
     The drive control signal generator  45 A is a functional unit for generating the drive control signal Sd for controlling the driving of the motor  20 . The drive control signal generator  45 A generates the drive control signal Sd based on the PWM command signal Sp 1  output from the speed controller  43 A and the PWM command signal Sp 2  output from the maximum air volume controller  44 A. 
     Specifically, when the PWM command signal Sp 2  is not output from the maximum air volume controller  44 A, the drive control signal generator  45 A generates a PWM signal having a duty ratio indicated by the PWM command signal Sp 1  output from the speed controller  43 A, and outputs the generated PWM signal as the drive control signal Sd. When the PWM command signal Sp 2  is output from the maximum air volume controller  44 A, the drive control signal generator  45 A generates a PWM signal having a duty ratio which is indicated, not by the PWM command signal Sp 1 , but by the PWM command signal Sp 2  output from the maximum air volume controller  44 A, and outputs the generated PWM signal as the drive control signal Sd. 
     The speed controller  43 A, the maximum air volume controller  44 A, the drive control signal generator  45 A, and the comparators  48 A and  49 A described above are implemented, for example, by program processing of a microcontroller (CPU). Note that the comparators  48 A and  49 A may be implemented by a dedicated logic circuit or the like. 
     Next, the flow of the air volume control of the fan  100 A will be described. 
       FIG. 13  is a flowchart showing the flow of processing related to air volume control by the motor drive control device  1 A according to the second embodiment. 
     First, when the speed command signal Sc is input from a host device to the control circuit  4 A, the target rotational speed acquisition unit  41 A of the control circuit  4 A acquires information on the target rotational speed Rtg from the speed command signal Sc (step S 1 A). 
     Next, the comparator  48 A of the control circuit  4 A determines whether the target rotational speed Rtg acquired in step S 1 A is larger than the rotational speed threshold value Rth (step S 2 A). 
     When the target rotational speed Rtg is smaller than the rotational speed threshold value Rth (step S 2 A: No), the control circuit  4 A performs the speed feedback control (step S 3 A). In other words, as described above, the drive control signal generator  45 A generates the drive control signal Sd based on the PWM command signal Sp 1  generated by the speed controller  43 A. As a result, the motor  20  operates so that the actual rotational speed Rr coincides with the target rotational speed Rtg. At this time, the maximum air volume controller  44 A does not generate the PWM command signal Sp 2 . 
     On the other hand, when the target rotational speed Rtg is larger than the rotational speed threshold value Rth in step S 2  (step S 2 A: Yes), the control circuit  4 A acquires the actual current value Ir of the motor  20  (step S 4 A). Specifically, as described above, the comparator  49 A acquires information on the actual current value Ir of the motor current calculated by the current value acquisition unit  46 A. 
     Next, the comparator  49 A determines whether the actual current value Ir of the motor  20  acquired in step S 4 A is larger than the current threshold value Ith (step S 5 A). 
     When the actual current value Ir is larger than the current threshold value Ith (step S 5 A: Yes), the control circuit  4 A performs the speed feedback control (step S 3 A). In other words, as described above, the drive control signal generator  45 A generates the drive control signal Sd based on the PWM command signal Sp 1  generated by the speed controller  43 A. As a result, the motor  20  operates so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg. 
     On the other hand, when the actual current value Ir is smaller than the current threshold value Ith (step S 5 A: No), the control circuit  4 A starts the maximum air volume control (step S 6 A). 
     In the maximum air volume control, the maximum air volume controller  44 A first reads the information  471 A of the rotational speed threshold value Rth from the memory unit  47 A (step S 7 A). 
     Next, the maximum air volume controller  44 A generates the PWM command signal Sp 2  so that the actual rotational speed Rr of the motor  20  coincides with the rotational speed threshold value Rth (step S 8 A). 
     Next, the control circuit  4 A generates the drive control signal Sd based on the PWM command signal Sp 2  (step S 9 A). Specifically, the drive control signal generator  45 A generates a PWM signal having a duty ratio indicated by the PWM command signal Sp 2  output from the maximum air volume controller  44 A in step S 9 , and outputs the generated PWM signal as the drive control signal Sd. 
     After steps S 3  and S 9 , the control circuit  4 A determines the presence or absence of an instruction for stopping the motor  20  (step S 10 A). When no instruction for stopping the motor  20  is given in step S 10 A (step S 10 A: No), the above-described processing (S 1 A to S 10 A) is repeatedly executed. On the other hand, when an instruction for stopping the motor  20  is received in step S 10 A (step S 10 A: Yes), the control circuit  4 A terminates the processing of the air volume control. 
       FIG. 14  is a view showing the relationship between the target rotational speed and the actual rotational speed of the motor  20  in the fan  100 A according to the second embodiment. 
     In  FIG. 14 , the horizontal axis represents the target rotational speed Rtg of the motor  20 , and the vertical axis represents the actual rotational speed Rr of the motor  20 .  FIG. 14  shows the principle of switching the control mode by the motor drive control device  1 A which focuses on the relationship between the target rotational speed Rtg and the actual rotational speed Rr. 
     As shown in  FIG. 14 , in the range where the target rotational speed Rtg indicated by the speed command signal Sc is smaller than the rotational speed threshold value Rth, the control circuit  4 A performs the speed control feedback control. As a result, the fan  100 A operates so that the actual rotational speed Rr of the motor  20  coincides with the target rotational speed Rtg. 
     On the other hand, in the range where the target rotational speed Rtg is larger than the rotational speed threshold value Rth, the control circuit  4 A performs the switching between the speed feedback control and the maximum air volume control according to the comparison result between the actual current value Ir of the motor  20  and the current threshold value Ith. 
     In other words, in the range where the target rotational speed Rtg is larger than the rotational speed threshold value Rth and the actual current value Ir of the motor  20  is larger than the current threshold value Ith, it can be estimated that the external pressure (pressure resistance) to the fan  100 A has been large. In this case, the control circuit  4 A performs the speed feedback control in order to ensure a necessary air volume. As a result, as indicated by reference sign  700 A, the fan  100 A operates so that the actual rotational speed Rr of the motor  20  coincides with the target rotational speed Rtg, thereby ensuring a necessary air volume. 
     On the other hand, in the range where the target rotational speed Rtg is larger than the rotational speed threshold value Rth and the actual current value Ir of the motor  20  is smaller than the current threshold value Ith, it can be estimated that the external pressure to the fan  100 A has been small and a sufficient air volume can have been secured. In this case, the control circuit  4 A performs the maximum air volume control while giving priority to the air volume quietness. In other words, as indicated by reference sign  701 A, the fan  100 A (motor  20 ) rotates at a constant rotational speed (with the rotational speed threshold value Rth) lower than the indicated target rotational speed Rtg. As a result, it becomes possible to suppress the air volume. 
       FIG. 15  is a view showing the relationship between the motor current and the rotational speed of the motor  20  in the fan  100 A according to the second embodiment. 
     In  FIG. 15 , the horizontal axis represents the rotational speed of the motor  20 , and the vertical axis represents the motor current of the motor  20 .  FIG. 15  shows a flow of switching the control mode by the motor drive control device  1 A which focuses on the relationship between the rotational speed (actual rotational speed Rr) and the motor current of the motor  20 . 
     As shown in  FIG. 15 , the fan  100 A according to the second embodiment performs normal speed feedback control in a range where the target rotational speed Rtg indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth. As a result, as indicated by reference sign  800 A, the fan  100 A operates so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg. 
     Thereafter, when the target rotational speed Rtg indicated by the speed command signal Sc exceeds the rotational speed threshold value Rth, the fan  100 A performs the maximum air volume control. As a result, as indicated by reference sign  801 A, the fan  100 A operates so that the rotational speed becomes constant (rotational speed threshold value Rth) regardless of the target rotational speed Rtg until the motor current exceeds the current threshold value Ith. 
     Thereafter, when the external pressure of the fan  100 A increases and the actual current value Ir of the motor current exceeds the current threshold value Ith, the fan  100 A performs the speed feedback control. As a result, as indicated by reference sign  802 A, the fan  100 A operates so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg. 
     When the target rotational speed Rtg lower than the rotational speed threshold value Rth is indicated by the speed command signal Sc, the fan  100 A returns to the normal speed feedback control again as indicated by reference sign  803 A. 
     As described above, in a state where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the fan  100 A according to the second embodiment operates so that the rotational speed of the motor  20  is constant regardless of the target rotational speed Rtg when the motor current is lower than the current threshold value Ith, and operates so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg when the motor current is higher than the current threshold value Ith. As a result, the fan  100 A can limit the maximum air volume in a region where the static pressure is low (for example, a range where the static pressure on the P-Q curves in  FIG. 9  is lower than Pb). 
     As described above, when the target rotational speed Rtg indicated by the speed command signal Sc is lower than the rotational speed threshold value Rth, the motor drive control device  1 A according to the second embodiment performs the speed feedback control for generating the drive control signal Sd so that the rotational speed (actual rotational speed Rr) of the motor  20  coincides with the target rotational speed Rtg, and when the target rotational speed Rtg is higher than the rotational speed threshold value Rth, based on the comparison result between the motor current of the motor  20  and the current threshold value Ith, the motor drive control device  1 A according to the second embodiment performs the switching between the speed feedback control and the maximum air volume control for generating the drive control signal Sd so that the rotational speed of the motor  20  does not exceed the rotational speed threshold value Rth. 
     According to this control method, as described above, in the range where the target rotational speed Rtg is lower than the rotational speed threshold value Rth, the motor  20  is driven so that the air volume of the fan  100 A increases in proportion to the indicated target rotational speed Rtg. On the other hand, in the range where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the motor  20  is driven so that the air volume of the fan  100 A changes according to the static pressure. 
     As described above, in the fan, the motor current and the external pressure are in a substantially proportional relationship. Therefore, in the fan  100 A according to the second embodiment, the current threshold value Ith is set as a reference value for determining whether the external pressure is in a high state or in a low state, and the current threshold value Ith is compared with the motor current of motor  20 , whereby it is estimated whether the external pressure is in the high state or not, and the motor control method is switched. 
     Specifically, in the state where the target rotational speed Rtg is higher than the rotational speed threshold value Rth, the fan  100 A performs the speed feedback control when the current flowing through the motor  20  is larger than the current threshold value Ith, and performs the maximum air volume control when the current flowing through the motor  20  is smaller than the current threshold value Ith. 
     As a result, when the external pressure of the fan  100 A is lower than a predetermined pressure (static pressure) corresponding to the current threshold value Ith, the fan  100 A suppresses the rotational speed of the motor  20  to be lower than the target rotational speed Rtg by the maximum air volume control, and operates so as to suppress the air volume. On the other hand, when the external pressure of the fan  100 A is higher than the predetermined pressure (static pressure) corresponding to the current threshold value Ith, the fan  100 A controls the rotational speed of the motor  20  so that the rotational speed of the motor  20  coincides with the target rotational speed Rtg by the speed feedback control, and operates so as to obtain a required air volume. 
     As described above, according to the fan  100 A to which the motor drive control device  1 A according to the second embodiment is applied, a sufficient air volume can be secured in a required operating range in a region where the static pressure is higher than a predetermined value, and the air volume can be suppressed regardless of an indicated target rotational speed Rtg in a region where the static pressure is lower than the predetermined value, whereby occurrence of noise and increase of power consumption of the fan  100 A can be suppressed. 
     Further, the motor drive control device  1 A according to the embodiment generates the drive control signal Sd in the maximum air volume control so that the motor  20  rotates at a constant rotational speed corresponding to the rotational speed threshold value Rth. 
     According to this control method, the motor  20  can be stably rotated at a constant rotational speed under the maximum air volume control, so that it is possible to further improve the stability and quietness of the operation of the fan  100 A under the maximum air volume control. 
     Note that the maximum air volume control by the motor drive control device  1 A according to the second embodiment is performed to control the air volume so that the maximum air volume is equal to a desired value according to the relationship between the rotational speed and the pressure loss of the motor  20  when the target rotational speed Rtg is higher than the rotational speed threshold Rth, and it is not necessarily performed to control the air volume so that the air volume is kept constant. Accordingly, this maximum air volume control differs from the conventional so-called constant air volume control in the control method and its effect. 
     Expansion of the Embodiments 
     The disclosure provided by the present inventors has been specifically described based on the embodiments, but it is needless to say that the disclosure is not limited to the above embodiments, and can be variously modified without departing from the subject matter of the disclosure. 
     For example, in the first embodiment, the case where the relational expression or table representing the characteristic  500  between the motor current and the rotational speed on the maximum air volume control line C is set as the correspondence relationship information  471  has been exemplified. However, the disclosure is not limited to this mode. For example, it is possible to use the characteristic  500  as a reference, perform a correction of adding or subtracting an offset amount to or from the reference, and use a relational expression or table representing the corrected characteristic as the correspondence relationship information  471 . 
     In the first embodiment, as shown in  FIG. 5A , the case where the rotational speed of the motor  20  is not changed in the range Mh and the range MI is exemplified, but the disclosure is not limited to this mode. For example, in the range Mh and the range MI, the rotational speed of the motor  20  may be reduced with an adjustment width smaller than the adjustment width of the range H and the range L. For example, when the adjustment range of the duty ratio of the drive control signal Sd in the range H and the range L is set to “±0.5%”, the adjustment range of the duty ratio of the drive control signal Sd in the range Mh and the range MI may be set to “±0.1%”. 
     In the first and second embodiments, the case where the speed command signal Sc is a PWM signal and the target rotational speed Rtg is indicated by the duty ratio of the PWM signal is exemplified, but the disclosure is not limited to this mode. For example, the speed command signal Sc may be an analog signal, and the target rotational speed Rtg may be indicated by the voltage level of the analog signal. 
     In the first and second embodiments, the case where the motor  20  is a three-phase brushless motor is exemplified, but the type of motor  20 , the number of phases of the motor  20 , etc. are not limited to this mode. For example, a single-phase brushless motor may be used. 
     The above-mentioned flowcharts show examples showing the operation, and are not limited to these flowcharts. In other words, the steps shown in each figure of the flowcharts are specific examples and are not limited to those of these flowcharts. For example, the order of some steps may be changed, other steps may be inserted between respective steps, or some steps may be performed in parallel.