Patent Publication Number: US-11378072-B2

Title: Air compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese patent application No. 2019-076607, filed on Apr. 12, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an air compressor that operates a compression mechanism by a motor. 
     BACKGROUND ART 
     While this type of air compressor is used by being connected to various machines, a working pressure (take-out pressure) and a consumption amount of compressed air are different depending on the machine to be used. For example, a spray gun that sprays paint by using the compressed air consumes a large amount of the compressed air because the spray gun is continuously used even though the working pressure is low. 
     When a machine that consumes a large amount of the compressed air is used as described above, an air compressor having a large discharge amount of the compressed air must be used. The reason is that for example, when the compressed air is not sufficient while using the spray gun, uneven painting of the painting is generated and thus a repainting work is required. 
     Therefore, in the machine that requires a large amount of the compressed air such as the spray gun, a large-type air compressor (for example, refer to JP-A-2003-239863) using an engine that discharges a large amount of the compressed air, can generate a larger amount of the compressed air than air to be consumed, and has a high filling speed is often used. 
     However, there is a problem with an engine-driven air compressor that is heavy and difficult to carry, has loud noise, and has a smell gasoline. 
     On the other hand, an air compressor in which a compression mechanism is operated by a motor (for example, refer to JP-A-2017-36692) is smaller and easier to carry than the engine-driven air compressor, and has less noise. When used at a location where there is no power source such as outdoor and a bridge, an engine-type generator can be used as the power source. However, since a power supply voltage is limited and a size of the motor is limited, there is a limit to an amount of compressed air that can be generated during the work. There is an air compressor that increases the amount of the compressed air that can be stored in a tank by increasing a pressure in the tank, but since a characteristic of the motor of the above-described air compressor is determined based upon a current value when the pressure in the tank reaches a high pressure, it cannot be said that the motor has a characteristic suitable for a case of a light load. Therefore, even though the air compressor is used for a machine such as a spray gun that uses a low air pressure, the generation of the compressed air cannot follow the use of the spray gun when the pressure in the tank becomes low, such that it is required to wait until the pressure in the tank becomes high and thus the workability is not good. 
     An object of the present invention is to allow a motor-driven air compressor to be used for a machine that requires a large amount of compressed air such as a spray gun by providing the motor-driven air compressor capable of increasing a discharge amount of the compressed air as compared with a related art. 
     SUMMARY OF INVENTION 
     According to an aspect of the present invention, there is provided an air compressor comprising: a motor; a compression mechanism that is driven by the motor and that is configured to generate compressed air; a tank that is configured to store the generated compressed air; a load acquisition part that is configured to acquire a load applied to the compression mechanism; and a control part that is configured to control a rotation of the motor, wherein the control part is configured to perform control for changing a TN characteristic of the motor in response to the load of the compression mechanism acquired by the load acquisition part. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external view of an air compressor; 
         FIG. 2  is a plan view of the air compressor; 
         FIG. 3  is a plan view of the air compressor from which a main body cover is removed; 
         FIG. 4  is a side view near an air outlet of the air compressor from which the main body cover is removed; 
         FIG. 5  is a block diagram illustrating an overview of a system of the air compressor; 
         FIG. 6  is a flowchart of field weakening control; 
         FIG. 7  is a flowchart of a process of setting the target number of rotations; 
         FIG. 8  is a diagram illustrating a change in a motor characteristic due to the field weakening control; 
         FIG. 9  is a diagram according to a first modification, and is a diagram illustrating the timing of mode switching; 
         FIGS. 10A and 10B  are diagrams according to a second modification, in which  FIG. 10A  is a plan view near an air outlet, and  FIG. 10B  is a side view near the air outlet; and 
         FIGS. 11A and 11B  are diagrams according to a third modification, in which  FIG. 11A  is a plan view near an air outlet, and  FIG. 11B  is a side view near the air outlet. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described with reference to the drawings. 
     An air compressor  10  according to the embodiment is a portable compressor, and as illustrated in  FIGS. 1 and 2 , the air compressor  10  includes a mechanism part covered by a main body cover  17  and two tanks  15  disposed below the mechanism part. 
     As illustrated in  FIG. 3 , the mechanism part includes a motor  11 , a fan  12 , a compression mechanism, and a control board (control part  30 ). 
     The motor  11  is an inner rotor type three-phase brushless DC motor in which a rotor is disposed inside an annular stator. The rotation of the motor  11  is controlled by a PWM signal outputted from the control part  30  which will be described later. The motor  11  includes a position sensor  36  and a thermistor  38  which will be described later. A current flowing through the motor  11  is supplied by converting an alternating current from an alternating current power source into a direct current. In the embodiment, an output of the air compressor  10  is 1.5 KW, and an upper limit of the alternating current supplied to the air compressor  10  is 15 A. Therefore, the motor  11  is controlled by the alternating current before being converted into the direct current with 15 A as an upper limit value. 
     The fan  12  is provided for cooling a heat-generating component such as the motor  11  by introducing cooling air into the inside of the mechanism part. The fan  12  is fixed to a rotating shaft of the motor  11 , and is configured to rotate integrally when the motor  11  is driven. 
     The compression mechanism is driven by the motor  11  to generate compressed air, and a well-known structure that compresses air introduced into a cylinder by reciprocating a piston can be used. The air compressor  10  according to the embodiment is a multi-stage compressor including two compression mechanisms of a primary compression mechanism  13  and a secondary compression mechanism  14 . That is, the air supplied from the outside is first compressed by the primary compression mechanism  13 . The air compressed by the primary compression mechanism  13  is introduced into the secondary compression mechanism  14 , and is further compressed by the secondary compression mechanism  14 . As described above, the air compressed with the two stages is sent to the tank  15  and stored. 
     The tank  15  is provided for storing the compressed air generated by the compression mechanism. The air compressor  10  according to the embodiment includes two tanks  15 . The two tanks  15  are disposed in parallel to each other along a longitudinal direction of the air compressor  10 . 
     The compressed air stored in the tank  15  is decompressed to any pressure by passing through a pressure reducing valve  16  and can be taken out to the outside from the air outlet. For example, the compressed air in the tank  15  can be supplied to an external device by connecting an air hose to which the external device such as a spray gun is connected to the air outlet. 
     In the embodiment, as illustrated in  FIG. 4 , two air couplers including a first air coupler  21  and a second air coupler  22  are vertically arranged as the air outlets. These air couplers are provided so as to protrude from the front of the main body cover  17  to the outside. The air coupler is a female coupler, and is configured to be easily attached and detached to and from corresponding male coupler. Therefore, the compressed air stored in the air compressor  10  can be configured to be taken out via the air hose by mounting the air hose mounted with the male coupler on the female coupler (the air outlet). For example, the first air coupler  21  is a coupler having a relatively small diameter corresponding to a device using a steady flow such as a spray, and the second air coupler  22  is a coupler for a large diameter hose suitable for the use of a device that consumes a large amount of the compressed air. 
     The first air coupler  21  is smaller and lighter than the second air coupler  22 , and is used for connecting a small spray gun. On the other hand, for example, the second air coupler  22  is used for the connection of an additional tank used for increasing the compressed air to be stored. When the additional tank is connected, the capacity of the compressed air increases, and the time for continuous work can be extended. Since the additional tank is effective in separating the drain generated during the air compression, the additional tank is often used for painting work requiring dry compressed air. When a mist separator is connected, since it becomes possible to supply the compressed air suitable for the painting by separating moisture, oil, and dust contained in the compressed air, a coupler on which the mist separator can be mounted may be provided as the second air coupler  22 . The second air coupler  22  connects a pneumatic tool such as a nailing machine, thereby making it possible to intermittently supply a large flow rate of the compressed air to the pneumatic tool. 
     At an architectural painting site, since a worker moves a lot and the feet of the worker are hooked on a hose drawn around the floor, there is a possibility that the air compressor  10  is unexpectedly pulled and falls down. Therefore, as illustrated in  FIG. 3 , the first air coupler  21  and the second air coupler  22  protrude along the longitudinal direction of the air compressor  10 . In other words, the axial directions of the first air coupler  21  and the second air coupler  22  are arranged so as to be equal to the longitudinal direction of the tank  15 . According to the above-described arrangement, even though the air hose connected to the air outlet is pulled, the air compressor  10  does not easily fall down. 
     As illustrated in  FIG. 4 , the first air coupler  21  and the second air coupler  22  are different in type and size. The second air coupler  22  larger than the first air coupler  21  is disposed below the first air coupler  21 . According to the above-described arrangement, the center of gravity is lowered and thus the air compressor  10  is hard to fall down. 
     Here, in the embodiment, the insides of the two tanks  15  communicate with each other, and the above-described pressure reducing valve  16  and the air outlet (the first air coupler  21  and the second air coupler  22 ) are provided in one of the two tanks  15 . 
     However, the invention is not limited thereto, and the pressure reducing valve  16  and the air outlet may be provided in both of the two tanks  15 . In the embodiment, as illustrated in  FIG. 3 , a connection part  23  capable of connecting the pressure reducing valve  16  and the air outlet is provided in both of the two tanks  15 . However, the number of components is reduced by providing the pressure reducing valve  16  and the air outlet only in one of the connection part  23 . 
     The connection part  23  is disposed inside the main body cover  17 . However, when the pressure reducing valve  16  and the air outlet are mounted on the connection part  23 , the pressure reducing valve  16  and the air outlet are required to protrude to the outside of the main body cover  17 . Therefore, in the main body cover  17 , an opening part for allowing the pressure reducing valve  16  and the air outlet to protrude is formed at a position facing the connection part  23 . The opening part is formed on both left and right sides respectively corresponding to the two connection parts  23 . 
     In the embodiment, the opening part facing the unused connection part  23  is covered by the outlet cover  18  as illustrated in  FIG. 2 . The outlet cover  18  is attachable and detachable to and from the main body cover  17 . When using the connection part  23  closed by the outlet cover  18 , the outlet cover  18  may be detached therefrom, and the pressure reducing valve  16  and the air outlet may be mounted on the connection part  23 . 
     The operation of the air compressor  10  is controlled by the control part  30  built in the air compressor  10 . Although not illustrated herein, the control part  30  is mainly configured with a CPU, and includes a ROM, a RAM, and an I/O. The CPU is configured to control various input devices and output devices by reading a program stored in the ROM. In the embodiment, as illustrated in  FIG. 3 , the control part  30  is configured with a control board disposed above the tank  15 . 
     As illustrated in  FIG. 5 , an operation switch  31 , a pressure sensor  33 , a current sensor  34 , a voltage sensor  35 , a position sensor  36 , and a thermistor  38  are provided as the input devices of the control part  30 . The input device is not limited to the above-mentioned input devices, and may include other input devices. Although details will be described later, in the embodiment, the pressure sensor  33 , the current sensor  34 , and the position sensor  36  function as a load acquisition part that acquires a driving load of the compression mechanism. 
     The operation switch  31  is various kinds of switches that can be operated by a user. Although not described in detail here, for example, a plurality of types of operation switches  31  such as a switch for turning on and off a power source and a switch for switching an operation mode may be provided. The operation switch  31  is disposed so as to be able to be pressed down on an operation panel  19  (refer to  FIG. 1 ) provided on the surface of the main body cover  17 . 
     The pressure sensor  33  is a tank internal pressure acquisition part that measures an internal pressure of the tank  15 . A pressure value detected by the pressure sensor  33  is transmitted to the control part  30 . The control part  30  controls the start or stop of the driving of the motor  11  based upon the pressure value acquired from the pressure sensor  33 . Specifically, an ON pressure which is a pressure value for starting the driving of the compression mechanism and an OFF pressure which is a pressure value for stopping the driving of the compression mechanism are predetermined, and for example, when the internal pressure of the tank  15  is lowered due to the use of the compressed air and the internal pressure of the tank  15  is lowered up to the preset ON pressure, the motor  11  is driven to fill the compressed air. When the internal pressure of the tank  15  reaches the preset OFF pressure while the motor  11  is being driven, the driving of the motor  11  is stopped. 
     The current sensor  34  is configured with an AC current sensor  34   a  that detects the alternating current from the alternating current power source serving as a power source of the air compressor  10 , and a DC current sensor  34   b  that detects the direct current supplied to the motor  11 . The AC current sensor  34   a  is provided for detecting the alternating current flowing from the alternating current power source to the air compressor  10 , and is used for performing monitoring so that the current flowing through the air compressor  10  does not exceed 15 A of an upper limit value. The DC current sensor  34   b  is provided for detecting a three-phase current value supplied to the motor  11 . The detection value of the DC current sensor  34   b  is transmitted to the control part  30 , and is used for the purpose of monitoring field weakening control which will be described later and the direct current flowing through an electronic component. The current sensor  34  functions as a motor load detection part that detects a load of the motor  11 . 
     That is, as a general characteristic of the motor  11 , the current value also gradually increases as the torque increases (refer to (2) in  FIG. 8 ). In a case where the motor  11  is incorporated in the air compressor  10 , since the torque of the motor  11  increases when the internal pressure of the tank  15  becomes high, the torque, that is, the internal pressure of the tank  15  can be estimated by referring to the current value of the DC current sensor  34   b . As a specific method of estimating the internal pressure of the tank  15 , for example, a method, in which a conversion table indicating a relationship between the current value of the DC current sensor  34   b  and the internal pressure of the tank  15  is stored in advance in the ROM, and the current value of the DC current sensor  34   b  is converted into the internal pressure of the tank  15  by using this conversion table, may be used. As another method of estimating the internal pressure of the tank  15 , a method, in which a calculation formula for converting the current value of the DC current sensor  34   b  into the internal pressure of the tank  15  is generated in advance, and the internal pressure of the tank  15  is estimated by substituting the current value of the DC current sensor  34   b  for this calculation formula, may be used. When the above-described conversion table and calculation formula are used, the DC current sensor  34   b  and the control part  30  function as the tank internal pressure acquisition part that acquires the internal pressure of the tank  15 . 
     The voltage sensor  35  is provided for detecting a primary side voltage value supplied to the motor  11 . The detection value of the voltage sensor  35  is transmitted to the control part  30  and used for the field weakening control which will be described later. 
     The position sensor  36  is provided for detecting a rotational position of the motor  11 . The position sensor  36  is configured with a Hall IC, and is configured to output a signal to the control part  30  when the rotation of the motor  11  (a rotor) is detected. The control part  30  can calculate the number of rotations (rpm) of the motor  11  by analyzing the signal from the position sensor  36 . 
     The thermistor  38  is provided for detecting a temperature of the motor  11 . The temperature detected by the thermistor  38  is used for correcting the control of the motor  11 . 
     The motor  11  detects a rotation angle of the motor  11  from winding resistance. The thermistor  38  may detect a temperature change in the winding resistance of the motor  11  and may correct the detection of the rotation angle of the motor  11  based upon the detected temperature change. 
     As illustrated in  FIG. 5 , the motor  11  and a display part  32  are provided as output devices of the control part  30 . The output device is not limited thereto, and may include other output devices. 
     The motor  11  serves as a power source for operating the compression mechanism as described above. The control part  30  controls the rotation of the motor  11  by PWM control. 
     A display part  32  is provided for displaying various information to the user. For example, there are display devices such as a 7-segment display, a liquid crystal screen, and an LED. The display part  32  according to the embodiment is provided on the operation panel  19  provided on the surface of the main body cover  17 . 
     Here, the control part  30  according to the embodiment is configured to perform control for changing the TN characteristic of the motor  11  in response to the internal pressure of the tank  15 . Specifically, the control part  30  is configured to change the TN characteristic of the motor  11  by the field weakening control. 
     In the motor-driven air compressor  10  of the related art, since the TN characteristic of the motor  11  is determined, there is a limit to increasing the number of rotations. In consideration of this point, when the TN characteristic of the motor  11  is changed in response to the internal pressure of the tank  15  (in response to the torque), the number of rotations of the motor  11  can be increased beyond an original characteristic of the motor  11 . 
     Accordingly, the number of rotations of the motor  11  can be increased at the time of a low load, thereby making it possible to increase the discharge amount of the compressed air. For example, when the spray gun is connected to the air compressor  10  and used, the internal pressure of the tank  15  is lowered when the remaining compressed air decreases. When the internal pressure of the tank  15  is lowered in this manner, the number of rotations of the motor  11  is increased, thereby making it possible to shorten the filling time of the compressed air by changing the TN characteristic of the motor  11  in accordance with the lowness of the internal pressure thereof. Next, when the compressed air is filled and the internal pressure of the tank  15  increases, the TN characteristic of the motor  11  is restored (returned to the original characteristic) in accordance with the increase of the internal pressure thereof, such that the motor  11  can be driven with optimum efficiency. Therefore, when the internal pressure of the tank  15  is low and the load is low, the number of rotations is increased to improve the discharge amount, and when the internal pressure of the tank  15  is high and the load is high, performance can be maintained by efficiently driving the motor  11 . 
     This field weakening control is executed by the control part  30  according to a flow of a process as illustrated in  FIG. 6 . The process illustrated in  FIG. 6  is executed every fixed time by being registered in a periodic handler. In the embodiment, the process illustrated in  FIG. 6  is executed every 125 μs. 
     First, in step S 100  illustrated in  FIG. 6 , a supply current to the motor  11  is acquired as the load of the motor  11  by using the DC current sensor  34   b . Next, the process proceeds to step S 105 . 
     In step S 105 , a current value acquired in step S 100  is subjected to dq conversion, thereby acquiring a d-axis current value Id and a q-axis current value Iq of a rotation coordinate system. Next, the process proceeds to step S 110 . 
     In step S 110 , a d-axis voltage value Vd and a q-axis voltage value Vq are calculated based upon Id and Iq acquired in step S 105 . Next, the process proceeds to step S 115 . 
     In step S 115 , a half of the supply voltage value to the motor  11  acquired by using the voltage sensor  35  is compared with absolute values of Vd and Vq calculated in step S 110 . When the latter is greater, the process proceeds to step S 120 . Otherwise, the process proceeds to step S 125 . 
     When the process proceeds to step S 120 , a command value of Id is calculated. Specifically, the command value of Id is calculated by multiplying a value obtained by subtracting the absolute values of Vd and Vq from the supply voltage value to the motor  11  by a predetermined proportional gain. The command value of the Id is a negative value. Next, the process proceeds to step S 130 . 
     When the process proceeds to step S 125 , 0 is set to the command value of Id. Next, the process proceeds to step S 130 . 
     In step S 130 , the field weakening control is executed by using the command value of Id. That is, a negative current is caused to flow through the d-axis by an amount of the command value of Id, whereby control for shifting an advance angle of the motor  11  in an advance direction is executed. However, when the command value of Id is 0, the field weakening control is not executed. 
     At this time, the command value of Iq is actually set with reference to various parameters, a voltage command value is calculated based upon the command value of Id and the command value of Iq, and the PWM control is executed by using a value obtained by converting the voltage command value into three phases of UVW. 
     When determining an output of the PWM, feedback control is executed so that the number of rotations of the motor  11  and the current value do not exceed a predetermined upper limit value. In the embodiment, the upper limit value of the number of rotations of the motor  11  is set to 3400 rpm, and the output is controlled so as not to exceed the upper limit value. In the embodiment, the upper limit value of the alternating current is set to 15 A, and the output is controlled so as not to exceed the upper limit value by detecting the current value with the AC current sensor  34   a.    
     Specifically, a process of setting the target number of rotations as illustrated in  FIG. 7  is executed. The process illustrated in  FIG. 7  is executed every fixed time by being registered in the periodic handler. In the embodiment, the process illustrated in  FIG. 7  is executed every 40 ms. 
     First, in step S 200  illustrated in  FIG. 7 , the number of rotations of the motor  11  is calculated. The number of rotations of the motor  11  can be calculated from the number of detections of the position sensor  36  in fixed time. After calculating the number of rotations of the motor  11 , the process proceeds to step S 205 . 
     In step S 205 , a direct current value is acquired by using the AC current sensor  34   a . Next, the process proceeds to step S 210 . 
     In step S 210 , it is performed to check whether or not the direct current value exceeds the upper limit value (15 A). When the direct current value exceeds 15 A, the process proceeds to step S 215 . On the other hand, when the direct current value is equal to or less than 15 A, the process proceeds to step S 220 . 
     When the process proceeds to step S 215 , the target number of rotations of the motor  11  is reduced by a predetermined amount. Accordingly, in the subsequent control of the motor  11 , control aiming at rotation at the target number of rotations is executed. Next, the process of setting the target number of rotations is terminated. 
     When the process proceeds to step S 220 , it is performed to check whether the direct current value is not near the upper limit value (equal to or greater than 14.5 A) and the number of rotations of the motor  11  calculated in step S 200  is less than the upper limit value (3400 rpm). When the direct current value is less than 14.5 A and the number of rotations of the motor  11  is less than 3400 rpm, the process proceeds to step S 225 . Otherwise, the process of setting the target number of rotations is terminated. 
     When the process proceeds to step S 225 , the target number of rotations of the motor  11  increases by a predetermined amount. Accordingly, in the subsequent control of the motor  11 , control aiming at rotation at the target number of rotations is executed. Next, the process of setting the target number of rotations is terminated. 
     According to the control described above, the target number of rotations is set as high as possible within a range where the alternating current does not exceed 15 A which is the upper limit value. 
     As can be seen with reference to (1) in  FIG. 8 , in the motor  11  of the embodiment, when the field weakening control is performed, the motor torque reaches 15 A of the upper limit value of the alternating current in the vicinity of 3 N·m (P 1 ). Accordingly, when the torque exceeds P 1 , it is not possible to perform the control of the number of rotations of the motor  11  by controlling the current value. However, in a torque region smaller than P 1 , since there is a margin until the motor torque reaches 15 A of the upper limit value of the alternating current, the field weakening control is performed by using the marginal current. 
     By performing the field weakening control as described above, the TN characteristic of the motor  11  is changed in response to the internal pressure of the tank  15 , and the number of rotations can be increased. 
     Under the above-described control, the motor  11  shows the characteristics as shown in  FIG. 8 .  FIG. 8  is a graph showing a TI characteristic (a characteristic indicating a relationship between the torque and the current) and a TN characteristic (a characteristic indicating a relationship between the torque and the number of rotations) of the motor  11 ; (1) shows the TI characteristic with the field weakening control; (2) shows the TI characteristic without the field weakening control; (3) shows the TN characteristic with the field weakening control; and (4) shows the TN characteristic without the field weakening control. In  FIG. 8 , when the motor  11  is incorporated in the air compressor  10 , the internal pressure (gauge pressure) of the tank  15  corresponding to the torque generated in the motor  11  is shown with a vertical line indicating 0 MPa and a vertical line indicating 4.4 MPa. 
     In the field weakening control according to the embodiment, the current value of the motor  11  is acquired (refer to step S 100  in  FIG. 6 ), and the internal pressure of the tank  15  is estimated based upon the acquired current value thereof. As the current value of the motor  11  increases (as the internal pressure of the tank  15  becomes high), the field weakening is configured to gradually become stronger (a degree of advancing an advance angle of the motor  11  becomes stronger). That is, the amount of decrease in the number of rotations according to the TN characteristic of the motor  11  is configured to increase so as to be stabilized at a fixed rotation (3,400 rpm in the embodiment) by the field weakening control. 
     As illustrated in (3), by performing the field weakening in this manner, the number of rotations of the motor  11  can be increased beyond the original characteristic of the motor  11  (refer to (4)). However, in the embodiment, since the number of rotations is controlled to be stabilized at 3400 rpm, the number of rotations is not increased beyond the original characteristic thereof. As the number of rotations increases, the current value increases more than the original characteristic of the motor  11 , but since the upper limit of the current value of the air compressor  10  of the embodiment is 15 A, control is performed so as not to exceed 15 A (refer to (1)). After reaching 15 A of the upper limit of the current value (a region where the torque is higher than P 1 ), as illustrated in (3), control is performed to reduce the number of rotations of the motor  11  so as to approach the number of rotations indicated by the original TN characteristic of (4). In other words, the control is performed so as to gradually reduce the number of rotations of the motor  11 , whereby the current value is maintained at 15 A even when the load increases. 
     In the embodiment, when the internal pressure of the tank  15  becomes about 0.8 MPa (refer to P 1 ), it is set to reach 15 A of the upper limit value of the current. That is, when the internal pressure of the tank  15  becomes about 0.8 MPa, the control of the motor  11  is configured to be changed. 
     Specifically, when the torque becomes higher than the line indicated by P 1  in  FIG. 8 , the current reaches 15 A of the upper limit of the current. As described above, when the torque is higher than P 1  (when the internal pressure of the tank  15  is higher than a predetermined value), the motor  11  is controlled so as to weaken the field weakening as the torque increases. On the other hand, when the torque is lower than P 1  (when the internal pressure of the tank  15  is lower than the predetermined value), the motor  11  is controlled so as to strengthen the field weakening as the torque increases. 
     In the embodiment, the internal pressure (P 1 ) of the tank  15  at which the control is switched is set to 0.8 MPa, and this setting is not limited to 0.8 MPa. However, it is desirable that the internal pressure of the tank  15  is set to reach 15 A in the range of 0.5 MPa to 1.5 MPa as a low load pressure zone. 
     As described above, the control part  30  according to the embodiment performs the control to change the TN characteristic of the motor  11  in response to the internal pressure of the tank  15 . According to such control, since the number of rotations of the motor  11  can be increased in accordance with the internal pressure of the tank  15 , the discharge amount of the compressed air can be increased even in the case of a small motor  11  driven air compressor  10 . 
     In the embodiment, the internal pressure of the tank  15  is estimated from the direct current flowing through the motor  11  detected by the DC current sensor  34   b , and it is also possible to estimate the internal pressure of the tank  15  from the alternating current flowing through the air compressor  10  detected by the AC current sensor  34   a . The internal pressure of the tank  15  may be directly acquired by using the pressure sensor  33 . Instead of using the current sensor  34 , the number of rotations of the motor  11  may be detected by using the position sensor  36 , thereby estimating the driving load of the air compressor  10  based upon the detected number of rotations thereof. 
     (First Modification) 
     A first modification is configured in such a manner that control is performed by switching between a normal mode and a following mode with reference to the internal pressure (the torque) of the tank  15 , and the mode is switched by a method different from the above-described embodiment. 
     The air compressor  10  according to the first modification includes: a normal mode in which the TN characteristic of the motor  11  is kept constant regardless of the internal pressure of the tank  15 ; and a following mode in which the TN characteristic of the motor  11  is changed in response to the internal pressure of the tank  15 . 
     Of the normal mode and the following mode, the normal mode is a control mode without the field weakening control (or a control mode in which an advance angle control is constant). In a pressure zone to which the normal mode is applied, since the TN characteristic of the motor  11  is kept constant, the TN characteristic of the motor  11  is not changed even though the internal pressure (the torque) of the tank  15  is varied. 
     On the other hand, the following mode is a control mode with the field weakening control (or a control mode in which the advance angle control is varied). In a pressure zone to which the following mode is applied, the TN characteristic of the motor  11  is changed in response to the internal pressure (the torque) of the tank  15 . A method of changing the TN characteristic is the same as that of the above-described embodiment, and the advance angle may be adjusted in accordance with the current value of the motor  11 . 
       FIG. 9  is a diagram illustrating a change in the internal pressure of the tank  15  when the spray gun is connected to the air compressor  10  according to the first modification and used. As illustrated in regions (a), (c), (e), and (g) in  FIG. 9 , when the spray gun is used, the compressed air is consumed and the internal pressure of the tank  15  is lowered. As illustrated in regions (b), (d), (f), and (h) in  FIG. 9 , when the compressed air is consumed to some extent and the internal pressure of the tank  15  is lowered up to the ON pressure, the compression mechanism is driven, such that the internal pressure of the tank  15  increases when the use of the spray gun is interrupted. However, since the internal pressure of the tank  15  is gradually lowered due to the intermittent use of the spray gun, finally, the compressed air may be not sufficient. 
     At this time, when the remaining amount of the compressed air decreases and the internal pressure of the tank  15  is lowered, the torque of the motor  11  is lowered, thereby making it possible to increase the number of rotations. However, when using the motor  11  optimized for a high load zone, there is a limit even though the number of rotations of the motor  11  increases in a low load zone, due to the characteristic of the motor  11 . 
     Therefore, in the modification, the number of rotations of the motor  11  can be increased by performing the field weakening control in the low load zone. 
     Specifically, in the modification, when the internal pressure of the tank  15  is higher than a predetermined level (refer to P 2  in  FIG. 9 ), efficient control is performed by utilizing the original characteristic of the motor  11  (without performing the field weakening control). On the other hand, when the internal pressure of the tank  15  is lowered below the predetermined level (P 2 ), the control for increasing the number of rotations of the motor  11  is performed by performing the field weakening control. Accordingly, as illustrated in  FIG. 9 , in a state before the internal pressure of the tank  15  is lowered up to P 2 , the control is executed in the normal mode, and when the internal pressure of the tank  15  is lowered below P 2 , the control is executed in the following mode. The predetermined level (P 2 ) is a pressure higher than 0.3 to 0.5 MPa which is the working pressure of the spray gun, and for example, is set to 1 MPa. The reason is that since the compressed air cannot be generated in time when the field weakening control is performed after the internal pressure of the tank  15  is lowered to the working pressure of the spray gun, a margin is provided to prevent the compressed air from running short. 
     When the internal pressure of the tank  15  becomes higher than a predetermined level (P 3 ) when the field weakening control is performed as described above, the field weakening control is terminated, and the control is configured to be switched from the following mode to the normal mode. This P 3  is set higher than P 2 , and set to, for example, 1.5 MPa. Since the internal pressure of the tank  15  gradually is lowered during the use of the spray gun, it can be estimated that the use of the spray gun is stopped in consideration the fact that P 3  higher than P 2  is detected. In other words, when it is estimated that the use of the spray gun is stopped, the mode is configured to be switched from the following mode to the normal mode in which efficiency is emphasized. 
     As described in the example of  FIG. 9 , when the compressed air is used and the internal pressure of the tank  15  is lowered beyond P 2  as illustrated in the region (e), the control is switched at the timing (T 1 ) exceeding P 2 . Accordingly, the field weakening control is executed, and the control for increasing the number of rotations is executed. 
     Then, when the compressed air is filled and the internal pressure of the tank  15  becomes higher than P 3  as illustrated in the region (h), the control is switched to the normal mode at the timing (T 2 ) exceeding P 3 . Accordingly, the field weakening control is released, and the control in which efficiency is emphasized is executed. 
     As a method of acquiring the internal pressure of the tank  15 , as described above, the method of estimating the internal pressure of the tank  15  from the current value of the motor  11  may be used, or the method of directly detecting the internal pressure of the tank  15  with the pressure sensor  33  may be used. The load of the air compressor  10  may be detected by detecting the number of rotations of the motor  11  by using the position sensor  36 . 
     The pressure values P 2  and P 3  to be used for switching the mode may be fixed values or variable values. When P 2  and P 3  are varied, P 2  and P 3  may be varied in response to a used amount of the compressed air. For example, the value of P 2  may be set to be high when the used amount of the compressed air is large by calculating the used amount of the compressed air from the detected value of the pressure sensor  33 . The values of P 2  and P 3  may be set to any values by a user using the operation panel  19 . According to the above-described configuration, the user can select any control in response to the tool (the spray gun) to be used and the amount of work. 
     According to the above-described configuration, even when the motor  11  optimized for the high load zone is used to quickly fill the tank  15  with high pressure air, the number of rotations in the low load zone can be increased, and even when the spray gun using low-pressure compressed air is used, air shortage is hardly generated. 
     Second Embodiment 
     As illustrated in  FIGS. 10A and 10B , the first air coupler  21  and the second air coupler  22  may have different lengths (protrusion amounts). For example, the second air coupler  22  larger than the first air coupler  21  may be disposed below the first air coupler  21 , and may protrude larger than the first air coupler  21 . According to the above-described configuration, since the air compressor  10  is hard to fall down, and further the connection position between the first air coupler  21  and the second air coupler  22  is offset, the air hose can be easily attached and detached. 
     Third Embodiment 
     As illustrated in  FIGS. 11A and 11B , the first air coupler  21  and the second air coupler  22  may be provided so as to have different axial directions. For example, an acute angle may be formed in the axial direction of the first air coupler  21  and in the axial direction of the second air coupler  22 . According to the above-described configuration, since the connection position between the first air coupler  21  and the second air coupler  22  is offset, the air hose can be easily attached and detached. 
     Fourth Embodiment 
     When detecting a fact that the internal pressure of the tank  15  is lowered below a predetermined value, the air compressor  10  may include a notification part for notifying the fact. As an example of the notification part, notification by voice from a speaker  37  or display on the display part  32  may be used. A solenoid valve for opening and closing a passage is provided in the passage for taking out the compressed air of the compression mechanism (for example, on the downstream side of the pressure reducing valve or the upstream side of the air coupler), and when the internal pressure of the tank  15  is lowered below the predetermined value, the passage of the compressed air may be shut off by the solenoid valve and thus the supply of the compressed air is stopped, thereby notifying the user of the fact that the internal pressure of the tank  15  is lowered. Accordingly, since it is possible to prevent painting from being performed in a state where the pressure is lowered, failure such as uneven painting can be prevented in advance. 
     When the pressure is lowered to a certain level (a first level), the notification is performed as described above, and when the pressure is lowered further than the first level and is lowered up to a second level, the supply of the compressed air to the air outlet may be shut off. 
     An external communication terminal (such as a cellular phone and a smartphone) may be linked with the air compressor  10 , and a signal may be transmitted to the communication terminal when the pressure is lowered, and the communication terminal may be used to notify that the pressure is lowered. According to the above-described notification method, information can be surely acquired even though the user works at a place away from the air compressor  10 . 
     According to an aspect of the present invention, there is provided an air compressor comprising: a motor; a compression mechanism that is driven by the motor and that is configured to generate compressed air; a tank that is configured to store the generated compressed air; a load acquisition part that is configured to acquire a load applied to the compression mechanism; and a control part that is configured to control a rotation of the motor, wherein the control part is configured to perform control for changing a TN characteristic of the motor in response to the load of the compression mechanism acquired by the load acquisition part. 
     According to the above invention, the control part performs control to change the TN characteristic of the motor in response to the load of the compression mechanism acquired by the load acquisition part. According to such control, since the number of rotations of the motor can be increased in accordance with the load of the compression mechanism, a discharge amount of the compressed air can be increased even in the case of a small motor-driven air compressor. 
     That is, in a motor-driven air compressor of a related art, since the TN characteristic of the motor is determined, there is a limit to increasing the number of rotations. In consideration of the above-described circumstance, according to the present invention, since the TN characteristic of the motor is changed in response to a driving load of the air compressor (in response to torque), it is possible to increase the number of rotations of the motor beyond a characteristic of an original motor. 
     As described above, it is possible to increase the number of rotations of the motor at the time of a low load, thereby increasing the discharge amount of the compressed air. For example, when the spray gun is connected to the air compressor and used, an internal pressure of the tank is lowered when the remaining compressed air decreases. In the present invention, when the internal pressure of the tank is lowered in this way, since the TN characteristic of the motor is changed in accordance with the lowness of the internal pressure thereof, the filling time can be shortened by increasing the number of rotations of the motor and by increasing the discharge amount of the compressed air. When the compressed air is filled and the internal pressure of the tank increases, the TN characteristic of the motor is restored (restored to an original characteristic) in accordance with the increase of the internal pressure thereof, thereby making it possible for the motor to be driven with optimum efficiency. Therefore, when the internal pressure of the tank is low and the load is low, the number of rotations is increased to improve the discharge amount, and when the internal pressure of the tank is high and the load is high, the performance can be maintained by efficiently driving the motor.