A method and apparatus for computing moment of inertia in a motor speed controller, and a speed control method and apparatus for motor

Abstract: In a motor speed controller for controlling an AC motor (6) to be driven at a variable speed, a signal proportional to a load torque and a signal proportional to an output torque of the AC motor are found by current detecting means (5) for detecting the primary current of the AC motor, and integrated over the speed changing period, and the integrated values are divided by the values corresponding to the amounts of speed change during the speed changing period so that the moment of inertia can be computed. In addition, there is provided means (8) for changing the rate of output frequency change caused by the output frequency command into a value according to the calculated moment of inertia. Moreover, there is provided a circuit for detecting a sudden change of the calculated moment of inertia, so that when a sudden change occurs, an abnormality signal is generated.

Description: This invention relates to speed control for driving AC motors at variable speeds by the change of output frequency, and particularly to a moment-of-inertia detecting method and apparatus for automatically detecting a resultant moment of inertia of the AC motor and a load connected thereto, and a speed control method and apparatus for driving the AC motor at the optimum acceleration or deceleration on the basis of the measured moment of inertia. When an AC motor is driven at a variable speed by a speed controller, the torque upon change of speed is greatly changed by the moment of inertia of the load connected to the AC motor. Therefore, overcurrent flows and the speed controller is tripped unless the speed is changed allowing for the moment of inertia of the load. A conventional method of computing the moment of inertia is described in Japanese Patent Laid-open Gazette 61-88780. This method computes the moment of inertia from the amount of integration of a torque proportional signal and the amount of change of revolution speed. In this prior art, if the moment of inertia is found and the rate of change of speed is decided, operation without overcurrent and trip can be realized, but the revolution speed must be detected. Thus, a speed detector is necessary. In addition, a vector control system without using the speed detector is disclosed (see Japanese Patent Laid-open Gazette 59-156191). In this system, a detector for, induced electromotive forces, which is, generally not used, is necessary for the general-use inverter, thus causing the construction to be complicated and the cost to increase. It is an object of the invention to detect a moment of inertia without using a speed detector and a detector for induced electromotive force. It is another object of the invention to provide an economical and highly reliable speed controller which is arranged to enable an operation at a proper acceleration or deceleration rate according to the calculated moment of inertia. These objects are solved in accordance with the features of the independent claims 1,6,13,17. Dependent claims are directed on preferred embodiments of the invention. A total moment of inertia of the AC motor and the load connected thereto can be found without use of any detector other than the current detector. When the maximum rate of speed change is changed into a value according to the previously found moment of inertia, the speed controller for the AC motor can be operated in the optimum condition without tripping. Moreover, the output of the inverter can be cut off when the moment of inertia is suddenly changed. Thus, the speed controller for the AC motor is simple in the construction, highly reliable, economical and highly safe. Fig. 1 is a block diagram of one embodiment of the invention; Fig. 2 is a vector diagram for a motor; Fig. 3 is a timing chart for the torque change relative to the frequency change; Fig. 4 is a flowchart for software; Fig. 5 is a flowchart for software in another embodiment of the invention; Fig. 6 is a block diagram of another embodiment of the invention; Fig. 7 is a block diagram of still another embodiment of the invention; Fig. 8 is a flowchart for software in the construction of Fig. 7; and Fig. 9 is a timing chart for the operation of the embodiment of Fig. 7 in which the frequency is changed twice. It is generally known that the total moment of inertia, GD 2 of a motor and a load on this motor is given by the following equation (1): where T 0 is the torque which the motor causes during accelerating (or decelerating), K 1 is the proportional constant, T L is the torque of the load at a constant speed, t 2 is the time shown in Fig. 3, and ΔN is the amount of change of the revolution rate The amount of change of the revolution rate is given by the following equation (2). ΔN = 120 pole number P of motor {(frequency f 1 after acceleration) - (frequency f 2 before acceleration)} From the equation (2), it will be seen that the amount of change of the revolution rate can be found from the amount of change of frequency which the inverter produces. As illustrated in Fig. 3, when the motor is accelerated from time t 0 , an acceleration torque T a is generated during the acceleration. The output torque T 0 is the sum of the load torque TL and the acceleration torque T a . (see Fig. 3). Therefore, if the motor is stopped from the acceleration, only the load torque T L remains, and thus the output torque is constant. The numerator of the equation (1) can be found by integrating the output torque and the load torque from when the acceleration starts to time t 2 when the output torque becomes constant, and subtracting the integration of the load torque from that of the output torque The denominator of the equation (1) is found from the equation (2). As a result, the moment of inertia GD 2 can be calculated. Similarly, the moment of inertia upon deceleration can be calculated. Here, it is necessary to obtain a signal proportional to the output torque. If the inverter is of the vector control system, this signal can be easily obtained. However, since the conventional vector control system inverter employs a sensor which is not used by the general-use inverter, so as to determine the torque current component, there is the problem as described above. Therefore, only the output current detector which is used in the general-use inverter is used to produce the output current component by vector control, and the moment of inertia is found on the basis of the obtained torque current component. When the moment of inertia GD 2 is found, the acceleration time t can be calculated from the following equation (3): t = GD 2 × (amout of change of speed ΔN) K 1 × (allowable output torque T) If the inverter is operated on the basis of the acceleration time calculated from the equation (3), the optimum operation can be made without trip. An embodiment of the invention will be described with reference to the accompanying drawings. Fig. 1 shows the construction of this embodiment. The current from a three-phase AC power supply 1 is converted by a rectifying circuit 2 into a DC current. The DC current is smoothed by a smoothing capacitor 3, and then converted by a reverse converter 4 into an arbitrary AC voltage of an arbitrary frequency. This AC voltage is supplied to a motor 6. In this case, a voltage type inverter is used for controlling the AC motor to change the revolution speed. That is, the reverse converter 4 is controlled by the logic section which is formed of a vector calculating portion A, a moment-of-inertia calculating portion B, a frequency acceleration or deceleration calculating portion 8 and a PWM waveform composer 14. Shown at 25 is a load. A voltage controlling portion 12 outputs voltage commands Vd, Vq and an internal phase angle δ on the basis of a frequency command f 1 * + f s , and currents id, iq. (For the operation of the vector calculating portion A, see Japanese Patent iaid-open Gazette No. 2-32788.) A coordinate converter 16 of the vector calculating portion A calculates a signal iq proportional to the output torque and a signal id proportional to the exciting current. The coordinate converter 16 makes d-q conversion from the following equation (4). The d-q coordinates are the rotating coordinates. If the phase of the output voltage V 1 from the inverter is represented by , and the phase difference between the induced voltage E 1 of the motor and the output voltage V 1 , or the internal phase angle by δ, then the internal phase angle δ and the inverter output voltage V 1 are given by the following equations (5), (6). V 1 = Vd·sin(δ) + Vq·cos(δ) = (-ω r ·La·iq+r 1 ·id)sin(δ)+(E 1 +r 1 ·iq+ω r ·La·id)cos(δ) δ = tan -1 {-(ω r ·La·iq+r 1 ·id)/ (E 1 +r 1 ·iq+ω r ·La·id)} Where, r 1 is the primary resistor, and L a is the leakage reactance. The above relations are shown in the vector diagram of Fig. 2. From the vector diagram of Fig. 2, it will be understood that in the coordinate conversion, the q-axis is in phase with the induced voltage E 1 , iq is the signal proportional to the torque and id is the signal proportional to the exciting current. Therefore, the output torque can be expressed by the value of iq. The primary voltage command V 1 in Eq. (5) is supplied to the PWM waveform composer 14, and then fed through an amplifier 15 to a power switching element of the reverse converter 4. The method for calculating the moment of inertia will be described below (since the moments of inertia for acceleration and deceleration are similarly found, the description is made only for acceleration). The frequency acceleration or deceleration calculating portion 8 functions to change the frequency command f 1 * at an arbitrary time and the rate of change of the frequency command f 1 * depending on the magnitude of the moment of inertia. The moment-of-inertia calculating portion B is formed of a torque current integrator 18, an accelerating end timing detector 17, a moment-of-inertia calculating portion 19, an accelerating signal generator 20 and a frequency change rate calculating portion 21. The moment-of-inertia calculator is operated at every certain time Δt to execute the calculation. The accelerating signal generator 20 produces an accelerating signal a when the frequency is changed. The torque current integrator 18 starts operation when supplied with the accelerating signal a from the accelerating signal generator 20, and adds the torque proportional signal iq at every certain time Δt. The accelerating end timing detector 17 monitors the torque proportional signal iq when supplied with the accelerating signal a from the accelerating signal generator 20. It detects the time when the torque proportional signal iq has decreased and reached a constant value, or when the motor has reached a constant speed. When the torque proportional signal iq becomes constant, the timing detector 17 supplies a signal b indicative of constant speed to the torque current integrator 18, thus stopping the adding operation. Also, it measures and produces a time t 2 from the accelerated state to constant speed of the motor. On the other hand, the frequency change rate calculating portion 21 calculates the change Δf (=f 1 -f 2 ) of the frequency f 1 * during the period from the rising-up of the accelerating signal a to the accelerating end timing t 2 , and supplies Δf to the moment-of-inertia calculating portion 19. The moment-of-inertia calculating portion 19 calculates the equations (1) and (2) to find the moment of inertia GD 2 . The operation will be mentioned in detail with reference to Fig. 3. When a speed setting device 7 changes the frequency from f 2 to f 1 , the frequency/acceleration or deceleration calculating portion 8 increases the command frequency during the time t 1 . Then, the motor starts to accelerate, and thus the load torque T L is added with the acceleration torque so that the output torque T 0 is produced. The torque current integrator 18 integrates the current iq proportional to this output torque T 0 relative to Δt. When the frequency command has reached f 1 after t 1 , the torque current detector 17 monitors the time when the torque current iq becomes constant (or only the load torque remains). This is because even if the output frequency of the inverter has reached f 1 , the motor does not reach the frequency. When the torque current iq becomes constant, the time t 2 from the start of acceleration to constant speed is measured. The integration of the torque current iq during this time is given by the following equation (7) On the other hand, the integration of only the load torque during the period of t 2 is given by the following equation (8): The amount of change of the revolution rate can be calculated from the following equation (9) since the command frequencies are known: ΔN = 120 P (f 1 - f 2 ) Therefore, the moment-of-inertia calculating portion 19 calculates the moment of inertia from the equation (10) which is obtained by modifying the equation (1). Thus, the moment of inertia GD 2 can be found. Fig. 4 shows the flowchart of the above operation. Use of software for this operation can be simply realized by the timer interruption of the flow shown in Fig. 4. The operation will be described with reference to Fig. 4. When the timer interruption is started at intervals of Δt, decision is made of whether the frequency command is changed or not (step S1). When the frequency command f 1 * is changed, the product of the torque current iq and Δt is found (step S2). During the time in which the frequency command is changing, iq×Δt is found and added to the previous value to produce Σiq×Δt (-S0-S1-S2-S0-). When the frequency change stops, the processing goes to step S3 where the torque current change is monitored (step S3, S4). Until the torque current becomes constant, the calculation of Σiq×Δt at step S2 is continued (-S0-S1-S3-S4-S2-S0). When the torque current becomes constant, t 2 is calculated (step S5) and the load torque iq cost ×t 2 is calculated (step S6). The moment of inertia is calculated from Eq. (10) by substituting Σiq×Δt found at step S2 and iq cost ×t 2 found at step S6 into Eq. (10) (step S7). While in the embodiment the output torque component T 0 and the load torque component T L are separately found from the start of acceleration to the end of acceleration, those components may be found at each sampling time. Fig. 5 is a flowchart for this operation. At step S8, Σ(iq×Δt - iq cost ×Δt) is calculated. Therefore, in this embodiment, steps S5 and S6 in Fig. 4 can be omitted. In this case, the load torque proportional signal iq cost is found from the torque proportional signal at constant speed and stored at step S16. These operations are concerned with the technique of computation, and thus can be performed by various different methods. Since the moment of inertia can be found as above, the frequency/acceleration or deceleration calculating portion 8 can decide the acceleration time for constant output torque T 0 from Eq. (3) on the basis of the found GD 2 . This means that the motor can be accelerated or decelerated so that the primary current of the motor is a predetermined constant current value. If the output torque is not constant at this time, it is suggested that the moment of inertia of the load has changed. In this case, the acceleration or deceleration time is again set to a new value. Also, when the output torque is suddenly changed, it is decided that the load becomes abnormal, and the inverter can be stopped. An abnormality detecting circuit 27 shown in Fig. 1 detects a sudden change of the moment of inertia, and produces an abnormality signal AL. Moreover, the abnormality detecting circuit supplies the abnormality signal AL to the amplifying circuit 15, stopping the operation of the speed controller. While in this embodiment, the motor speed is supposed to become constant from the fact that the output torque proportional signal iq becomes constant, the revolution speed of the motor can always be detected by use of a slip frequency calculating portion 10 shown in Fig. 1. Since the current iq proportional to the output torque is known, the slip frequency fs is calculated from the equation (11): fs = M 2πT 2  2 iq where T 2 is the secondary time constant,  2 is the secondary magnetic flux, and M is the mutual inductance. By use of this equation, it is possible to correct the frequency f 1 * for fs and find the correct revolution rate of the motor. Thus, when the load torque T L varies at high and low speeds, the moment of inertia can be more precisely calculated. Fig. 6 shows another embodiment. This embodiment is different from the embodiment of Fig. 1 in that a torque current integrator 31 and a moment-of-inertia calculating portion 26 have different constructions from those in the embodiment of Fig. 1. In other words, in Fig. 1, the torque current integrator 18 calculates the separate load torque component and output component and supplies them to the moment-of-inertia calculating portion 19, whereas in Fig. 6 the torque current integrator 31 calculates Σ(iq×Δt-iq cost ×Δt) and supplies it to the moment-of-inertia calculating portion 26. A description will be made of an application of the invention to a general-purpose inverter of the so-called V/F control system which is controlled so that the ratio between the output voltage and the output frequency is maintained to have a certain relation (see Fig. 7 for the operation). In this case, when the frequency is changed twice and the term of the load torque T L is eliminated, the following equation (12) is satisfied: T 1 , T 2 the first torque, second torque Δf 1 , Δf 2 the first and second amounts of frequency change Δt 1 , Δt 2 ... the first and second amounts of time change K 2 .... the proportional constant In the V/F control which is widely used for the general-purpose inverter, the component proportional to the torque cannot be directly measured. However, since the output torque (T 1 , T 2 ) is approximately proportional to the primary current of the motor, GD 2 can be found from the measurement of the primary current as expressed by the equation (13): I T1 , I T2 ... the primary currents of the motor at time T 1 , T 2 K 3 ..... the proportional constant According to the equation (13), the moment of inertia can be found without knowing the load torque T L . A method of finding the moment of inertia for the inverter of V/F control will be described with reference to Figs. 7 to 9. In the V/F control, the vector current iq proportional to the torque cannot be known. The moment-of-inertia calculating portion 30 calculates the moment of inertia GD 2 from the equation (12) by substituting three pieces of information: the motor primary current i 1 from the current detector 5, the amount of change of frequency Δf from the frequency/acceleration or decelerating calculating portion 28 and the amount of change of time Δt. In order to eliminate the term of load torque T L , the frequency is necessary to be changed twice as described above. Fig. 8 is a flowchart for the operation, and Fig. 9 shows the change of frequency and current. The current is measured at each sampling time Δt, and integrated when the frequency is changed. If the frequency has been changed twice, the GD 2 is calculated from the equation (14) which is a modification of Eq. (13). To satisfy Eq. (14), acceleration is made twice for different rates of frequency change. The acceleration in twice is not necessary to be a special accelerating operation for the moment of inertia, but may be a normal accelerating operation. Thus, the moment of inertia can be found at each accelerating operation except the first one. The decelerating operation may of course be a normal decelerating operation. Further, a voltage controlling portion 29 calculates and outputs an output voltage V on the basis of V/f ratio predetermined by a frequency command f*. According to this embodiment, since the torque proportional signal is approximated to the primary current, it will be known that the moment of inertia can be calculated. In this case, similarly the speed or induced electromotive force feed-back is not necessary, and the moment of inertia can be calculated. That is, there is the effect that it can be realized to make the optimum accelerating and decelerating operation matched with the load. According to this invention, since the total moment of inertia of the motor and the load connected thereto can be automatically calculated by use of only the sensor which is generally used in the general-purpose inverter, the construction is not complicated. In addition, the motor can be accelerated or decelerated during the optimum time by use of the calculated moment of inertia. If the moment of inertia is suddenly changed, it is decided that the load has caused an abnormality, and the inverter can be stopped, thus the safety being improved. Thus, the control apparatus for the motor is simple in construction, highly reliable, economical, and very safe.

Claims: A moment-of-inertia detecting apparatus in a motor speed controller for controlling an AC motor (6) connected to a load (25) to be driven at a variable speed according to a speed command (N*), said speed controller having means (8) for producing an output frequency command (f 1 *) which is changed with the change of said speed command, said moment-of-inertia detecting apparatus comprising: current detecting means (5) for detecting a primary current of said AC motor; and moment-of-inertia detecting means (A, B) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the amount of change of output frequencies for each of at least two operating conditions of said AC motor which are caused by said output frequency command and in which the rates of change of output frequencies are different, and a detected signal from said current detecting means in each of said operating conditions. An apparatus according to claim 1, characterized in that said moment-of-inertia detecting means is moment-of-inertia detecting means (A, B) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of an amount of change of output frequencies for two operating conditions of either accelerating or decelerating operating condition and a constant speed operating condition in which the rate of change of output frequency is 0, and of a detected signal from said current detecting means. An apparatus according to claim 1, characterized in that said moment-of-inertia detecting means is moment-of-inertia detecting means (A, B) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of amounts of change of output frequencies for at least two accelerating or decelerating operating conditions in which the rates of change of output frequencies are different, and of a detected signal from said current detecting means. An apparatus according to claim 1, characterized in that said moment-of-inertia detecting means comprises: first torque detecting means (A, 17, 20, 21, 22) for detecting a signal proportional to the torque of said AC motor at a constant speed on the basis of a current detected signal (iu, iw) from said current detecting means; second torque detecting means (A, 17, 20, 21, 23) for detecting a signal proportional to the torque of said AC motor at a changing speed on the basis of a current detected signal (iu, iw) from said current detecting means; and thereby a total moment of inertia of said AC motor and said load connected thereto is detected on the basis of the detected signal from said first torque detecting means, and the detected signal from said second torque detecting means, and the rates of change of said output frequencies. An apparatus according to claim 1, characterized in that said means (8) for producing the output frequency command changed with the change of said speed command, comprises means (7, 28) for changing the output frequency at least twice at different rates; and said moment-of-inertia detecting means comprises means (30) for detecting a total moment of inertia of said AC motor and said load connected thereto from the mean value of the primary current of said AC motor and rates of frequency change (Δf 1 , Δf 2 ) in the speed change periods (Δt 1 , Δt 2 ) according to said output frequency change in twice. A motor speed controller for controlling an AC motor (6) connected to a load (25) to be driven at a variable speed according to a speed command (N*), characterized in comprising: current detecting means (5) for detecting the primary current of said AC motor; means (8) for changing an output frequency command (f 1 *) with the change of said speed command and producing said output frequency command; moment-of-inertia detecting means (26, 30) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the amounts of change of output frequency at least in two operating conditions which are caused by said output frequency command and in which the rates of change of output frequency are different, and a detected signal from said current detecting means; and control means (8, 28) for controlling the change of said output frequency command in accordance with the detected moment-of-inertia. A motor speed controller according to claim 6, characterized in that said moment-of-inertia detecting meals is moment-of-inertia detecting means (A, B) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the amounts of change of output frequency in two operating conditions, or in either accelerating or decelerating operating condition and a constant-speed operating condition in which the rate of output frequency change is C, and a detected signal from said current detecting means. A motor speed controller according to claim 6, characterized in that said moment-of-inertia detecting means is moment-of-inertia detecting means (A, B) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the amounts of change of output frequency at least in two accelerating or decelerating operations in which the rates of output frequency change are different, and a detected signal from said current detecting means. A motor speed controller according to claim 6, characterized in that said moment-of-inertia detecting means comprises: first torque detecting means (A, 17, 20, 21, 22) for detecting a signal proportional to the torque of said AC motor at a constant speed on the basis of a current detected signal (iu, iw) from said current detecting means; and second torque detecting means (A, 17 20, 21, 23) for detecting a signal proportional to the torque of said AC motor at a changing speed on the basis of the current detected signal (iu, iw) from said current detecting means, thereby a total moment of inertia of said AC motor and said load connected thereto is detected on the basis of the detected signal from said first torque detecting means, and the detected signal from said second torque detecting means, and the rates of change of said output frequencies, A motor speed controller according to claim 6, characterized in that said means (8) for producing the output frequency command changed with the change of slid speed command, comprises means (7, 28) for changing the output frequency at least twice at different rates; and said moment-of-inertia detecting means comprises means (30) for detecting a total moment of inertia of said AC motor and said load connected thereto from the mean value of the primary current of said AC motor and rates of frequency change (Δf 1 , Δf 2 ) in the speed change periods (Δt 1 , Δt 2 ) according to said output frequency change in twice. A motor speed controller according to claim 6, characterized in further comprising means (27) for detecting a sudden change of said moment of inertia detected at each time of acceleration or deceleration and producing an abnormality signal. A motor speed controller according to claim 6, characterized in further comprising means (15, 27) for detecting a sudden change of said moment of inertia detected at each time of acceleration or deceleration and cutting off the output of an inverter. A method of detecting a moment of inertia of a load (25) of an AC motor (6) in a motor speed controller for controlling the AC motor connected to the load to be changed in output frequency and driven at a variable speed in accordance with a speed command (N*), characterized by comprising: a first step (S1, S10, S11) for judging the change of the output frequency; a second step (S2, S8, S14, S15) for integrating an output torque proportional signal (iq) obtained from the primary current of said AC motor every sampling time (Δt) when it is judged that there exists the change of the output frequency; a third step (S4, S10, S11) for judging that the output torque proportional signal (iq) becomes almost a definite value when it is judged that there doesn't exist the change of the output frequency; and a fourth step (S7, S9, S13) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the rates (Δf) of change of the output frequencies from said first step to said third step and the integrated value obtained from said second step. A method according to claim 13, characterized in that prior to said first step there is provided a fifth step (S16) for storing a load torque proportional signal (iq cost) obtained by detecting the primary current during the operation of said AC motor at a uniform speed, and said second step is provided with; a step (S8) for obtaining a first product of the output torque proportional signal and the sampling time, a step (S8) for obtaining a second product of the load torque proportional signal and the sampling time, a step (S8) for obtaining the difference between the first product and the second product, and a step (S8) for integrating a difference value every sampling time. A method according to claim 13, characterized in that prior to said first step there is provided a fifth step (S16) for storing a load torque proportional signal (iq cost) obtained by detecting the primary current during the operation of said AC motor at a uniform speed, and said second step is provided with; a step (S8) for obtaining the difference between the output torque proportional signal and the load torque proportional signal, difference a step (S8) for obtaining the product of a difference value and the sampling time, and a step (S8) for integrating a difference value every sampling time. A method according to claim 13, characterized in that said first step, second step and third step are at least twice performed against the change of two output frequencies having different change ratios of the output frequencies (S10-S14, S11-S15), and said fourth step (S13) is performed after second time. A method of controlling an AC motor (6) connected to a load (25) to be changed in output frequency and driven at a variable speed in accordance with a speed command (N*), characterized by comprising: a first step (S1, S10, S11) for judging the change of the output frequency; a second step (S2, S8, S14, S15) for integrating an output torque proportional signal (iq) obtained from the primary current of said AC motor every sampling time (Δt) when it is judged that there exists the change of the output frequency; a third step (S4, S10, S11) for judging that the output torque proportional signal (iq) becomes almost a definite value when it is judged that there doesn't exist the change of the output frequency; a fourth step (S7, S9, S13) for detecting a total moment of inertia of said AC motor and said load connected thereto on the basis of the rates (Δf) to change of the output frequencies from said first step to said third step and the integrated value obtained from said second step, and a fifth step for controlling the change of said output frequency in accordance with the moment-of-inertia detected in said fourth step. A method according to claim 17, characterized in that prior to said first step there is provided a fifth step (S16) for storing a load torque proportional signal (iq cost) obtained by detecting the primary current during the operation of said AC motor at a uniform speed, and said second step is provided with; a step (S8) for obtaining a first product of the output torque proportional signal and the sampling time, a step (S8) for obtaining a second product of the load torque proportional signal and the sampling time, a step (S8) for obtaining the difference between the first product and the second product, and a step (58) for integrating a difference value every sampling time. A method according to claim 17, characterized in that prior to said first step there is provided a fifth step (S16) for storing a load torque proportional signal (iq cost) obtained by detecting the primary current during the operation of said AC motor at a uniform speed, and said second step is provided with; a step (S8) for obtaining the difference between the output torque proportional signal and the load torque proportional signal, difference a step (S8) for obtaining the product of a difference value and the sampling time, and a step (S8) for integrating a difference value every sampling time. A method according to claim 17, characterized in that said first step, second step and third step are at least twice performed against the change of two output frequencies having different change ratios of the output frequencies (S10-S14, S11-S15) and said fourth step (S13) is performed after second time.

Inventors: FUJII HIROSHI, IBORI SATOSHI, KAMBARA TAKATSUGU, NAKAMURA ATSUHIKO, FUJII, HIROSHI, IBORI, SATOSHI, KAMBARA, TAKATSUGU, NAKAMURA, ATSUHIKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H02P7
IPC codes: H02P27, H02P21