Control system for an induction motor

What is claimed is:

1. A control system for an induction motor for independently commanding torque current components and exciting current components of a stator current group sent to the stator of an induction motor, and for changing the amplitude and the frequency of the stator current group so as to control the induction motor comprising;

a torque detector for detecting an output torque of the induction motor, and

torque control means for changing the amplitude of the stator current group based on a command torque of the induction motor and the amplitude of a predetermined exciting current component, and for changing the frequency of the stator current group based on the amplitude of the exciting current component, the command torque and the output of the torque detector so as to control the output torque of the induction motor.

2. The control system for an induction motor according to claim 1, further comprising;

a current detector for measuring or estimating the stator current group,

a dq-axis current converter for converting the output of the current detector into a two-phase alternating current of a predetermined d-axis stator current in a d-axis direction and a q-axis stator current in a q-axis direction whose phase differs by 90.degree. from the d-axis direction,

a rotor flux estimator for estimating a d-axis rotor flux in the d-axis direction and a q-axis rotor flux in the q-axis direction of the induction motor,

wherein the torque detector estimates the output torque of the induction motor from the d-axis stator current, the q-axis stator current, the d-axis rotor flux and the q-axis rotor flux.

3. The control system for an induction motor according to claim 1, wherein the torque control means changes the amplitude and frequency of the stator current group based on the command torque of the induction motor and the amplitude of the predetermined exciting current component, compares the command torque with the output of the torque detector so as to correct the frequency of the stator current group.

4. A control system for an induction motor for independently commanding a torque current component and an exciting current component of a stator current group supplied to a stator of an induction motor, and changing an amplitude and a phase of the stator current group to control an output torque of the induction motor, comprising;

a current detector for measuring or estimating the stator current group,

a dq-axis current converter for converting the output of the current detector into a two-phase alternating current of a predetermined d-axis stator current in a d-axis direction and a q-axis stator current in a q-axis direction whose phase differs by 90.degree. from the d-axis direction,

a rotor flux estimator for estimating a d-axis rotor flux in the d-axis direction and a q-axis rotor flux in the q-axis direction of the induction motor,

a first speed estimator for calculating a first numerator portion of an estimated speed from the d-axis stator current, the d-axis rotor flux and a constant unique to the induction motor, and dividing the first numerator portion of the estimated speed by the q-axis rotor flux so as to estimate the rotational speed of the induction motor,

a second speed estimator for calculating a second numerator portion of an estimated speed from the q-axis stator current, the q-axis rotor flux and a constant unique to the induction motor, and dividing the second numerator portion of the estimated speed by the d-axis rotor flux so as to estimate the rotational speed of the induction motor,

an estimated speed switch for switching between output values of the first and second speed estimators so as to determine the rotational speed of the induction motor, and

torque control means for determining the amplitude of the torque current component based on a command torque of the induction motor, and changing the phase of the stator current group based on the amplitude of the torque current component, the amplitude of a predetermined exciting current component and the output of the estimated speed switch so as to control the output torque of the induction motor.

5. The control system for an induction motor according to claim 4, further comprising a rotational speed control means for comparing a command speed of the induction motor with the output of the estimated speed switch so as to compute the command torque of the induction motor,

wherein the torque current component and the exciting current component of the stator current group supplied to the stator of the induction motor are independently commanded, and the amplitude and phase of the stator current group are changed to control the rotational speed of the induction motor.

6. The control system for an induction motor according to claim 5, wherein the rotational speed control means includes torque current command value generating means for comparing the command speed of the induction motor with the output of the estimated speed switch so as to change the amplitude of the command value of the torque current component, phase converting means for changing the phase of a command value of the stator current group based on the amplitude of the command value of the torque current component, the amplitude of a command value of the predetermined exciting current component and the output of the estimated speed switch, dq-axis current command value generating means for generating the command values of the d- and q-stator currents based on the amplitude of the command value of the torque current component, the amplitude of a command value of the exciting current component and the output of the phase converting means, current control means to output d- and q-axis control signals so that the output of the dq-axis current converter matches the corresponding command value of the dq-axis current command value generating means, and control signal distributing means for convening the output of a current control means into a control signal of a stator current output unit which outputs the stator current group, and

wherein the rotor flux estimator estimates the d- and q-axis rotor fluxes based on the output of the dq-axis current converter, the output of the current control means and a constant unique to the induction motor.

7. The control system for an induction motor according to claim 4, wherein the estimated speed switch compares the absolute value of one of the d- and q-axis rotor fluxes with a predetermined threshold, switches to the output of the first speed estimator if the absolute value is smaller than the threshold, and switches to the output of the second speed estimator if the absolute value is greater than the threshold so as to determine the rotational speed of the induction motor.

8. The control system for an induction motor according to claim 4, wherein the estimated speed switch switches to the output of the first speed estimator if the absolute value of the d-axis rotor flux is smaller than that of the q-axis rotor flux, and switches to the output of the second speed estimator if the absolute value of the d-axis rotor flux is greater than that of the q-axis rotor flux so as to determine the rotational speed of the induction motor.

9. The control system for an induction motor according to claim 4, wherein the estimated speed switch switches between the output of the first speed estimator and that of the second speed estimator based on the phase of the stator current group so as to determine the rotational speed of the induction motor.

10. The control system for an induction motor according to claim 4, wherein the estimated speed switch compares the absolute values of the d- and q-axis rotor fluxes with a predetermined threshold, and outputs the information that the induction motor is stopped if the absolute values of the d- and q-axis rotor fluxes are smaller than the predetermined threshold.

11. The control system for an induction motor according to claim 4, wherein the torque control means includes phase converting means for determining the amplitude of a command value of the torque current component based of the command torque of the induction motor and changing the phase of a command value of the stator current group based on the amplitude of the command value of the torque current component, the amplitude of a command value of the predetermined exciting current component and the output of the estimated speed switch, stator current command value generating means for generating the command value of the stator current group based on the amplitude of the command value of the torque current component, the amplitude of a command value of the exciting current component and the output of the phase converting means, and current control means to output a control signal group so that the output of the current detector matches the corresponding command value of the stator current command value generating means, and

wherein the rotor flux estimator includes dq-axis control signal converter for converting the control signal group of the current control means into a d-axis control signal in the d-axis direction and a q-axis control signal in the q-axis direction so as to estimate the d- and q-rotor fluxes based on the output of the dq-axis current converter, the output of the dq-axis control signal converter and a constant unique to the induction motor.

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FIELD OF THE INVENTION

The present invention relates to a control system for an induction motor capable of controlling the output torque and rotational speed of an induction motor, and to an electric power steering apparatus for a vehicle which generates a suitable auxiliary steering force corresponding to the steering force by means of the control system so as to implement good steering feeling.

A power steering apparatus utilizing a motor has recently been noted because the fuel consumption is less than that of hydraulic operation or the like. However, the electric power steering apparatus according to the prior art uses a motor having a permanent magnet such as a DC motor. Therefore, if the wiring to the motor is short-circuited for some reason, the steering becomes very heavy due to dynamic braking, since the motor performs dynamic braking as a generator. In the worst case, steering cannot be performed. Japanese Laid-Open Patent No. 4-137465 has disclosed that a clutch is provided between a motor and a steering shaft in order to take countermeasures for safety at the time of failure.

The induction motor has a simple structure, so the production costs are lower and maintenance can easily be carried out. However, controllability is poor. Accordingly, the induction motor is not suitable for precisely obtaining a position, speed or torque control. For example, in the case where the torque control is performed by an inverter control according to the prior art, the controllability is much worse than that of a DC motor. In particular, torque cannot easily be obtained in the low-speed area. For this reason, a driving torque cannot be obtained when starting the motor, so the application of the induction motor is restricted. Consequently, the induction motor is not suitable for an electric power steering apparatus which requires a great torque at the time of starting.

However, vector control has been realized with the advancement of electronics. By using vector control, the poor controllability, including starting torque characteristics, has been overcome, so that the controllability can be obtained that is almost the same as that of the DC motor. Accordingly, the application to speed control and the like has been expanded. However, if vector control is applied to the induction motor, a speed sensor such as an encoder for detecting the rotational speed of the induction motor is required when performing torque control. Consequently, the induction motor is less advantageous than other motors with respect to size and cost. As an example in the application of speed control, a method for estimating the rotational speed of the induction motor from the measured current value or the like without using a speed detector has been investigated.

An example of a control system for an induction motor according to the prior art will be described below.

Japanese Laid-Open Patent No. 1-214287 has disclosed a method for estimating the rotational speed of an induction motor using vector control, in which a rotor flux is obtained from a stator current and a stator voltage, and a rotational speed is estimated from the rotor flux and the integral value thereof.

There is an example in which a three-phase induction motor is regarded as a two-phase induction motor by three-phase to two-phase conversion in order to estimate a rotational speed. The basic formula of the two-phase induction motor is expressed in the following equation (1). ##EQU1## wherein i.sub.1d and i.sub.1q are d-axis and q-axis stator currents which flow on the stator side, v.sub.1d and v.sub.1q are voltages, .phi..sub.2d and .phi..sub.2q are d-axis and q-axis rotor fluxes on the rotor side. R.sub.1 and L.sub.1 are a resistance and an inductance on the stator side, and R.sub.2 and L.sub.2 are a resistance and an inductance on the rotor side. M is a mutual inductance, .theta. is an rotational angle of a motor, and p is the number of pole pairs. It has been known that a rotor phase resistance R.sub.2 which greatly varies with the change in temperature, is removed from equation 1 and the rotational speed is estimated from equation 2. By this method, the rotational speed can be obtained without the influence of the change in rotor phase resistance. ##EQU2##

The following method can be used. More specifically, the prior art for speed control is applied to estimate the rotational speed of the induction motor. By using the speed thus estimated, speed control and torque control can be performed by vector control.

For a method using a motor having a conventional permanent magnet, however, there are the following problems. Although countermeasures can be taken against failure by means of a clutch such as an electromagnetic clutch, there is a chance that the clutch will malfunction. By using the clutch, the structure becomes more complicated and larger. In addition, costs may be increased.

Japanese Laid-Open Patent No. 1-214287 has disclosed a control system for an induction motor whose structure is simple. However, there are problems in that the rotary speed is estimated depending on a constant and the precision in estimation of the rotational speed is poor. According to the method (equation 2) in which the rotor phase resistance is removed, the precision in estimation is considerably poorer when the denominator approaches 0 and the response is poor due to smoothing by a filter or the like.

In order to solve the above-mentioned problems, the electric power steering apparatus of the present invention controls a motor, having no permanent magnet, in correspondence to the steering force and generates the auxiliary steering force on a steering system.

A control system for an induction motor according to the present invention comprises a current detector for measuring or estimating the stator current group supplied to the stator of an induction motor; a dq-axis current converter for converting the output of the current detector into a two-phase alternating current of a predetermined d-axis stator current in a d-axis direction and a q-axis stator current in a q-axis direction whose phase differs by 90.degree. from the d-axis direction, a rotor flux estimator for estimating a d-axis rotor flux in the d-axis direction and a q-axis rotor flux in the q-axis direction; a first speed estimator for calculating a first numerator portion of an estimated speed from the d-axis stator current, the d-axis rotor flux and a constant unique to the induction motor, and dividing the first numerator portion of the estimated speed by the q-axis rotor flux so as to estimate the rotational speed of the induction motor; a second speed estimator for calculating a second numerator portion of an estimated speed from the q-axis stator current, the q-axis rotor flux and a constant unique to the induction motor, and dividing the second numerator portion of the estimated speed by the d-axis rotor flux so as to estimate the rotational speed of the induction motor; an estimated speed switch for switching between output values of the first and second speed estimators so as to determine the rotational speed of the induction motor; and torque control means for determining the amplitude of the torque current component based on a command torque of the induction motor, and changing the phase of the stator current group based on the amplitude of the torque current component, the amplitude of a predetermined exciting current component and the output of the estimated speed switch so as to control the output torque of the induction motor.

The present invention can implement an electric power steering apparatus in which an additional mechanism such as a clutch is not required, the structure is simple and steering can still be performed in the event of failure.

Further, the present invention can implement a control system for an induction motor in which the output torque and rotational speed of the induction motor can always be controlled with precision and the response is good.

By using the control system, an electric power steering apparatus which does not need the speed detector of the motor can easily be implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1 is a block diagram showing the entire structure of an electric power steering apparatus according to a first embodiment of the present invention,

FIG. 2 is a block diagram showing the entire structure of an electric power steering apparatus according to a second embodiment of the present invention,

FIG. 3 is a block diagram showing the entire structure of an electric power steering apparatus according to a third embodiment of the present invention,

FIG. 4 is a block diagram showing the entire structure of a control system for an induction motor according to a fourth embodiment of the present invention,

FIG. 5 is a conceptional diagram showing the change in rotor flux with time of an induction motor,

FIG. 6 is a conceptional diagram showing a method for switching a speed estimator according to the fourth embodiment of the present invention,

FIG. 7 is a conceptional diagram showing another method for switching a speed estimator according to the fourth embodiment of the present invention,

FIG. 8 is a block diagram showing the entire structure of a control system for an induction motor according to a fifth embodiment of the present invention,

FIG. 9 is a block diagram showing the entire structure of a control system for an induction motor according to a sixth embodiment of the present invention,

FIG. 10 is a block diagram showing the entire structure of a control system for an induction motor according to a seventh embodiment of the present invention, and

FIG. 11 is a block diagram showing the entire structure of an electric power steering apparatus according to an eighth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electric power steering apparatus according to an embodiment of the present invention will be described with reference to the drawings.

In FIG. 1 showing the entire structure of an electric power steering apparatus according to a first embodiment of the present invention, an induction motor is indicated at 100, a steering wheel is indicated at 102, a steering shaft is indicated at 104, a steering gear is indicated at 106, a wheel is indicated at 108, a steering force sensor is indicated at 110, a reduction gear is indicated at 112, and an electric controller is indicated at 114.

The operation of the electric power steering apparatus having such a structure will be described with reference to FIG. 1.

In a steering apparatus that does not generate auxiliary steering force, a steering wheel 102 provided on a steering shaft 104 is operated so that a wheel 108 is steered through a steering gear 106. In the electric power steering apparatus according to the present embodiment, an induction motor 100 is coupled to the steering shaft 104 through a reduction gear 112. When the induction motor 100 is driven, an auxiliary steering force is generated. An electric controller 114 controls the induction motor 100 in response to the output of a steering sensor 110 provided on the steering shaft 104. Consequently, the desired auxiliary steering force can be obtained.

When an induction motor having no permanent magnet is used as in the present embodiment, it is in a free condition even if the wiring to a motor is short-circuited for some reason. Consequently, the electric power steering apparatus works as a steering apparatus which cannot reduce an auxiliary steering force. Therefore, it is not necessary to use a clutch as a safety mechanism which separates a motor from a steering shaft. The clutch is required for a DC motor having a permanent magnet, because the motor performs dynamic braking as a generator in the case of short-circuit and steering cannot be operated in the worst case. Consequently, there are some problems with respect to safety.

As described above, when the induction motor is used to generate the auxiliary steering force of the electric power steering apparatus, the safety mechanisms that are not necessary for original functions can be omitted. In addition, the size can be made smaller and the structure becomes more simple.

While the induction motor is used in the present embodiment, a motor having no permanent magnet such as a reluctance motor can also be used.

A second embodiment of the present invention provides an electric power steering apparatus which can easily respond according to a torque command when the torque command is given from a steering sensor to an induction motor.

An electric power steering apparatus according to the second embodiment of the present invention will be described with reference to the drawings.

FIG. 2 is a block diagram showing the entire structure of an electric power steering apparatus according to a second embodiment of the present invention.

In FIG. 2, an induction motor is indicated at 100, a steering wheel is indicated at 102, a steering shaft is indicated at 104, a steering gear is indicated at 106, a wheel is indicated at 108, a steering force sensor is indicated at 110, a reduction gear is indicated at 112, a control system for a motor is indicated at 200, a current commander is indicated at 202, a current controller is indicated at 204, a torque current commander is indicated at 206, a three-phase AC converter is indicated at 208, a voltage commander is indicated at 210, a PWM (pulse width modulation) inverter is indicated at 212, and an electric detector is indicated at 214a, 214b and 214c.

The operation of the electric power steering device having such a structure will be described with reference to FIG. 2.

The steering apparatus is the same as that of the first embodiment. When the induction motor 100 connected to the steering shaft 104 through the reduction gear 112 is driven, an auxiliary steering force is generated. The operation of the control system for a motor 200 for controlling the induction motor 100 in correspondence to the output of the steering force sensor 110 attached to the steering shaft 104 will be described in the present embodiment.

First of all, the operation of the current commander 202 will be described. In most cases, a three-phase induction motor is used. The three-phase induction motor can be regarded as a two-phase induction motor by three-/two-phase conversion. A torque .tau. generated by the two-phase induction motor is expressed in equation 3: ##EQU3## wherein i.sub.1d and i.sub.1q are stator d- and q-shaft currents, i.sub.2d and i.sub.2q are rotor d- and q-shaft currents, L.sub.2 is a rotor self-inductance, M is a mutual inductance, and p is the number of pole pairs. I.sub.1d and I.sub.1q are exciting and torque currents which are DC currents. The relationship between i.sub.1d, i.sub.1q and I.sub.1d, I.sub.1q is expressed in equation 4: ##EQU4## wherein .theta..sub.0 is an electric phase angle.

It is assumed that a necessary auxiliary steering force command .tau.* is inputted from the steering sensor by steering. In this case, it is enough that the induction motor gives the auxiliary steering force to the steering shaft. Accordingly, the torque current commander 206 outputs the torque current command value I.sub.1q * given by equation 5. ##EQU5##

The operation of the three-phase AC converter 208 will be described. The slip speed .omega..sub.s is calculated by means of the exciting current command value I.sub.1d * and the torque current command value I.sub.1q * according to equation 6, ##EQU6## wherein R.sub.2 is a stator phase resistance. When the rotational speed of the induction motor is low, the slip speed .omega..sub.s is integrated to approximate to the electric phase angle .theta..sub.0. In the case of strict control, the rotational speed .omega..sub.m of the induction motor is measured or estimated. The rotational speed .omega..sub.m thus obtained is added to the slip speed .omega..sub.s, and the sum is integrated to obtain the electric phase angle .theta..sub.0. Accordingly, the exciting current command value I.sub.1d * and the torque current command value I.sub.1q * are converted into two-phase stator current command values i.sub.1d * and i.sub.1 * having a phase difference of 90.degree. according to equation 7, in a similar manner to equation 4. ##EQU7## The two-phase stator current command values i.sub.1d * and i.sub.1 * are converted into three-phase stator current command values i.sub.1a *, i.sub.1b * and i.sub.1c * according to equation 8. ##EQU8##

The operation of the current controller 204 will be described. Current feedback control is performed in such a manner that actual stator currents i.sub.1a, i.sub.1b and i.sub.1c follow stator current command values i.sub.1a *, i.sub.1b * and i.sub.1c * respectively. For example, the actual stator alternating current is detected by the current detectors 214a, 214b and 214c, and a voltage command value v.sub.1z (z=a, b, c) obtained according to equation 9 is output from the voltage commander 210. ##EQU9## The PWM inverter 212 sends, to the induction motor 100, a signal having a pulse width corresponding to the voltage command value which is a control signal sent from the voltage commander 210, so as to make a current flow. If three-phase stator alternating currents i.sub.1a, i.sub.1b and i.sub.1c sent to the induction motor 100 are added, a value of 0 is obtained as in equation 10.

i .sub.1a +i .sub.1b +i .sub.1c =0 (10)

It is possible to detect two of three currents and calculate the residual current from the two current values which are detected. Thus, the stator alternating current sent to the induction motor can be controlled to be converted into the desired command value. Using current control, the desired torque can be generated by the induction motor. Consequently, it is possible to implement a response corresponding to a command sent from the steering sensor.

While the rotational speed of the induction motor can be detected by a speed sensor fixed to the induction motor, it may be estimated from a current value and the like sent to the induction motor. In addition, the rotational speed of the induction motor can be obtained from the gear ratio of the reduction gear 112 and the steering angle which is measured.

In the case where the winding of the induction motor is short-circuited between winding terminals or partly short-circuited in the motor, the load impedance of a driving circuit greatly decreases so that excessive current flows. Consequently, an output-stage semiconductor may be broken. In such a case, the control is interrupted so as not to send an output current from the current controller 204. If the current flow to the motor is interrupted, the induction motor does not generate braking force and is in a free condition even though the winding is short-circuited. For this reason, the auxiliary steering force is not generated but steering can still be performed manually. In this respect, the induction motor is different from a motor having a permanent magnet. In the case where a direct current flows for some reason, for example, the output-stage semiconductor of the current controller 204 being short-circuited, a great braking force is generated so safety problems exist.

A third embodiment of the present invention provides an electric power steering apparatus capable of stopping the current flow to an induction motor when a direct current is sent to the induction motor.

The electric power steering apparatus according to the third embodiment of the present invention will be described with reference to the drawings.

FIG. 3 shows the entire structure of the electric power steering apparatus according to the third embodiment of the present invention.

In FIG. 3, an induction motor is indicated at 100, a steering wheel is indicated at 102, a steering shaft is indicated at 104, a steering gear is indicated at 106, a wheel is indicated at 108, a steering force sensor is indicated at 110, a reduction gear is indicated at 112, a control system for a motor is indicated at 200, a current commander is indicated at 202, a current controller is indicated at 204, a current detector is indicated at 214a, 214b and 214c, stop means for malfunction is indicated at 300, a malfunction detector is indicated at 302, and a switch is indicated at 304.

The operation of the power steering apparatus and the control system 200 is the same as in the second embodiment. According to the present embodiment, the stop means for malfunction 300 is provided. The operation of the stop means for malfunction 300 will be described.

The malfunction detector 302 checks currents output from respective current detectors and measures the time over which they have the same polarity, i.e., the time necessary for the polarity to change from positive to negative or from negative to positive. If the time over which the polarity of at least one current does not change is greater than a predetermined constant value, it is decided that DC components are flowing due to some failure so that braking occurs. The switch 304 is turned off so as to stop the braking operation. Consequently, even though the DC components may flow for some reason, the current flow to the induction motor is interrupted so that steering can be maintained.

In general, steering is controlled to be harder for safety reasons when running on the highway. For this purpose, a direct current flows to perform dynamic braking. In the case where a DC command is given, the malfunction detector 302 compares the time in which the polarity of the actual current value output from the current detector is not changed with the time in which the polarity of a corresponding current command value output from the current commander 202 is not changed. If the time in which the polarity of the actual current value is not changed is longer than the time in which the polarity of the current command value is not changed by a predetermined constant value or more, the switch 304 is turned off.

While a switch for cutting off a current is provided following the control system 200 in the present embodiment, it may be provided in a part of the control system in which a stator current command value is given or following the voltage commander.

The electric power steering apparatus has been described in the above-mentioned embodiment. Also in the case where the induction motor of the electric power steering apparatus is controlled, the information for the rotational speed of the induction motor is necessary. However, if a speed sensor for measuring the rotational speed of the induction motor is used, the size of the unit is increased and costs become higher. A control system for an induction motor having no speed sensor will be described in the following embodiment.

The control system for an induction motor according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 4 shows the entire structure of a control system for an induction motor according to a fourth embodiment of the present invention.

In FIG. 4, an induction motor is indicated at 100, two-/three-phase converters are indicated at 120 and 122, a rotor flux estimator is indicated at 124, a speed estimator A is indicated at 126, a speed estimator B is indicated at 128, an estimated speed switch is indicated at 130, a speed controller is indicated at 132, a rotary/static coordinate converter is indicated at 134, a two-/three-phase converter is indicated at 136, a slip frequency computing unit is indicated at 138, an adder is indicated at 140, an integrator is indicated at 142, a voltage commander is indicated at 210, a PWM inverter is indicated at 212, and current detectors are indicated at 214a, 214b and 214c.

FIG. 5 is a conceptional diagram showing the change in rotor flux with time of the induction motor.

FIG. 6 is a conceptional diagram showing a method for switching the speed estimator according to the fourth embodiment of the present invention.

FIG. 7 is a conceptional diagram showing another method for switching the speed estimator according to the fourth embodiment of the present invention.

The speed control of the control system for a motor having such a structure will be described with reference to FIGS. 4, 5 and 6.

In the same manner as the control system for an induction motor according to the prior art and the second embodiment, a signal having a pulse width corresponding to a voltage command value, which is a control signal sent from the voltage commander 210, is sent from the PWM inverter 212 to the induction motor 100. In this case, three-phase stator alternating currents i.sub.1a, i.sub.1b and i.sub.1c sent to the induction motor 100 are detected by the current detectors 214a, 214b and 214c.

The speed controller 132 calculates a torque current command value I.sub.1q * from a rotational speed command value .omega..sub.m * and a rotational speed estimated value .omega..sub.me of the induction motor according to equation 11. ##EQU10## In the case of control according to equation 11, if the precision in estimating the rotational speed value is high, it is possible to make the rotational speed of the induction motor follow the rotational speed command value so that speed control can optionally be performed. The slip frequency computing unit 138 calculates a slip speed .omega..sub.s, using equation 12, from an exciting current command value I.sub.1d * and a torque current command value I.sub.1q * which are predetermined constant values. ##EQU11## The slip speed .omega..sub.s and the rotational speed estimated value .omega..sub.me are added by the adder 140. Then, a value thus obtained is integrated by the integrator 142, so that an electric phase angle .theta..sub.0 is obtained. Two-phase stator current command values having a phase difference of 90.degree. are obtained from equation 13 using the exciting current command value I.sub.1d *, the torque current command value I.sub.1q * and the electric phase angle .theta..sub.0 by means of the rotary/static coordinate converter 134. ##EQU12## The two-phase stator current command values are then converted into three-phase stator current command values i.sub.1a *, i.sub.1b * and .sub.1c * using equation 14 by means of the two-/three-phase converter 136. ##EQU13##

Further, current feedback control is performed by the voltage commander 210 in such a manner that the actual stator alternating current follows the stator current command value. For example, a voltage command value v.sub.1z (z=a, b, c) obtained using equation 15 is output from the current detectors 214a, 214b and 214c. Consequently, the stator alternating current sent to the induction motor can be controlled to have the desired command value. ##EQU14##

As described above, the control system for an induction motor has the same structure as a vector controller having a speed sensor according to the prior art. A structure in which the rotary speed of the induction motor is estimated will be described below.

The three-/two-phase converter 120 converts the outputs of the current detectors 214a, 214b and 214c into two-phase alternating currents i.sub.1d and i.sub.1q according to equation 16. ##EQU15## The three-/two-phase converter 122 converts three-phase voltage command values v.sub.1z (z=a, b, c) into two-phase AC voltages v.sub.1d and v.sub.1q according to equation 17. ##EQU16## Rotor fluxes .phi..sub.2d and .phi..sub.2q can be estimated from the two-phase alternating currents and the two-phase AC voltages by means of the rotor flux estimator 124 according to equations 18 and 19. ##EQU17## In the same manner as the prior art, two equations for estimating the rotational speed .omega..sub.me of the induction motor can be obtained from the basic equation (13) of the induction motor which is a two-phase model. ##EQU18## According to equations 20 and 21, a denominator may have a value of 0 in a similar manner to the prior art. In this case, the precision in estimating the rotational speed is poor in the vicinity of 0. In the case where the induction motor has no current flow, i.e., the induction motor stops or a speed is not controlled, the rotor flux .phi..sub.2q which is a denominator of equation 20 and the rotor flux .phi..sub.2d which is a denominator of equation 21 have a value of 0. When the induction motor is driven, the rotor flu