JPH0667273B2 - How to restart the induction motor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for controlling an induction motor by a PWM inverter, and restarts a speed control or torque control system that does not use a speed sensor, that is, a normal coasting operation. The present invention relates to the improvement of the restarting method after the restart or after the recovery from the momentary power failure.

[Conventional technology]

The method of controlling an induction motor by a PWM inverter, which is generally called by the name of TG-less or PG-less speed control or torque control, is a method of controlling the current, without using a speed sensor such as TG (tacogene) PG (pulse generator). The feature is that the deviation is controlled by calculating torque, speed, etc. from the voltage signal.

For example, the torque control method to which the present invention is applied is described in a paper "New High Speed Torque Control Method for Induction Motor Based on Instantaneous Slip Frequency Control" on page 9, No. 1, page 9 of the Institute of Electrical Engineers of Japan B. .

In this paper, the motor input voltage is detected and integrated in the control circuit to obtain the motor magnetic flux. That is,
This is a so-called magnetic flux calculation type control method, and selects an inverter output voltage that follows a magnetic flux command given the length of the magnetic flux vector and draws a circular locus.

Further, the motor generated torque is calculated as a vector product of the magnetic flux and the motor input current, and the inverter output voltage whose magnitude follows the given torque command is selected. The control is performed so that the instantaneous values of the magnetic flux and the torque are kept within a predetermined error, and the inverter output voltage is updated at high speed every moment.

FIG. 2 is a block diagram of a torque control system that employs the output voltage detection method of the PWM inverter proposed by the present applicant in Japanese Patent Application No. 61-99228 in the control method described in the above paper. Power is supplied from the voltage source 1 to the three-phase induction motor 6 via the switch 4, the positive bus 1a and the negative bus 1b and the three-phase PWM inverter 3. The control circuit 7 processes the command and the detected current and voltage signals, and generates an energization signal for the switching element of the PWM inverter 3.

The PWM inverter 3 is composed of six arms, each of which has a transistor and a diode connected in antiparallel, and can be represented by three changeover switches Su, Sv, Sw as shown in the figure.

Current detector 5u, 5v, 5w from each output terminal of PWM inverter 3
Power is supplied to the three-phase induction motor through the voltage detector, and the voltage detector 2 is connected between the positive and negative buses on the DC side, so that each phase current and each phase voltage can be detected from these detectors and switch state variables described later. There is.

The primary terminal voltage and current of the three-phase squirrel cage induction motor And the secondary current is Then, the voltage equation is However, the symbol Is a vector display of the quantity converted to the straight axis, the horizontal axis, that is, the d and q axes. For example, Is the d-axis component is V ₁ d and the q-axis component is v ₁ q, Indicated by Is similarly defined. The left side of represents the case where both the d and q axis components are 0, and in the case of the squirrel cage rotor, the secondary voltage is as follows.

The constant in the formula is R ₁ ; Primary winding resistance L ₁₁ ; Primary inductance R ₂ ; Secondary winding resistance L ₂₂ ; Secondary inductance M: Mutual inductance m is rotational angular velocity, p is differential operator, j is vector Represents the product.

On the other hand, as the definition of magnetic flux, the primary magnetic flux Is Expand the first line of the expression Substituting expressions and rearranging If you integrate both sides That is, the primary magnetic flux of the electric motor is obtained by the integral calculation of the equation.

The changeover switches Su, Sv, Sw may fall to the positive bus bar 1a side or to the negative bus bar 1b side, and do not take intermediate positions. When the former is the state 1 and the latter is the state 0, the output states of the inverter can all be expressed by the switch state variable table shown below.

Here, k is a number indicating the state of the changeover switch, and there are only eight of these. Also Is the switch state variable expressed by the d and p 2-axis components, and the actual d and q
The axial voltage v ₁ d, v ₁ q is the same as the voltage V of the DC voltage source 1 Multiply by Can be expressed as

The switch state variable table is shown in FIG.
The parentheses next to v ₁ indicate the states of the changeover switches Su, Sv, Sw, and represent the voltage vector that steps clockwise by 60 ° as k increases.

Note that k = 0 and k = 7 are called zero vectors and coincide with the origin in the figure. k = 0 and k = 7 are the output of the inverter, respectively, and the changeover switches Su and S of FIG.
v, Sw fall to all positive bus 1a side, or negative bus 1d
The line voltage of the induction motor 6 is 0 in all, although there is a difference whether it falls to the side, which is a three-phase short-circuit mode. Also, u,
The reference axes of the v and w phases are k =
It corresponds to the direction of 1, k = 3, k = 5.

The instantaneous torque T is the primary magnetic flux of the formula And primary current It is calculated by the formula as the vector product of.

Where φ ₁ d, φ ₁ q and i ₁ d, i ₁ q are the primary magnetic flux, respectively. And primary current These are the components when is decomposed into the d and q axes.

Blocks 701 and 703b indicate the primary terminal voltage based on the states of the changeover switches Su, Sv, Sw and the voltage V of the DC voltage source 1 detected by the voltage detector 2. Is a block for calculating a switch state variable table and an equation.

Block 702 detected by current detector 5u, 5v, 5w 3
This block converts the phase currents iu, iv, iw into the d, q2 axis components by the following equation.

This primary current Is multiplied by the primary winding selection resistance R ₁ in block 703a and the primary terminal voltage is multiplied in block 704. To primary winding resistance R ₁ and primary current Subtract the product of.

The block 705 is a block for integrating the magnetic flux according to the formula The d and q biaxial components φ ₁ d and φ ₁ q are obtained, and in block 710, the magnetic flux vector length φ ₁ is obtained by the following equation.

Further, in block 710, as shown in the magnetic flux state diagram of FIG. Depending on which region the clockwise rotation angle θ with respect to the d-axis of the vehicle is divided by 60 ° of 30 °, 90 °, 150 °, 210 °, 270 °, 330 ° as a boundary line The control flag fθ is generated as follows: −30 ° ≦ θ <30 °; fθ = I 30 ° ≦ θ <90 °; fθ = II 90 ° ≦ θ <150 °; fθ = III 150 ° ≦ θ <250 °; fθ = IV 210 ° ≤ θ <270 °; fθ = V 270 ° ≤ θ <330 °; fθ = VI Fig. 6 is a state diagram of the hysteresis comparator. The magnetic flux vector length φ ₁ is the magnetic flux command value ▲ φ ^* Error margin Δφ for ₁ ▼
Using, A control flag fφ for controlling so that That is, the magnetic flux vector length φ ₁ increases and is the upper limit. Control flag fφ = 0 for demagnetization is generated, and the magnetic flux magnetic flux vector length φ ₁ is decreased, which is an adjustment. Control flag fφ = 1 for instructing the magnetization is generated.

Thus, the magnetic flux vector length φ ₁ is controlled so as to draw a limit cycle in the direction of the arrow shown in FIG. 6, but in reality, the magnetic flux vector length φ ₁ calculated by the equation in block 706. Is subtracted from the magnetic flux command value ▲ φ ^* ₁ ▼ in block 708, and a control flag fφ = 1,0 is generated in block 711 according to the state control diagram of FIG.

The limit cycle of the magnetic flux shown in FIG. 6 is the primary magnetic flux with respect to FIG. This corresponds to the fact that the head of the vector is controlled to always exist in the illustrated torus part.

The control flag fφ according to FIG. 6 and the control flag fθ described with reference to FIG. 4 are combined, and for example, fθ = 1, fθ = I
If the control flag of is set, the area is −30 ° ≦ θ
Since it means the magnetization mode at <30 °, the primary magnetic flux Primary voltage to be integrated into vector The vector will have an outward component of the circle,
From the figure, only k = 1, 2, or 6 may be selected.

Block 707 is a block which calculates the calculated instantaneous torque T a vector product between the output of the block 702, 705 by the equation, by subtracting the instantaneous torque T from the torque command T ^* at block 709, determined by the torque command T ^* and the formula A control flag f _τ is generated in block 712 according to the state control diagram of FIG. 7 so that the difference from the instantaneous torque T can be suppressed within a predetermined error limit.

FIG. 7 is a state diagram of the three-value hysteresis comparator, and the torque deviation T ^* -T is the upper limit value ΔT ₁ (ΔT ₁ >
When 0) is reached, the acceleration mode control flag f _τ = 1 is generated. When the motor is accelerated and the torque deviation is T lower limit value-Δ
When T ₂ (ΔT ₂ > 0) is reached, the zero vector mode control flag f _τ = 0 is generated, and the torque gradually decreases, the stock difference increases again, and when the upper limit value ΔT ₁ is reached, the mode shifts to the acceleration mode. Draw a limit cycle that goes around the hysteresis loop in the upper half of Fig. 7 in the direction of the arrow.

When this is expressed in the time domain, the instantaneous torque T fluctuates as shown in the torque waveform diagram of FIG. 5, and it reciprocates within the band of the upper and lower deviations ΔT ₁ + ΔT ₂ across the torque command T ^* .

Next, when the motor is performing regenerative braking, a hysteresis loop in the lower half of FIG. 7 is drawn, and when the torque deviation reaches the negative lower limit value ΔT ₁ (ΔT ₁ > 0), the deceleration mode Generate the control flag f _τ = −1. After that, the limit cycle in the direction of the arrow is repeated as in the case of power running. Thus, block 712 outputs the control flag f _τ = 1,0, -1.

Bunlock 713 is output from blocks 710,711,712 3
Number of control flags fθ, fφ, a block for determining the most suitable inverter output voltage to each combination of f _tau, 4
Primary magnetic flux explained in the figure Of the vector length and rotation direction of these three control flags f
It is controlled by θ, fφ, and f _τ .

For example, when the control flags fθ = 1 and fφ = I as described above, the voltage vector is calculated according to k in the switch state variable table. , It is possible to select one of k = 1, 2, and 6 as the voltage vector, but if the control flag f _τ = 1 at this time, a vector having a component rotating clockwise k = 2, that is, the output voltage vector Is selected. If f _τ = -1, When f _τ = 0, it is a zero vector, Is selected.

The following switching table shows three control flags f
The values of the number k of the output voltage vector are shown for all combinations of θ, fφ, and f _τ. By referring to this switching table in block 713 every operation cycle, a switching signal is sent to the inverter 3, Instantaneous control of magnetic flux and torque is performed.

The inverter frequency is the primary magnetic flux in Fig. 4. It can be considered as the rotational speed of the vector, but this is not given from the outside but occurs as a result of integrating the voltage vector by the formula.

[Problems to be solved by the invention]

In the technique described above, since the rotation speed sensor is not used for calculating the speed and torque, the target speed for restart cannot be determined if an instantaneous power failure occurs. Since the speed feedback signal can be used even if the switch 4 is opened due to the momentary power failure in the conventional torque control using the rotation speed signal of the electric motor, the voltage-type inverter frequency at the time of restoration is the speed feedback signal. Can be determined from However, in the control method described above, when the switch 4 is opened due to an instantaneous power failure, the primary voltage And primary current Also becomes zero or does not show the value at the normal time, so that the inverter frequency at the time of restoration cannot be calculated.

Therefore, in order to deal with such a case, the motor is stopped once and then restarted, or a weak induced voltage due to the residual magnetic flux of the motor is detected and amplified by a high-gain amplifier, so that It has been common to obtain a rotation frequency, generate an inverter frequency corresponding to this, and restart.

The present invention has been made to make it possible to easily perform a restart during a normal coasting operation or a restart after restoration from an instantaneous power failure without performing such a complicated operation.

[Means for solving problems]

In order to perform synchronous pull-in quickly with a small shock, the upper limit value ΔT ₁ and the lower limit value ΔT ₂ of the operating point of the block 712, which is a torque hysteresis comparator, are reduced and the initial value of the control flag, which is the output, is forced to f _τ. = 1 so that the magnetic flux can be calculated. Therefore, the voltage detector 2 is connected to the switch 4
Than the DC voltage source 1 side. By doing so, the voltage feedback signal is effective even if the switch 4 is opened, so that the virtual primary magnetic flux generated if the switch 4 is closed. Is calculated in the calculator. By doing so, the magnetic flux calculation can be started before the switch 4 is closed.

At this time, the magnetic flux command value ▲ φ ^* ₁ ▼ is set to a small value, and when the normal DC voltage source voltage V is applied, the primary magnetic flux The rotation speed of the vector is at least set to a value not lower than the maximum inverter frequency, the torque command T ^* is set to a sufficiently small value, and the switch 4 is closed after the power failure is released.

Until the synchronous pull-in phenomenon ends, the magnetic flux command value ▲ φ ^* ₁ ▼,
The torque command T ^* and the upper limit value ΔT ₁ and the lower limit value ΔT ₂ of the operating point of the block 712 of the hysteresis comparator are restricted to small values, and the restriction is released when the synchronous pull-in is completed.
If the power failure time becomes long and the voltage drops to near zero, the magnetic flux calculation becomes impossible, but in such a case, the residual magnetic flux of the motor also drops and the shock due to synchronous pull-in disappears. Therefore, you can start it normally.

[Work]

Before closing the switch 4, the calculated primary magnetic flux The rotation speed of the vector is set to be necessarily higher than the actual rotation speed of the electric motor. When the switch 4 is closed, the control flag f _{τ is set to} 1 at first, so that the magnetizing current flows through the motor to generate the instantaneous torque T, and the difference from the torque command T ^* is immediately the lower limit value −ΔT. _When it exceeds ₂ , the control flag f _τ = 0 and the inverter generates a zero vector, and the primary magnetic flux Stop vector rotation.

Then, the instantaneous torque T decreases, and the control flag f _τ becomes 1 again.
And the primary magnetic flux The vector rotates. Repeat this operation to 1
Secondary magnetic flux The average rotation speed of the vector matches the rotation speed of the electric motor,
Complete synchronization.

During this time, since the torque command T ^* is restricted to a small value, the shock of the synchronous pull-in is suppressed to a small value. Details of this operation will be described in the following embodiments.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of a method of restarting an induction motor according to the present invention. The same reference numerals as those in FIG. 2 indicate the same functional parts, and the portions different from FIG. 2 are torque commands T ^* , The magnetic flux command value ▲ φ ^* ₁ ▼ is suppressed to a small value for a predetermined time, and a block is added to change the upper limit value ΔT ₁ and the lower limit value ΔT ₂ of the operating point of the block 712 which is a torque hysteresis comparator. The detector 2 is moved from the switch 4 to the DC voltage source 1 side.

Reference numeral 11 is a restart command generator that issues a restart command, and generates a signal when the power is restored from an instantaneous power failure or when restarting during normal coasting operation. Reference numeral 12 is a restart time limiter that generates a logic signal that becomes active only for a predetermined time.The torque error width controller 13, the magnetic flux command controller 14, and the torque command controller 15 are shown below.
Performs a series of operations for restarting the motor only when the restart timer 12 is active.

The torque error width controller 13 is for changing the upper limit value ΔT ₁ and the lower limit value ΔT ₂ of the operating point of the block 712, which is a torque hysteresis comparator, to small values, and increases only when the restart time limiter 12 is active. , The lower limit values ΔT ₁ and ΔT ₂ are made small so that the hysteresis comparator can generate the control flag f _τ = 1 even for a small torque command T ^* as described later.

The torque command controller 15 blocks the torque command T ^* given from the outside only while the restart time limiter 12 is active, and instead gives a small torque command T ^* ′ provided internally as an input to the control circuit 7. Is.

The magnetic flux command controller 14 blocks the magnetic flux command value ▲ φ ^* ₁ ▼ given from the outside only while the restart timer 12 is active,
Instead, a small magnetic flux command value included inside ▲ φ ^* ₁ ▼ ′
Is given as an input to the control circuit 7.

When the motor restart signal is issued from the restart command generator 11, the control circuit 7 replaces the magnetic flux command value ▲ φ ^* ₁ ▼ and the torque command T ^* given from the outside by the above operation, and synchronizes at the restart. Small magnetic flux command value for pulling in ▲ φ
^* ₁ ▼ ', torque command T ^* ' are given. If such a preparation is made, the voltage detector 2 detects the voltage V of the DC voltage source 1 even if the switch 4 is open, so that the virtual primary magnetic flux generated if the switch 4 is closed. Is calculated.

At this time, since the magnetic flux command value ▲ φ ^* ₁ ▼ ′ is small, the primary magnetic flux The rotation speed of is faster than usual. The magnitude of the magnetic flux command value ▲ φ ^* ₁ ▼ 'is set so that this rotation speed becomes higher than the maximum operating frequency of the inverter.

After returning from the momentary power failure, make the above preparations and then close the switch 4.

FIG. 8 is a motor torque speed curve for explaining the synchronous pull-in operation of the present invention, where the horizontal axis represents torque and the vertical axis represents motor rotation speed. Now, the operating point at the time of a momentary power failure occurs is P, the electric motor by momentary power failure will lose the generated torque, the operating point and moved to P _0.

The point P ₀ with the passage of time is moved toward the upper vertical axis at the origin 0, considered to the instantaneous not move the point P ₀ is. Therefore, before the occurrence of an instantaneous power failure, that is, the normal magnetic flux command value ▲ φ ^* ₁ ▼
Torque speed curve C with maximum torque Tm corresponding to
On the other hand, the torque speed curve C ₀ ′ corresponding to the small magnetic flux command value ▲ φ ^* ₁ ▼ ′ for restart is a curve with a high synchronous rotation speed N ₀ ′ and a small maximum torque Tm ′.

Now, in order to perform the synchronous pull-in without shock, the torque speed curve C ₀ passing through the no-load operating point P ₀ of the electric motor may be realized by changing the inverter frequency. The principle of realizing the tor speed curve C ₀ with no load is described below, in which the motor bears the load only after the synchronous pull-in is completed.

When the switch 4 is opened after the momentary power failure occurs and the current no longer flows in the main circuit, the motor torque becomes zero and the calculated value of the instantaneous torque T becomes zero, but the magnetic flux can be calculated and the switch 4 is switched on. Virtual primary magnetic flux equivalent to the magnetic flux generated in the induction motor 6 when closed Is calculated.

However, since the calculated value of the instantaneous torque T is zero as described above, the feedback to the block 709 is also zero, and the hysteresis comparator of the block 712 always operates the power running control flag f _τ = 1 even for a small torque command T ^* . Keep issuing.

On the other hand, since the magnetic flux command value ▲ φ ^* ₁ ▼ 'is given a small value, the virtual primary magnetic flux generated by the calculation The locus of the arrowhead of the vector draws a circle with a small radius that almost follows the magnetic flux command value ▲ φ ^* ₁ ▼ '.

At this time, between the magnetic flux vector length φ ₁ , the inverter output voltage maximum value v ₁ , and the magnetic flux vector rotation speed, that is, the frequency f ₁ , There is a relational expression, and the frequency becomes higher as the magnetic flux is weakened. However, in the formula, the maximum value of the inverter output voltage refers to a state in which the power running control flag f _τ = 1 is always output, and does not include the zero vector, and only the voltage vector that produces the maximum acceleration torque in the reference rotation direction. Indicates the voltage value when the is output.

Torque speed curve for Figure 8 curve C ₀ 'small flux command value ▲ φ ^* ₁ ▼', the synchronous rotational speed N ₀ 'is less flux vector length phi ₁ to phi _{1 expression'} obtained by substituting It is determined by the frequency f ₁ ′ and the number of poles P _N of the induction motor 6. However, here the magnetic flux vector length φ ₁ ′ is the magnetic flux command value ▲
It is the calculated magnetic flux vector length obtained by φ ^* ₁ ▼ ′.

The curve C ₀ ′ can be considered as a virtual torque speed curve of the induction motor 6 at the moment when the switch 4 is turned on.

When the switch 4 is turned on, a current flows through the electric motor, and a calculation instantaneous torque T is generated. At the moment the switch 4 is turned on, the magnetic flux inside the motor has not yet risen, and the motor does not immediately generate torque, but it generates torque as the magnetic flux rises, and the point P ′ on the curve C ₀ ′. Try to accelerate slightly along the curve C ₀ ′.

However, the torque command T ^* ′ is given a small value, and the upper limit value ΔT ₁ and the lower limit value ΔT ₂ of the operating point of the torque hysteresis comparator of the block 712 are also set small, so that the control flag f _τ is immediately set. Changing from 1 to 0, the inverter outputs a zero vector.

When a zero vector is output, the primary magnetic flux The vector stops and the instantaneous torque T decreases. Stopping the magnetic flux vector for a certain period of time means that the average rotational speed, that is, the frequency of the magnetic flux vector becomes smaller than f ₁ ′ in the equation.

When the instantaneous torque T decreases due to the output of the zero vector, the torque hysteresis comparator again generates the acceleration control flag f _τ = 1. Since the inverter outputs a non-zero voltage, the magnetic flux vector rotates again and the instantaneous torque T also increases. Similarly, the torque hysteresis comparator control repeatedly outputs the flag f _τ = 1 and 0 alternately.

That is, the magnetic flux vector is rotated by the frequency of the equation,
By repeating stopped by the output of zero vector, reaches the frequency f ₀ corresponding to the point P ₀ as the average value However, t ₀ and t ₁ are the times during which the inverter outputs a zero vector and a non-zero vector in the unit time, respectively.

In this way, the speed of the electric motor hardly changes, the curve C ₀ ′ descends in the direction of the arrow a, reaches the curve C _0, and the synchronization is performed.

Since the motor torque at this time is the torque at the point P ₀ ′ in FIG. 8, it is equal to the torque at the intersection of the line segment P ₀ P and the curve C ₀ ′, and is the maximum during the process of the curve C ₀ ′ descending. Torque Tm ′
Reach

Therefore, in order to reduce the shock of synchronization, the magnetic flux command value ▲ φ ^* ₁ ▼ ′ and the torque command T ^* ′ are reduced to make the maximum torque Tm ′ on the torque speed curve C ₀ ′ as small as possible. , The lower limit values ΔT ₁ and ΔT ₂ of the torque hysteresis comparator must be made sufficiently small so that the zero vector is easily generated.

If the torque hysteresis comparator lower limit value Δ
When T ₁ and ΔT ₂ are excessively large, the conversion cycle of the control flag f _τ = 1,0 becomes long, and therefore the curve C ₀ ′ may go beyond the target curve C ₀ and reach the curve C ₀ ″. In this case, since the inverter frequency becomes lower than the motor frequency, the brake torque is generated, and the action of pulling up the curve C ₀ ″ in the direction of the curve C ₀ occurs, but the time required for synchronization becomes longer and the shock also becomes larger.

In addition, the frequency f ₁ ′ determined by the magnetic flux command value ▲ φ ^* ₁ ▼ ′
Is not set to a value that is not lower than the inverter amount high frequency that can be reached at least in the operating state, the motor frequency may be higher than the inverter frequency,
May cause brake torque. In such a case, the frequency of the inverter is the given magnetic flux command value ▲ φ ^* ₁
Since it is possible to go down but not go up to the frequency determined by ▼ ', synchronization is performed by decelerating the motor to the rotation speed corresponding to the frequency f ₁ ′ determined by the magnetic flux command value ▲ φ ^* ₁ ▼'. . This is not preferable in terms of control because it causes extra deceleration.

〔The invention's effect〕

According to the method for restarting an induction motor according to the present invention, a special detector is provided for restarting after a momentary power failure of a torque control system of an induction motor by an inverter that does not use a speed sensor, or restarting during coasting. It is possible to perform synchronous pull-in with less shock without using.

In particular, when the control circuit is digitized, the restart command generator 11 to the torque command controller 15 in the block diagram shown in FIG. It can be extremely simple in comparison.

[Brief description of drawings]

1 is a block diagram showing an embodiment of a method for restarting an induction motor according to the present invention, FIG. 2 is a block diagram showing a conventional technique, FIG. 3 is an output voltage vector diagram of an inverter according to a switch state variable, FIG. Fig. 4 is a magnetic flux state diagram showing an instantaneous control method of the primary magnetic flux vector of the electric motor, Fig. 5 is a torque waveform diagram of the electric motor, Fig. 6 is a state diagram of a magnetic flux hysteresis comparator, and Fig. 7 is a three-valued hysteresis of torque. The state diagram of the comparator, FIG. 8 is a torque speed curve of the electric motor for explaining the synchronous pull-in operation of the induction motor according to the present invention. 1 ... DC voltage source, 2 ... voltage detector, 3 ... PWM inverter, 4 ... switch, 5u, 5v, 5w ... current detector, 6 ...
Induction motor, 7 Control circuit, 11 Restart command generator, 12 Restart timer, 13 Torque error width controller,
14 …… Magnetic flux command controller, 15 …… Torque command controller.

Claims (1)

[Claims] 1. A DC voltage source, a PWM inverter, an induction motor, a DC voltage source voltage detector, and a motor current detector, and at least a variable voltage variable frequency AC voltage from the DC voltage source to the induction motor via the PWM inverter. And a switch state signal that commands the output voltage of each phase of the inverter, a DC voltage source voltage detection value, and a motor current detection value are used to calculate a motor instantaneous magnetic flux vector signal and an instantaneous torque signal, and the magnetic flux calculated value is calculated. Of the torque and its command value exceeds a predetermined allowable range, a hysteresis comparator that generates a first control flag for commanding a magnetic flux increase or a magnetic flux decrease, and a deviation between the calculated torque value and its command value Command to increase / decrease the torque or maintain the current state depending on whether the torque exceeds the specified allowable range or is within the specified allowable range. And a three-value hysteresis comparator that generates a second control flag, and a third control flag that indicates in which circular arc area the current circumference is divided from based on the magnitude and sign of the magnetic flux vector component. And a switching position of the inverter that generates a voltage vector for optimizing the torque response by designating a combination of these three control flag values. In the restart method at the time of momentary power failure or motor coasting in the one in which the magnetic flux vector is controlled so as to draw an approximate circular locus, the DC voltage source voltage detector is connected to the power source side with respect to the switch. , A time limit setting means for ensuring a sufficient time for restarting the motor, and a magnetic flux command and a torque command within the time limit of the time limit setting means. And a means for forcibly changing the torque hysteresis value of the ternary hysteresis comparator that generates the second control flag to a value that is weaker than that during normal operation, and the inverter frequency during restart during normal operation After setting the frequency higher than the maximum frequency that can occur, the switch is closed and synchronous pull-in is performed, and after the time set by the time limit setting means is returned to the normal magnetic flux command, torque command, and torque hysteresis value, to restore normal operation. Characteristic method of restarting the induction motor.