JPH0612956B2 - Control device for variable voltage variable frequency inverter for induction motor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control device for a variable voltage variable frequency inverter for an induction motor, and more particularly to a control device for a variable voltage variable frequency inverter for an induction motor that causes an electric vehicle to power and regeneratively brake.

Recently, variable speed control of an induction motor using a variable voltage variable frequency inverter has been widely applied to fields where a DC motor has been conventionally used because of the merit of easy maintenance of an induction motor having no commutator.

For example, it is being put to practical use in applications where a large torque is required at the time of starting, such as an electric vehicle, and a wide range of speed control is required.

The induction motor has a constant speed characteristic of rotating around a synchronous speed that is essentially determined by the power supply frequency and the number of poles.
Therefore, in order to continuously change the rotation speed of the induction motor, it is necessary to control by changing the frequency of the power supply. Further, the magnitude of the torque generated by the induction motor is determined by the magnetic flux created in the air gap by the stator and the current flowing through the rotor. Therefore, torque control must be accomplished by controlling the magnetic flux and rotor current.

When an induction motor is used for driving a vehicle, a method of keeping the slip frequency constant and controlling the electric current constant is generally performed because of the demand for constant torque control. In the induction motor, when the slip frequency is kept constant and the output frequency of the power supply is raised, the equivalent impedance increases. Therefore, in order to keep the current constant, the power supply voltage must be increased in proportion to the increase in the power supply frequency. The need for constant torque control in an electric vehicle is due to a demand for a constant acceleration and a good ride comfort.

Various control methods have heretofore been considered as control methods for increasing the voltage in proportion to the increase in frequency as described above. As a typical example of this, there is a pulse width modulation (PWM) inverter control method that controls the average voltage applied to the induction motor by modulating the pulse width of the output voltage.

FIG. 1 shows a configuration example in which an induction motor for an electric vehicle is driven by a conventional PWM inverter control device. DC power is supplied from a DC overhead wire 1 via a pantograph 2 to an inverter 3 that generates a variable voltage variable frequency. The three-phase alternating current converted into a predetermined voltage and frequency by the inverter 3 is supplied to the induction motor 4 to drive it. An AC current transformer 5 is provided on the power line that connects the inverter 3 and the induction motor 4, and the detected current is rectified by the rectifier 6.
To the DC current I _Mf .

The gate control of the inverter 3 is performed using a pulse train obtained by comparing a plurality of DC voltages corresponding to a sine wave voltage with a triangular wave, and the frequency of the triangular wave is switched according to the rotation frequency of the induction motor 4. ing.

The frequency generated by the pulse generator 7 directly connected to the induction motor 4 is input to the frequency / digital converter 8,
Here, it is converted into a digital quantity f _D1 and input to the microcomputer 9. On the other hand, the slip frequency pattern f _SP
Is a digital quantity f by the analog / digital converter 10.
_It is converted to _SD and input to the microcomputer 9.

In the microcomputer 9, digital quantities f _D1 and f _SD
And are added at the time of power running of the electric vehicle and subtracted at the time of regeneration, and the result is output to the digital / frequency converter 11 as a digital amount f _ID . With this digital / frequency converter 11, f _ID is a frequency f that is an integral multiple of the inverter frequency.
Converted to _I. The counter 12 is operated by this frequency f _I, and the output of the counter 12 is given as the addresses of the read-only memories (ROM) 13-1 to 13-4. Triangular wave data is written in the ROM 13-4, and this output is input to the digital / analog converter 14 and converted into a triangular wave.

On the other hand, the direct current I _Mf, which is the output of the rectifier 6 corresponding to the electric current of the electric motor detected by the alternating-current AC device 5, is input to the subtractor 15, where it is subtracted from the electric motor current I _MP , The difference is converted by the plurality of resistors 16 into a plurality of DC voltages that are approximated by a sine wave. These plural DC voltages are respectively inputted to the comparator 17, and the comparator 1
It is compared with the triangular wave by 7 and is output as a plurality of pulses having different widths from the respective comparators 17. These pulses having different widths are respectively inputted to the data selectors 18 of the respective phases, and based on the outputs of the ROMs 13-1 to 13-3 inputted to the data selector 18, U,
The V and W phases are rearranged into PWM pulse trains shifted by 120 degrees. The pulse train of each of these phases is applied to the gate circuit of the inverter 3 through the gate transformer 19,
Control the gate.

Thus, the frequency of the power source supplied to the induction motor 4 is controlled by the microcomputer 9, and the voltage is controlled by the output of the subtractor 15 so that the motor current I _Mf becomes substantially equal to the motor current pattern I _MP. . At this time, the voltage-frequency ratio is substantially constant, the current supplied to the induction motor 4 is constant, and the generated torque is controlled to be constant.

By the way, when the induction motor 4 is driven by such a pulse-width-modulated square wave voltage, the current flowing through the induction motor 4 does not become a perfect sine wave, but contains a considerable harmonic component. In order to reduce this harmonic component, the frequency of the triangular wave is increased so that the frequency of the inverter 1
It can be reduced by increasing the number of pulses in the cycle and smoothing by the reactance of the induction motor 4.

On the other hand, however, it is not desirable for the semiconductor control element such as a thyristor used in the main circuit of the inverter 3 to have an extremely high frequency to cause an increase in switching loss. Further, since the thyristor does not have an ideal switch function and generally requires a commutation period of about 100 to 200 μS, the inverter 3 can output as the number of pulses in one cycle of the inverter frequency increases. The maximum output voltage becomes smaller. FIG. 2 shows an example of a PWM pulse train for explaining this situation. When the pulse width Δt, which is the pulse width, approaches 100 to 200 μS, the output voltage cannot be further increased. Therefore, in order to further increase the output voltage, the number of pulses in one cycle of the inverter frequency must be reduced.

Therefore, conventionally, control has been performed to switch the number of pulses according to the inverter frequency. In the conventional example shown in FIG. 1, the pulse number switching signal from the microcomputer 9 is changed to R
A method is adopted in which an appropriate one is selected from a plurality of triangular waves (each having a different frequency) output to the OM 13-4 and written in the ROM 13-4.

FIG. 3 shows a flow chart of the pulse number switching and frequency control program used in the above conventional example. Switching frequency _f 1 of the pulse _{number, f} 2 ......... _{f n} is excisional at step 101. At step 102, the rotation frequency f _D1 of the induction motor 4 and the slip frequency pattern f _SD are read. Next, in step 103, it is determined whether power running or regenerative. In the case of power running, step 10
4, the frequency calculation of f _ID = f _D1 + f _SD is performed,
In the case of regeneration, in step 105, f _ID = f _D1 −f
Frequency calculation of _SD is performed. Next, step 106.
When f _D1 is smaller than f ₁ , the number of pulses 27 is selected. If f _D1 is larger than f _1, it is judged in step 107 whether f _D1 is larger than f ₁ , and if it is larger, the number of pulses 15 is selected. Otherwise, the same judgment is repeated. , The optimum number of pulses is selected according to the rotation frequency f _D1 of the induction motor 4.

By the way, in the above-mentioned conventional technique, there is a problem that the ripple of the current flowing through the induction motor increases when the number of pulses is switched.

In view of the above-mentioned drawbacks of the prior art, an object of the present invention is to provide a control device for a variable voltage variable frequency inverter for an induction motor, which reduces the ripple of the current flowing through the induction motor to be controlled.

The present invention provides means for selecting a pulse mode capable of supplying the induction motor with an AC voltage at the same level as during power running in a frequency band higher than the frequency of the AC voltage during power running during regeneration of the induction motor. It is intended to reduce the current ripple of.

The operation of the present invention will be described below.

In the above-mentioned conventional example, the number of pulses is switched corresponding to the same inverter frequency during power running and during regeneration. That is, the number of pulses is 271 with respect to the inverter frequency f ₁ .
Switching of 5 corresponds, which means that it is the same during powering and during regeneration.

On the other hand, the equivalent circuit for one phase of the induction motor 4 is as shown in FIG. 4, and the direction of the current I ₁ flowing through the equivalent circuit is opposite between during power running and during regeneration. In the figure, the direction of the current I ₁ is indicated by a solid arrow during power running and by a broken arrow during regeneration. Further, the control for making the ratio of the voltage and frequency supplied to the induction motor 4 constant is for finally making the exciting current I ₀ constant and making the field flux constant. However, when the exciting current I ₀ is too large, the field iron core saturates,
Inrush current (overcurrent) flows due to the decrease in inductance. If it is too small, the utilization efficiency of the electric motor will be deteriorated. As described above, since the direction of the current I ₁ is opposite during power running and during regeneration, the terminal voltage V ₁ > V ₀ during power running, the terminal voltage V ₁ <V ₀ during regeneration, and the field magnetic flux. as shown in FIG. 5 shows the characteristics of the motor voltage to the inverter frequency when the constant, the inverter output voltage of the pulse mode a towards the power running V ₁ is becomes higher than V ₁ of the regenerative. In the figure, reference numeral 20 indicates the terminal voltage (motor voltage) V ₁ during power running and reference numeral 21 during regeneration.

In this way, if the number of pulses is switched at the same inverter frequency during power running and regeneration as in the conventional case, the ripple of the motor current will increase for the following reasons. That is, PW
The minimum slit width of the M-modulated pulse train is narrower during power running, and there is a margin for the commutation period of the thyristor during regeneration, because the minimum slit width is larger.

Therefore, this means that at the inverter frequency f ₁ >f> f ₂ , PWM modulation can be performed with the triangular wave of the pulse mode A, but it is modulated with the triangular wave of the pulse mode B having an inverter frequency one step lower than this. Therefore, the ripple (peak current value) of the induction motor 4 increases due to the increase in the harmonic component contained in the PWM modulation pulse train. This not only causes internal loss of the motor,
Since the inverter must have the ability to commutate the peak current, it has a large capacity (thyristor, etc.).
Is required, and there is a drawback that the cost is increased.

Therefore, in order to solve the above-mentioned drawbacks of the prior art found by the present inventors, an AC voltage of the same level as during power running is supplied to the induction motor during regeneration in a frequency band higher than the frequency of the AC voltage during power running. The pulse mode that can be used is selected.

An embodiment of a control device for a variable voltage variable frequency inverter for an induction motor according to the present invention will be described below with reference to the drawings using the same reference numerals for the same parts as the conventional example.

FIG. 6 is a block diagram showing the configuration of the control device for the variable voltage variable frequency inverter for the induction motor of this embodiment.
The frequency of the power supplied to the induction motor 4 is controlled by the microcomputer 9, the voltage is controlled by the output of the subtractor 15, and the configuration of each component is exactly the same as that of the conventional example, so the description thereof will be omitted. The difference from the conventional example of this embodiment is that in the pulse number switching program of the microcomputer 9,
This is because the power-running time and the regenerative time are distinguished from each other, and the inverter frequency at the time of pulse switching is selected at the time of regenerative power to be higher than that at the time of power-running.

FIG. 7 shows a flow chart of the pulse number switching and frequency control program of the above embodiment. Step 2
At 01, the rotation frequency f _D1 of the induction motor 4 and the slip frequency pattern f _SD are read. In step 202, it is determined whether power running or regenerative. In the case of power running, in step 203 f _ID = f
The frequency calculation of _D1 + f _SD is performed. Next, step 20
Proceed to 4 during power running of the number of pulses of the switching frequency f _11, f _21, ...
... f _n1 is set. Thereafter, at step 205, it is judged if the rotation frequency f _D1 of the induction motor 4 is smaller than f ₁₁ , and if it is smaller, the number of pulses 27 is selected. If not smaller, go to step 206 and determine whether f _D1 is larger than f ₁₁ ; if larger, the number of pulses is 1
5 is selected. If not, the next step is determined by the same step, and the optimum number of pulses corresponding to the rotation frequency f _D1 of the induction motor 4 is selected. If not powering at step 202, that is, go to step 207 during regeneration,
The frequency calculation of f _ID = f _D1 -f _SD is performed, and then, in step 208, the switching frequency f ₁₂ , f ₂₂ ... Of the pulse number during regeneration.
... f _n2 is set. After that, if it is determined in step 209 that f _D1 is smaller than f ₁₂ , the pulse number 27 is selected, and if not, the process proceeds to step 210 and f _D1
There optimal number of pulses corresponding to the rotation frequency f _D1 of the induction motor 4 repeats the same step when the pulse number 15 is not likely to be selected if it is determined whether or larger than f ₁₂ larger is selected.

As described above, in this embodiment, in each of steps 204 and 208, a pulse switching frequency set suitable for power running and regeneration is set, and the set value of pulse switching frequency during regeneration is set higher than that for power running. There is. For example, from the inverter frequency f ₁₁ that switches from 27 pulses to 15 pulses during power running, from 15 pulses to 27 pulses during regeneration.
The inverter frequency f ₁₂ for switching to the pulse has a higher frequency value.

In this embodiment, since the inverter frequency for switching the pulse number during regeneration is set higher than that during power running, PWM modulation is performed using the triangular wave in the pulse mode most suitable for regeneration to control the inverter 3. PW
There is an effect that the ripple of the current of the induction motor 4 can be reduced by preventing the increase of the harmonic component contained in the M modulation pulse train. Therefore, not only the internal harmonic loss of the induction motor 4 can be reduced, but also the ripple is small and the peak current can be suppressed low, so that the commutation capability of the inverter 3 can be designed low, and the induction motor 4 and the inverter 3 can be designed. It has the effect of making it smaller and lighter and cheaper.

In addition, as the modulation method of the present embodiment, an example of using a DC voltage that gives an output voltage pattern and a triangular wave as a carrier for PWM pulse control has been described, but the present invention is not limited to this. It can be applied to all inverters that control the number of pulses and the pulse width.

As described above, the control device for a variable voltage variable frequency inverter for an induction motor according to the present invention has an effect of reducing the ripple amount of the current flowing through the induction motor to be controlled.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration example of a conventional variable voltage variable frequency inverter controller for an induction motor, FIG. 2 is an explanatory diagram showing an example of a PWM modulation pulse train, and FIG. 3 is shown in FIG. FIG. 4 is a program flow chart of the microcomputer, FIG. 4 is an equivalent circuit diagram of one phase of the induction motor 4 shown in FIG. 1, and FIG. 5 is a relationship between the frequency of the inverter shown in FIG. 1 and the output voltage. The diagram shown in FIG. 6 is a block diagram showing the configuration of an embodiment of a control device for a variable voltage variable frequency inverter for an induction motor to which the present invention is applied,
FIG. 7 is a program flow chart of the microcomputer shown in FIG. 3 ... Inverter, 4 ... Induction motor, 9 ... Microcomputer, 13-1 to 13-4 ... Read-only memory, 15 ... Subtractor, 16 ... Resistor, 17 ... Comparator, 18 ... … Data selector.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Koji Jimbo, 1070 Ige, Katsuta-shi, Ibaraki Hitachi, Ltd. Mito Plant (72) Inventor Masahiko Masamoto 3-1-1, Saiwaicho, Hitachi, Ibaraki Hitachi, Ltd., Hitachi Research Laboratory (72) Inventor, Tokunosuke Tanamachi, 3-1-1 Sachimachi, Hitachi City, Ibaraki Hitachi, Ltd., Hitachi Research Laboratory (56) Reference: JP-A-51-136131 (JP, A) ) JP-A-54-63218 (JP, A)

Claims (2)

[Claims] 1. An induction motor to be controlled is supplied with a pulse width-modulated AC voltage, and the number of pulses included in one cycle of the AC voltage is switched in response to a change in the frequency of the AC voltage. In the controller of the variable voltage variable frequency inverter for an induction motor, which performs control to make the ratio of the AC voltage and the frequency substantially constant, a frequency band higher than the frequency of the AC voltage during power running during regeneration of the induction motor. 2. A control device for a variable voltage variable frequency inverter for an induction motor, comprising means for selecting a pulse mode capable of supplying the induction motor with an AC voltage at the same level as during power running. 2. The selection of the pulse mode according to claim 1, wherein the frequency of the alternating voltage when switching the number of pulses included in one cycle of the alternating voltage is the same as the switching of the same number of pulses. A control device for a variable voltage variable frequency inverter for an induction motor, wherein a frequency during regeneration is set to be higher than a frequency of an AC voltage supplied during power running of the induction motor.