JPS5889089A - Controlling method for variable voltage and variable frequency inverter of induction motor - Google Patents

Abstract

PURPOSE:To reduce the current ripple of an induction motor by selecting the frequency of an inverter which switches a PWM modulation pulse number to become higher at the regenerating time than that at the power driving time. CONSTITUTION:The frequency of a power source which is supplied to an induction motor 4 is controlled by a microcomputer 10, and the voltage is controlled by the output of a subtractor 15. The power driving time and the regenerating time are distinguished during the pulse number switching program of the microcomputer 10, thereby selecting the frequency of the inverter at the time of switching the pulse at the regenerating time higher than the power driving time. Accordingly, a PWM modulation is performed by employing a triangular wave of the pulse motor mostly adapted for the regenerating time, the inverter 3 is controlled, thereby preventing the harmonic component contained in the PWM modulation pulse train from increasing.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling a variable voltage, variable frequency inverter for an induction motor, and more particularly to a method for controlling a variable voltage, variable frequency inverter for an induction motor that powers an electric vehicle and performs regenerative braking.

Recently, variable speed control of induction motors using variable voltage variable frequency inverters has been increasingly applied to fields where DC motors have traditionally been used, due to the ease of maintenance of induction motors that do not have commutators.

For example, they are being put into practical use in applications such as electric cars that require large torque at startup and also require wide range speed control.

An induction motor has a low speed characteristic of rotating around a synchronous speed essentially determined by the frequency of the power supply and the number of poles.

Therefore, in order to continuously change the rotational speed of the induction motor, it is necessary to control the power supply by changing its frequency. Further, the magnitude of the torque generated by the induction motor is determined by the magnetic flux created by the stator in the air gap and the current flowing through the rotor. Therefore, torque control must be achieved by controlling magnetic flux and rotor current.

When an induction motor is used to drive a vehicle, a method is generally used in which the sliding frequency is kept constant and the current is controlled to be constant due to the requirement for constant torque control.

In an induction motor, when the slip frequency is held constant and the output frequency of the power source is increased, the equivalent impedance increases.

Therefore, in order to keep the current constant, the power supply voltage must also increase in proportion to the increase in the power supply frequency.

Furthermore, it is necessary to perform constant torque control in electric vehicles.
This is due to the need to maintain constant acceleration and improve riding comfort.

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

FIG. 1 shows an example of a configuration for driving an induction motor for an electric vehicle using a conventional PWM inverter control method. DC power is supplied from a DC overhead wire 1 via a pantograph 2 to an inverter 3 that generates a variable voltage and variable frequency. Three-phase alternating current converted to a predetermined voltage and frequency by this inverter 3 is supplied to an induction motor 4 to drive it.

An AC current transformer 5 is provided on the power line connecting the inverter 3 and the induction motor 4, and the detected current is input to a rectifier 6 to be converted into a DC current IM+.

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

The frequency generated by the pulse generator 7 directly connected to the induction motor 4 is input to a frequency/digital converter 8,
Here, it is converted into a digital quantity fD1 and input to the microcomputer 9. On the other hand, the slip frequency pattern fB
P is converted into a digital quantity f8D by the analog/digital converter 10 and input to the microcomputer 9.

In the microcomputer 9, the digital quantities 'ot and f8
D is added when the electric car is running, subtracted during regeneration, and the result is output to the digital/frequency converter 11 as a digital quantity rro. This digital/frequency converter 11 converts f[D into a frequency f1 that is an integral multiple of the inverter frequency.

This frequency f! The output of the counter 12 is stored in the read-only memory (几QM) 13-
It is given as an address from 1 to 13-4. I'LOM13
-4 is written with triangular wave data, and this output is input to the digital/analog converter 14 and converted into a triangular wave.

On the other hand, the DC current L1, which is the output of the rectifier 6 and corresponds to the motor current detected by the AC current transformer 5, is input to the subtracter 15, where it is subtracted from the motor current IMF, and the difference is The resistor 16 converts the voltage into a plurality of DC voltages approximated to a sine wave. These multiple DC voltages are each input to a comparator 17, and this comparator 1
7, the pulses are compared with the triangular wave, and outputted from each comparator 17 as a plurality of pulses with different widths.

Each of these pulses with different widths is transmitted to the data selector 1 of each phase.
Based on the outputs of the ROMs 13-1 to 13-3, which are input to the data selector 18,
, V, and W phases are rearranged into PWM pulse trains shifted by 120 degrees. These pulse trains of each phase are applied to the gate circuit of the inverter 3 through the gate transformer 19 to control the gate.

The frequency of the power supplied to the induction motor 4 is thus controlled by the microcomputer 9, and the voltage is controlled by the output of the subtractor 15 so that the motor current 1wt is approximately equal to the motor current IMP. At this time,
The ratio of voltage and frequency is approximately 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 with 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 slight harmonic component. . In order to reduce this harmonic component, the frequency of the triangular wave is increased, and the frequency of the inverter 43 is 1.
It can be reduced by increasing the number of pulses during the cycle and smoothing it by the reactance of the induction motor 4.

However, on the other hand, for semiconductor control elements such as thyristors used in the main circuit of the inverter 3'', it is undesirable to make the frequency extremely high and cause an increase in switching loss. Since it has no function and generally requires a commutation period of 100 to 200 μs, the larger the number of pulses in one cycle of the inverter frequency, the smaller the maximum value of the output voltage that the inverter 3 can output. Fig. 2 shows an example of a PWM pulse train to explain this situation, and when Δt, which is the slit width between pulses, approaches 100 to 200 μs, the output voltage cannot be increased any further. , to further increase the output voltage, the number of pulses in one cycle of the inverter frequency must be reduced.

For this reason, conventional 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 R,
A method is adopted in which a suitable one is selected from among a plurality of triangular waves (each having a different frequency) outputted to the ROM 13-4 and written in the ROM 13-4.

FIG. 3 shows a flowchart of the pulse number switching and frequency control program used in the conventional example. In step 101, the pulse number switching frequency f,, f2...f. is set. At step 102, the rotation frequency fD of the induction motor 4 and the slip frequency pattern fsD are read. Next, in step 103, it is determined whether it is power or regeneration, and if it is power, step 1o4. The frequency calculation of frn=fol+'sn is performed.

In the case of regeneration, in step 105 flD2fD, -fl
A frequency calculation of 1D is performed. Next, step 10
6 and if fDl is smaller than fl, the number of pulses 27 is selected. If fD, is larger than fl, then in step 107 fDI73: f, ! ') It is determined whether the number of pulses is large, and if it is large, the number of pulses 15 is selected, and if not, the same determination is repeated and the optimal number of pulses according to the rotation frequency fD1 of the induction motor 4 is selected. Ru.

In this way, in the conventional example, switching of the number of pulses was carried out corresponding to the same inverter frequency during both driving and regeneration. That is, the inverter frequency f corresponds to a change in the number of pulses of 27015, and this is the same during both drive and 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 (1) flowing through the equivalent circuit is opposite between during power travel and during regeneration.

In the figure, current 1. The direction of the arrow is shown by a solid line arrow when the engine is running, and by a broken line arrow when it is regenerating. Further, the control to keep the ratio of the voltage and frequency supplied to the induction motor constant is ultimately to keep the excitation current Io constant and the four fluxes of the field constant. However, excitation electrician. If is too large, the field core will be saturated and the inductance will drop, causing an inrush current (overcurrent) to flow. If it is too small, the efficiency of using the electric motor will deteriorate. As mentioned above, since the direction of the current I is opposite during the forward movement and during regeneration, the terminal voltage V + during the forward movement
＞V o, and the terminal voltage V during regeneration becomes <V.

Therefore, the VI during power travel needs to be higher than that during regeneration.

Therefore, when the number of pulses is switched at the same inverter frequency during power and regeneration as in the conventional case, as shown in Fig. 5, which shows the characteristics of the motor voltage with respect to the inverter frequency when the field magnetic flux is constant. The inverter output voltage in pulse mode A is higher during power generation than during regeneration. In the figure, the reference numeral 20 indicates the terminal voltage (motor voltage) (2) at the time of power travel, and the reference number 21 indicates the terminal voltage (motor voltage) (2) at the time of regeneration. Therefore, the minimum slit width of the PWM modulated pulse train is narrower during the regeneration period, and there is a margin for the commutation period of the thyristor because the minimum slit width is larger during the regeneration period.

Therefore, this means that fl>f>f2. At an inverter frequency of Due to the increase, the ripple (peak current value) of the induction motor 4 increases. This not only causes internal losses in the motor, but also requires the inverter to have the ability to commutate peak currents, which requires large capacitance elements (such as thyristors), resulting in increased costs.

SUMMARY OF THE INVENTION In view of the above drawbacks, it is an object of the present invention to provide a method for controlling a variable voltage variable frequency motor for an induction motor, which reduces the current flow through the induction motor to be controlled.

The present invention focuses on the fact that, when the excitation current of the induction motor to be controlled is the same, the motor terminal voltage is higher during powering than during regeneration, and PWM modulation ().

The inverter frequency for switching the number of regenerations is selected so that it is higher during first regeneration than during regeneration, thereby reducing the current ripple of the induction motor.

Hereinafter, an embodiment of the control method for a variable voltage variable frequency inverter T for an induction motor according to the present invention will be explained with reference to the drawings, using the same reference numerals for the same parts as in the conventional example.

FIG. 6 is a block diagram showing the configuration of a control device for a variable voltage variable frequency inverter for an induction motor according to this embodiment.

The frequency of the power supply supplied to the induction motor 4 is controlled by the microcomputer 10, the voltage is controlled by the output of the subtracter 15, and the configuration of each component is completely the same as that of the conventional example, so a description thereof will be omitted. The difference between this embodiment and the conventional example is as follows.
In the pulse number switching program of the microcomputer 10, a distinction is made between the current time and the regeneration time, and the inverter frequency at the time of pulse switching is set to be higher than the frequency during the regeneration time.

FIG. 7 shows a flowchart of the pulse number switching and frequency control program of the above embodiment. Step 2
At 01, the rotational frequency fDI of the induction motor 4 and the sliding frequency pattern f8D are read. In step 202, it is determined whether it is power or regeneration, and if it is power, then f in step 203.
A frequency calculation of xo-=foH+fsD is performed. Next, the process proceeds to step 204, and the switching frequency fll+f21+...f, 1 for the number of pulses during the ka-row is set. Thereafter, in step 205, it is determined whether the rotational frequency fD of the induction motor 4 is smaller than '11, and if it is smaller, <response number 27 is selected. If not, the process goes to step 206, where it is determined whether fDl is larger than fIt, and if it is larger, the number of pulses 15 is selected. If not, the same steps are performed to determine the next rotational frequency fD of the induction motor 40.

The optimum number of pulses is selected according to the Step 202
If it is not row 5, that is, in the case of regeneration, the process goes to step 207, where the frequency calculation of fID=fJ'SO is performed.
Next, in step 208, the switching frequency of the response number '12+f22...f during regeneration is determined. 2 is set. After that, if it is determined in step 209 that fol is smaller than f,2, the S-Rus number 27 is selected; otherwise, the process goes to step 210, where it is determined whether fDI is larger than fX2; Number of pulses 15
If it is not likely to be selected, similar steps are repeated to select the optimum number of pulses according to the rotational frequency fDI of the induction motor 4.

In this way, in this embodiment, in steps 204 and 208, the pulse switching frequency suitable for the power travel and regeneration is set, and the set value of the pulse switching frequency for the regeneration is set to be higher than that for the power travel. It's expensive. For example, the inverter frequency ftt changes from 27 pulses to 15 pulses during power generation, and from 15 pulses to 2 pulses during regeneration.
The inverter frequency f12, which is switched to 7 pulses, has a higher frequency value.

In this embodiment, since the inverter frequency for switching the number of pulses during regeneration is set higher than that during drive, PWM modulation is performed using the triangular fatigue pulse mode most suitable for the first regeneration, and the inverter 3 By controlling PW
(M modulation) has the effect of preventing an increase in harmonic components included in the pulse train and reducing ripples in the current of the induction motor 4.

Therefore, not only can the internal harmonic loss of the induction motor 4 be reduced, but also the ripple is small and the peak current is kept low, so the commutation capacity of the inverter 3 can be designed to be low, and the induction motor 4 and the inverter 3 It has the effect of making it smaller and lighter, making it more valuable.

It should be noted that although the modulation method of this embodiment has been explained using an example in which a DC voltage that provides an output voltage pattern and a triangular wave is used as a carrier wave for PWM pulse control, the present invention is not limited to this. It can be applied to all inverters that control the number of pulses and pulse width.

As described above, the method of controlling a variable voltage variable frequency inverter for an induction motor according to the present invention has the effect of reducing ripples in the current flowing through the induction motor that is the object of control.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a configuration example of a conventional control device for a variable voltage variable frequency inverter for an induction motor, Fig. 2 is an explanatory diagram showing an example of a PWM modulation pulse train, and Fig. 3 is a block diagram showing an example of a control device for a conventional variable voltage variable frequency inverter for an induction motor. Figure 4 is an equivalent circuit diagram for one phase of the induction motor 4 shown in Figure 1, and Figure 5 shows the relationship between the frequency and output voltage of the inverter shown in Figure 1. 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 flowchart 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, 1
7...Ratio'44-'lIjFigure-45,-;'ruf* 17m

Claims (1)

[Claims] 1. Supply an alternating current modulated by a plurality of pulses with different widths to an induction motor to be controlled, and calculate the number of pulses included in one cycle of the alternating current in response to a change in the frequency of the alternating current. In a method for controlling a variable voltage variable frequency inverter for an induction motor, which performs control to maintain a constant ratio between the frequency and voltage of the alternating current by switching the number of pulses included in one cycle of the alternating current, For an induction motor, characterized in that the frequency of the current is set so that the frequency during regeneration is higher than the frequency of the alternating current supplied when the induction motor is in motion, when switching the same number of pulses. Control method for variable voltage variable frequency inverter.