JPH0612959B2 - Induction motor controller with pulse width modulation inverter - Google Patents

Description

The present invention relates to a control system for an induction motor using a pulse width modulation inverter, and more particularly to improvement of switching control of the number of pulses included in a half cycle of an inverter output voltage.

As is well known, the pulse width modulation inverter can control the voltage by the inverter itself, so that a desired voltage can be obtained at a desired frequency, and the output can operate the induction motor at a variable speed.

FIG. 1 is a circuit configuration of a conventional induction motor control system using a pulse width modulation inverter, in which 1 is a DC power supply and 2 is a DC power supply.
Is a pulse width modulation inverter composed of switching elements UP to WN such as thyristors, and 3 is an induction motor.

In FIG. 1, a rotation frequency _n of the induction motor 3 is detected by a detection circuit 6, and a slip frequency command value _s from a slip frequency command circuit 10 is added to this by an adder / subtractor circuit 11 during power running and subtracted during regeneration. This is the inverter 2
Output frequency of ₀ (= _n ± _s ). Modulation circuit 5
Then, in response to the output of the adder / subtractor circuit 11, the triangular wave generating circuit 51 and the sine wave generating circuit 52 change the frequency to ₀ (= _n ±
_s ) is an integer multiple of 3 and the triangular wave voltage and frequency is ₀ (=
_(n ± _s ) three-phase sine wave voltage is generated in synchronization,
The comparison circuit 53 compares the triangular wave voltage and the sine wave voltage for each phase. By the output of the comparison circuit 53 of the modulation circuit 5, the switching elements UP to WN of the inverter 2 are turned on / off in a predetermined order via the gate signal processing circuit 4. The pulse width modulation operation at this time will be described with reference to FIG. 2 by taking the case where the frequency of the triangular wave voltage is three times the frequency of the sine wave voltage as an example.

The triangular wave generating circuit 51 and the sine wave generating circuit 52 shown in FIG.
The triangular wave voltage and the three-phase sine wave voltage as shown in (a) are generated, and the triangular wave voltage and the sine wave voltage are compared by the comparison circuit 53 for each phase, and the inverter as shown in FIG. The ON / OFF signals of the switching elements UN, VN and WN of No. 2 are output. Although the credits for the switching elements UP, VP and WP are not shown, the signals shown in FIG. 2B are inverted. When the output signal of the comparison circuit 53 as shown in FIG. 2B is applied to the switching elements UN, VN, WN through the gate signal processing circuit 4 to perform the on / off operation, the switching elements UN, The voltages across the VN and WN elements are as shown in (c), (d) and (e) of FIG.
Although the voltage across each element of the switching elements UP, VP and WP is not shown, it is the voltage of (c), (d) and (e) in FIG. 2 inverted. From the voltage across UN of (c) and the voltage across VN of (d) in FIG.
The voltage between V (= voltage between U and V of the induction motor 3) is shown in FIG.
As shown in (f), the number of pulses included in the half cycle of the sine wave voltage, that is, the half cycle of the inverter output voltage is 3
The width of the central pulse is different from the widths of the other pulses. Although the V-W voltage and the W-U voltage of the inverter 2 are not shown, the U-voltage in FIG.
They are delayed by 120 degrees and 240 degrees from the -V voltage, respectively.

As can be seen from the above description, the width of each pulse included in the half cycle of the inverter output voltage, that is, the output voltage of the inverter 2 is the ratio of the _peak value e _cp of the sine wave voltage to the _peak value e _3P of the triangular wave voltage e _cp / e _3P (hereinafter referred to as modulation degree)
By changing, also the number of pulses by varying the ratio _3/0 of the frequency ₀ of the frequency ₃ and sine wave voltage of the triangular wave voltage can be controlled respectively. The modulation degree is obtained by comparing the command value of the current command circuit 8 with the value of the current flowing through the induction motor 3 detected by the detection circuit 7, and the deviation thereof causes the sine wave generation circuit 52 through the modulation degree control circuit 9. It is controlled by changing the _peak value e _cp of the sinusoidal voltage at the output of the. The number of pulses is switched by changing the frequency ₃ of the triangular wave voltage output from the triangular wave generating circuit 51 via the pulse number switching circuit 12 according to the magnitude of the inverter output frequency.

However, it was observed that when the number of pulses was switched, the current I _M of the induction motor 3 transiently changed, torque fluctuations occurred, and the control system was adversely affected.

The present invention has been made in view of the above-mentioned conventional drawbacks, and an object thereof is to provide a control system of an induction motor by a pulse width modulation inverter capable of reducing torque fluctuation when switching the number of pulses.

The present invention is characterized in that the degree of modulation is changed before and after the switching of the number of pulses.

When the cause of the torque fluctuation at the time of switching the number of pulses was investigated, the following facts were clarified.

That is, the relationship between the modulation degree γ and the magnitude V _M of the fundamental wave component of the output voltage (line voltage) of the inverter 2 is calculated by the inventor to be 3rd for pulses such as 9 and 5 pulses.
As shown in the figure, they do not match.

Therefore, when the current of the induction motor 3 is constantly controlled as described above and the number of pulses is changed from 9 to 5, for example,
_Assuming that the modulation degree γ is at point (a) in FIG. 3, the magnitude V _M of the fundamental wave component of the output voltage of the inverter 2 is (a) in FIG.
In order to rapidly increase from the point to the point (b) as shown by the dotted line in (a) of FIG. 4, the current I _M of the induction motor 3 is also changed to (b) of FIG.
Try to increase like the dotted line. After that, the control system operates and the modulation degree γ is controlled from point (a) of FIG. 3 to (c) as shown in (c) of FIG. 4, resulting in V _M and I _M Changes as shown by the solid lines in (a) and (b) of FIG. 4, and settles at a constant value before the number of pulses is switched.

As described above, when the number of pulses is switched, the magnitude V _M of the fundamental wave component of the output voltage of the inverter 2 transiently changes abruptly, so that the current I _M of the induction motor 3 also changes abruptly to cause torque fluctuation. .

Therefore, in the present invention, the number of pulses is switched according to the inverter output frequency, and the number of pulses is switched in PWM modulation having a mode in which the number of pulses is plural and at least one pulse has a width different from other pulses. At times, the degree of modulation is changed so that the magnitude of the fundamental wave component of the inverter output voltage does not change even in a transient state. For this purpose, for example, a modulation factor ratio is set such that the fundamental wave components of the inverter output voltage for each number of pulses are equal, and the modulation factor at the time of pulse number switching is changed according to this ratio. It is desirable to do.

FIG. 5 shows a circuit of an embodiment of the present invention,
The difference from the conventional example of FIG. 1 is that the pulse switching circuit 1
Since the output of the modulation degree control circuit 9 is controlled by the output of 2, the other symbols and the pulse width modulation operation of the inverter 2 are the same as those in FIG.

The operation of the present invention shown in FIG. 5 will be described with reference to FIGS. 6 and 7.

In FIG. 5, the pulse number switching circuit 12 changes the number of pulses to 5, for example, when the number of pulses is 9 and the modulation degree γ is at point (a) in FIG. 6 (this is the same as FIG. 3 described above). When a command to switch is issued, the command controls the output of the modulation degree control circuit 9 so that the modulation degree γ shifts to the point (b) in FIG. 6 as shown in (c) in FIG. Te, the size V _M of the fundamental wave component of the output voltage of the inverter 2 does not change as in FIG. 7 (b). Then, the current I _M of the induction motor 3 does not change as shown in (b) of FIG. 7 and the torque fluctuation does not occur.

The output of the modulation degree control circuit 9, that is, the modulation degree γ is controlled by a pulse number switching command (for example, from 9 pulses to 5 pulses) from the pulse number switching circuit 12, and the fundamental wave component of the output voltage of the inverter 2 is controlled. Specifically how the magnitude V _M is prevented from changing the output that is related modulation factor γ in the modulation degree control circuit 9 for the deviation ΔI between the detection value of the current of the induction motor 3 and the command value of the current command circuit 8 For example, it may be changed between 9 pulses and 5 pulses. That is, the relationship between ΔI and γ is expressed by 9 pulses γ = K ₉ · ΔI (1) 5 pulses γ = K ₅ · ΔI (2) (However, K ₉ and K ₅ are different constants. So that K ₉ > K ₅ ). That is, different modulation factors γ are taken for the same current deviation input ΔI.

Also the relationship between gamma and _{V M} from FIG. 6, 9 Pulse _{V M = G 9 · γ .........} (3) 5 Pulse _{V M = G 5 · γ .........} (4) ( although, _{G 9 G 5} is a constant _and, when G 9 _<G 5), to ensure that _{V M} does not change during the pulse number switch is (3) = (4) from the _{equation, K 9 · G 9 = K} 5・ G ₅ ............ The constants K ₉ and G ₉ or their ratio may be set so as to satisfy (5).

As described above, according to the present embodiment, the magnitude V _M of the fundamental wave component of the output voltage of the inverter 2 at the time of switching the number of pulses.
Since the modulation degree γ is changed so that does not change, the current of the induction motor 3 does not change, the torque does not fluctuate, and the control system is not adversely affected.

When changing the modulation factor γ when switching the number of pulses,
In the illustrated embodiment, the crest value of the sine wave voltage of the sine wave generating circuit 52 of the modulation circuit 5 is changed, but this is the crest value of the triangular wave voltage of the triangular wave generating circuit 51, or both the sine wave voltage and the triangular wave voltage. It goes without saying that the peak value of can be changed.

As described above, according to the present invention, when the number of pulses is switched, the modulation factor is changed so that the magnitude of the fundamental wave component of the inverter output voltage does not change, so the current of the induction motor does not change, and It is possible to provide a control system for an induction motor using a pulse width modulation inverter that does not cause torque fluctuation and does not adversely affect the control system.

[Brief description of drawings]

FIG. 1 is a conventional circuit diagram, FIGS. 2 to 4 are operation explanatory diagrams of FIG. 1, FIG. 5 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 7 and FIG. 7 are explanatory diagrams of the operation of FIG. 1 ... DC power supply, 2 ... Inverter, 3 ... Induction motor, 4 ... Gate signal processing circuit, 5 ... Modulation circuit, 6 ...
... Rotation frequency detection circuit, 7 ... Current detection circuit, 8 ... Current command circuit, 9 ... Modulation degree control circuit, 10 ... Slip frequency command circuit, 11 ... Addition / subtraction circuit, 12 ... Pulse number switching circuit, γ ...... modulation, V M _...... magnitude of the fundamental component of the inverter output voltage.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Narita 3-1-1, Sachimachi, Hitachi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Shigetoshi Okamatsu 1070 Ichige, Katsuta, Ibaraki Stock Hitachi, Ltd. Mito Factory (56) References Japanese Patent Laid-Open No. 5091717 (JP, A)

Claims (1)

[Claims] 1. A pulse width modulation inverter fed from a DC power supply, an induction motor energized by the inverter, and a half cycle of the output voltage of the inverter for controlling the output frequency and output voltage of the inverter. Means for switching the number of pulses included for each output frequency band of the inverter;
A pulse width modulation means having a function of controlling one pulse to have a width different from that of another pulse, wherein a means for changing the modulation degree of the pulse width modulation before and after switching the number of pulses is provided. A control device for an induction motor using a characteristic pulse width modulation inverter.