JPH0446568A - Control system of multiple pwm inverter - Google Patents

Abstract

PURPOSE:To reduce the higher harmonic of a low order, a current ripple, thrust pulsations or torque pulsations at the time of half bridge one-step operation by changing the carrier frequency of a half bridge inverter by output frequency and setting carrier frequency at the time of starting at a value higher than that at the time of multiple operation. CONSTITUTION:A carrier-frequency command circuit 6 is installed, and carrier frequency is set at a value higher than that at the time of multiple operation on the operation only of a half bridge inverter. Accordingly, the higher harmonic of a low order can be decreased from output currents, and thrust pulsations can be reduced when a linear motor is used as load and torque pulsations when a rotary machine is used as load in the same extent as those at the time of multiple operation respectively.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a control system for a multiplex PWM inverter that drives a linear motor or a rotary motor [Prior Art] Examples include the technique described on pages 915 to 916 of Proceedings (1978). This relates to a multiplex inverter in which the AC output ends of a half-bridge inverter without an output transformer and a full-bridge inverter with an output transformer are connected in series.

When multiple inverters are operated, the apparent frequency (equivalent carrier frequency) of the multiple inverters can be made higher than the actual carrier frequency of each unit inverter by phase difference operation in which an equal phase difference is provided for the triangular wave carriers of each stage. Therefore, harmonics appearing in the output current appear near multiples of this equivalent carrier frequency and are not seen near the original carrier frequency, making it possible to reduce low-order harmonics.

On the other hand, in the low output frequency region at startup, V/f (ratio of output voltage to frequency) increases due to the negative resistance, and the output transformer saturates, making it necessary to operate a full-bridge inverter. I can't. Therefore, all full-bridge inverters are stopped and only half-bridge inverter stages without output transformers are operated.

[Problem to be solved by the invention]

Therefore, at startup with a low output frequency, harmonics are generated near frequencies that are integral multiples of the carrier frequency, and low-order harmonics increase, so the overall distortion rate of the output current increases as shown in Figure 2. There was a problem in that the thrust force or torque pulsation increased.

[Means to solve the problem]

In order to achieve the above object, the present invention is characterized by providing means for changing the carrier frequency of the half-bridge inverter depending on the output frequency, and setting the carrier frequency at startup to be higher than at the time of multiplex operation.

[Operation] The carrier frequency setting means of the half-bridge inverter operates to increase the carrier frequency in a low output frequency region. As a result, lower harmonics during half-bridge stage operation are reduced, and current ripple, thrust pulsation, or torque pulsation can be reduced.

〔Example〕

FIG. 1 shows an embodiment of the present invention, showing one phase of a multiplex inverter constituted by a half-bridge inverter and a full-bridge inverter, and its control circuit. Generally, it is composed of one stage of half-bridge inverter and multiple stages of full-bridge inverters, but here the number of multiplexed stages is two, and only one stage of full-bridge inverters is shown. 1 is one phase of the full bridge inverter, and 11-14 GTO thyristors (Gate Tur
n off Thyristor), and each G
It consists of 15 to 18 diodes connected in antiparallel to the TO thyristor. 2 is one bill of half-bridge inverter, and similarly C3TO thyristor 21°2
2, and anti-parallel diodes 23 and 24. 3 is a DC side circuit, and a neutral line is taken from a midpoint between DC voltage sources 31 and 32 having the same output voltage. 4 is the output transformer of the full bridge inverter. 5 is an output voltage command circuit that distributes the command voltage effective value E' given to the multiplex inverter according to the output voltage ratio of each stage, and 6 is a carrier frequency command value f to each stage according to the output frequency command value f'. A carrier frequency command circuit 71.74 provides a modulated wave signal e' from the output frequency command value f' and the command voltage effective values E'l and E'2 given from 5.
Modulated wave generation circuit that creates l and e'2, 72.73.7
5 are carrier frequency command values f'cl and f'c2 and carrier phase command value θ'1. A carrier generation circuit that generates carriers from θ′a2°0′bz.

81,. 82.86 is a comparator that compares the magnitude of the carrier with the modulated wave signals e1 and +2 and generates a pulse signal based on the comparison, and 83 is a polarity inverter that inverts the polarity of the voltage command value given to the negative arm of the full bridge inverter. The circuits 84, 85.87 are comparators 81, 82 .
G of the inverter at each stage based on the pulse signal from 86
This is a gate amplifier that drives T.O.

Next, the operation will be described.

When the output frequency of the multiplex inverter is equal to or higher than a certain frequency Fs, multiplex operation is performed using a full-bridge inverter or a full-bridge inverter and a half-bridge inverter. By performing phase difference operation to give each carrier an equal phase difference according to the number of stages,
The apparent frequency (equivalent carrier frequency) becomes higher than the actual carrier frequency, and lower harmonics are reduced.

However, if the output frequency is lower than Fs at startup,
Since the V/f (ratio of output voltage to frequency) is large, the output transformer 4 is saturated and the full-bridge inverter cannot be operated. Therefore, at startup when the output frequency is below Fs, the half-bridge inverter operates alone. Therefore, harmonics appear near frequencies that are multiples of the carrier frequency, increasing current distortion.

To solve this problem, carrier frequency command circuit 6
When operating the half-bridge inverter alone, the carrier frequency is set higher than the carrier frequency during multiplex operation to reduce lower harmonics from the output current. - For example, in FIG. 3, when the carrier frequency during multiple operation is FCm, the total distortion rate of the output current is do%. When the half-bridge inverter is operated alone at the same carrier frequency Fc+a, the total distortion rate increases to dx%. Therefore, when operating the half-bridge inverter alone, the carrier frequency of the half-bridge inverter is set to F.
By setting cs, the overall distortion rate is the same as during multiple operation d
It can be suppressed to 0%. In this case, the carrier frequency fc1 of the half-bridge inverter and the carrier frequency fcz of the full-bridge inverter are set to 5 with respect to the output frequency f, for example, as shown in FIG. 4.

〔Effect of the invention〕

According to the present invention, by setting the carrier frequency when the half-bridge inverter is operating alone to be higher than when the full-bridge inverter is operating in multiple modes, the overall distortion factor of the output current at startup is reduced. This makes it possible to reduce thrust pulsations in the case of a heavily loaded near motor and torque pulsations in the case of a rotary machine at the time of startup, to the same extent as in the case of multiple operation.

[Brief explanation of drawings]

Fig. 1 is a system diagram of an embodiment of the present invention, Fig. 2 is a characteristic diagram of the total distortion factor of output current with respect to output frequency, and Fig. 3 is a carrier diagram during single half-bridge inverter operation and full-bridge inverter multiplex operation. FIG. 4 is a characteristic diagram of the overall distortion factor of the output current with respect to the frequency. FIG. 4 is an explanatory diagram of an example of setting the carrier frequency with respect to the output frequency. Fig. 2 Fig. 3

Claims (1)

[Claims] 1. The AC output ends of a half-bridge inverter without an output transformer and a single-phase full-bridge inverter equipped with the output transformer are connected in series, and the half-bridge inverter and the full-bridge inverter are connected in series. A multi-voltage PWM inverter equipped with a control circuit that performs pulse width modulation control to adjust the output voltage to a predetermined value, characterized in that the carrier frequency of the half-bridge inverter is increased in accordance with a decrease in the inverter frequency. Control method for multiple PWM inverter. 2. The multiple PWM inverter control method according to claim 1, wherein the carrier frequency of the half-bridge inverter is set to be higher than the carrier frequency of the full-bridge inverter.