JPS62138068A - Controller for pwm inverter - Google Patents

Abstract

PURPOSE:To reduce magnetic noises without increasing loss generated in an inverter by changing the frequency of a carrier signal in response to the load currents of the inverter. CONSTITUTION:The load currents of an induction machine 2 detected by current detectors 12a-12c fitted onto three-phase output lines for an inverter 1 are full-wave rectified by a full-wave rectification circuit 5, and smoothed by a filter 6 for smoothing. An output from the filter 6 is passed through an upper and lower limit limiter 7, and carrier frequency is determined on the basis of a previously set characteristic pattern. The analog signal of the carrier frequency is transmitted over comparators 10a-10c through a V/F converter 8 and an integrator 9. Triangular wave signals for modulation from the integra tor 9 and since wave control signals from a control-signal arithmetic circuit 4 are inputted to the comparators 10a-10c, and signals displaying switching timing are transmitted over base drive circuits 11a-11c.

Description

[Detailed description of the invention] [Technical field to which the invention pertains]

The present invention relates to a control device that can reduce magnetic noise generated from an induction machine when the induction machine is driven by a so-called PWM inverter (an inverter that varies one frequency of voltage and (current) using a pulse width modulation method).

[Prior art and its problems]

One of the problems when driving an induction machine with a voltage-type PWM inverter is magnetic noise generated from the induction machine. This is due to the carrier frequency of the carrier signal for pulse width modulation, and in current PWM inverters, where the carrier frequency is often selected to be around 1 to 3KH2, this is particularly important because it is in a frequency band that is audible to the human ear. It is considered a problem. One possible countermeasure to this problem is to raise the carrier frequency and remove it from the human audible range, but this method also increases the switching loss of the power conversion elements used in the inverter almost in proportion to the carrier frequency. Therefore, disadvantages such as an increase in the size of the cooling body of this power conversion element and an increase in cost occur.

[Purpose of the invention]

This invention solves the above-mentioned problems in a variable speed drive system for an induction motor which performs frequency control using a PWM inverter, and eliminates the problem of induction motors that cause human discomfort and confusion without increasing the loss of one member generated inside the inverter. An object of the present invention is to provide a control device that can reduce magnetic noise.

[Key points of the invention]

The main point of this invention is that in a variable speed drive system for an induction machine that uses a PWM inverter and performs frequency control, the carrier frequency is raised to reduce magnetic noise when the load is light, and when the load is heavy, the carrier frequency is increased and the magnetic noise is reduced. Due to noise from other machines, etc.
Since the magnetic noise is almost inaudible, the carrier frequency is lowered to prevent an increase in the number of losses generated inside the inverter. In other words, the main point of the present invention is that in a PWM inverter that performs pulse width modulation using a modulating carrier signal (such as a triangular wave signal for 51xl) (for example, using an induction motor as a load), The inverter is equipped with a means for varying the frequency according to the load current of the inverter (a current detector, a full-wave rectifier circuit, a smoothing filter, a lower limiter, a V/F converter, an integrator, etc.). be.

[Embodiments of the invention]

The present invention will be described in detail below based on FIGS. 1(A) and 1(B). Figure (A) is a block circuit diagram as an embodiment of the device of the present invention, and Figure (B) is a characteristic diagram for explaining the operation of the main part of Figure (8).
” indicates the relationship with the charging current. In each figure, the same reference numeral indicates the same or the part corresponding to 411. In Fig. 1 (8), the control signal is Front 1 ri 1 circuit 4 is P
Three sine wave signals (i.e., the amplitude is proportional to the output voltage of inverter 1, and the output 3-phase sine wave signals of the same frequency as the comparator 10a provided corresponding to each phase
Give ~10c. The comparators 103 to 10c use this sine wave control signal and the modulation signal 3 as a modulation carrier signal output from the integrator 9.
A base drive circuit 11 is provided corresponding to each of the three phases by inputting the angular wave signal 9A and determining the point at which both signals meet.
2 to IIC as a signal indicating opening/closing timing. As a result, the base drive circuits 11a to 11c supply a Hess current for opening/closing driving to each of the three-phase transistors (not shown) in the PWM inverter 1. On the other hand, a current detector t2a-12C installed on the 3-phase output line of inverter 1
The load current of the inductor 2 detected by is subjected to full-wave rectification in a full-wave rectifier circuit 5 and smoothed by a smoothing filter 6 to obtain a load current value ■il■. From this load current value ■11■, in the upper and lower limiter 7, as shown in Fig. 1 (
A carrier frequency 4fi f c is determined based on a preset characteristic pattern as shown in B), and an analog signal of this carrier frequency value fe is used to convert each positive and negative half wave through the V/F converter 8. A square wave of the frequency [C with the same period is created, and the integrator 9 integrates this square wave to obtain the modulation triangular wave signal 9A. In this case, the carrier frequency fc changes depending on the load current, as shown in the characteristic of the a load current value (11) versus the carrier frequency value fc in FIG. 1(B). Next, as an example, in a PWM inverter using power transistors as power conversion elements, the carrier frequency at light load is 5KH2, and at maximum load is 2.5Kli2.
5 KH□ regardless of the load, as before.
Let's compare the loss generated by the power transistor at maximum load when it is constant. (Occurrence of power transistors) ■Stationary one-member loss and turn-off loss account for a large amount of loss, among which steady loss is
We focus on the turn-off loss, since it is unrelated to the carrier frequency. Turn-off loss P when carrier frequency fc is 5KH2. , F is the following formula % Formula %: Rector current. On the other hand, when the carrier frequency becomes 1/2, the collector current i
c increases the ripple, set the ripple rate at 82 to 5 to 1.2
Assuming that, 2.5KH2 becomes 1.4. Therefore, the collector current at 2.5KH2 is (1,4/1.2) ic compared to the collector current ic at 5KH2, and 2.
The turn-off loss POFF1 at the time of 5 and 82 is expressed by the following 2. Poryl= A-(1,4/1.2)2・tc"-(
1/2) fc""0.68PoFt In other words, if we adopt a control method that lowers the carrier frequency fc to 1/2 during load compared to the conventional control method where the carrier frequency fc is constant, the turn-off loss will be 68%. This means that it can be reduced.

【Effect of the invention】

As is clear from the following explanation, according to the present invention, when the load is light, the carrier frequency is raised to reduce the magnetic noise generated by the induction machine, and as the load current increases, the carrier frequency is lowered. Therefore, the generation of power conversion elements in the inverter device (ignition can be reduced).
Since the carrier frequency changes continuously with respect to the load current, the magnetic noise also changes continuously, and when the load current is large, the magnetic noise also increases, but this is masked by the noise from other machines that are the load. Therefore, it is possible to obtain the effect that humans no longer feel uncomfortable or confused.

[Brief explanation of drawings]

FIG. 1(A) is a block circuit diagram as an embodiment of the device of the present invention, and FIG. 1(B) is a characteristic diagram illustrating the operation of the main part of FIG. 1(A). 1: PWM inverter, 2: induction machine, 3: frequency setter, 4: control signal calculation circuit, 5: full wave rectifier circuit, 6: smoothing filter, 7: upper and lower limiter, 3: V/F converter,
9+Rt divider, l□a~10C: Comparator, 1la-1i
ck"-base drive circuit, 12a to 12(,: current detector, ■ie■: fJ, current value, rc: 4-
, +・Rear frequency (Hani). Tuna management group 1j11 Fang 1 figure

Claims (1)

[Claims] 1) P that performs pulse width modulation using a carrier signal for modulation
A control device for a PWM inverter, comprising means for varying the frequency of the carrier signal according to a load current of the inverter.