JPS62135276A - Current limiting method for pwm inverter - Google Patents

Abstract

PURPOSE:To limit currents positively against overcurrents, and to improve the continuation of operation by providing a feedback mode between a reflux mode and an overcurrent detecting level and simultaneously turning an inverter transistor OFF in the feedback mode. CONSTITUTION:An AC power supply is fed to an induction motor 17 through a converter 14 and an inverter consisting of transistors 1-6. Currents to the induction motor 17 are detected by an AC machine 16, arithmetically operated by a computing element 18, compared with reference voltage by comparators 19-21 and given to a sine-wave PWM controller 22. The PWM controller 22 controls the transistors 1-6 through a base drive circuit 23. An overcurrent detecting level is set by a rheostat RH, a current limiting by a reflux mode is used mainly, and only a singular point where currents do not attenuate at the reflux mode is utilized for a current limiting by a feedback mode.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for limiting current in a PWM (Pulse Width Modulation) inverter.

[Conventional technology]

A current flow diagram representing the current limiting means in the reflux mode in the conventional example is shown in FIG.

FIG. 6 is a current waveform diagram and a PWM pulse waveform diagram.

1 to 6 are inverter transistors made into a three-phase bridge, and connected current die A-door to 12 are connected in antiparallel to them.
The output line of the inverter is connected to the primary winding of the induction motor 17.

Current limiting in the freewheeling mode is shown in FIG. 5, when the current limiting circuit is activated, it turns off transistors 1, 2 and 3 (or turn A) and turns off transistors 4, 5 and 6.

1, when the current limiting circuit operates 1, it cuts the PWM pulse width and throttles the output voltage of the inverter.

Therefore, when the transistor 2.3 is turned off and the transistor /I 5.6 is turned off, the current 11 during operation is as follows: transistor 1 → load 17 → diode 9 -> route of transistor 1 to reflux.

The current at this point decreases as shown in Figure 6 (a).
3. PWM pulses p1 and D2 shown in Fig. 6(b)
Inverter current 11 flows in response to p,
The part indicated by the broken line of pb is the PWM pulse width (p1+pa, p2+pb) when no current restriction is applied.

[Problems solved by the invention and still dozing off] In this way, the current limitation in the free-flow mode is such that at the moment when the PWM = commercial waveform reaches the current limit level, 1) the W M voltage is cut l-1,, induced Connect the electric motor 17 to the flywheel diode door ~ 12T4. In order to avoid U-circuiting and a gradual decrease in the flywheel current 12, the peak of the current waveform is cut off (the waveform is j).

Therefore, there is little disturbance in the current waveform, and the current is limited without being attenuated by 1 Herc.

However, since the flywheel diodes 7 to 12 short-circuit the induction electric current of the induction motor 17, depending on the polarity of the induced voltage and the IJ direction of the flywheel current 12, the flywheel'? The r3 flow 12 may be in the increasing direction.

When this mode is entered, the current limit in the reflux mode is not applied and the overcurrent detection level is reached, making it impossible to continue operation.

SUMMARY OF THE INVENTION An object of the present invention is to provide a current limiting method for a PWM inverter that overcomes the drawbacks of the conventional method and improves operation continuity even more than the conventional method.

(Means for Solving the Problems) The present invention provides a feedback mode between the above-mentioned freewheeling mode and the overcurrent detection level, and simultaneously turns off the inverter transistors in the feedback mode.

[Effect]

Current limiting by feedback mode is current limiting (feedback mode)
In FIG. 3, which shows a current flow diagram at the time of operation, when the current limiting circuit (U shown in the figure) operates, 1-transistors 1, 2, .
3, J: and 4, 5.6 are turned off at the same time.

However, since all the transistors are base-blocked to the same 11.1, the current attenuation is shown as the current 12 in Figure 4.
As in, the current quickly becomes zero current. Note that 15 is a smoothing power supply (DC power supply section) provided in the DC section.

Since the current is actually attenuated, there is no unique mode in which the current does not decrease in freewheeling mode, but because the current waveform is greatly disturbed from the sine wave, the current is limited in a state where the torque is greatly attenuated.

The present invention takes advantage of the characteristics of current limiting in the above two modes, sets the current limiting level to the limiting level in the freewheeling mode (the limiting level in the feedback mode), and mainly limits the current in the freewheeling mode. Utilizing the current limitation by the feedback mode only at the singular point where the current does not attenuate, improves operational continuity without applying any torque that disturbs the W M current waveform.

(Embodiment) A block diagram showing a circuit configuration according to an embodiment of the present invention is shown in FIG.

In all drawings, the same reference characters represent the same or corresponding parts.

13 is negative 6! 14 is an inverter that performs AC/DC conversion, 1G is a current transformer that detects the load current flowing into the induction motor 17, and 18 is a current detector that operates the detected current. Mine equipment, 19-21 are comparators,
22G sine wave PWM suppressor that controls to send out a sine wave output voltage; 23 (base drive circuit that turns on and off inverters 1 to 1 to 6;
vo is a constant DC voltage source, R1+ is a variable resistor, R1-
R3 is a fixed resistor.

When the overcurrent detection level is set by the variable resistor RH, the current limit level is determined by the fixed resistors R1, R2, and R3 in a free-circulation mode or a feedback mode.

2 is an overcurrent detection signal, 2 is a current limit detection signal in feedback mode, and ◎ is a current limit detection signal that is applied to J5t in freewheeling mode.

Figure 2 shows a time chart of the base drive waveform during current limiting operation in freewheeling mode and a time chart of the base drive waveform during current limiting operation in feedback mode.
Accepted.

f, is 11791 frequency (pulse), and U phase is transistor 1. The U phase corresponds to 1-transistor 4, and the same applies hereafter.

Moreover, by the control means of the present invention, 211.2 in the shaded area
13,214. 212.221 of the satin-finished part with many dots scattered from the off state to the off state,
223. . . . represents a transition from an OFF state to an OFF state.

〔Effect of the invention〕

Thus, according to the present invention, the following remarkable effects can be obtained.

(i) Effective torque can be maintained by reliable current limiting operation with less disturbance of the current waveform during sudden acceleration/deceleration of induction motors other than fault currents such as load short circuits and earth faults, and overcurrents in overload 11Fi. It is possible to continue operation without damage.

(ii) Since the switching frequency of the power transistor is also prevented from becoming high in the current limiting section, it is possible to prevent the switching loss of the power 1 transistor from increasing.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the circuit configuration according to an embodiment of the present invention, Fig. 2 does not show its operation, but shows the timing of the voltage waveform from -1 to -1, and Fig. 3 shows the feedback mode. Figure 4 shows the current flow diagram when the current is limited by J
The time charts of FIGS. 5 and 6 are explanatory diagrams of conventional examples. 1-6...Transistor, 7-12...Diode, 13...AC power supply, 14...Converter, 15...
... flattening capacitor (DC power supply part), 16 ... current transformer, 17 ... induction motor, 18 ... path current detection calculator,
19-21... Comparator, 22... Sine wave P W M
f, 11 control, 23...Base drive circuit, ■,
... Constant DC voltage source, Rl-1... Variable resistor, R
1-R3...Fixed resistor. Applicant's agent Fujio Fuji Figure 1 Figure 3 Figure 4 Figure 5 Figure 6

Claims (1)

[Scope of Claims] 1. Connect the positive arm and negative arm of the DC section, lead an output line from the connection point to the load, connect a multiphase bridge, and connect a diode in antiparallel to the transistor of each arm. In a PWM inverter, when the load current reaches the first stage current limit, the transistors in all positive arms of the inverter are turned on or off, the transistors in all arms of the load are turned off or on, and the diode is removed from the load. When the load current reaches the second stage of current limiting, where the load current still increases, all transistors of the inverter are turned off and the charge is transferred from the load to the DC power supply via the diode. A current limiting method for a PWM inverter, characterized in that when the load current reaches a third stage of current limiting in which the load current is further increased, an AC power source that supplies power to the inverter is cut off.