JPS591072B2 - sine wave inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sine wave inverter for driving an AC motor;
In particular, the present invention relates to an AC motor and a driving sine wave inverter with improved operation under heavy loads.

Recently, pulse width modulation type sine wave inverters, which control the ratio between the voltage applied to the load and its frequency to a constant value, have come to be used as power sources for driving AC motors. Figure 1 is a block diagram of a circuit that drives an induction motor using a conventional sine wave inverter. Rectifier bridge circuit for rectification, 3 is a smoothing capacitor,
This is a feedback bridge circuit consisting of a sine wave inverter made up of 4-pin transistors Q1 to Q6 and a 5-pin rectifier D7 to D12. 6 is a time constant circuit into which the speed command signal Vs is input; T is a voltage/frequency conversion circuit that generates a pulse signal with a frequency corresponding to the output voltage of the time constant circuit 6; 8
is a ring counter that distributes the output pulses of the voltage/frequency conversion circuit into three phases.

The output pulse of the ring counter 8 is input to the next stage sine wave oscillator 9 and converted into a sine wave signal. As a sine wave oscillator, for example, a type that first obtains a rectangular wave that alternately inverts positive and negative for each pulse using a continuous pulse train that is the output of a ring counter, and then converts this rectangular wave into a sine wave by passing it through a secondary time constant circuit. You may also use one. Therefore, a sine wave corresponding to the frequency of the ring counter 8 can be obtained. The sine wave oscillator 9 is supplied with a signal obtained by converting the output of the time constant circuit 6 into an absolute value in an absolute circuit 10 and then multiplying the output by a constant circuit 11. As a result, the output signal of the sine wave oscillator 9 becomes a sine wave signal having a peak value corresponding to the magnitude of the speed command signal. 12 is a sawtooth wave generation circuit, which generates a sawtooth wave.

Reference numeral 13 denotes a pulse width modulation circuit, into which two signals, a sine wave from the positive wave oscillator 9 and a sawtooth wave from the sawtooth wave generation circuit 12, are input, and the two signals are compared at the cross point to determine whether the sawtooth wave is better than the sine wave. During the period when the sawtooth wave is higher than the sine wave, it outputs a signal that controls the energization of the transistors Ql, Q3, and Q5 of the sine wave inverter 4, and when the sawtooth wave is higher than the sine wave, it outputs a signal that controls the energization of the transistors Q2, Q4, and Q6. do.

The operation of the sine wave inverter configured in this way will be briefly explained. FIG. 2 is a waveform diagram for one arbitrary phase of a three-phase inverter, and since the other two phases are similar except that the phases are different, only one phase will be discussed. FIG. 2a shows a sine wave signal having a frequency and peak value corresponding to the magnitude of the speed command signal Vs, which is the output signal of the sine wave oscillator 9 in FIG. This is a comparison diagram of the pulse width modulation circuit 13 in the pulse width modulation circuit 13, and the period when the sawtooth wave is lower than the sine wave is shown in the rectangle above the center line, as shown in FIG. A rectangular wave below the line is generated, and the width of the rectangular wave changes periodically along the time axis. A rectangular wave above the center line is applied to the base of, for example, a transistor Q1 among the transistors constituting the sine wave inverter 4,
When the lower rectangular wave is applied to the base of transistor Q2 of the inverter, the connection point C between transistors Q and Q2
The temporal average of the output voltage gives a sine wave voltage as shown in FIG. 2c. As shown in Figure 2d, the phase of the load current due to this voltage is delayed due to the inductive load, and when transistor Q1 is on, the current increases, and when transistor Q2 is on, the current decreases, resulting in a zigzag trajectory. The time average changes sinusoidally as shown by the dotted line. In this way, the three-phase signal outputs a sine wave with a phase shift of 120 degrees. By the way, the conventional device shown in FIG. 1 is equipped with an overcurrent detection device 14 on the output side of each phase, and when the V1 load current exceeds a limit value of 1L, the overcurrent detection device 14 issues an overcurrent detection signal 0LS. , as shown in FIG. 2e, at time t! 1, the rectangular wave immediately falls, turning off the transistor Q1 and turning on the transistor Q2 to reduce the current (FIG. 2f). Next, at timing t/2 when the rectangular wave next rises, the transistor Q1 is turned on again.
When the transistor Q2 turns off and the current increases, and the load current exceeds the limit value 1L at time t/3, the overcurrent detection device 14 again outputs the overcurrent detection signal 0L.
S is generated, and the current is reduced by the same action as described above, so that the load current does not always exceed the limit value 1L, and each transistor is protected from overload. The conventional device as described above does not stop energizing the induction motor even when a heavy load is applied to the induction motor and the load current increases.
Configured to protect sine wave inverters. However, when an overcurrent is detected, the load current immediately begins to fall from the limit value 1L, as shown in FIG. 2f. Therefore, the hatched area in Figure 2 (f) is the area where the current cannot flow, even though the load current may flow, and the temporal average value of the load current is much lower than the current limit value. v will be low. For this reason, the induction motor has the disadvantage that the torque generated in the motor is insufficient, and the induction motor stops because it cannot withstand a heavy load. The present invention is a new invention that improves the conventional drawbacks as described above,
The purpose of this is to raise the load current to the limit value when the load current increases in a sine wave inverter that has a sine wave oscillator, pulse width modulation circuit, and overcurrent protection circuit. The goal is to obtain a circuit device that can do this.

In order to achieve the above object, the present invention includes an AC motor drive device that drives an AC motor with a variable frequency sine wave inverter using power transistors, and generates a variable voltage signal having a predetermined relationship with a speed command signal. has a pseudo sine wave voltage with a variable frequency and a frequency that is a positive multiple of the variable frequency,
A pulse width modulation means changes the temporal duty of the output voltage of the sine wave inverter in a substantially sinusoidal manner by comparing sawtooth waves having a constant peak value, and a pulse width modulation means that detects the current of each phase of the sine wave inverter An overcurrent detection means that outputs an overcurrent generation signal when an overcurrent occurs, and a signal that controls the pulse width modulation means and suppresses an increase in load current for a certain period of time when an overcurrent occurs. and a triangular wave generating means for stopping the rising operation of the sawtooth V wave of the phase during a period in which the pulse width modulating means suppresses an increase in the load current of the phase in which the overload has occurred, During the period when the pulse width modulation means suppresses the increase in the load current of the overloaded phase, the intersection point of the sawtooth V wave and the pseudo sine wave is delayed in time to increase the time duty for turning on the transistor of the phase. The present invention provides a sine wave inverter that compensates for a drop in output voltage of the sine wave inverter when an overcurrent occurs.

Next, one embodiment of the present invention will be described in detail using the drawings.

FIG. 3 is a block diagram showing an embodiment of the present invention;
In the figure, 21 is a time constant circuit, and 22 is an adder, which compares the speed command signal Vs with the output voltage of a speed generator, which will be described later, and outputs the difference.

23 is a proportional-integral type amplifier circuit that increases the output voltage from the adder 22; 24 is a voltage/frequency conversion circuit that generates a pulse signal with a frequency corresponding to the output voltage of the amplifier circuit 23;
25 is an amplifier circuit that converts the output voltage of the amplifier circuit 23 into an absolute value and multiplies it by K. 26 is a frequency divider circuit that divides the pulse signal output from the voltage/frequency conversion circuit 24. For example, the pulse signal frequency is divided by 12. Divide the frequency by 1.

A sine wave generating circuit 27 generates a sine wave having a frequency that is 1/24 of the pulse signal frequency generated from the voltage/frequency conversion circuit 24 and having a peak value corresponding to the output voltage of the amplifier circuit 25.

28 is a triangular wave generation circuit which generates a sawtooth wave in synchronization with the output of the voltage/frequency conversion circuit 24, and also stops increasing the output voltage of the sawtooth wave for a period specified by a signal from a timer circuit, which will be described later. Instructions lifted? After that, the output voltage continues to rise again.

A comparison circuit 29 compares the sine wave output from the sine wave generation circuit 27 and the sawtooth wave output from the triangular wave generation circuit 28.

30 is a sine wave inverter 4 based on the output of the comparison circuit 29.
At the same time, the current of the induction motor 1 is reduced by the output signal of the comparison circuit described later. 31 is a timer circuit, 32 is a current detection circuit, 33 is a comparison circuit, and 34 is a speed generator that detects the rotation speed of the induction motor 1.

Next, the operation of the embodiment will be explained. Rotate induction motor 1 at a predetermined rotation speed? A speed command signal Vs is given from the input circuit to control the speed.

The signal V8 is smoothed by the time constant circuit 21 and then sent to the adder 2.
2. The rotational speed signal of the induction motor 1 rotating at the old rotational speed is inputted to the other input end of the adder 22 with its polarity reversed.
The two input signals are added to the adder 22. Then, the difference between these two signals is output from the adder 22.
After the output from the adder 22 is amplified by the amplifier circuit 23,
It is input to the voltage/frequency conversion circuit 24. Note that FIG. 4a shows the output voltage of the amplifier circuit 23. The voltage/frequency conversion circuit 24 outputs a pulse signal (FIG. 4b) having a frequency proportional to the magnitude of the input voltage. The frequency dividing circuit 26 receives the pulse signal, divides the frequency of the pulse signal into 1/24, and outputs a rectangular wave whose first half is positive and whose second half is negative, as shown in FIG. 4c. Further, when the sine wave generator 27 receives the pulse signal and receives the first pulse, it outputs a voltage that becomes (EmXsinlf) with respect to the voltage Em, as shown in FIG. 4d. Then, when the second pulse is input, it outputs a voltage that becomes (Em
360, a negative pseudo sine wave is output. However, this switching between positive and negative is performed by a signal from the frequency dividing circuit 26. Further, the value of the voltage Em is changed in proportion to the output of the amplifier circuit 25 because well-known variable voltage/variable frequency control is performed. On the other hand, the triangular wave generation circuit 28 outputs a sawtooth wave synchronized with the pulse signal (FIG. 4b), and the comparator circuit 29 compares the sawtooth wave with the pseudo sine wave shown in FIG. A positive signal is output when the sawtooth wave is lower than the wave, and a negative signal is output when the opposite is true (Fig. 4e). The output of comparator 29 is input to pulse width modulation circuit 30. In the pulse width modulation circuit 30, for example, regarding one phase of the U phase, the comparator 2
When the output of the induction motor 1 is positive, the transistor Q1 is turned on, and when the output is negative, the transistor Q2 is turned on. Similarly, the induction motor 1 rotates at a rotation speed that receives a new command, and the rotation speed is fed back to the input side, so that the induction motor 1 is automatically controlled so as to always rotate at a rotation speed according to the speed command signal V8. Assume that during such a normal operating state, a heavy load is applied to the induction motor 1 for some reason, the rotational speed decreases, and a large current flows, for example, to the U phase.

This current value is immediately detected by the current detection circuit 32 and input to the comparison circuit 33. As shown in FIG. 5a, when the value exceeds the limit value 1L, the output of the comparator circuit 33 immediately becomes 61'', and this signal
and input to the pulse width modulation circuit 30? It will be done. The pulse width modulation circuit 30 cuts off the input signal from the comparator circuit 29 so that the output 61'' of the comparator circuit 33 swings in the negative direction immediately at the time T when the overcurrent occurs, as shown in FIG. 5b. , turns off transistor Q, and turns on transistor Q2.

Therefore, the current flowing through the U phase gradually begins to decrease as shown in FIG. 5a. On the other hand, the timer circuit 31 receiving the output "1" from the comparator circuit 33 sends a command to the triangular wave generating circuit 28 to keep the peak value of the sawto b wave constant for a time T, as shown in FIG. 5C. .

When time T has elapsed from time T, a release signal is issued from timer circuit 31 to pulse width modulation circuit 30. At this time T2, as shown in FIG.
),. The U-phase current begins to increase (Figure 5a). If a large load is still applied to the induction motor 1 and it dies, and the U-phase current exceeds the limit value 1L again, the same operation as described above is repeated and the U-phase current is limited. At time T3, when is the sawtooth v wave set? However, when the U-phase current exceeds the limit value 1L at time T4, the current limiting operation as described above is performed again.

In such a current limiting operation, the U-phase current is set to the limit value 1.
This is repeated as many times as necessary until L is no longer exceeded, after which normal operation returns. Note that in the above embodiment, regardless of the output frequency of the voltage/frequency conversion circuit 24, the timer circuit 3
1, the operating time T is kept constant, but it is changed according to changes in the output frequency of the frequency conversion circuit 24 (as the frequency becomes higher, the operating time T becomes smaller, and as the frequency becomes lower, the operating time T becomes larger). ) can also be done. As described above in detail, according to the present invention, when a large load is applied to the induction motor, the current flowing through the induction motor can be increased almost to the limit value.
The torque of the induction motor can be increased compared to a sine wave inverter equipped with a conventional overcurrent protection device.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a circuit that drives an induction motor using a sine wave inverter, Fig. 2 a to f are waveform diagrams for any one phase of a three-phase inverter used in a sine wave inverter, and Fig. 3 4 is a block diagram showing an embodiment of the present invention, and FIGS. 4a to 4e and 5a to 5c are waveform diagrams of various parts in the embodiment of the present invention. In the figure, 1 is an induction motor, 2 is a rectifier bridge circuit, 3 is a smoothing capacitor, 4 is a sine wave inverter, 5 is a feedback bridge circuit, 6 is a time constant circuit, 7 is a voltage frequency conversion circuit, 8
is a ring counter, 9 is a sine wave oscillator, 10 is an absolute value circuit, 11 is a constant circuit, 12 is a sawtooth wave generation circuit, 13
is a pulse width modulation circuit, 21 is a time constant circuit, 22 is an adder, 23 is an amplifier circuit, 24 is a voltage/frequency conversion circuit, 25
is an amplifier circuit, 26 is a frequency divider circuit, 27 is a sine wave generation circuit,
28 is a triangular wave generation circuit, 29 is a comparison circuit, 30 is a pulse width modulation circuit, 31 is a timer circuit, 32 is a current detection circuit,
33 is a comparison circuit, and 34 is a speed generator.

Claims (1)

[Claims] 1. In an AC motor drive device that drives an AC motor with a variable frequency sine wave inverter using power transistors, a variable voltage/variable frequency pseudo sine wave voltage having a predetermined relationship with a speed command signal; , pulse width modulation means for changing the temporal duty of the output voltage of the sine wave inverter in a substantially sinusoidal manner by comparing sawtooth waves having a frequency that is a positive multiple of the variable frequency and having a constant peak value; overcurrent detection means that detects the current of each phase of the sine wave inverter and outputs an overcurrent occurrence signal when overcurrent occurs;
A timer circuit that controls the pulse width modulation means when an overcurrent occurs and outputs a signal that causes the means to suppress an increase in load current for a certain period of time, and a timer circuit that outputs a signal that controls the pulse width modulation means to suppress an increase in the load current for a certain period of time; and triangular wave generation means for stopping the rising operation of the sawtooth wave of the phase during the period when the increase is suppressed, and the sawtooth wave during the period when the pulse width modulation means suppresses the increase in the load current of the phase in which the overload has occurred. By temporally delaying the intersection of the wave and the pseudo sine wave and turning on the transistor of the relevant phase, the time duty of the output voltage is increased and the drop in the output voltage of the sine wave inverter when an overcurrent occurs is compensated for. Features of sine wave inverter. 2. The sine wave inverter according to claim 1, wherein the operating time of the timer circuit is varied as the operating frequency of the sine wave inverter changes.