JPH06113574A - Starter for semiconductor motor - Google Patents

Abstract

PURPOSE:To prevent fluctuation of r.p.m. by detecting AC three-phase motor current and setting a current increase rate and a final limit current based on the initial current at the time of starting. CONSTITUTION:When a start command is fed to a current reference pattern generating circuit(PG) 18 and a phase control circuit 20, the PG 18 generates a current reference signal iref which increases from a level set by an initial current setting circuit 15 at a rate set by an increase rate setting circuit 16. A signal detected through any one of current detectors 10-12 is fed through a rectifying circuit 13 and a low-pass filter 14 to produce a current feedback signal (if) and the difference DELTAi from the current signal iref is fed to a current control circuit 19. The current control circuit 19 produces a phase control signal Ec from the difference DELTAi. The phase control circuit 20 increases the voltage to be applied on a motor when the signal Ec is positive whereas lowers the voltage to be applied on the motor when the signal Ec is negative thus equalizing the feed back amount if to the current reference iref.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor motor starting device for starting a three-phase AC motor.

[0002]

2. Description of the Related Art As is well known, in a three-phase AC motor, an overcurrent of 6 to 8 times the rated current flows at the time of direct insertion start, which increases mechanical stress on the load and causes a decrease in power supply voltage. Cause problems. In order to eliminate such inconvenience, a semiconductor motor starting device using a semiconductor switching element such as a thyristor or a triac has been conventionally used. FIG. 3 shows a conventional semiconductor motor starting device.

In FIG. 3, reference numerals 1 to 6 denote semiconductor switching elements such as thyristors, which are connected in antiparallel to each of the three phases to supply electric power to the three-phase AC motor 7. Reference numeral 9 is a phase control pattern generation circuit for generating a phase control signal for turning on / off the semiconductor switching elements 1 to 6 based on a start / stop command. In such a semiconductor motor starting device, when a start command is input to the phase control pattern generation circuit 9 from the outside, the phase control pattern generation circuit 9 has a predetermined characteristic in the applied voltage at the time of starting the three-phase AC motor. Thus, the position control signal pattern for firing the semiconductor switching elements 1 to 6 is generated. For example, as shown in FIG. 4A, after the start command of the voltage applied to the three-phase AC motor 7, the firing phases of the semiconductor switching elements 1 to 6 are changed so as to increase linearly. In this way, the overcurrent at the start is controlled by gradually increasing the voltage applied to the three-phase AC motor 7. After the start, the semiconductor switching element 1
6 to 6 are all fired, and the voltage applied to the three-phase AC motor 7 is equal to 100% of the three-phase AC power supply voltage. Further, when a stop command is input to the phase control pattern generating circuit 9, the voltage applied to the three-phase AC motor 7 is generally controlled so as to decrease in reverse to that at the start.

[0004]

In the above semiconductor motor starter, the rotation speed of the three-phase AC motor 7 reaches the rated speed before the voltage applied to the three-phase AC motor 7 reaches 100% of the three-phase power supply voltage. If it becomes, depending on the load,
As shown in (b), the rotation speed vibrates near the rated speed. This vibration causes a large mechanical stress in the power system connected to the three-phase AC motor 7 and also in the motor itself, sometimes causing a big problem such as damage to the power system. The cause of this vibration is that a large amount of harmonics (5th, 7th, 11th, 13th harmonics of the power source fundamental frequency) are included in the voltage applied to the motor whose phase is controlled by the semiconductor switching elements 1 to 6. This is because In other words, the three-phase AC motor 7 tries to increase the rotation speed up to the synchronous speed of the fifth harmonic, which is most included in the power supply, but when the synchronous speed of the fundamental wave is exceeded, the motor becomes a generator and power is supplied to the power supply side. To regenerate the braking force on the motor. Although the rotation speed of the motor is lower than the rated speed due to inertia, it tries to increase again to the synchronous speed of the fifth harmonic contained in the voltage. The same phenomenon is repeated thereafter, and finally, when the voltage applied to the motor becomes equal to the power supply voltage, that is, when it becomes a sine wave that does not include harmonics, the vibration of the rotational speed subsides and the rated speed Be stable during driving. The present invention has been made to solve the above problems, and an object of the present invention is to provide a semiconductor motor starter capable of eliminating vibration of the motor rotation speed.

[0005]

In a semiconductor motor starting device of the present invention, a semiconductor AC switch having semiconductor switching elements connected in anti-parallel is connected between a three-phase AC power source and a three-phase AC motor. In a semiconductor motor starting device for controlling a phase of a semiconductor switching element at the time of starting the three-phase AC motor to change a voltage applied to the three-phase AC motor, an electric current of the three-phase AC motor is detected to start an initial operation at the time of starting. It is characterized in that a means for setting the current, a means for setting the rate of increase of the current from the initial current, and a means for setting the final limit current are provided.

[0006]

According to the semiconductor motor starting device of the present invention, the current of the three-phase AC motor is controlled in the set pattern. Therefore, when the three-phase AC motor reaches the rated speed and the current drops, the set current will flow. Since the voltage applied to the three-phase AC motor rises and becomes equal to the AC power supply voltage, it is possible to eliminate the vibration of the rotation speed of the three-phase AC motor.

[0007]

1 is a block diagram of a semiconductor motor starting device showing an embodiment of the present invention. The same parts as those in FIG. 3 are designated by the same reference numerals, and detailed description thereof will be omitted.

In FIG. 1, 1 to 6 are semiconductor switching elements, 7 is a three-phase AC motor, 10 to 12 are current detectors for detecting the current of the three-phase AC motor 7, for example, a current detector such as CT, 13 is a current detector. A rectifying circuit for rectifying the outputs of the capacitors 10 to 12, 14 is a low-pass filter, 15 is an initial current setting circuit, 16 is a rising rate setting circuit for setting a rising rate of current, 17 is a limit current circuit for setting a final current, 1
Reference numeral 8 is a current reference pattern generation circuit that generates a current reference pattern from the signals of the setting circuits 15 to 17, and reference numeral 19 is an output iref from the current reference pattern generation circuit 18 and a current field background signal which is an output signal of the low pass filter 14. A current control circuit that generates a signal Ec that determines the phase from the difference from the port if, and 20 is a phase control circuit that generates a gate in each thyristor from the signal Ec.

Now, when the start command is input to the current reference pattern generation circuit 18 and the phase control circuit 20,
The current reference pattern generation circuit 18 increases the value from the value set by the initial current setting circuit 15 as shown in FIG.
The current reference signal ire rising at the rising rate set in 6.
generate f. The signals detected by the current detectors 10 to 12 pass through the rectifier circuit 13 and the low-pass filter 14 to become the current feedback signal if, and the difference Δi = (iref−if) from the current signal iref is input to the current control circuit 19. . The current control circuit 19 changes the phase control signal Ec from Δi.
Is output.

There are various control methods such as P control, PI control, PID control, etc. as known in the conventional control technology, but here, P control which is simple proportional control will be described. The phase control signal Ec is Eref> if (that is, Ec> 0 when the feedback amount is smaller than the reference value. Conversely, iref <if, that is, Ec> 0 when the feedback amount is larger than the reference value. .

Therefore, the phase control circuit 20 increases the voltage applied to the motor when the signal Ec is positive, and the signal Ec.
When the voltage applied to the motor is lowered when c is negative, the feedback amount if is controlled to be equal to the current reference iref.

Therefore, as shown in FIG. 2 (c), the motor current is in accordance with the current reference until the motor rotation speed reaches the rated value. The solid line in FIG. 2 shows the case where the time required to start the motor is long with respect to the setting, and the dotted line shows the case where the time is short. When the rotation speed of the motor approaches the rated value, the motor current drops to a current determined by the load. Since the current reference is increasing or fixed to the limit current, when the motor current starts to decrease, the phase control signal Ec becomes Ec> 0, and FIG.
As shown in (a), the voltage applied to the three-phase AC motor 7 rises in a short time. In other words, when the motor reaches the rated speed, the voltage applied to the three-phase AC motor 7 rises to 100% of the current voltage in a short time. Therefore, the period in which the voltage applied to the three-phase AC motor 7 contains harmonics should be shortened. It is possible to eliminate the vibration of the rotation speed.

[0013]

As described above, according to the present invention, the motor current can be controlled to be equal to the current reference.
When the motor current becomes smaller than the current reference, the voltage applied to the motor quickly becomes equal to the power supply voltage, and the period in which harmonics occur can be shortened, and mechanical stress applied to the motor and load power system due to vibration of the motor rotation can be reduced. It can be lost. Also, since the motor current is increased linearly, not only can an extremely smooth start be made possible, but the initial current, current increase rate, and limit current can be set individually, so a start most suitable for the load can be achieved. It becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram of a semiconductor motor starting device showing an embodiment of the present invention.

2A and 2B are waveform diagrams for explaining the operation of the semiconductor motor starter according to the present invention, in which FIG. 2A is a waveform diagram of a motor applied voltage, FIG. 2B is a waveform diagram of a motor rotation speed, and FIG. Waveform diagram, (d) Waveform diagram of current reference

FIG. 3 is a block diagram of a conventional semiconductor motor starting device.

FIG. 4 is a waveform diagram illustrating the operation of a conventional semiconductor motor starting device.

[Explanation of symbols]

1 to 6 are semiconductor switching elements, 7 is a three-phase AC motor, 10 to 12 are current detectors, 13 is a rectifier circuit, 14
Is a low pass filter, 15 is an initial current setting circuit, 16 is a rising rate setting circuit, 17 is a limit current setting circuit, 18 is a current reference pattern generation circuit, 19 is a current control circuit, 20
Indicates a phase control circuit.

Claims (1)

[Claims] 1. A semiconductor AC switch in which semiconductor switching elements are connected in anti-parallel is connected between a three-phase flow power source and a three-phase AC motor, and the semiconductor switching elements are phase-controlled when the three-phase AC motor is started. In the semiconductor motor starting device that changes the voltage applied to the three-phase AC motor, means for detecting the current of the three-phase AC motor and setting the initial current at the time of starting, and setting the rate of increase of the current from the initial current And a means for setting a final limit current.