JPS6013400B2 - AC motor control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a control device for an AC motor, and particularly to a device that can immediately detect stalling of an AC motor and perform necessary control.

As a control device for an AC motor, a control system is often used that converts input power into a desired variable frequency using a mist pressure type inverter or a current type inverter, and then adjusts this variable frequency to drive the AC motor. ing.

FIG. 1 shows an example of a control device for an AC motor in which a variable frequency power source is constructed using a current type inverter.

In the figure, commercial frequency power from an input power source 11 is rectified by a rectifier 12 and input to an inverter circuit 14 through a DC reactor 13. This inverter circuit receives a signal from a control circuit, which will be described later, and converts input power into square wave alternating current power. In this case, the frequency f and voltage V of the AC output power are controlled so that the ratio of the two is constant as shown in FIG.
Start and accelerate 6. When the acceleration progresses and the AC motor 16 reaches the maximum frequency operation, the switch 17 is closed and the switch 1 is closed.
5 and switch to commercial frequency power from the input power source 11. In the above, the phase control system of the rectifier 12 is configured as follows. That is, the speed setting variable resistor 18
The voltage reference value signal obtained through the input limiter 19 is led to the comparator 21 and compared with the output of the voltage detector 20 that detects the output voltage of the inverter circuit 14. The output of this comparator passes through the voltage control circuit 22 and is then inputted to one input terminal of the comparator 23 as a current reference value signal. A current transformer 24 is connected to the other input terminal of the comparator 23.
, an output current detection value signal of the input power source 11 obtained by the overload relay 25 and the current detector 26 is input. A comparison output of these two signals is led to a rectifier phase control circuit 28 via a current control circuit 27 to control the conduction angle of the rectifier 12. Further, the frequency control system of the inverter circuit 14 is configured as follows. That is, the voltage reference value signal obtained through the input limiter 19 described above is transmitted to the oscillator 29.
to control its oscillation frequency. This oscillation output is converted into a gate pulse by the ring counter 30, and is inputted to the control terminal of the inverter 4 via the pulse amplification circuit 31 to control its output frequency. When the AC motor 16 stalls for some reason, the output current 1 increases rapidly after the stall point P, as shown in FIG. 3, and the current is limited. After reaching the value L, the flow continues at that value.

In addition, in the figure, N exemplifies how the speed of the electric motor 16 changes. If such a state continues, the overload relay 25 will eventually operate, and its contact circuit will open the power supply circuit or cut off the gate circuit of the rectifier 12 to protect the motor from burnout due to stalling. However, the above-mentioned overload relay and the integral circuit called the amplifier thermal relay, which is often used as an alternative, require a considerable amount of time to start operating after the motor stalls, and during that time, a large current flows. This increases power loss and requires a variable frequency power supply with a large capacity. Furthermore, when operating an AC motor by switching from a variable frequency power supply to a commercial power supply, if the switching is performed in a stalled state, a large current comparable to that of a direct start will flow into the AC motor, causing disturbance to the power supply system. There are drawbacks such as large heat loss occurring in the AC motor and shortening its lifespan. The present invention has been made to eliminate the above-mentioned drawbacks of conventional devices.

That is, the present invention aims to provide a control device for an AC motor that does not have the above-mentioned disadvantages by using a stall detection circuit that compares a voltage detection value with a voltage reference value and detects a stall based on the error signal. purpose. 4 or less,
The invention will now be described in detail with reference to the illustrated embodiments. FIG. 4 shows an example of a stall detection section according to the present invention.

In the same figure, the voltage detection value signal and the voltage reference value signal input to the terminals FBK and REF are led to the comparator 21 in the same way as in the case of FIG.
2, but at the same time it is input to the stall detection circuit 32. In this stall detection circuit 32, the voltage reference value signal and the voltage output value signal are inputted to an intensifier 35 through resistors 33 and 34, respectively.

The gain of this multiplier 35 is determined by the feedback resistor 36 and the resistor 33.
, 34, the slight error between the voltage reference value signal and the voltage detection value signal can be greatly amplified and displayed in the output of the multiplier 35. This error signal is input to an intensifier 39 through a resistor 37. Reference numeral 40 denotes a variable resistor for setting an error reference value, the output of which is input to the multiplier 39 through a resistor 41.

This multiplier 39 has a voltage diode 4 connected in parallel to it.
2 constitutes a switching circuit, and a multiplier 35
When the output becomes larger than the error reference value, the transistor 43 is made conductive. As a result, the coil of the relay 44 is energized, and its contact circuit is activated to generate a stall detection signal. As shown in FIG. 5, the stall detector 32 configured in this manner has two input terminals connected to the voltage detector 20 and the output side of the input limiter 19, respectively, and a relay 4.
4 contacts 45 are inserted between the speed setting variable resistor 18 and the input limiter 19.

In addition, the structure of the parts other than the above in FIG. 5 is the same as that in FIG. 1. In the circuit shown in FIG. 5, when the AC motor 11 stalls, the stall detection circuit 32 detects the stall, and the saddle point 45 opens to lower the voltage reference value.

As shown in FIG. 6, when the contact opens during the acceleration of the AC motor, the output voltage V and frequency f of the inverter 14 gradually decrease. When the voltage reference value becomes the detected voltage value at that time, the stall state of the AC motor 16 is released, the contact 45 closes, and the AC motor is accelerated again. In the embodiment shown in FIG. 7, the output signal of the stall detection circuit 32 is input to the control terminal of the input limiter 19 as well as to the control terminal of the voltage control circuit 22.

In addition, in FIG. 7, the configuration of the parts other than the above is the same as that of FIG. 1. In the circuit shown in FIG. 7, when the AC motor 11 stalls, the stall detection circuit 32 detects the stall and issues a control signal to the input limiter 19 and the voltage control circuit 22.

As shown in FIG. 8, if a stall is detected at time t3 while the AC motor is accelerating, the input limiter 19 that received the signal from the stall detection circuit will continue to operate until another control signal arrives. During this time, voltage V and frequency f are kept constant. At the same time, the current limit value peak associated with voltage control is 1 as shown in Figure 9.
・Click up and increase the speed of the AC motor. As a result, when the speed of the AC motor reaches the specified value, the input limiter 19 controls the voltage V and frequency f again at time Z intervals, and returns the current reference value signal IL to the original level 1,
Reduce to. As is clear from the above explanations of several embodiments, the present invention compares the voltage detection value input to the AC motor with the voltage reference value to find the error between the two, and then calculates the error between the two. If the current limit value is exceeded, it is determined that this is a stall, and a control signal is sent to the input terminal and control terminal of the input limiter to stop or suppress the operation of the variable frequency power supply, and the current limit value is increased by 2. By doing this, you can recover from a stall.

Therefore, according to the present invention, it is possible to reduce power loss due to overcurrent flowing when the AC motor stalls, and it is possible to prevent the installed capacity of the variable wave number power source from increasing unnecessarily. In addition, since the input power source is not switched from the variable frequency power source to the commercial frequency power source when the AC motor is stalled, the life of the AC motor will not be shortened.
Furthermore, when a stall occurs, it can be detected without delay and countermeasures can be taken, so stable operation can be maintained for three years. In addition, in the above explanation, when the AC motor stalls, the error between the detected voltage value and the reference value is controlled using an input limiter, etc., and the AC motor is started and accelerated again with both values at the same level. As described above, the present invention is not limited to this example, and the main circuit of the AC motor may be directly opened and closed to control the operation.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing an example of a control device for a subordinate AC motor, Fig. 2 is a characteristic curve diagram of voltage V and frequency f in the device shown in Fig. 1, and Fig. 3 is a diagram showing an example of a control device for a subordinate AC motor. Characteristic curve diagram of the rotational speed and current of the AC motor when a stall occurs in the device shown in Figure 1, Figure 4
5 is a circuit diagram showing an example of application of the circuit shown in FIG. 4, FIG. 6 is an explanatory diagram of the operation of the circuit shown in FIG. FIG. 7 is a circuit diagram showing another example of application of the circuit shown in FIG. 4, and FIGS. 8 and 9 are diagrams for explaining the operation of the circuit shown in FIG. 5. 11... Input power supply, 12... Rectifier, 13... DC reactor, 14... Inverter, 15, 17... Switch, 16... AC motor, Tortoise 8... Variable resistor for speed setting, 19. ...Input limiter, 20...Voltage detection circuit, 2
1, 23... Comparator, 22... Voltage limiter, 24...
... Current transformer, 25 ... Overload relay, 26 ... Current detection circuit, 27 ... Current control circuit, 28 ... Rectifier phase control circuit, 29 ... Oscillator, 30 ... Ring Counter, 31... Pulse intensification circuit, 32... Stall detection circuit,
33, 34, 36, 37, 41...Resistance, 35, 39
...Intensifier, 40...Variable resistor for error setting, 42.
... Constant voltage diode, 43 ... Transistor, 44
．． ．．・IJO Leh. Figure 2 Groove 3 Figure Crocodile Figure 6 - Figure Crocodile 4 Figure 8 Figure 9 Figure 5 Figure 7

Claims (1)

[Scope of Claims] 1. A control device for starting and accelerating an AC motor using a variable frequency power source, including a voltage detection circuit that detects the output voltage of the variable frequency power source, a voltage detection value signal of the voltage detection circuit, and a voltage reference. A stall detection circuit is provided, which includes a comparator that compares the value signal and determines the error between both signals, and a circuit that determines that the AC motor has stalled when the error exceeds a set value. A control device for an AC motor, characterized in that the operation of the variable frequency power source is stopped or suppressed by an output. 2. The output of the stall detection circuit lowers the level of the voltage reference value signal and lowers the frequency of the variable frequency power supply to continue operation of the AC motor, and when the voltage detection value signal reaches the same level as the voltage reference value signal, Claim 1 characterized in that it is determined that the stall has been released and the AC motor is accelerated again.
A control device for an AC motor as described in . 3. Claim 1, characterized in that the frequency of the variable frequency power source is held constant by the output of the stall detection circuit, and the acceleration of the AC motor is sustained by increasing the limit value of the output current of the variable frequency power source. AC motor control device.