JPS6035918B2 - AC motor control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device for variable speed operation of an AC motor that is supplied with power via a power conversion device that outputs an AC voltage of variable voltage and variable frequency. It's about prevention.

As power conversion devices used for variable speed operation of synchronous motors and induction motors, indirect frequency conversion current source inverters and direct frequency conversion cycloconverters are known.

FIG. 1 is a block diagram showing an example of a conventional configuration of a control system for a case in which an induction motor is operated at a variable speed using a current source inverter.

1 is a commercial three-phase AC power supply, 2 is a converter configured with a controlled rectifier, 3 is an inverter configured with a controlled rectifier, 4
is an induction motor which is the load of the inverter 3; 5 is a current smoothing reactor; 6 is a converter phase shifter that controls the output voltage of the converter 3; 7 is a current detector composed of a current transformer and a rectifier; 8 and 9 is a voltage detector composed of a current control computing unit and a voltage control computing unit that control a converter phase shifter, a 1-channel transformer, and a rectifier;
1 is an inverter control circuit consisting of a logic circuit and an increase circuit for controlling the thyristor of the inverter 3; 12 is a voltage/frequency converter; and 13 is a speed setting capable of arbitrarily setting the load voltage and inverter output frequency. It is a vessel.

The torque generated by the induction motor 4 is equal to the output voltage V of the inverter 3.
ac, current is given as a function of lac or ldc, output frequency, slip frequency, etc.

In the control system shown in Fig. 1, the frequency of the inverter 3 is controlled by open loop control using the speed setting value e, and the load voltage is controlled by comparing the detected value of the output voltage with the voltage reference value (speed setting value e,). Since closed-loop control is performed and the load power factor (ratio of magnetizing current to generate magnetic flux and secondary current that contributes to torque generation) is not controlled, under certain load conditions, the torque generated by the induction motor and It is known that an unstable phenomenon in which the output voltage and current of the inverter oscillates at a low frequency, so-called "disturbance", occurs. It is known to connect an actuator as a dummy load to the inverter in order to prevent such disturbances.

By lowering the output impedance of the current source inverter equivalently, when a disturbance occurs in the control system such as a change in the motor load, the ability of the motor primary current to follow this disturbance is improved, resulting in an unstable phenomenon. The aim is to suppress the However, such conventional solutions require additional components in the main circuit, and have drawbacks such as increasing the capacity of the power converter by the dummy load.

An object of the present invention is to return to the origin of the instability phenomenon caused by adding an element to the main circuit component and to prevent such instability phenomenon in a control manner.

The present invention achieves the above object by detecting signs of disturbance due to changes in the internal magnetic flux of the motor, and damping the changes in the internal magnetic flux by changing the output frequency of the power converter according to the rate of change in the internal magnetic flux. This has been achieved.

FIG. 2 is an equivalent circuit for one phase of an induction machine, where R, is the primary resistance, and 1, is the primary leakage inductance.

R2 is a secondary resistance, and when the slip is S, its primary side converted value is R2/S. 12' is 2 on the primary side
The next leakage inductance, M', is the primary-side equivalent mutual inductance.

A magnetizing current iM that generates magnetic flux and a secondary current i2 that contributes to torque generation are superimposed and flow as a primary current i. The present invention proposes that the main cause of instability phenomena that appear as low-frequency vibrations in the torque generated by an electric motor, the output voltage and output current of a current-source inverter is the disturbance caused when the load torque (for example, an electric motor) fluctuates. The corresponding response is the high output impedance of the current source inverter and its output current (1
The reason for this is that the current (second current) is hindered by the fixed frequency. In other words, when the secondary current changes due to load torque fluctuations, the primary current cannot immediately follow the change, so the magnetizing current changes by that amount, resulting in a change in the internal magnetic flux or internal induced voltage. It is conceivable that the mutual interference between the secondary current and the magnetizing current further develops into the mutual interference between the internal induced voltage and the inverter output current, and that this appears as the unstable phenomenon described above. It can be done. Here, the present invention provides an advantage that the magnitude of the internal magnetic flux caused by the mutual interference between the secondary current and the magnetizing current is i! The frequency of the primary current is changed in a direction that suppresses changes in the internal magnetic flux in response to changes in the internal magnetic flux.

For example, for a change in the increasing direction of magnetic flux, when the motor is in the motor operation mode, that is, when the slip S is positive, the magnetic flux differential value d! ? l/at is superimposed on the frequency command value with a polarity that increases the inverter output frequency. Since changes in the rotational speed of the motor are delayed due to inertia, an increase in the inverter frequency results in an increase in the slip S and further a decrease in the secondary resistance R2/S. The decrease in secondary resistance R2/S increases the secondary current, which results in a decrease in magnetizing current and a decrease in magnetic flux.

On the other hand, when the magnetic flux tries to decrease, the inverter frequency decreases to suppress the decrease in the magnetic flux. On the other hand, when the electric motor is also in the generator operation mode, the correspondence between the increase/decrease in the magnitude of the magnetic flux and the increase/decrease in the inverter frequency for suppressing it is opposite to that described above.

This is because in the generator mode, the slip S has a negative polarity, and in order to increase the magnitude of the slip S, it is necessary to reduce the inverter frequency (when the slip S is positive, the slip (To increase the speed, increase the inverter frequency as described above.) Therefore, the polarity when superimposing the magnetic flux differential value dl/dt on the frequency command value needs to be switched between the motor operation mode and the generator operation mode. FIG. 3 shows an embodiment of the present invention in which the magnetic flux loss is determined by integrating the motor terminal voltage, and the discrimination between the two operating modes is made based on the level of the firing angle control signal of the power supply side converter.

In FIG. 3, the same components as in FIG. 1 are given the same reference numerals, and a description thereof will be omitted. According to this embodiment, the motor terminal voltages VR, Vs, VT detected insulated via the transformer 14 are detected by the magnetic flux detector 20.
1 is derived, and the magnitude of the magnetic flux i0l is calculated here. That is, the three-phase voltages VR, Vs, and VT are first
The two-phase converter 15 converts it into two-phase voltages VQ and V8. And each output voltage VQ, V8 is integrator 16
The two-phase magnetic flux is integrated by ? Q, OB
is converted into a signal corresponding to
l0l=ノJ2 Q+? According to the relational expression 2 3, a signal corresponding to the magnetic flux 1 is obtained, and this is sent to the differentiating circuit 18.
The differential signal dl◇l/dt is guided by the polarity switch 22 to the adder 19 at the input section of the voltage/frequency converter 12 with the polarity matched to the operating mode each time, and is converted to the original frequency command value. The operating mode is determined by comparing the level of the output voltage EQ of the current control calculator 8, which is a firing angle command signal that determines the output voltage of the converter 2, with the reference level in the comparator 21. This is done by comparing the firing angle control signal EQ.
When the value of is zero, the firing angle of converter 2 is 900 and the converter DC output voltage is zero. This is the boundary between forward conversion operation and inverse conversion operation of converter 2, and the motor operation mode. Converter 2 performs forward conversion operation, and converter 2 performs forward conversion operation when in generator operation mode.
performs an inverse conversion operation, so in this case, the comparator 21
The operation mode is determined by the polarity of the firing angle control signal. This makes it possible to determine the operating mode of the motor without detecting the rotational speed of the motor, the output voltage of converter 2, or the input voltage of converter 2. By selecting the polarity relationship for each mode as described above, it is possible to give a damping effect to the change in internal magnetic flux, so a damping effect is produced in the mutual interference between the secondary current and the magnetizing current, resulting in a wide speed control range. Instability phenomena can be prevented over a long period of time. Magnetic flux? Alternatively, the detection circuit 1 and 1 may be replaced with various other known ones. For example, the magnetic flux ◇ may be directly detected using a Hall element, or the detected motor terminal voltage value may be divided by the value equivalent to the inverter output frequency. And magnetic flux l? It is also possible to obtain the equivalent of l.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a known configuration example of a control device for an AC motor, FIG. 2 is an equivalent circuit diagram of an induction motor, and FIG. 3 is a block diagram showing an embodiment of the device of the present invention. 1...AC power supply, 2...Converter,
3. ．．・．． ．． - Inverter, 4... Induction motor, 5... Current smoothing reactor, 6... Phase shifter for converter, 7... Current detector, 8...
... Current control computing unit, 9... Voltage control computing unit, 10... Voltage detector, 11... Inverter control circuit, 12... Voltage/frequency conversion vessel, 13
... Speed setting device, 14 ... Motor terminal voltage detector, 15 ... 2-phase/3-phase converter, 16 ...
...Integrator, 17... Arithmetic unit, 18...
... Differential circuit, 19 ... Adder, 20 ...
・Magnetic flux detector, 21... Comparator, 22...
...Polarity switch. Spear 1 picture Spear Z picture Next 3 pictures

Claims (1)

[Scope of Claims] 1. In a control device for variable speed operation of an AC motor supplied via a power conversion device that outputs an AC voltage of variable voltage and variable frequency, the internal magnetic flux of the AC motor is determined by a circuit that calculates A first signal is derived by differentiating the magnitude of the internal magnetic flux, and the first signal is outputted with the following polarity depending on whether the AC motor is in motor operation mode or generator operation mode. superimposing the converter output frequency on a second signal commanding it, i.e. with a polarity such that when in the motor operating mode, the output frequency is increased with an increasing change in the magnitude of the internal magnetic flux; An alternating current characterized in that when in the generator operation mode, the first signal is superimposed on the second signal with a polarity such that the output frequency is decreased as the magnitude of the internal magnetic flux increases. Electric motor control device. 2. The power converter is an indirect frequency converter consisting of a power supply side converter, a motor side converter, and a DC intermediate circuit interposed between both converters, and controls the power supply side converter to determine the operation mode of the AC motor. 2. The AC motor control device according to claim 1, wherein a signal for commanding the angle is used.