JPS62152393A - Control method for induction motor - Google Patents

Abstract

PURPOSE:To perform a cooperative operation of a turbine with an induction motor and save power cost, by continuously controlling the output of the induction motor in response to torque by means of slip frequency from zero to rated value. CONSTITUTION:Shifting from a condition where in a non-exciting state the rotor of a motor 3 is rotating by a turbine to a condition where the motor 3 is loaded, then the torque is operated with a subtractor 7 dividing a power command 17 by the number of revolutions fm of the motor and a slip frequency fs is found which is equivalent to its torque with a slip frequency operating machine 8. This slip frequency fs is added at a frequency adder 9 to the number of revolutions fm, finding the frequency of the inverter unit. The motor 3 is then controlled so that the load may be carried by the motor 3.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a power control method for an induction motor driven by an inverter device, and particularly relates to a control method for an induction motor in a system that operates in conjunction with a turbine. be.

[Technical background of the invention and its problems]

Conventionally, gas turbines and steam turbines were often used as the drive machines for large, high-speed compressors and pumps, such as 3000KW-800Orpm, but this was because there was no electric motor suitable for such high-speed, large-capacity drives. This is because of this. In recent years, electric motor drives have been desired due to the soaring prices of oil and gas, and the development of large-capacity inverters and ultra-high-speed induction motors using large power semiconductor devices (Toshiba Review, November 1984 issue) has also progressed, making it difficult to put them into practical use. I've reached a stage.

Conventionally, turbines have been used to drive high-speed compressors, and they have been used in chemical plants, etc. Turbine drives have relatively low system efficiency and also have noise and environmental pollution problems. In addition, due to the surplus of nighttime electricity in recent years, nighttime electricity rates are said to be 1/2 to 1/3 cheaper than during the day, and are cheaper than the fuel costs for turbines (gas and oil). By using cheap electricity at night, chemical plants and the like can use existing steam equipment to create an energy-saving system that drives compressors and the like by using a turbine and an electric motor driven by an inverter device. In such a system, how to control the load carried by the electric motor can be a problem in stable operation of the system.

[Purpose of the invention]

Therefore, the present invention was made in view of the above-mentioned problem, and an object of the present invention is to provide an induction motor control method that stably controls the load power carried by the motor in a combined operation system of a turbine and an electric motor driven by an inverter device. purpose.

[Summary of the invention]

The present invention calculates torque by dividing the power command by the detected rotation speed of the induction motor, calculates the slip frequency corresponding to the torque, and adds this slip frequency to the rotation speed to increase the frequency of the inverter device. In addition, the power is controlled by controlling the power supply voltage of the motor using the rotational speed as a voltage reference.

[Embodiments of the invention]

An embodiment of the present invention is shown in FIG. FIG. 1 shows an AC power supply 1, an inverter device 2, an induction motor 3, a compressor 4, a turbine 5, a speed detector 6, a divider 7, a slip frequency calculator 8, and an adder 9. , an inverter controller 10, a power calculator 11, a power controller 12, a frequency/voltage converter 13, a voltage controller 14, a current controller 15, and a phase controller 16. The power reference 17 of the output of the induction motor 3 is used as an input signal, and this power reference L7 is used as the output f, n of the speed detector 6 of the induction motor 3.
The value divided by is the torque reference for the induction motor 3. In addition, in order to correct changes in the characteristics of the motor due to temperature, etc., the output power of the induction motor 3 is calculated from the terminal voltage Vm of the induction motor 3 and the input current 1 frequency f1, and the power is calculated from the difference between this value and the power reference 17. A torque reference correction signal is calculated by the device 12, and a slip frequency command f is calculated from this torque reference correction signal and the torque reference.

Generally, the torque T and output P of an induction motor can be approximated by the following equations.

vm Goose T = 1f, (τl ) − (1) P =
2πf,,T,(2)
Kl constant, vm: motor terminal voltage, fffi: rotational frequency, f,: slip frequency Normally, f of an induction motor is sufficiently small compared to f, , so equation (1) can be further modified by vIn! T = K+ f-(-r-) -(
3), and according to equations (2) and (3), P is 1/m
At t P=2π, f, f-(,,-1)
- Without (4), therefore, the ratio of Vm and f□ vrrV
If ′f is kept constant, equation (4) becomes p = K, f, fm ・(
51m t K,=2π becomes +(7−), and P can be controlled by f. Furthermore, by giving an output reference Pref as a command value of f, and driving the induction motor at a frequency of f, +f□, the output of the induction motor can be continuously controlled.

The torque reference correction signal described above is a signal for correcting errors caused by approximation of equations (1) to (3). Frequency adder 9
The induction motor rotation speed f1. A slip frequency command f is added to l, and the inverter controller 10 operates the inverter device 2 at frequencies f, , +f. By using this, the induction motor 3 can be driven at a slip frequency corresponding to the power standard 17. On the other hand, the induction motor rotation speeds f and n are converted by the frequency/voltage converter 131C to the motor terminal voltage reference V.
The voltage controller 14, the current controller 15, and the phase controller 16 automatically control the output voltage of the inverter device 2 (=induction motor terminal voltage Vm) to be equal to the reference value Vmr. Therefore, by this automatic control system, the ratio vrrV between the terminal voltage Vm and the rotational frequencies f and l is
f□ is controlled to be constant.

The time chart in FIG. 2 shows a method for causing the electric motor 3 to carry a load from a state where the rotor is still being rotated by the turbine and the electric motor 3 is not energized. When a start command is received, the inverter device is operated at the same frequency as the rotation speed,
In addition, after the output voltage of the inverter device has been increased to a predetermined level or higher or to a rated value, control is performed such that the inverter device is controlled to carry electric power by adding the slow and slip frequencies.

Furthermore, when disconnecting the load from the motor, the voltage supply may be stopped after the slip frequency is reduced to zero.

Another embodiment of the invention is shown in FIG. 19 is a PWM inverter, and 20 is a controller for PWM controlling the inverter. In this case, the PWM inverter 19 controls both the terminal voltage Vm and the frequency of the induction motor.

The inverter device 2 in FIG. 1 and the inverter 19 in FIG. 3 may be either voltage source inverters or current source inverters, and their configurations are well known and will not be described here.

Further, when the loss of the induction motor 3 can be ignored or when the control accuracy of the output of the induction motor does not need to be high, the calculation of the power calculator 11 can be replaced by the calculation of the input power of the induction motor.

〔Effect of the invention〕

According to the present invention, the output of the induction motor can be continuously controlled from zero to the rated value by all υ frequencies. This enables cooperative operation of the turbine and induction motor, reducing power costs. Furthermore, since the control method of the present invention performs control using the slip frequency, no large stress is applied to the shafts of the induction motor, turbine, or compressor.

Furthermore, since the compressor can be driven by either the single-induction motor or the turbine, system redundancy is possible and an energy-saving system with improved reliability can be provided.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the invention, FIG. 2 is a time chart showing the operation of FIG. 1, and FIG. 3 is a block diagram showing another embodiment of the invention. 1... AC power supply 2... Rectifier/inverter 3... Induction motor 4... Compressor 5... Steam turbine 6... Speed detector 7... Divider
8...Slip frequency, wave number calculator 9...Frequency adder 10...Inverter controller 10'...P
WM inverter controller 11...power calculator 12...power controller 13
...Frequency/voltage converter 14...Voltage controller 15...Current controller 16...
-Phase controller t7... Power standard 18... DC power supply 19... PWM inverter agent Patent attorney
Written by Nori Chika Yudo Hiroshi Mimata Figure 1 Figure 2 Prd Figure 3

Claims (1)

[Claims] In a device having a load driven by either an induction motor or a turbine energized by an inverter device, a torque command is calculated by dividing the power command carried by the induction motor by the rotation speed of the induction motor, A slip frequency is calculated from this torque command, and a signal obtained by adding the rotation speed of the induction motor and the slip frequency is used to control the output frequency of the inverter device, and the output voltage of the inverter device is controlled using the rotation speed as a voltage reference. A method for controlling an induction motor, characterized in that: