Device for controlling an AC motor

• US 5,206,575 A
• Filed: 11/09/1990
• Issued: 04/27/1993
• Est. Priority Date: 11/10/1989
• Status: Expired due to Fees

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Mochikawa, Hiroshi, Nakamura, Yoshinobu
• Primary Examiner(s): Shoop, Jr., William M.
• Application Number:
  US07/610,248
• Time in Patent Office: 900 Days
• Field of Search: 318/729, 318/807
• US Class Current: 318/807
• CPC Class Codes: H02P 23/26 Power factor control [PFC]
• CPC Class Codes: H02P 27/08 with pulse width modulation
• CPC Class Codes: H02P 6/28 Arrangements for controlling current H02P6/10 takes precedence

Abstract

In the A.C. electric motor control device of this invention, with the object of improving response to abrupt speed adjustments and load fluctuations, and of improving stability of control under low speed operation, a prescribed current of the inverter circuit is detected and the power factor or power is controlled in accordance with the amount of change in this current.

Specifically, a prescribed current of the inverter circuit is detected and the power factor or power is calculated from this detected current value. Using the result of this calculation, together with a command value, control quantities are calculated that determine the output voltage and output frequency of said inverter circuit. Control signals for the switching elements of said inverter circuit are then generated in accordance with these calculated control quantities.

Claims

1. A control device for an AC motor, comprising:

a source of DC power;

inverter means for converting the DC power into AC power and supplying the AC power to the AC motor;

current detection means for detecting a current flowing from said DC power source to the inverter means and for generating a signal representative of the detected current that is determined from both of a detected current Ia occurring at a first time at least before a changeover of phase of an output of the inverter means and a detected current Ib occurring at a second time at least after the changeover of phase of the output of the inverter means;

power factor calculation means connected to the current detection means for calculating a power factor of the AC motor from the detected current and for outputting a corresponding power factor signal;

voltage frequency calculating means for receiving as input signals at least the calculated power factor and a speed setting signal, for calculating control signals in accordance with the input signals, and for outputting the control signals; and

control means for receiving the control signals and for generating drive signals to control the inverter means based on the control signals.

2. A control device for an AC motor according to claim 1, wherein said current detection means includes:

DC current detection means for detecting the current flowing in a DC side of said inverter means;

sampling and hold signal generating means for generating a sampling and hold signal from said control signals and said drive signals, wherein said drive signals invert at a prescribed electric angle determined by the output frequency of said inverter means; and

current sampling and hold means for sampling and holding the DC current signal from said DC current detection means under control of said sampling and hold signal.

3. A control device for an AC Motor according to claim 1, further comprising:

mean value calculation means for calculating a mean value of the signal from said current detection means;

wherein said voltage frequency calculation means includes means for generating a voltage signal and frequency signal based on at least one of the power factor signal and the calculation result of said mean value calculation means.

4. A control device for an AC motor according to claim 1, wherein said power factor calculation means includes means for calculating the power factor of said AC motor from current values detected before and after the point of phase switching detected by said current detection means.

5. A control device for an AC motor, comprising:

inverter means provided with switching elements for converting DC voltage from a DC power source to AC voltage and for supplying the converted AC voltage to said AC motor;

current detection means for detecting a current supplied from the inverter means to the AC motor and for generating a signal representative of the detected current that is determined from both of a detected current Ia occurring at a first time at least before a changeover of phase of an output of the inverter means and a detected current Ib occurring at a second time at least after the changeover of phase of the output of the inverter means;

power calculation means for calculating a power passed to said AC motor from a command value and the detection signal of the current detection means; and

control means for generating control signals and supplying the signals to the power calculation means and for supplying drive signals based on the signals from the power calculation means to the inverter means.

6. A control device for an AC motor, comprising:

a source of DC power;

inverter means for converting said DC power into AC power and for supplying a converted AC voltage to the AC motor;

current detection means for detecting a current flowing from said DC power source to said inverter means and for generating a signal representative of the detected current flow that is determined from both of a detected current Ia occurring at a first time at least before a changeover of phase of an output of the inverter means and a detected current Ib occurring at a second time at least after the changeover of phase of the output of the inverter means;

power factor calculation means connected to the current detection means for calculating the power factor of the AC motor from the detected current and for outputting a corresponding power factor signal;

adder means for receiving the power factor signal and for adding to it a predetermined signal;

proportional integrating means for proportionally integrating the output of the adder means;

voltage frequency calculation means for receiving a predetermined reference signal and for generating a reference voltage signal; and

control means for receiving the output of the proportional integrating means, the reference voltage signal, and the predetermined reference signal and for outputting a control signal to control the inverter means.