METHOD AND MEANS FOR LIMITING THE MAXIMUM HORSEPOWER OUTPUT OF A SELF-EXCITED ALTERNATING CURRENT MOTOR

• US 3,769,564 A
• Filed: 08/30/1972
• Issued: 10/30/1973
• Est. Priority Date: 08/30/1972
• Status: Expired due to Term

• Current Assignee: Magnetek Incorporated Ste
• Original Assignee: Charles E. Rettig
• Inventors: Rettig, Charles E.
• Primary Examiner(s): Rubinson, Gene Z.
• Application Number:
  US05/284,856
• Time in Patent Office: 426 Days
• Field of Search: 318/227,230,231 321/5
• US Class Current: 318/803
• CPC Class Codes: H02P 27/045 whereby the speed is regulated by measuring the motor speed and comparing it with a given physical value
• CPC Class Codes: H02P 27/047 V/F converter, wherein the voltage is controlled proportionally with the frequency
• CPC Class Codes: H02P 27/048 using AC supply for only the rotor circuit or only the stator circuit
• CPC Class Codes: H02P 27/06 using dc to ac converters or inverters H02P27/05 takes precedence

Abstract

A method and means for controlling the horsepower output of a self-excited alternating current motor of the induction or synchronous reluctance type. In connection with an induction motor, the method includes the steps of generating a slip frequency in the motor proportional to the desired output torque condition while simultaneously establishing the magnitude and phase displacement of the stator current at preselected values, and, at rotor speeds at predetermined magnitude, simultaneously adjusting the slip frequency in the motor to maintain a constant horsepower output as motor speed increases. Analogous steps are carried out to control a torque of a synchronous reluctance motor. The means comprises a motor control having a frequency control means for providing a stator current of a desired frequency and phase displacement to the motor, a power supply means for regulating the magnitude of the stator current to a predetermined value, a speed control means, and a means for limiting the horsepower output of the motor at motor speeds above a predetermined value.

Claims

1. A motor control interposable between an electrical energy source and the stator winding of a self-excited alternating current induction motor for controlling the torque developed by the motor and for limiting the maximum horsepower output of the motor, said control comprising:

a. first signal means for generating a torque command signal indicative of the desired motor output torque condition;

b. second signal means coupled to the alternating current motor for providing a signal indicative of the rotor frequency of the motor rotor;

c. frequency control means connectable to the stator winding of the motor for energizing the stator winding with a cyclical current, said frequency control means being coupled to said second signal means for establishing the frequency of the cyclical current in accordance with said rotor frequency signal, said frequency control means being further coupled to said first signal means for altering the established frequency of the cyclical current and the slip frequency of the motor in accordance with the torque command signal;

d. power supply means connectable to the electrical energy source and coupled to said frequency control means for providing a regulatable current to the frequency control means, said power supply means being coupled to said first signal means for regulating the magnitude of the stator current as a predetermined function of the torque command signal; and

e. means for limiting the output horsepower, said limiting means having an input coupled to the stator winding to receive the stator voltage and an output limiting command signal coupled to said frequency control means, said limiting command signal indicative of a rotor frequency and horsepower output in excess of a predetermined value, said frequency control means responding to said limiting command signal by increasing the slip frequency to maintain a maximum horsepower output and torque per ampere.

2. The motor control according to claim 1 wherein said output horsepower limiting means is further defined as a transformer means coupled to the motor for receiving the stator voltage, a rectifier means coupled to said transformer means for rectifying the signal indicative of the stator voltage and a variable resistance means coupled to said rectifier means for producing said limiting command signal proportional in magnitude to the output voltage of said rectifier means, said limiting command signal coupled to said frequency control means for altering the slip frequency of the motor above the magnitude it would otherwise have been only in accordance with the torque command signal.

3. The motor control as claimed in claim 2 and further including a bistable means coupled between said rectifier means and said variable resistance means, said bistable means conducting at a predetermined voltage corresponding to said predetermined rotor speed, said variAble resistor means being operative whenever said bistable means is in a state of conduction.

4. The motor control as claimed in claim 3 and further including a filter means coupled between said transformer means and said rectifier means.

5. The device as claimed in claim 2 wherein said variable resistor means comprises an optically coupled isolator.

6. A method of producing a desired torque condition and a maximum torque per ampere and horsepower output in a self-excited alternating current motor having a stator winding energized by a stator current having a frequency characteristic, said motor having a motor flux controllable to a desired level by the magnitude of the stator current, said motor also including a rotor the rotary condition of which is definable in terms of a rotor frequency which is referenced to the stator winding, and said motor being capable of having a slip frequency therein, wherein said method comprises steps of:

a. establishing the frequency of the stator current at a value equal to the algebraic summation of the existing rotor frequency and a slip frequency proportional to the torque being produced;

b. concurrently establishing the magnitude of the stator current at a value tending to maintain the motor flux at the desired level during the production of motor torque; and

c. concurrently increasing the slip frequency, whenever the rotor frequency exceeds a predetermined value, to limit the maximum horsepower output of the motor to a substantially constant value for all rotor frequencies in excess of said predetermined frequency.