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METHODS AND APPARATUS FOR CONTROLLING ELECTRIC MOTORS

IP.com Disclosure Number: IPCOM000009647D
Original Publication Date: 2000-Jan-01
Included in the Prior Art Database: 2002-Sep-06
Document File: 6 page(s) / 189K

Publishing Venue

Motorola

Related People

Dah-Lain Almon Tang: AUTHOR [+2]

Abstract

The present invention relates generally to con- trol systems, and more particularly, to methods and apparatus for controlling electric motors.

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Technical MO-LA @ Developments

 METHODS AND APPARATUS FOR CONTROLLING ELECTRIC MOTORS

by Dah-Lain Almon Tang and Dan Teng

INTRODUCTION

  The present invention relates generally to con- trol systems, and more particularly, to methods and apparatus for controlling electric motors.

BACKGROUND OF THE INVENTION

  Conventional induction motors have been wide- ly used for controlling various types of machinery, such as, for example, washing machines, blowers, fans, and compressors. These induction motors usu- ally include a stator and a rotor. The rotor is mount- ed within the stator for rotation and is coupled to a shaft. When the motor is energized, the rotor rotates and turns the shaft at a predetermined speed.

  The stator of the motor typically has a plurality of stator windings. The stator windings are connect- ed to a power converter by one or more supply lines. The power converter can usually generate voltages of different frequencies which are applied to the sta- tor windings of the stator to control the speed and torque of the motor.

  In order to control the rotational speed of the motor, various control techniques, such as, for example, PID (proportional/integral/differential) control, field oriented control, and adjustable fre- quency control have been utilized. In PID control techniques, a measured rotational speed of the motor is usually compared to a desired motor speed to gen- erate a speed error signal. The speed error signal is provided to a PID speed controller. The PID speed controller usually employs a transfer element having a transfer function or response characteristics G,(s) which can be illustrated by the following equation:

G,(s) = Kp + Ki/S + K&

where Kp, Ki and K,j are control constants.

  However, conventional PID speed controllers usually have a high gain when the speed controller has large error and a low gain when the speed con- troller has a small error. As a result, me PID speed controllers may not be able to rapidly change the speed of the motor to the desired motor speed. For example, when the measured speed of the motor is less than the desired speed, it may take a relatively long time to increase the speed of the motor to the desired speed. In addition, when the PID speed con- trollers are required to respond quickly, the PID speed controllers may become unstable.

  The control constants of the conventional PID speed controllers are also usually set as fLved values on the basis of the design values of the motor con- stants. The deviation between the control constants and the actual motor constants may result in calcula- tion errors of the drive signals. Thus, optimal con- trol performance may be difftcult to obtain especial- ly when the magnitude of the load on the motor varies significantly. The control constants of the PID speed controllers may further have to be changed or modified whenever the motor associated with the PID speed controller is replaced.

SUMMARY OF THE INVENTION

  The present invention provides...