Abstract
Squirrel-cage induction motors have been the first choice for constant-speed drive systems because of their superb reliability, robustness and cost effectiveness. DC motors, on the other hand, with transistors/thryistors as the power amplifier have been used for speed and position control applications especially in high-performance manufacturing machines where the main requirements are fast response and high position accuracy. With the recent development of frequency inverters, it is now possible to control speed and position of AC motors at a competitive cost. This paper investigates the performance of three-phase squirrel-cage induction motors for position control. It is shown that by optimising the control parameters, an acceptable position accuracy can be achieved. The inherent nonlinearities and motor inductance limit the speed of response. However, utilising the speed-torque characteristics, the reliability of critical components can be improved significantly which makes A C motors an ideal choice for application where the speed of response is not as critical as the position accuracy and reliability. Three control techniques of on-off, multi-step and PID are reported and their performances are compared for an A C induction motor. The integral term of a PID controller ensures high position accuracy and stiffness at steady-state, whereas the derivative term provides some improvement on the transient response and prevents overshoot which is not acceptable for many industrial applications.
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