Identification of Thermally Safe Load Cycles for An Induction Motor Position Servo

Abstract
Advances in the available performance of induction motor-inverter systems permit these drive systems to meet progressively more demanding performance criteria. High-performance servos require the induction motor to produce high torque and power. The severely repetitive load cycles stress the motor's insulation with high operating temperatures. The traditional rating of the induction motor based on ac line operation is not useful under these circumstances. In spite of the complexity of the thermal problem, an approximate thermal calculation procedure will allow servo designers and users alike to predetermine thermally safe load cycles for induction motor servo drives. Study is directed to the formulation of a simplified thermal model of vector-controlled induction motor servos rated between 1 and 15 kW and to the subsequent use of this model to predict thermally safe load cycles. A set of normalized curves relating duty cycle and torque is presented for the available classes of motor insulation. Theoretical results are supported by experimental verification on an existing vector-controlled induction motor servo system.

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