Torque and slip behavior of single-phase induction motors driven from variable-frequency supplies

Computations show that, unlike the three-phase induction motor, the single-phase induction motor's slip is not constant with changes in frequency at a constant load torque. A constant 'volts per hertz' law is found to provide nearly rated torque without excessive slip over a portion of the upper speed range. but the maximum available torque decays rapidly below about 50% of the base frequency. The torque-slip behavior of the single-phase induction motor under variable frequency operation provides insights to possible scalar control laws for optimizing performance at all speeds. Several possible open loop control strategies are examined using computer simulations on a sample 0.5 hp single-phase induction motor. Experimental results on the actual motor show excellent agreement with the analysis and simulation. These experiments verify that an adjustable-frequency power supply can be used for speed control of the single-phase induction motor if the motor's unique operating characteristics are recognized and taken into consideration.<
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