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

Adjustable-frequency drives have not been widely used with single-phase induction motors. 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 over a portion of the upper speed range, but the maximum available torque decays rapidly below about 50% of the base frequency. The behavior of the single-phase induction motor under variable-frequency operation is studied, providing 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 0.5-hp single-phase induction motor. Experimental results show excellent agreement with the analysis and simulation. These experiments provide proof 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 accounted for.<<ETX>>