Adjustable-frequency drives have not been widely used with single-phase induction motors. The open-loop constant "volts per hertz" control law used at all but very low speeds for many three-phase induction motor drives cannot be used analogously with the single-phase motor. 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.
