Comparison of variable speed drives for single-phase induction motors

Different possibilities of employing a single-phase induction motor in a variable speed drive are surveyed in this paper. Motor features prove to depend mostly on the elimination or keeping the running capacitor in series with the auxiliary winding, as well on the chosen voltage-frequency profile (in open-loop control). The market requirement for low-cost topologies affects the drive performance, since a capacitor-less motor appears to be the best solution, but it requires a higher voltage for the auxiliary winding. The permanent split-capacitor motor offers a good compromise if the V/Hz dependency is in accordance with the maximum efficiency or the minimum pulsating torque optimisation criteria. Finally, inverter topologies suitable for low-cost, single-phase induction motor drive systems are analysed and tested, both in simulations and experiments. It is concluded that the elimination of the running capacitor provides an advantage only if the stator asymmetry is smaller or inexistent, as is the case of the symmetrical motor. Therefore, by keeping the running capacitor in series with auxiliary winding and compensating for the increasing of its reactance at low speeds, may lead to a competitive solution in application areas with no tough demands for high performance, such as pumps and fans.