New method of low-speed commutation of an inverter-fed synchronous motor

The naturally commutated, inverter-fed synchronous machine cannot use rotational voltages for natural commutation at very low speeds, and the usual technique to overcome this is to pulse the d.c. input to the inverter. A new technique is described which makes use of transformer action between rotor and stator, the rotor being supplied from halfwave rectified a.c. Commutation is automatic and requires simpler control circuits.