How can the speed of a synchronous motor be checked to verify synchronous operation under load?
A – Observation of the rotor under a stroboscopic light source synchronized with the supply voltage will show a stationary pattern if the rotor is running at synchronous speed. A continuously moving pattern indicates that the rotor is “slipping sync,” probably from loading more than pullout torque capability. An oscillating pattern indicates a variation of torque angle caused by a varying load or voltage.
