Control system for an electronically commutated motor includes circuitry for integrating the back emf of an unenergized motor winding to determine motor rotor position. However, integration should not occur on a field collapse voltage which precedes the back emf. A zero-approaching detector enables the integration of the back emf only when a zero-approach occurs at the ending of the field collapse voltage. The integration is also inhibited from being spuriously enabled by zero approaches for a predetermined length of time after commutation and prior to the ending of the field collapse voltage. A system for controlling the average voltage cyclically applied to a load such as an electronically commutated DC motor includes circuitry for generating a direct function of the applied voltage. The function is compared to a reference by a comparator which indicates when the function reaches the reference. The end of each voltage cycle is signaled when the function of the applied voltage reaches a predetermined value. The external application of voltage to the load is terminated when the function of the applied voltage reaches the reference, and each voltage cycle is terminated when the function of the applied voltage reaches the predetermined value.