Abstract: A motor controller controls a motor's rotational speed using sensed back electromagnetic force (back EMF) of the motor. The sensed back EMF is presumed to contain an error component caused by series resistance in the motor coil. A velocity sense signal is continually generated representing the sum of the measured back EMF and an error-cancelling quantity equal to the current in the coil scaled according to a gain-control signal stored in the controller. During the first part of a calibration operation, the coil current is set to zero until a time T.sub.1 at which the coil current is substantially zero and the motor velocity is substantially constant. The value of the velocity sense signal is stored at the time T.sub.1, this value representing the zero-current back EMF of the coil.