In a permanent-magnet (PM) brushless dc motor, the waveform of back electromotive force (emf) is related to the rotor position; hence, the back emf can be used for position sensorless control. However, in practical implementation, the terminal voltage or phase voltage is used instead, as the back emf is difficult to be sensed directly. Thus, detection error of the rotor position can occur. This paper documents the calculations and analyses on the detection error and the motor commutation angle, and presents an error compensation circuit to ensure proper commutation. Finite-element field simulation and experimental results are also given to verify the calculations as well as the compensation circuit.