a method and system for providing sensorless brushless dc motor control using predictive switch timing requires connecting a stator coil in a bridge configuration , applying a positive excitation voltage across the coil for a predetermined time period , deactivating the excitation voltage , and monitoring the voltage generated due to electro - motive force across the coil . the polarity of v emf changes when the rotor has moved a known distance — typically 90 °. after detecting a polarity change , a negative excitation voltage is applied across the coil , deactivated , and v emf monitored to detect a polarity change . this sequence is repeated to maintain the rotation of the rotor . the motor is preferably set into motion using a start - up routine , which also determines the predetermined time period used during steady - state operation .