Bipolar/unipolar-drive back-emf communication sensing method

Apparatus to commutate the drive voltages of a brush loss DC motor without sensors has the rotor position and velocity determined by detecting the polarity reversal of the Back-EMF of an undriven leg when compared to a transformed reference. The Back EMF detection is determined by a resistive divider 56, 116 coupled to a centre tap of the motor and further resistive dividers 53-55 and 113-115 coupled to respective legs of the motor winding, in a network 93a, which is coupled to Back EMF comparators 42-44. A line 99 enables selection of unipolar or bipolar operation by means of a power switch 96 and NAND gates 27a. 28a and 29a. In an extra measure of efficiency, the apparatus includes a commutation delay timing circuit 95 for optimization of motor operation, and it minimizes the participation of a microprocessor or other microcontroller.