Bidirectional Universal Motor

Published in Electronic Design February, 1980

This article describes a circuit I designed where a logic signal controls the direction of a universal AC motor, taking advantage of the little-known fact that a motor of this type can be rewired to run in either direction at about 85% of the normal power output even though only the positive or negative half cycle is used. The reason for this is that the motor's back EMF is the main determinant of current flow. Equally important in this design is the use of a current-mode (Norton) amplifier where the signal voltages are substantially beyond the amplifier's power rails, a situation in which an op amp cannot be used.