Skip to Main Content
A brushless d-c motor system with a unique position-sensing device has been developed. This position-sensing device works on the principle of varying magnetic reluctance and is called a reluctance switch. The reluctance switch consists of a magnetic structure excited by a high-frequency oscillator. Sensing windings on the stator sense the position of the salientpole rotor which is mechanically attached to the motor rotor. The reluctance switch, in addition to the commutation function, makes simple the incorporation of speed control by pulsewidth modulation or position control. Other unique control functions can be obtained with modifications. The developed motor is a torque motor intended for operation in space. Solid lubricant bearings and materials reflect this intended environment. The motor utilizes a permanent-magnet rotating field with a three-circuit stationary armature. The armature is driven from a three-phase transistor bridge which in turn is driven by the rectified, filtered signals from the reluctance-switch sensing windings. Reversibility and voltage speed control were requirements. This paper covers the design of the reluctance switch and the control circuits for the motor. Ripple torques and magnetic cogging torque are discussed. The unique design features of the motor are covered. The incorporation of position control, speed control, and other unique control functions are emphasized. The motor was developed at the Westinghouse Aerospace Electrical Division on Contract No. NAS5-3934 for the NASA Goddard Space Flight Center.