Magnetic amplifier speed control for a D-C motor

MAGNETIC AMPLIFIERS constitute rugged and maintenance-free elements for the conversion of a constant a-c voltage into an adjustable d-c voltage. They can be used for adjustable voltage drives of d-c shunt motors when no reversibility and no regeneration is required. The operation of a self-saturating magnetic amplifier with d-c output connected to the armature of a d-c motor is in many respects similar to that of controlled thyratron rectifiers. In both cases, the speed-torque characteristic at a fixed firing angle is not flat as with a smooth armature supply voltage, but curved at low load like that of a series motor. This is caused by the a-c content in the armature voltage and the rectifying property of the supply. In the lower power range, single-phase magnetic amplifiers offer economical advantages over polyphase amplifiers. Since for single-phase supply the distortion of the motor characteristics is most severe, economical means are required which restore the original shunt-motor characteristics.