Abstract :
THE MOST commonly available adjustable speed drives of today use the d-c motor. The speed can be controlled by armature voltage variation, field current variation, or a combination of both. Where the load torque is essentially constant throughout the speed range, armature voltage control is used. The second method, motor field adjustment, is most economical where the load power is substantially constant over a speed range not wider than 4 to 6. When an extremely wide speed range is desired with reduced torque at the higher speeds, the combination of both armature and field control gives good results. At low speeds, the motor field is usually maintained at a constant level and the motor speed is varied by changing the armature voltage. When rated armature voltage is reached, the speed can be further increased by weakening the field. The armature voltage, in any case, may be supplied from a Ward-Leonard generator, magnetic amplifiers, tubes, or semiconductors.
