A 220 V, dc series motor has an armature current of 86 A, armature resistance of 0.2 ?, and a field resistance of 0.1 ?. The terminal voltage of the motor can be adjusted by means of solid-state devices. Ignore the field saturation.
a. When the motor runs at 600 rpm, the armature current is 20 A. A 0.1 ?
resistance is added in shunt to the field windings. If the load torque remains
constant, calculate the speed of the motor.
b. The resistance in shunt of the field windings is removed. The motor is used to
lower the same mechanical torque described in (a) by a countercurrent braking
method. The desired speed in the countercurrent braking is 10 rpm. The braking
current is twice the rated value. Calculate the armature voltage of the motor.
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