Question:

A 4- pole DC motor is operated as a separately excited motor. 

When the motor is externally driven at 1800 rpm with rated field current and zero armature current, the armature voltage is 250V DC. 

When the motor is supplied from a 250VDC supply with the same field current, and is providing rated mechanical output torque to a load, the input current is 50A, and the motor speed has dropped to 1701 rpm.

(a)  Calculate the armature resistance for the motor.

(b)  If the motor rotational losses are 2 kW at these operating conditions, calculate the efficiency of the motor in this load situation.

(c)  Calculate the actual mechanical output torque of the motor under these load conditions.

(d)  If the motor is required to rotate at 900 rpm, with a reduced armature current of 20 A, calculate the armature voltage needed to achieve this operating condition. Assume the field current is unchanged.

(e)  Discuss the two possible ways of achieving variable speed operation for a DC machine. Show the torque speed characteristics of a DC motor operating under both these types of speed control strategies, and discuss how they relate to the concepts of the "constant torque" and "constant power" regions of variable speed operation. Discuss the advantages and disadvantages of each speed control strategy.