4. Does the speed of a separately-excited DC motor increase or decrease when the

Question

4. Does the speed of a separately-excited DC motor increase or decrease when the armature current increases? a It increases. b. It decreases. c It stays the same because speed is independent of motor current. d. The speed will oscillate around the previous value. 5. The armature resistance R, and constant K, of a DC motor are 0.5 and 5 r/min/V, respectively. A voltage of 200 V is applied to this motor. The no-load armature current is 2 A. At full load, the armature current increases to 50 A. What are the no-load and full-load speeds of the motor? a. n NOLOAD b. n NOLOAD = 1005 r/min, n FULLLOAD =-995 r/min, n FULL LOAD- = 1000 r/min, n FULLLOAD- 875 r/min 875 r/min c. 1 NoLoAD d. The speeds cannot be calculated without constant K2.

Explanation / Answer

4. option(b)

We know that speed of dc motor is proportional to back emf / flux i.e Eb / . When load is increased back emf Eb and flux decrease due to armature resistance drop and armature reaction respectively .However back emf decreases more than so that the speed of the motor slightly decreases with load.

In simple words,

when load increases, armature current should increase.

But when load increases, back emf/ flux will decrease.

As we know back emf/flux is directly proportional to the speed of the separately excited DC motor. So, the speed will decrease.

5. option(c)

By substituting given values in the formulae of no-load and full load speeds, we can get option c.

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