Page 662 - Basic Electrical Engineering

29. A 500 V shunt motor takes a current of 5 A on no-load. Calculate the efficiency of the motor when
it takes 100 A. Take R = 0.5 Ω and R = 250 Ω.
f
a

[Ans 85.4 per cent]

30. A shunt motor takes 125 A at 400 V at 1000 rpm at a particular load. If the total torque remains
unchanged, Calculate the speed and armature current when the magnetic field is reduced 80 per cent
of its original value. Take R = 0.25 Ω.
a

[Ans 1224 rpm, 156.25 A]

31. A dc shunt generator has a field resistance of 60 Ω and armature resistance of 0.03 Ω. As a
generator, the machine delivers 40 kW at 240 V when driven at a speed of 450 rpm. Calculate the
speed of the machine when running as a motor taking 40 kW of power input at 240 V.

[Ans N=424 rpm]

C. Multiple Choice Questions

1. The expression for EMF induced in a dc machine is

a.

b.

c.

d.

2. Which of the following statements is not true for a dc machine?

a. EMF induced is directly proportional to air-gap flux
b. EMF induced is directly proportional to number of armature conductors
c. EMF induced in inversely proportional to number of parallel paths of the armature
conductors