Page 661 - Basic Electrical Engineering

B. Numerical Problems

22. The wave-connected armature of a two-pole 200 V generator has 400 conductors and runs at 300
rpm. Calculate the useful flux per pole.

[Ans ϕ = 10 mWb]

23. The induced EMF in a dc machine while running at 500 rpm is 180 V. Calculate the induced EMF
when the machine is running at 600 rpm. Assume constant flux.

[Ans E = 216 V]

24. A 250 V shunt motor draws 5 A while running on no load at 1000 rpm. Calculate the speed of the
motor when it is loaded and draws a current of 50 A. The armature circuit resistance is 0.2 Ω and
field circuit resistance in 250 Ω.

[Ans N = 964 rpm]

25. A dc shunt machine has armature resistance of 0.5 Ω and field resistance of 750 Ω. When seen as a
motor on no load at 500 V, the line current drawn is 3 A. Calculate the efficiency of the machine
when it operates as a generator with an output of 2 kW at 500 V.

[Ans η = 89.6 per cent]

26. A four-pole wave-connected dc armature has 50 slots with 10 conductors per slot. The armature is
rotated at 1000 rpm of the useful flux per pole is 30 mwb, calculate the amount of EMF induced.

[Ans 500 V]

27. A six-pole armature has 410 wave-connected conductors. The flux per pole is 0.02 wb. Calculate
the speed at which the armature must be rotated so as to generate 400 V.

[Ans 975 rpm]

28. A 200 V dc shunt motor having an armature resistance of 0.2 Ω and field resistance of 100 Ω draws
a line current of 50 A at full-load at a speed of 1500 rpm. What will be its speed at half load?

[Ans 1539 rpm]