Department of Electrical Engineering
BEEG-1001 Session:2021-22 Tutorial sheet4
Basic Electrical Engineering Odd Semester Module II
DC/AC Machine
1.
An 8-poles DC machine has 500 armature conductors and useful flux of 0.05 Wb.
What will be the emf induced, if it is lap connected and runs at 1200 rpm? What must
be the speed at which it is to be driven to produce the same emf if it is wave-
connected?
[Ans: 500V, 300 rpm]
2.
A 250 V DC shunt motor takes 30 A current while running at full load. The resistance
of armature and field windings are 0.1Ω and 200 Ω respectively. Determine the back
emf produced in the motor, when it runs on full load.
[Ans: 247 V]
3.
A 250 V DC shunt motor having an armature resistance of 0.25Ω carries an armature
current of 50A and runs at 750 rpm. If the flux is reduced by 10%, find the new
speed. Assume that the torque remains the same.
[Ans: 828 rpm]
4.
A series motor has an armature resistance of 0.2 Ω and a series field resistance of
0.3Ω. It is connected to a 240 V supply and at a particular load runs at 24 rev/s when
drawing 15 A from the supply.
(a)
Determine the generated e.m.f. at this load.
(b)
Calculate the speed of the motor when the load is changed such that the current
is increased to 30 A. Assume that this causes a doubling of the flux.
[Ans: (a) 232.5 V, (b)11.6 rev/s]
5.
A separately-excited generator develops a no-load e.m.f. of 150 V at an armature
speed of 20 rev/s and a flux per pole of 0.10 Wb. Determine the generated e.m.f.
when (a) the speed increases to 25 rev/s and the pole flux remains unchanged, (b)
thespeed remains at 20 rev/s and the pole flux is decreased to 0.08 Wb,and (c) the
speed increases to 24 rev/s and the pole flux is decreased to 0.07 Wb.
[Ans: 187.5 volts, 120 volts, 126 volts]
6.
A three-phase 2-pole motor is to have a synchronous speed of 6000 rev/min.
Calculate the frequency of the supply voltage.
[Ans: 100 Hz]
7.
A 3-phase, 6-poles, 50 Hz induction motor has a slip of 1% at no load and 3% at full
load. Find (a) the synchronous speed,(b) the no load speed,(c) the full-load speed,(d)
the frequency of rotor current at standstill and (e) the frequency of rotor current at
full load.
[Ans: 1000 rpm, 990 rpm, 970rpm, 50Hz, 1.5 Hz]
8.
If the electromagnetic force in the stator of an 8 poles induction motor has a
frequency of 50Hz, and that in the rotor 3/2 Hz, at what speed is the motor running
and what is the slip?
[Ans: 728 rpm, 3%]
9.
When supply voltage to a 3-phase induction motor is reduced by 10%, find the
reduction in torque.
[Ans: 19%]
