Problem Set - 3
DC Generators
1. The armature of a 4-pole lap wound machine has 48 slots. There are 10 conductors per
slot. The axial length of the armature is 10.8 cm, and its diameter is 25 cm. The pole
arc is 15 cm and the flux density per pole is 1.2 T. If the armature speed is 1200 rpm,
determine
(a) the induced emf per coil ( Ans: 15.5 V (Double layer winding, 12 coils per parallel
path )
(b) the induced emf per turn ( Ans: 3.11 V (5 turns per coil) )
(c) the induced emf in each conductor ( Ans: 1.552 V (2 conductors per turn)
(d) what is the frequency of the induced emf in the armature winding? ( Ans: 40 Hz)
2. A 50 kW, 250 V, 600 rpm, separately excited generator has an armature resistance of 0.05
Ω. The generator supplies the rated load at its rated voltage. The field winding has a
resistance of 100 Ω and is connected across a 200 V source. The rotational loss is 5 kW.
Determine
(a) the efficiency ( Ans: 87.72 % )
(b) the applied torque ( Ans: 907.2 Nm )
(c) and the voltage regulation when the armature reaction is zero. ( Ans: 4 % )
3. A 4 pole dc shunt generator with a shunt field resistance of 100 Ω and an armature
resistance of 1 Ω has 400 lap connected conductors in its armature. The flux per pole is
20 mWb. The rotational loss is 500 Watts. If a load resistance of 10 Ω is connected across
the armature and the generator is driven at 1200 rpm, compute
(a) the terminal voltage ( Ans: 144.145 V )
(b) the efficiency ( Ans: 68.42 % )
(c) voltage regulation ( Ans: 11 % )
4. A 25 kW 125 V separately excited DC machine is operated at a constant speed of 3000
rpm with a constant field current such that the open circuit armature voltage is 125 V.
The machine is observed to be acting as a generator with a terminal voltage of 124 V and
a terminal power of 24 kW. The armature resistance is 0.02 Ω. Calculate the speed of the
generator. ( Ans: 3069 rpm )
5. In a 110 V compound generator, the resistance of the armature, shunt and series windings
are 0.06, 25 and 0.05 Ω respectively. The load consists of 200 lamps each rated at 55 W,
100 V. Find the total emf and armature current, when the machine is connected for
(a) long shunt ( Ans: 112.54 V, 114 A )
(b) short shunt ( Ans: 112.35 V, 114.22 A )
(c) How will the ampere turns of the series windings be changed, if a diverter of resistance
0.1 Ω is connected across the series field in the long shunt case? ( Ans: 66 % )
Ignore armature reaction and brush drop.
6. The shunt field current of a 125 V, 60 kW dc generator has to be increased from 3.5 A to
4 A to produce flat-compounding from no load to full load respectively. Each field pole
has 1500 turns. Calculate
(a) the number of series field turns per pole. ( Ans: 1.56 )
Assume short shunt connection.
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