# R-Nov 2013 Electrical Machines-I

```SET - 2
CODE: GR11A2016
GR 11
II B Tech I Semester Regular Examinations,GR11D5104
Nov, 2013
Electrical Machines-I
GR11D5104
GR11D5104
(Electrical and Electronics Engineering
)
GR11D5104
Max Marks: 75
GR11D5104
Time: 3 hours
1). a
b
All questions carry equal marks
*****
With the help of sketches, describe constructional features of a DC generator.
An 8 pole DC generator running at 1200 rpm with a flux of 25 mWb per pole
generates 440 V. Calculate the number of conductors, if the armature is i) lap wound
ii) wave wound.
[8]
[7]
2). a
b
Derive an expression for the torque developed by a DC motor.
A 4 pole, 220 V shunt motor has 540 lap wound conductors. It takes 32 A from the
supply mains and develops output power of 5.595 kW. The field winding takes 1 A.
The armature resistance is 0.9 ohms and the flux per pole is 30 mWb. Calculate i)
speed ii) armature torque iii) shaft torque.
[8]
[7]
3). a
Describe the Hopkinson’s test for obtaining the efficiency of two similar shunt
motors.
Two identical DC machines when tested by Hopkinson’s method gave the following
test results: Field currents are 2.5 A and 2 A. Line voltage is 220 V. Line current
including both the field currents is 10 A. Motor armature current is 73 A. The
armature resistance of each machine is 0.05 ohms. Calculate the efficiency of both
the machines.
[8]
From the fundamentals, derive the expression for the EMF equation of a single phase
transformer.
A transformer on no load takes 1.5 A at a power factor of 0.2 lagging when
connected across 230 V, 50Hz supply. The ratio between the primary and secondary
turns is 3.Calculate the value of primary current when secondary is supplying a
current of 40 A at a power factor of 0.8 lagging. Neglect the voltage drop in the
winding. Draw the relevant phasor diagram.
[8]
Derive the expression for saving in copper of auto transformer compared to two
winding transformer.
A 600 KVA, single phase transformer has an efficiency of 92% both at full load and
half load at unity power factor. Determine its efficiency at 60% of full load at 0.8
power factor lag.
[8]
Why a starter is necessary for DC motor and explain the working principle of a
3-point starter?
Describe a suitable method for determining the efficiency of two similar DC series
machines.
[8]
Explain the effect of variations of frequency and supply voltage on iron losses.
Discuss various possible poly phase connections in transformers.
[8]
[7]
b
4). a
b
5). a
b
6). a
b
7). a
b
*****
[7]
[7]
[7]
[7]
```