Hall Ticket No Question Paper Code: AEE004
INSTITUTE OF AERONAUTICAL ENGINEERING
(Autonomous)
Four Year B.Tech III Semester End Examinations (Regular) November, 2018
Regulation: IARE R16
DC MACHINES AND TRANSFORMERS
Time: 3 Hours Max Marks: 70
Answer ONE Question from each Unit
All Questions Carry Equal Marks
All parts of the question must be answered in one place only
UNIT I
1. With the help of neat diagram obtain the expression for the energy stored in a magnetic system
for a simple attracted armature type relay.
The magnetic flux density on the surface of an iron face is 1.6 T which is a typical saturation
level value for ferromagnetic material. Find the force density on the iron face.
2. Explain the 'field energy' and 'co-energy' in magnetic system.
Discuss in detail the production of mechanical force for an attracted armature relay excited by
an electric source.
UNIT II
3. Making use of Faraday's law derive the equation for the emf induced in a DC Machine.
The armature of 6 pole DC generator has a wave winding containing 664 conductors. Compute
the generator emf when flux per pole is 0.06 Weber and the speed is 250 rpm. At what speed
must be the armature an emf of 250 V if the flux per pole is reduced to 0.058 Weber.
4. Derive the emf equation of self excited and separately excited DC generators.
A 8 Pole DC shunt generator with 778 wave connected armature conductors and running at 500
rpm supplies a load of 12.5 Ω resistances at terminal voltage of 50 V. The armature resistance
is 0.24 Ω and field resistance is 250 Ω. Find the armature current, induced emf and the flux per
pole.
UNIT III
5. Explain the working of 4 point starter for a dc shunt motor. Draw the diagram of connections
showing OLC and NVC and mention advantages and disadvantages compare to point starter.

A DC shunt motor runs at speed of 1000 rpm on no load taking a current of 6A from the supply
when connected to 220V DC supply .Its full load current is 50A. Calculate its speed on full load
.Assume Ra=0.3 Ω and Rsh 110 Ω.
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6. Explain the internal and external characteristics of DC shunt motor.
A 500 V DC shunt motor running at 700 rpm takes an armature current of 50 A. Its effective
armature resistance is 0.4Ω. What resistance must be placed in series with armature to reduce
the speed to 600 rpm, the torque remaining constant?
UNIT IV
7. Derive an expression for the emf of an Ideal transformer.
A 20 KVA, 2500/250 50 Hz, 1-phase transformer has the following test results.
O.C. test (l.v. side): 250 1.4 105 W
S.C. test(h.v. side): 104 8A, 320 W
Compute the efficiency at full-load and 0.8 lagging power factor.
8. How do you find equivalent parameters of a given transformer, explain in detail?
Two transformers A and B are joined in parallel to the same load. Determine the current delivered
by each transformer, given open circuit emf 6600 V for A and 6400 V for B. equivalent leakage
impedance in terms of the secondary (0.3+j3)Ω for A and (0.2+j1)Ω for B. the load impedance
is
UNIT V
9. With the help of circuit diagrams, explain any two types of three phase transformer connections.

A 500 KVA, 3-phase, 50 Hz transformer has a voltage ratio (line voltage) of 33/11 KV and is
delta/star connected. The resistance per phase is: high voltage side 35 low voltage side 0.876
Ω and the iron loss is 3050 W. Compute the value of efficiency at full load and one-half of full-load
respectively at 0.8 power factor lagging.
10. Explain in detail about the operation of Auto transformers and derive the expression for copper
savings.
Explain in detail about the Scott connection in Three phase transformer.