SET - 2
CODE: GR11A2016
GR 11
GR11D5104
II B. Tech I Semester Supplementary Examinations,
May/June 2015
Time: 3 hours
GR11D5104
GR11D5104
Electrical Machines - I
GR11D5104
(Electrical and Electronics Engineering)GR11D5104
Max Marks: 75
Answer any FIVE questions
All questions carry equal marks
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1). a
Explain in detail the commutation of D.C Generator. How Commutation in DC
Machine is improved?
[8]
b
A lap wound D.C generator has 8 poles, 120 slots with 8 conductors in each slot.
Flux per pole is 0.04 Wb. Find
i. Induced emf generated at 600 RPM.
ii. What should be speed at rotation if induced emf is to be 500 V?
[7]
2). a
Explain four methods by which Flux is varied to control speed of a DC Series Motor,
with neat sketches.
[8]
b
A DC Shunt Motor drives a centrifugal pump whose torque varies as the square of
the speed. The motor is fed from a 200V supply and takes 50A when running at
1000rpm. What resistance must be inserted in the armature circuit in order to reduce
the speed to 800rpm? The armature and field resistances of motor are 0.1 ohm and
100 ohm respectively.
[7]
Explain Hopkinson’s Test in detail for DC Machines.
[8]
b
A 400V DC shunt motor takes 5A at no-load its armature resistance 0.5ohm and
shunt field resistance is 200ohm. Estimate the KW output and efficiency when the
motor takes 50A on full load.
[7]
4). a
What is an Ideal Transformer? Derive an expression for induced emf in a
transformer in terms of frequency, the max value of flux and the number of turns on
the windings.
[8]
b
A 25 kva single phase transformer has 300 turns on primary and 50 turns on
secondary winding. The primary is connected to 1200V, 50Hz mains determine
i.
Primary and Secondary Current on Full Load
ii.
Voltage Ratio and Current Ratio
iii.
Secondary emf
[7]
3). a
SET - 2
CODE: GR11A2016
GR 11
GR11D5104
5). a
GR11D5104
GR11D5104
GR11D5104
Discuss essential and desirable conditions to be fulfilled forGR11D5104
operating two single
phase transformers in parallel with equal voltage ratios. Draw the Phasor Diagram
[8]
A ∆-∆ bank consisting of three 10Kva, 2300/230V transformers supplies a load of
20KVA. If one transformer is removed, find for the resulting V-V connection
a. KVA load carried by each transformer
b. Percent of rated load carried by each transformer
c. Total KVA rating of the V-V bank
[7]
Explain with neat sketches the principle of operation of a D.C. Motor.
[8]
b
What are the various losses in D.C Machine and derive the condition for maximum
efficiency.
[7]
7). a
Draw the Phasor Diagram of a loaded single phase transformer and derive from this
equivalent circuit of the transformer referred to primary.
[8]
Compare D/D and V/V connections for a 3-phase Transformer in all aspects.
[7]
b
6). a
b
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