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Posted By: kondapalli Member Level: Gold Posted Date: 06 Jun 2008
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2007 Jawaharlal Nehru Technological University B.Tech Applied Electronics and Instrumentation ELECTRICAL TECHNOLOGY setno2 Question paper
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1. (a) Explain how a.c. voltage generated is converted to D.C. voltage in a generator?
(b) What is the main purpose of laminating the armature core of a D.C. Generator.
(c) A 4-pole, long shunt, lap wound generator supplies 25kw at a terminal voltage
of 500 V. The armature resistance is 0.03
, series field resistance is 0.04
and
shunt field resistance is 200
. The brush drop may be taken as 1 V. Determine
the e m f generated. [5+3+8]
2. (a) Explain various power stages of in a D.C. Motor.
(b) What will happen when a D.C. Series Motor is started without a load con-
nected to it?
(c) A 200V d.c. series motor runs at 750 rpm when taking a current of 30A. The
resistance of the armature is 0.5
and that of field is 0.3
. If the current
remains constant, calculate the resistance necessary to reduce the speed to
250rpm. [4+4+8]
3. (a) Explain the principle of operation of transformer. Derive its e. m. f. equation.
(b) A 1-phase transformer has 180 turns respectively in its secondary and primary
windings. The respective resistances are 0.233
and 0.067
.
Calculate the equivalent resistance of
i. the primary in terms of the secondary winding,
ii. the secondary in terms of the primary winding, and
iii. the total resistance of the transformer in terms of the primary. [8+8]
4. Write short notes on:
(a) OC and SC tests on transformers.
(b) Losses in transformers. [10+6]
5. (a) Sketch torque-speed characteristics of an induction motor working at rated
voltage and frequency, deriving necessary expressions.
(b) A 3-phase, 50 Hz, 400 V, wound-rotor induction motor runs at 960 r.p.m.
at full-load. The rotor resistance and standstill reactance per phase are
0.2
and1
respectively. If a resistance of 1.8
is added to each phase of
the rotor at standstill, what would be the ratio of starting torque with full
voltage and the added resistance to the full-load torque under normal running
conditions? [8+8]
6. (a) Derive e.m.f equation for an alternator and explain distribution factor and
pitch factor used in e.m.f. Equation.
(b) Write the expression showing the relationship between speed frequency and
no. of poles of a synchronous machine. The speed of rotation of the turbine
driving an alternator is 166.7 r.p.m. What should be the no. of poles of the
alternator if it is to generate voltage 50HZ. [10+6]
7. (a) A 3-f, 6600v; 50Hz; Y connected synchronous motor takes 50 A current.
The resistance and synchronous reactance per phase are 1 ohm and 20
re-
spectively. Find the power supplied to the motor and induced e.m.f. for a
powerfactor of
i. 0.8 lagging and
ii. 0.8 leading.
(b) Derive expressions for distribution factor and pitch factor. [8+8]
8. (a) Give the description of A.C tachometer and mention its applications.
(b) Write a short note on shaded pole type servo-motor. [8+8]
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