GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER- III (New) EXAMINATION — WINTER 2019
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Subject Code: 2130904 Date: 5/12/2019Subject Name: DC Machines and Transformer
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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MARKS
Q.1 (a) Explain principle of energy conservation. 03
(b) List out parts of D.C machine and briefly discuss any two. 04
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(c) An 8-pole dc generator has 500 armature conductors and 07has a useful flux per pole of 0.065 wb. What will be the emf
generated if it is lap connected and run at 1000 rpm? What
must be the speed at which it is to be derived to produce the
same emf if it is wave wound?
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Q.2 (a) Define following terms: 03
(1) Mechanical and Electrical angle
(2) Pole pitch
(3) pitch factor
(b) Explain polarity test of transformer. 04
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(c) A single phase transformer is designed to operate at 07240/120V,50 Hz. Calculate the secondary no load voltage
and its frequency if the h.v side of the transformer is
connected to (a) 240 V , 40 Hz (b) 120 V, 25 Hz (c) 120 V,50
Hz (d) 480 V, 50 Hz (e) 240 V d.c.
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OR
(c) Discuss retardation test on D.C machine. 07
Q.3 (a) Distinguish between singly excited and doubly excited 03
magnetic systems.
(b) A 4- pole, lap wound, long shunt, dc compound generator has 04
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useful flux per pole of 0.07wb. The armature consists of 220turns and resistance per turn is 0.004ohm. Calculate the
terminal voltage if the resistance of shunt and series field is
100 ohm and 0.02 ohm respectively; when the generator is
running at 900 rpm with armature current of 50A. also
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calculate the power output in KW for the generator(c) Explain Equivalent circuit of transformer and draw 07
generalized phasor diagram.
OR
Q.3 (a) Explain comparison between simplex lap and wave winding. 03
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(b) Explain Equalizer connection. 04(c) Define voltage regulation of a transformer. Describe the 07
method to find out voltage regulation of a transformer using
open circuit and short circuit tests.
Q.4 (a) Explain O.C & S.C. test on 1-F transformer 03
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(b) What is the necessity of starter in DC motor? Explain 3-point 04starter for DC motor.
(c) Explain the methods to improve commutation and explain any one method.
OR
Q.4 (a) Derive condition for maximum efficiency for I - F transformer. 03
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(b) Enlist different speed control methods of DC shunt motor. 04Explain any one method.
(c) Draw the vector diagrams and winding connections for the 07
Following transformer connections.
(a) Dz6 (b) Yz11 (c) Yd11
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Q.5 (a) Explain the load characteristics of DC shunt generator. 03
(b) Explain Sumpner’s test for testing of a transformer. 04
(c) A 600 KVA single phase transformer has an efficiency of 92 07
% both at full load and half load at unity power factor.
Determine its Efficiency at 60 % of full load at 0.8 power
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Factor lag.OR
Q.5 (a) Describe parallel operation of transformer. 03
(b) What are the advantages and disadvantages of Swinburne test? 04
(c) Derive an expression for saving of copper when auto 07
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