Enrolment No.
GUJARAT TECHNOLOGICAL UNIVERSITY
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BE - SEMESTER-V (OLD) EXAMINATION - WINTER 2018
Subject Code:150901 Date: 04/12/2018
Subject Name: Electrical Machine - I1
Time: 10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
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- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
Q.1 (a) Draw connection diagrams, phasor diagrams and clock representations for Dyl and Yd11 connections of three phase transformer. [07]
(b) Explain Scott connection in detail also explain how we can obtain a balanced two phase supply from a balanced three phase supply by using this connection. [07]
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Q.2 (a) State and explain the essential and desirable conditions for parallel operation of two three phase transformers. [07]
(b) Explain the open delta connection (V-V connection) in details. [07]
OR
(b) 120 KVA, 3-phase, 50 Hz, 6600/400-V transformer is delta connected on H.V. side and star connected on L.V. side. The resistance of HV winding is 4.5 ohm/phase and of LV winding is 0.04 ohm/phase. If it’s full load efficiency is 95% at 0.85 p.f. (lag), Calculate (i) Iron losses & copper losses at full load. (i1) Efficiency of transformer at half load and 0.8 p.f. (lead) [07]
Q.3 (a) A 20 hp, 400 V , 50 Hz, three phase star connected induction motor has the following test data: [07]
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No-load test: 400 V, 11 A, p.f=0.2
Blocked rotor test: 100 V, 25 A, p.f=0.4.
The stator and rotor copper losses are equal at standstill. Draw the circle diagram and find out the line current, speed, efficiency and p.f at full load.
(b) Why induction motor is taking a large current at starting? Give the list of various starters used for induction motors and also explain any one of them in detail. [07]
OR
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Q.3 (a) Derive the expression of torque developed by a three phase induction motor. Also obtain the condition for maximum torque. [07]
(b) Derive the equivalent circuit of a three phase induction motor step by step. [07]
Q.4 (a) Give the list of various methods of controlling the speed of three phase induction motor. Explain anyone method in detail. [07]
(b) Explain in detail about the terms crawling and cogging in three phase induction motor. [07]
OR
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Q.4 (a) With the help of double revolving field theory explain why the single phase induction motor is not a self starting. [07]
(b) Explain the construction and working principle of double cage induction motor. [07]
Q.5 (a) Explain the construction and operating principle of Schrage motor. [07]
(b) Draw the constructional features and operating characteristics of shaded pole induction motor. Also state its application. [07]
OR
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Q.5 (a) Explain the operating principle, construction and applications of induction generator. [07]
(b) Explain the effect of unbalanced supply voltage and variation in frequency on the operation of three phase induction motor. [07]
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