GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER- III(OLD) EXAMINATION — SUMMER 2019
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Subject Code: 131701 Date: 11/06/2019
Subject Name: Electrical Machines
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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Q.1 (a) Explain different losses in transformer and derive the expression for condition for maximum efficiency of transformer. (07)
(b) Explain the working principle of 1-phase transformer under no-load condition with vector diagram. (07)
Q.2 (a) What is cogging and crawling? Differentiate between slip ring induction motor and squirrel cage induction motor. (07)
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(b) An ideal 25K VA transformer has 500turns on the primary winding and 40turns on secondary winding. The primary is connected to 3000V, 50Hz supply. Calculate: 1.primary and secondary currents on full load 2. Secondary EMF 3.maximum core flux. (07)
OR
(b) What is slip? Explain torque-slip characteristics of 3-phase induction motor. (07)
Q.3 (a) Explain the working principle, construction and applications of capacitor start capacitor run induction motor. (07)
(b) Describe the construction and working of a double-cage induction motor. (07)
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OR
Q.3 (a) List out starting methods of poly phase induction motor. Explain any one method in detail. (07)
(b) Explain No-load and Blocked rotor test on three phase induction motor. (07)
Q.4 (a) Explain Swinburne’s test of DC-machine. (07)
(b) Explain different type of DC generator according to its field winding. (07)
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OR
Q.4 (a) Explain ZPF method for finding regulation in Alternator. (07)
(b) Explain the operating principle of synchronous motor. And also explain why synchronous motor is not started? (07)
Q.5 (a) Explain hunting effect in synchronous motor. (07)
(b) What is the elementary concept of rotating machines? Explain electromechanical conversion. (07)
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OR
Q.5 (a) Define (i) Pitch factor. (ii) Distribution factor for alternator (07)
(b) An 8 pole armature has 96 slots with 8 conductors per slot. It is driven at 600 RPM. The useful flux per pole is 10mwb. Calculate the induced emf in armature winding when it is: (1) lap connected (2) wave connected. (07)
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