1
Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
BE SEMESTER-III(OLD) • EXAMINATION WINTER 2016
Subject Code:130904 Date:09/01/2017
Subject Name:Electrical Machines-1
Time: 10:30 AM to 01:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

Explain in detail the working and construction of Induction motor
07

Draw and explain the characteristics of DC generator and state their application.
07
Q.2

Explain with neat diagram SC and OC test on a transformer and how parameters of equivalent circuit can be calculated with the help of this test
07

Derive the relation for starting and running torque for a Induction motor. Also draw and explain the torque-slip characteristics.
07
OR

Draw and explain the vector diagrams for actual transformer on-load.
07
Q.3

Write a short note on auto-transformer. Give its applications and derive the relation for saving of copper compared to 2-winding transformer.
07

Explain the voltage build-up process in a self excited DC shunt generator. Also explain critical resistance.
07
OR
Q.3

Write short note on pitch factor and distribution factor of an alternator.
07

Explain the method of determining voltage regulation of an alternator by synchronous impedance method.
07
Q.4

Explain the conditions necessary for parallel operation of two alternators. Also explain any one method for synchronizing the two alternators.
07

With a neat diagram explain the power stages in a DC motor
07
OR
Q.4

Explain with a neat circuit diagram Swinburne's test to find the efficiency of a DC motor. Also state it's limitation.
07

A 3-phase, 50-Hz, 4-pole induction motor has a slip of 4%. Determine the frequency of rotor emf and speed of motor if rotor has a resistance of 1Ω and standstill reactance of 4Ω, Calculate the power factor of the motor(i) at standstill and at a speed of 1400rpm.
07
Q.5

Explain with a neat diagram working of a 3-point starter.
07

A short shunt compound wound DC generator supplies a load current of 50-A at 220-V. The generator has the following winding resistance: Armature winding resistance 0.5Ω, Series field resistance 0.2Ω, shunt field resistance 100Ω. Calculate the generated e.m.f. Assume brush drop 1-V per brush.
07
OR
Q.5

State and explain the necessary condition for parallel operation of two single phase transformer.
07

State different methods of speed control of DC motors. Explain Ward Leonard method of speed control in detail.
07