Exam Details
| Subject | electrical machine-iii | |
| Paper | ||
| Exam / Course | pddc | |
| Department | ||
| Organization | Gujarat Technological University | |
| Position | ||
| Exam Date | November, 2018 | |
| City, State | gujarat, ahmedabad |
Question Paper
1
Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER-VI EXAMINATION WINTER 2018
Subject Code:X60901 Date: 16/11/2018
Subject Name: Electrical Machine-III
Time: 02:30 PM TO 05:00 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 how to obtain efficiency of a D.C. machine by conducting brake test. State its advantages and disadvantages.
07
Explain direct loading test to determine regulation of an alternator.
07
Q.2
Derive an equation for the e.m.f. induced in an alternator. Also derive expressions for the pitch factor and distribution factor.
07
Describe the Hopkinson's test for the two identical shunt motors indicating how the efficiency of each machine on full load is obtained.
07
OR
The Hopkinson's test on two d.c. shunt machines gave the following results on full load. Line voltage is 220 volts, line current excluding field currents is 15 Amps. Motor armature current 72 Amps. The field currents are 1.5 Amps and 1.0 Amp for generator and motor respectively. The armature resistance of each machine is 0.2 Ω. Calculate the efficiency of each machine.
07
Q.3
Explain field test to find efficiency of a d.c. series machines with diagram.
07
Derive the expression for the output power of cylindrical rotor alternator connected to infinite bus in terms of excitation voltage, bus bar voltage and load angle.
07
OR
Q.3
Explain the concept of two reaction theory in a salient pole synchronous machine with a phasor diagram.
07
A 2300 50 Hz, 3-phase star connected alternator has an effective armature resistance of 0.2 Ω. A field current of 35 A produces a current of 150 A on short circuit and an open circuit e.m.f. 780 V (line). Calculate the voltage regulation at 0.8 p.f. lagging and 0.8 p.f. leading for the full load current of 25 A.
07
Q.4
Compare the various methods of finding the regulation of an alternator.
07
With a neat circuit diagram explain how an alternator is synchronized with bus bars.
07
OR
Q.4
Explain the construction and working principle of 3-phase synchronous motor.
07
Draw and explain the experimental set up to obtain V-curves of synchronous motor.
07
Q.5
Explain construction and working principle of auto synchronous motor.
07
Explain the brushless D.C. motor.
07
OR
Q.5
Explain reluctance motor and hysteresis motor.
07
Explain the significant of short circuit ratio.
07
Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
PDDC SEMESTER-VI EXAMINATION WINTER 2018
Subject Code:X60901 Date: 16/11/2018
Subject Name: Electrical Machine-III
Time: 02:30 PM TO 05:00 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 how to obtain efficiency of a D.C. machine by conducting brake test. State its advantages and disadvantages.
07
Explain direct loading test to determine regulation of an alternator.
07
Q.2
Derive an equation for the e.m.f. induced in an alternator. Also derive expressions for the pitch factor and distribution factor.
07
Describe the Hopkinson's test for the two identical shunt motors indicating how the efficiency of each machine on full load is obtained.
07
OR
The Hopkinson's test on two d.c. shunt machines gave the following results on full load. Line voltage is 220 volts, line current excluding field currents is 15 Amps. Motor armature current 72 Amps. The field currents are 1.5 Amps and 1.0 Amp for generator and motor respectively. The armature resistance of each machine is 0.2 Ω. Calculate the efficiency of each machine.
07
Q.3
Explain field test to find efficiency of a d.c. series machines with diagram.
07
Derive the expression for the output power of cylindrical rotor alternator connected to infinite bus in terms of excitation voltage, bus bar voltage and load angle.
07
OR
Q.3
Explain the concept of two reaction theory in a salient pole synchronous machine with a phasor diagram.
07
A 2300 50 Hz, 3-phase star connected alternator has an effective armature resistance of 0.2 Ω. A field current of 35 A produces a current of 150 A on short circuit and an open circuit e.m.f. 780 V (line). Calculate the voltage regulation at 0.8 p.f. lagging and 0.8 p.f. leading for the full load current of 25 A.
07
Q.4
Compare the various methods of finding the regulation of an alternator.
07
With a neat circuit diagram explain how an alternator is synchronized with bus bars.
07
OR
Q.4
Explain the construction and working principle of 3-phase synchronous motor.
07
Draw and explain the experimental set up to obtain V-curves of synchronous motor.
07
Q.5
Explain construction and working principle of auto synchronous motor.
07
Explain the brushless D.C. motor.
07
OR
Q.5
Explain reluctance motor and hysteresis motor.
07
Explain the significant of short circuit ratio.
07
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