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Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
DIPLOMA ENGINEERING SEMESTER -III• EXAMINATION SUMMER 2017
Subject Code: 330903 Date: 05 -05 2017
Subject Name: ELECTICAL MACHINE-1
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.
4. Use of programmable Communication aids are strictly prohibited.
5. Use of only simple calculator is permitted in Mathematics.
6. English version is authentic.
Q.1

Explain singly and doubly excited magnetic field systems.
07



07

(i)Draw Dy1 Dy11 connection vector diagrams for three phase transformer
(ii)Why transformer cannot be connected to DC supply?

03

Dy1 Dy11



03
Q.2

Draw and explain load characteristic of D.C. shunt and series generator.
07



07

List and explain various losses occurring in D.C. generator.




OR

A 110 volt, shunt generator has full load current of 100A, shunt field resistance of 55Ω and constant losses of 500W.If the Full load efficiency is 88% ,find armature resistance .Also find the load current corresponding to maximum efficiency.
07

110V 100 A
55 Ω 500 W . 88%

Q.3

Explain various speed control methods of D.C. shunt motor
07



07

A dc shunt drives a centrifugal pump whose torque varies as square of the speed. The motor is fed from a 230V supply takes 60 A when running at 1200 rpm. What resistance must be inserted in the armature circuit in order to reduce the speed to 850 rpm The armature and field resistance of the motor are 0.1 and 95 Ω respectively.

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230 ,1200 850 0.1 95

OR
Q.3

Explain the method to determine the efficiency of D.C. shunt machine by "Swinburne Test"
07



07

Drive the torque equation of D.C. motor.




Q.4

Derive the condition for maximum efficiency of Transformer
07



07

The following are the test result of a 230/600 single phase ,8.5KVA transformer
OC test 230V,2.1A, 295W LV side
SC test: LV is shorted 28V,12A, 160 W ,HV side
Find the full load efficiency and regulation at 0.8 lagging power factor


8.5
230V,2.1A, 295W LV
LV 28V,12A, 160 W ,HV
0.8

OR
Q.4

Explain O.C. and S.C. test of 1-phase transformer
07



07

Derive the E.M.F. equation of single phase Transformer




Q.5

Draw and Explain vector diagram of Ø transformer for lagging power factor load.
07




07

(i)Compare between banks of 3 single phase transformers with three phase transformer
State the condition for parallel operation of 3 phase transformers.
04
03


.

04
03
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OR
Q.5

A transformer has a primary winding of 500 turns and secondary of 100turns .When the load current on secondary is 70A at 0.8 power factor lagging, the primary current is 20 A at 0.707 power factor lagging. Determine the no load current of the transformer.
07


500 100 . 70 A ,0.8 20 A 0.707
07

State and explain the accessories of three phase transformer.