1

GATE EE 2002

Subjective

+5

-0

A $$415$$ $$V,$$ $$2$$ pole, $$3$$ phase, $$50$$ $$Hz,$$ star connected, non-salient pole synchronous motor has synchronous reactance of $$2\,\Omega $$ per phase and negligible stator resistance. At a particular field excitation, it draws 20 A at unity power factor from a $$415$$ $$V,$$ $$3$$ phase, $$50$$ $$Hz$$ supply. The mechanical load on the motor is now increased till the stator current is equal to $$50$$ $$A.$$ The field excitation remains unchanged. Determine:

$$(a)$$ the per phase open circuit voltage $${E_0}$$

$$(b)$$ the developed power for the new operating condition and corresponding power factor.

$$(a)$$ the per phase open circuit voltage $${E_0}$$

$$(b)$$ the developed power for the new operating condition and corresponding power factor.

2

GATE EE 2001

Subjective

+5

-0

A $$50$$ $$kW$$ synchronous motor is tested by driving it by another motor. When the excitation is not switched on, the driving motor takes $$800$$ $$W.$$ When the armature is short-circuited and the rated armature current of $$10$$ $$A$$ is passed through it, the driving motor requires $$2500$$ $$W.$$ On open-circuiting the armature with rated excitation, the driving motor takes $$1800$$ $$W.$$ Calculate the efficiency of the synchronous motor at $$50\% $$ load. Neglect the losses in the driving motor.

3

GATE EE 2001

Subjective

+5

-0

Two identical synchronous generators, each of $$100$$ $$MVA,$$ are working in parallel supplying $$100$$ $$MVA$$ at $$0.8$$ lagging $$p.f.$$ at rated voltage. Initially the machines are sharing load equally. If the field current of first generator is reduced by $$5\% $$ and of the second generator increased by $$5\% ,$$ find the sharing of load ($$MW$$ and $$MVAR$$) between the generators.

Assume$${X_d} = {X_q} = 0.8\,\,p.u.$$ no field saturation and rated voltage across load. Reasonable approximations may be made.

Assume$${X_d} = {X_q} = 0.8\,\,p.u.$$ no field saturation and rated voltage across load. Reasonable approximations may be made.

4

GATE EE 2000

Subjective

+5

-0

A $$2300$$ $$V,$$ $$3$$-phase synchronous motor driving a pump is provided with a line ammeter and a field rheostat. When the rheostat is adjusted such that the $$ac$$ line current is minimum. The ammeter reads $$8.8$$ $$A.$$ What is the power being delivered to the pump, neglecting losses? How should the rheostat be adjusted so that the motor operates at $$0.8$$ leading power factor? How many $$kVARs$$ is the motor supplying to the system at this new power factor?

Questions Asked from Synchronous Machines (Marks 5)

Number in Brackets after Paper Indicates No. of Questions

GATE EE Subjects

Electromagnetic Fields

Signals and Systems

Engineering Mathematics

General Aptitude

Power Electronics

Power System Analysis

Analog Electronics

Control Systems

Digital Electronics

Electrical Machines

Electric Circuits