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B. Tech(SEM V) Third Sessional Test 2014-15

EMEC-II

TIME: 3 Hr. Total Marks: 100

Note: Attempt all questions

1. Attempt any fourquestions 5*4=20

a) Draw and explain the equivalent circuit diagramand phasor diagram of a synchronous machine.

b) What do you know about voltage regulation of analternator? Name various mathod of obtaining the

same. Explain any one of the following.

c) How does the parallel operation of two alternators

differ from the condition when an alternator runswith an infinite bus bar? Discuss about

synchronizing power

d) A 2200 V, 50 Hz, 3 phase star connected

alternator has an effective resistance of 0.5 ohmper phase. A field current of 30 A produced the

full load current of 200 A on short circuit and aline to line emf of 1100 V on open circuit.

Determine

i. The power angle of alternator when itdelivers ful oad at 0.8 pf(lagging)

ii. The SCR of the alternator

(Following Paper ID and Roll No. to be filled in your Answer Book)

Pa er ID : Roll No.

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e) Explain power angle characteristics of cylindrical

rotor alternatorf)

A three phase star cnnected alternator is rated at

1500 kVA, 12000V. the armature effectiveresistance and synchronous reactance are 2 ohmand 35 ohm respectively per phase. Calculate the

percentage regulation for a load of 1200kW atpower factor of 0.8 lagging.

2. Attempt any fourquestions 4*5=20

a) Explain that a 3 phase synchronus motor is not self-starting. Discuss the use of damper winding in a

synchronous motor.

b) From phasor diagram of the salient pole synchronous

machine, show that:

2sin11

2

sin 2

dqd XX

V

X

VEoutputpower

c)

Two single phase alternator operating in parallel haveinduced emfs on open circuit of 2200and 22010V and respective reactances of j3 and j4. Calculatei) terminal voltage ii) currents and iii) power delivered

by each of the alternators to a load of resistance 6

d) What is hunting and causes of hunting in synchronous

machines? How is it minimised? What are the

advantages and disadvantages of hunting insynchronous machines?

e) Discuss the operation of synchronous motor at constantload variable excitation. Also discuss V curves and

inverted V curves.

f) A 1000 kVA 11000 V 3 phase star connected

synchronous motor has an armature resistance andreactance per phase of 3.5 and 40 respectively.

Determine the induced emf and angular retardation of

the rotor when fully loaded at a) unity power factorb)0.8 pf lagging c) 0.8 leading

3. Attempt any twoquestions 2*10=20

a)

Develop an equivalent circuit diagram for 3 phaseinduction motor. Show in circuit that R2(1-s)/s is the

electrical equivalent of mechanical load on motor,where R2is rotor resistance.

A 3 phase star connected 400V 50 Hz 4 pole induction

motor has following per phase parameters in ohmsreferred to stators

R1= 0.15 X1= 0.45 R2

=0.12 X2=0.45 Xm=

28.5. Compute stator current and power factor when

motor is operated at rate voltage and frequency with

s=0.04.

b) Derive expression for torque in 3 phase inductionmotor. How is this affected by the rotor parameters?

Draw suitable slip torque characteristics at

subsynchronous, supersynchronous and reversedirection speeds and explain their physical meaning.

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c) A 400v 3 phase 50 Hz 4 pole induction motor (starconnected) takes a line current of 10 A with 0.86 pf

lagging. Its stator losses are 5% of input. Rotor copperlosses are 4% of input to rotor, and mechanical losses

are 3% of the input of rotor. Calculate i) slip and rotorspeed ii) torque developed in rotor iii) shaft torque.

4. Attempt any twoquestions 2*10=20

a) Why starter is needed for operation of 3 phase

induction motor? List different starting method of slipring and squirrel cage IM and explain any one of it. An

induction motor has staring current of six times the fullload current and a full load slip of 0.05. find in pu of

full load values the line current and starting torque withi) Direct Starting ii) Star-Delta Starting.

b) Explain deep bar and double cage rotor in squirrel cageinduction motor. The impedances at standstill of the

inner and outer cages of a double cage rotor are

(0.01+j0.5) and (0.05+j0.1) respectively. Thestator impedances are assumed to be negligible.

Calculate the ratio of torques due to two cages:i) at starting ii) when running with slip of 5%

c)

The rotor of a 4 pole 50 Hz slip ring induction motorhas a resistance of 0.25 per phase and runs at 1440

rpm at full load. Calculate the external resistance perphase which must be added to lower the speed to 1200

rpm, the torque being same as before.

5. Attempt any twoquestions 2*10=20

a) Write short notes on any two-i. Stepper motor

ii. Shaded pole motoriii.

Method of speed control of single phase

induction motor

b) Explain two field revolving theory for a single phase

induction motor. Give its torque speed characteristics.Why this induction motor does not have any starting

torque? Show that this motor can run in either direction

if once started

c) A 125 kW 4 pole 110V 50 Hz single phae inductinmotor delivers rated output at a slip of 6%. The copper

loss at full load is 25 watts. Calculate the full loadefficiency and rotor copper loss caused by backwardfield. Rotational loss may be 25 watts. Neglect stator

loss

.

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