ac mc question paper.docx

7/28/2019 ac mc question paper.docx

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MODULE 3

PART A

Each question carries 3 marks.

1. Briefly explain the working of an alternator with figure?

2. List the advantages of stationary armature.

3.Draw the schematic diagram of alternator.

4.Why alternator is also known as synchronous generator?

5.The armature of an 8 pole,3 phase,50Hz alternator has 18 slots &10 conductors/slot .A flux of 0.04 Wb

is entering the armature from one pole .calculate the induced emf per phase.

6.what is the relevance of synchronous impedance in an alternator?

7.Define voltage regulation ?. list the methods to determine voltage regulation.

8.explain the principle of operation of synchronous motors?

9.why synchronous motor is not a self staring motor?

10.list some of the advantages and disadvantages of 3 phase induction motor

11. explain squirrel cage rotor & wound rotor?

12.What is the relevance of slip in an induction motor?

13.why synchronous motor never run on synchronous speed?

14.Derive an expression of rotor frequency in terms of main supply frequency and slip.

15.Explain the terms slip &slip frequency .

16. list 6 applications of 3 phase induction motor.

PART B

Each question carries 5 marks

1..Draw the equivalent circuit for a 3 phase induction motor and explain how mechanical power

developed is taken care in equivalent circuit.

2.Explain the construction and working of alternator?

3.explain the methods of determining voltage regulation?


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4..derive the expression for developed torque in a 3phase induction motor and find the condition for

maximum torque.

5..neatly sketch the torque slip characteristics of a 3phase induction motor indicating there in the

starting torque , maximum torque & operating region.

6..explain with suitable diagrams, how rotating mf is produced in a 3phase induction motor.

7..describe no load test & blocked rotor test on an induction motor.

8.state various methods of starting of a 3phase induction motor .

9.state & explain the various methods of speed control of 3 phase induction motors.

10.explain different types of 1 phase motors and list some of its applications

11.compare between synchronous and induction motor.

Part c

Each questions carry 12 marks

1.A 3 phase induction motor is wound for 4 poles and is supplied from 50 Hz system .Calculate

a) the synchronous speed

b) speed of the motor when slip is 4 % &

c) the rotor current frequency when the motor runs at 600 rpm.

2.The power input to the rotor of a 440V, 50Hz 3 phase ,6 pole induction motor is 60 kW.It is observed

that the rotor emf makes 90 complete cycles per minitue.calculate

a) the slip

b) the rotor speed

c)the rotor copper loss

d)mechanical power developed

3.Find the synchronous impedance &reactance of an alternator in which a given field current producesan armature current of 200A on shortcircuit and a generated emf of 50V on open circuit. The

armature resistance is 0 .1ohm.To what induced voltage must the alternator be excited If it is to deliver

a load of 100A at a p.f of 0.8 lagging with a terminal voltage of 200V.


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4.A 3.5MVA, 3-Pase ,Y connected alternator rated at 4160 V at 50 Hz has the open circuit characteristics

given by the following data:

Field current(Am) 50 100 150 200 250 300 350 400 450

E.M.F.(Volts) 1620 3150 4160 4750 5130 5370 5550 5650 5750

A field current of 200A is found necessary to circulate full load current on short circuit of the alternator.

calculate by (i) synchronous impedance method and (ii) ampere turns method the full load voltage

regulation at 0.8 pf lagging .Neglect resistance .comment on the results obtained.

5.A 3 phase induction motor having a star connected rotor has an induced e.m.f of 80 V between slip

rings at standstill on open circuit .the rotor has a resistance and reactance per phase of 1 ohm and 4ohm

respectively .calculate current/phase and p.f when (a) slip ring are short circuited (b) slip-rings are

connected to a star-connected rheostat of 3ohm per phase.

6.A 3 phase ,slip ring induction motor gives a reading of 55V across slip-ring s on opencircuit when at

rest with normal stator voltage applied .the rotor is stator connected and has an impedance of (0.7 +j5)

ohm per phase. find the rotor current when the machine is (a) at standstill with the slip-rings joined to a

star -connected starter with a phase impedance of (4+j3)ohm and (b) running normally with a 5 % slip.

7.A 746kW ,3 phase ,50Hz ,16 pole induction motor has a rotor impedance of ( .02+j0.15)ohm at

standstill .full load torque is obtained at 360 rpm .calculate (i) the ratio of maximum to full load torque

(II) the speed of maximum torque and (iii) the rotor resistance to be added to get maximum starting

torque.

8.estimate the stator current ,equivalent rotor current ,efficiency ,output and p.f at a slip of 5% for a

motor having the following data :

Stator impedance : 1+j3

Rotor stand still impedance : 1+j2

No load shunt impedance :10 +j50

Volts/phase :250V

ac mc question paper.docx

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