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SLIATE
SRI LANKA INSTITUTE OF ADVANCED TECHNOLOGICAL EDUCATION
(Established in the Ministry of Higher Education, vide in Act No. 29 of 1995)
Higher National Diploma in Engineering (Electrical)
Second Year, First Semester Examination – 2014
EE 2104 Electrical Machine A
Instructions for Candidates:
Answer Five Questions out of six
No. of questions :06
No. of pages
: 03
Time
:3 hours
01.
State True or False.
(i)
(ii)
Transformer efficiency is low since the losses in transformer are generally high.
Main two types of losses in transformers are iron loss and copper loss in which copper loss
is constant and iron loss is varying.
When the input to an electrical machine is mechanical energy and the output is electrical
energy then the device is called as motor.
One of the functions of a commutator is converting alternating voltage into direct voltage
and vice versa.
Armature reaction is the effect that the magnetic field produced by the armature current,
has on the magnetic field produced by the field system.
Rotor resistance of induction motor is affected by slip.
The maximum torque occurs in an induction motor when rotor resistance equals to the
rotor reactance.
Squirrel cage induction motors are explosion proof.
In synchronous motors the stator is wound for the same number of poles as the rotor poles.
The synchronous motor has direct voltage applied to the rotor winding and a three phase
voltage is applied to the stator winding.
(20 Marks)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
(ix)
(x)
02.
(i)
(ii)
With the help of characteristic curves, describe the characteristics of separately excited
D.C. generator.
(10 marks)
Is it possible to get the speed above the normal by using the field rheostat method on D.C.
shunt motors, discuss.
(3 marks).
(iii) What are the advantageous of field rheostat method?
(2 marks).
(iv) A 1000kW, 500 V, 8 pole separately excited dc generator runs at 250 rpmand it has 2000
lap connected conductors. Full load copper loss is 20kW. Calculate the useful flux per pole
(5 marks).
03.
(i)
Explain that the shunt generators are self-protective against accidental short circuit.
(5 marks).
(ii) Why is it not advisable to fully load a dc shunt generator until its voltage builds up to the
rated value?
(5 marks).
(iii) It is not advisable to run the dc series motors without load. Give the reason. (5marks).
(iv) A 4 pole dc shunt generator with lap winding armature has armature and field resistances
of 100 Ω and 0.1 Ω respectively. It supplies power to 25 lamps rated for 200 V, 40 W each.
Calculate the total armature current and the generated electromotive force by allowing a
contact drop of 1 V per brush. (Assume two brushes are available.)
(5 marks).
04.
a)
(i) The primary of a single transformer having a turns ratio 1:2 is connected to a 10 V dc
source while its secondary is connected to a 10 Ω resister. What is the value of current in
the resister? Explain the reason .
(4 marks)
(ii) Why cores of transformers are laminated.
(1 marks)
(iii) Why low voltage winding is placed first on the core and then high voltage winding in case
of a core type transformer.
(1 marks)
b)
Discuss the repercussion on the following while operating the two transformers in parallel.
(i)
(ii)
Wrong polarity connections of two transformers.
Un-identical voltage ratings of both primaries and secondaries.
(2 marks).
(2 marks).
c)
(i)
(ii)
(iii)
(iv)
What are the equivalent components can be determined from the short circuit test of
transformer.
(1 marks).
Explain the procedure of conducting the short circuit test.
(4 marks).
Draw the equivalent transformer circuit as referred to primary winding.
(2 marks).
Draw the vector diagram for the above equivalent circuit.
(3 marks).
HNDE Electrical Machine A 2014 1st semester
Page 2
05.
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
What is the requirement for keeping air gap between stator core and rotor and why this air
gap is made very small?
(2 marks).
Explain, the squirrel cage induction motors are not suitable to start against heavy loads.
(4 marks).
What is the purpose of damper winding in the synchronous motors?
(1 mark).
Briefly explain the function of damper winding in synchronous motor.
(2 marks).
Synchronous motors are operated at unity power factor, lagging power factor, leading
power factor, briefly explain the first two conditions.
(6 marks).
How will you increase the starting torque in the slip ring motors by adding external
resistances to the rotor circuit.
(3 marks).
Draw the complete rotor circuit diagram of slip ring motor after connecting a star
connected external resistances.
(2 marks).
06.
a) Write short notes on
(i) Rotor of synchronous motors.
(ii) Pull out torque of synchronous motors.
b)
(i)
(ii)
c)
(i)
(ii)
(iii)
(iv)
(2 marks).
(2 marks).
What are the advantages of slip ring motors?
(3 marks).
What will happen to the efficiency of a transformer when copper loss is equal to iron loss?
r
(1 mark).
An 8 pole lap wound armature rated at 500 rpm is required to generate 250 V. The useful
flux per pole is 0.05 Wb. If the armature has 100 slots calculate the number of conductors
per slot.
(3 marks).
What is back emf in a dc motor?
(2 marks).
What are the two types of compound wound motors which assist and opposes each fields
of motors. Give brief explanation.
(4 marks).
Write 3 starting methods of three phase squirrel cage induction motors.
(3 marks)
HNDE Electrical Machine A 2014 1st semester
Page 3