Cochin University of Science and Techology (CUST) 2007-4th Sem B.Tech Electrical and Electronics Engineering electrical machines - Question Paper

B.TECH IV SEMESTER exam (SUPPLEMENTARY) 2007


ELECTRICAL MACHINES


1. a) What is the principle of operation of a dc generator? Why is a commutator and brush
arrangement necessary for the operation of a dc generator?

b) A 4pole lap wound shut generator supplies to 50 lamps of 100w, 200V every. The
field and armature resistances are 50ohm and 0.2ohm respectively. Allowing a brush
drop of 1V per every brush, compute the subsequent 1) armature current 2) current
per path 3) generated emf

OR

2. a) Derive the emf formula of a dc generator.

b) A 4pole generator supplies a current of 143A. It has 492 armature conductors-
1) wave wound 2) lap wound. When delivering full load, the brushes are provided an
true lead of 10degree. compute the demagnetizing ampere-turns per pole. The
field winding is shunt connected and take 10A. obtain the number of extra shunt field
turns to neutralize the demagnetization.

3. a) Enumerate the reasons which reason terminal voltage under load conditions to be
various from the terminal voltage under no load conditions for dc shunt generator.

b) A separately excited dc generator running at 1000rpm gave the subsequent o c c
Field current (A) – 1.2 2.4 3.6 5.0 6.2 7.4
EMF (V) - 140 280 375 440 490 520

compute 1) The voltage developed when the machine is excited as a shunt
generator with a field resistance of 80ohm at 1000rpm.

2) The speed at which the machine just fails to excite.

3) The additional resistance which should be inserted in the field circuit to
decrease the voltage to 360V at 1000rpm.

OR

4. a) Draw and discuss the external characteristics of shunt, series and compound
generators.

b) 2 dc shunt generators with emfs of 120V and 115V, armature resistances of
0.05ohm and 0.04ohm and field resistances of 20ohm and 25ohm respectively are in
parallel supplying a total load of 25KW. How do they share the load?

5. a) Draw the characteristics of dc shunt and series motors. Use these curves to discuss
the applications for which these motors are used.

b) A 4pole 250V, wave connected shunt motor provide 10KW when running at 1000rpm
and drawing armature & field currents of 60A & 1A respectively. It has 560
conductors. Its armature resistance is 0.2 ohm. Assuming a drop of 1V per
brush.Calculate 1) total torque 2) useful torque 3) useful flux/pole 4) rotational
losses 5) efficiency.
OR

6. a) define how a swin burne’s test is conducted on DC machines. State its advantages
and disadvantages.

b) A 10KW,240V, dc shunt motor draws a line current of 5.2A while running at no
load speed of 1200rpm from a 240 DC supply. It has an armature resistance of
0.25ohm and a field resistance of 160ohm. Estimate the efficiency of the motor
when it delivers rated load.

7. a) Draw the phasor diagram of the transformer on lagging pf load and discuss it.

b) The high voltage and low voltage winding of a 2200/220V single phase 50Hz
transformer has resistances of 4.8ohm and 0.04ohm and reactances 2ohm and
0.018ohm respectively. The low voltage winding is connected to a load having an
impedence of (6+j4)ohm. Determine 1) current in 1.v.winding and h.v.winding 2)
load voltage 3) power consumed by the load.
OR

8. a) Develop the equivalent circuit of a single phase transformer.

b) A single phase transformer is connected to a 230V, 50Hz supply. The net cross
sectional area of the core is 60sq.cm. The number of turns in the primary is 500 and
in the secondary is 100. Determine 1) transformation ratio 2) emf induced in the
secondary winding 3) maximum value of flux density in the core.

9. a) discuss with circuit diagrams, the open circuit and short circuit tests to be carried
out in the laboratory for the determination of parameters of a single phase
transformer. The subsequent readings were found from OC & SC tests on eight KVA
400/120V 50Hz transformer: OC test : 120V,4A,75W
SC test : 9.5V,20A,110W
compute 1) voltage regulation and efficiency for 0.8 pf lagging, full load
2) the efficiency at half full load 0.8 pf lagging load.

OR

10. a) Write short notes on : 1) Y-Y connections 2) V-V connections

b) 2 single phase transformers with equal turns have impedence of (0.6+j4) and
(0.8+j10)ohm with respect to the secondary. If the operate in parallel determine
how they will share total load of 120KW at 0.8 pf lagging.

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Earning:   Approval pending.