Part - A(Objective test items)- july 2007
Q:1. The relationship between power, voltage and current can be represented by a divided triangle.
Draw a power triangle for DC circuit.
Ans:1. p/v/i
Q:2. Identify the fault in the battery charging circuit shown.
Ans:2. Polarity of battery is wrongly connected to the rectifier supply.
Q:3. What will happen if a permanent magnet moving coil instrument is connected to AC supply?
Ans:3. NO deflection or NO readings.
Q:4. In a serial lamp set the number of lamps used are generally 5% more than the calculated number
of lamps fora given voltage.
Ans:4. To compensate for higher voltage fluctuations.
Q:5. What are the possible causes of a food mixer running hot at normal speed?
Two causes are
a) sparking at commutator
b) Over voltage.
What are other two causes.
Ans:5. a) Overloading.
b) Partial short in the field or armature.
Q:6. Earth tester is mainly used for measuring earth electrode resistance.
Given two more uses of the earth tester.
Ans:6. a) Earth continuity test.
b) Soil Resistivity.
Q:7. What is mutual induction?
Ans:7. Changing magnetic flux of one coil induces emf in an other electrically isolated coil.
Q: 8. You find a coil with core in contactors.
What is the main function?
Ans:8. Switch ON / OFF the supply.
Q:9. What is the material used for the core of a high frequency transformer?
Ans:9. Ferrite core.
Q:10. More retentivity magnetic material is to be used for transformers core.
Name the material is used for the core in power transformers?
Ans:10. Silicon steel.
Q:11. A mechanism is incorporated in circuit breakers to switch off the circuit breaker at fault cindition
either automatically or manually at the desired time.
What is the mechanism known as?
Ans:11. Trip mechanism.
Q:12. The input voltage of a regulator circuit is different from the output voltage.
What can you state about the value of the input voltage compared to the output voltage?
Ans:12. Higher or lower than output voltage and is not constant.
Q:13. a) How many contactors are there in fully automatic star-delta starters?
b) Name the contactors in fully automatic star-delta starters?
Ans:13. a) Automatic star-delta starters have three contuctors.
b) One main conductor with OL relay, one star contactor and one delta contactor.
Q:14. inspite of no electrical fault in the fan motor and its wiring circuit, it is still running slow, even by
applying rated voltage. The blades are not deformed and ther is no wobbling.
What are the reasons?
Ans:14. i) No lubrication / Dry lubrication / Resistive bearing.
ii) Bearing fault.
Q:15. Domestic mixers / liquidizer generally use universal motor, and it has two windings.
a) What are these windings?
b) How they are connected together?
Ans:15.a) Armature winding and field winding.
b) The two windings are connected in series.
Q:16. In figure part of a fluorescent lamp circuit is shown.
a) What is the name of this part?
b) How it is connected initially at the time of starting?
Ans:16. a) Starter b) In series with the filament.
Q:17. You have to replace a thermostat in a geyser.
How do you connect the thermostat with the heating element?
Ans:17. In series with the heating elements.
Q:18. What is the function of a fuse with respect to vaule of current in a circuit?
Ans:18. The fuse must open the circuit, if the current is greater than the reted value of the circuit current.
Q:19. The charging curve of a RC circuit is shown in the figure.
The value og vc is 90 v.
Calculate
a) the value v c11 fot 1t,
b) the value vc2 for 2t,
Ans:19. a) vc1=90x63/100= 56.7v b) vc2=90x86/100=77.4v
Q:20. Among various type of bulbs one type of bulb should not be handled by bare hands even
while replacing the bulb.
What is the type of lamp?
Ans:20. Hologen lamps.
Part - B - july 2007
Q:1. a) Describe the purpose of earting?
b) A battery operated circuit requires 6v and a capacity of 4 Ah. Cells rated at 1.5v and 2 Ah
are available to do the job.
Calculate the required arrangement of cells.
c) State the different types of singale phase watt meter.
Ans:1. a) The purpose of earthing is to provide protection to personnel, equipment and circuits by
eliminating the possibility of dangerous or excessive voltage.
b) No of cells in series= vr/vp.c=6v/1.5v=4cells
No of parallel row= Ahr/Ahp.c=4Ah//2Ah=2 rows
c) There are three types of wattmeters,they are
(i) Dynamometer type
(ii) Induction type
(iii) Electrostatic type.
Q:2. a)Explain the purpose of the fuse in a circuit?
b) A four pole generator having a simplex wave wound armature has 51 slots,each slot contanining
20 conductors.
What will be the voltage generated in the machine, when driven at 1500 rpm.assuming the flux
per pole is 7.0 mWb?
Ans:2. a) A fuse is a safety device used for the purpose of protecting a circuit against excess current. In
the event of excessive current, the fuse element melts and opens up the circuit there by protecting
equipment machine from damage.
b) Data given
Z=51x20=1020
P=4
N=1500 RPM
Q=7x10-3wb
A=2 (wave winding)
We know E=thita ZN/60xP/A=7x10-3squrex1020x1500/60x4/2=357v
Q:3. a) List the conditions to be fulfilled to connect two transformer in parallel.
b)When the load is removed from an AC generator, its terminal voltage rises from 480v at ful load
to 660v at no load.
Calculate the voltage regulation.
c) What is the function of centrifugal switch in single phase AC motor?
Ans:3. a) To operate two or more transformer in parallel, the following conditions to be fuldilled. They are
(i) Voltage ratings of transformer must be same.
(ii) Impedance rating of transformer must be same.
(iii) Polarity of the transformer must be same.
(iv) Phase sequence of the transformer must be same.
b) Data given
No load voltage=660v
Full load voltage=480v
% Regulation = Vnl - Vfl/Vflx100
= 660-480/480x100=37.5%
c) The centrifugal switch is located inside the motor and is connected in series with the starting
winding. The function of the centrifugal switch is to disconnect starting winding after the rotor
has reached 75 to 80% of the rated speed.
Q:4. a) State the necessity of a star-delta starter for a 3-phase squirrel cage induction motor.
b) State the definition of a circuit breaker and list the various types of circuit breakers.
c) A 400v, 3-phase, 8 pole, 50 Hz squirral cage motor has a rated full load speed of 720 rpm.
Determine the synchronous Speed.
Ans:4 a) Squirral cage motor during starting time it will take 5-6 times of full load current for a few seconds.
This starting current will cause too much fluctuations in the power lines and disturb the other loads.
On the other hand, if all the squirral cage motors connected to the power lines are started at the
same times, they may momentarily over load the power lines.
Because of these reasons, the applied voltage to the squirral cage motors needs to be reduced
during the starting periods and regular supply could be given when the motor picks up its speed.
For This purpose only star-delta starters are required to start the squirral cage motors.
b) Definition:
A circuit breaker is a device capable of making and breaking a circuit under normal conditions
as well as under abnormal conditions such as those of short circuit.
Types:
(i) Air break circuit breaker.(ACB).
(ii) Oil circuit breaker.(OCB)
(iii) Air blast circuit breaker.(ABCB)
(iv) Miniature circuit breaker.(MCB)
(v) Earth leakage circuit breaker.(ELCB)
c) Solution:
Synchronous speed Ns=120xf/p=120x50/8=750 rpm
b) A four pole generator having a simplex wave wound armature has 51 slots,each slot contanining
20 conductors.
What will be the voltage generated in the machine, when driven at 1500 rpm.assuming the flux
per pole is 7.0 mWb?
Ans:2. a) A fuse is a safety device used for the purpose of protecting a circuit against excess current. In
the event of excessive current, the fuse element melts and opens up the circuit there by protecting
equipment machine from damage.
b) Data given
Z=51x20=1020
P=4
N=1500 RPM
Q=7x10-3wb
A=2 (wave winding)
We know E=thita ZN/60xP/A=7x10-3squrex1020x1500/60x4/2=357v
Q:3. a) List the conditions to be fulfilled to connect two transformer in parallel.
b)When the load is removed from an AC generator, its terminal voltage rises from 480v at ful load
to 660v at no load.
Calculate the voltage regulation.
c) What is the function of centrifugal switch in single phase AC motor?
Ans:3. a) To operate two or more transformer in parallel, the following conditions to be fuldilled. They are
(i) Voltage ratings of transformer must be same.
(ii) Impedance rating of transformer must be same.
(iii) Polarity of the transformer must be same.
(iv) Phase sequence of the transformer must be same.
b) Data given
No load voltage=660v
Full load voltage=480v
% Regulation = Vnl - Vfl/Vflx100
= 660-480/480x100=37.5%
c) The centrifugal switch is located inside the motor and is connected in series with the starting
winding. The function of the centrifugal switch is to disconnect starting winding after the rotor
has reached 75 to 80% of the rated speed.
Q:4. a) State the necessity of a star-delta starter for a 3-phase squirrel cage induction motor.
b) State the definition of a circuit breaker and list the various types of circuit breakers.
c) A 400v, 3-phase, 8 pole, 50 Hz squirral cage motor has a rated full load speed of 720 rpm.
Determine the synchronous Speed.
Ans:4 a) Squirral cage motor during starting time it will take 5-6 times of full load current for a few seconds.
This starting current will cause too much fluctuations in the power lines and disturb the other loads.
On the other hand, if all the squirral cage motors connected to the power lines are started at the
same times, they may momentarily over load the power lines.
Because of these reasons, the applied voltage to the squirral cage motors needs to be reduced
during the starting periods and regular supply could be given when the motor picks up its speed.
For This purpose only star-delta starters are required to start the squirral cage motors.
b) Definition:
A circuit breaker is a device capable of making and breaking a circuit under normal conditions
as well as under abnormal conditions such as those of short circuit.
Types:
(i) Air break circuit breaker.(ACB).
(ii) Oil circuit breaker.(OCB)
(iii) Air blast circuit breaker.(ABCB)
(iv) Miniature circuit breaker.(MCB)
(v) Earth leakage circuit breaker.(ELCB)
c) Solution:
Synchronous speed Ns=120xf/p=120x50/8=750 rpm
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