1. For the same rating machines, which of the following statement is correct regarding the efficiency (η)?
a) η of low speed machine > η of high speed machine
b) η of low speed machine < η of high speed machine
c) η of low speed machine = η of high speed machine
d) Any of the mentioned
Explanation: The amount of conductor and iron materials required for a machine of given rating is inversely proportional to its speed. Also, more iron and conductor would entail more losses.
2. No load rotational losses in electrical machine consists of _____________
a) Friction and windage losses
b) Stator core, friction and windage losses
c) Rotor core, friction and windage losses
d) No load core, friction and windage losses
Explanation: The sum of friction and windage loss under no load is called no load rotational losses.
3. If the dimension of a single phase transformer is made sqrt(3) times less than the original structure, then what is new no load current rating of the transformer?
a) 1/sqrt(3)
b) sqrt(3)
c) 3
d) 1/3*sqrt(3)
Explanation: The no load current of the transformer is directly proportional to the dimension of the transformer.
4. For a single phase, 50 Hz transformer, if the open circuit test is conducted at 40 Hz. What is the variation in the no-load power factor of the transformer?
a) Decreases
b) Increases
c) Remains constant
d) None of the mentioned
Explanation: If the frequency reduces then flux density increases, so the flux and then the magnetizing current. When magnetizing current increases power factor angle increases so the power factor decreases
5. The desired voltage regulation for a single phase, 50 Hz transformer is zero. Its pu ohmic drop is 4% and pu reactive drop is 4%. The required power factor angle for the operation in degrees is?
a) 45
b) 90
c) 30
d) 60
Explanation: The power factor angle is, tan(phi)=pu resistive drop/pu reactive drop
= 4/4 = 1.
6. While conducting lab experiments on DC machine loading, the students are given a machine with its rating as 100 kW maximum rating. Then the _____________
a) machine can’t be overloaded after 100 kW
b) machine can be overloaded after 100 kW
c) machine can be overloaded by 5% for a small time duration
d) machine can be overloaded by 10%
Explanation: As the maximum rating is specified and operation condition are also same, it cannot be overloaded beyond the maximum.
7. A 3-phase, five limbed shell type transformer, star connected, 200V, 50 Hz, 100 kVA will have ___________
a) no third harmonic current in its phase but has sinusoidal flux
b) third harmonic current in its phase but has flat-topped flux
c) no third harmonic current in its phase but has flat-topped flux
d) third harmonic current in its phase but has sinusoidal flux
Explanation: Star connection in three phase has no closed path for the flow of the third harmonic current.
8. For a separately excited dc machine of 25 kW, 250 V and armature resistance of 0.25 ohms, is running at 3000 rpm with supply of 255 V. The electromagnetic power produced at the armature in kilo watts is?
a) 5
b) 10
c) 0.5
d) 8
Explanation: The current in armature, Ia = 255-250/0.25 = 20 A
Power = Eb*Ia = 250*20 = 5000 W.
9. If two alternators A and B are operated in parallel having per unit synchronous reactance as 0.02 ohms and 0.05 ohms respectively.Then what the ratio of contributing loading of machine A to B?
a) 2.5
b) 0.4
c) 1
d) can not be operated in parallel
Explanation: The load shared by the alternators is inversely related to their pu reactance.
10. A 200V, 10 hp, 4 poles, 60 Hz, Y-connected induction motor had full load slip of 5%. Then the rotor speed of the motor with respect to stator is?
a) 1710 rpm
b) 1800 rpm
c) 90 rpm
d) 3510 rpm
Explanation: Synchronous speed, Ns = 120*f/P = 120*60/4 = 1800 rpm
Rotor speed = (1-slip)*Ns = 0.95*1800 = 1710 rpm.