Rating and Loss Dissipation MCQs ( Electrical Machines ) MCQs – Competitive Electrical Machines MCQs
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Electrical Machines MCQs – Rating and Loss Dissipation MCQs ( Electrical Machines ) MCQs
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Losses and Efficiency
1. For the given traction application using a dc series motor for a starting time ’t’ is applied. If this method is replaced by a series-parallel control, giving 50% time for each series and parallel. The saving in the starting energy of _________
a) 25%
b) zero
c) 50%
d) 75%
Answer: a
Explanation: Initially let the energy utilized was 100% for time t.Then for 50% duration for series, energy used is 50%. For parallel operation for the same circuit, the resistance gets halved assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only utilized. Hence total saving is 25%.
2. If two series motors are identically coupled. One is running as motor while other as generator. For this combination, the iron losses and frictional losses are found identical for _________
a) identical speed and excitation
b) identical speed
c) identical rating and construction
d) none of the mentioned
Answer: a
Explanation: For same iron and frictional losses, same frequency should be there. For that same speed must be run.
3. Hysteresis loss in a dc machine is _____ of rate of flow of air and _____ on frequency of _____
a) independent, dependent, magnetic reversal
b) independent, dependent, operation
c) dependent, independent, magnetic reversal
d) none of the mentioned
Answer: a
Explanation: Hysteresis loss in a dc machine is independent of rate of flow of air and depends on frequency of magnetic reversal.
4. In dc machine iron losses cause _________
a) heating in core
b) loss in efficiency
c) rise in temperature of ventilating air
d) all of the mentioned
Answer: a
Explanation: The iron losses cause heating of the core which causes reduction inefficiency and cooling air gets heated up.
5. For squirrel cage and slip ring induction motor, cooling methods is efficient in _________
a) squirrel cage induction motor
b) slip ring induction motor
c) both of the motors
d) none of the mentioned
Answer: a
Explanation: As the squirrel cage induction motor has more ventilation and more space as compares to slip ring induction motor, there will be more efficient ways to cool down the squirrel cage motor.
6. If one of the phases of the supply breaks down, then the connected three phase induction motor _________
a) continues to run as a single phase induction motor, provided load does does not increase beyond 57.7%
b) stops operating after few seconds
c) continues to run as single phase induction motor
d) continues to run as two phase induction motor
Answer: a
Explanation: When one of the phases breaks down then the other two phases will supply the rated current but the load should be reduced to 57.7%.
7. If one of the phases of RYB supply gets broken, then the temperature rise of the induction motor _________
a) remains same as before
b) reduces as compared to the normal operating temperature
c) reduces to temperature rise of corresponding single phase induction motor
d) increases
Answer: a
Explanation: The current flowing in the armature will remain same so to maintain the flux requirements of the machine. Hence the temperature rise will also remain same as the current remains same.
8. A three phase induction motor is connected to the infinite bus operating at the normal conditions. There occurs an unbalancing in the supply, leading to _________
a) unequal heating losses
b) stopping of motor
c) increase in lower
d) none of the mentioned
Answer: a
Explanation: The unbalancing in the supply phase will create an unbalanced distribution of the current in the phases, thereby unequal heating.
9. A peaky voltage supply is given to the 3 phase power transformer of the connection power system. These results in _________
a) reduction in iron losses
b) reduction in copper losses
c) increase in iron losses
d) reduction in noise
Answer: a
Explanation: The flux wave will be sinusoidal in nature as the emf is peaky in nature. Sine wave has least losses when compared to other wave forms.
10. The iron losses in a saturated three phase alternator is lesser than the non-saturated three phase alternator.
a) True
b) False
Answer: a
Explanation: The non-saturated reactance is greater than saturated reactance. So the reactive losses or iron losses will reduce due lesser contribution of the same.
11. The hydroelectric plants in the industry are best suited with _________
a) closed circuit air cooling
b) hydrogen gas
c) direct water
d) all of the mentioned
Answer: a
Explanation: The closed circuit is so used to reuse the air in the system so that to save water again in the cooling system.
Losses and Efficiency – 2
1. A consideration of the power losses in electrical machines is essential for which of the following reasons?
a) Operating cost
b) Temperature rise
c) Voltage drops
d) All of the mentioned
Answer: d
Explanation: A machine with lower efficiency has more losses, and therefore increased operating cost. Also, losses cause heating of the machine and therefore, its temperature rise. Similarly, voltage drop IR is associated with ohmic loss.
2. To determine the efficiency of the machine, direct load test is not advantageous because of which of the following reasons?
a) Cost of providing large inputs
b) Difficulty of dissipating the large outputs
c) Both cost of providing large inputs and difficulty in dissipating the large outputs
d) None of the mentioned
Answer: c
Explanation: Also, a small error in the measurement of either output or input causes the same amount of error in the computed efficiency.
3. Which of the following statements are correct regarding brush contact losses?
a) In DC machine: proportional to armature current
b) In synchronous machine: neglected
c) In induction machine: neglected
d) Any of the mentioned
Answer: d
Explanation: There is brush contact loss at the contacts between the brushes and commutator (in DC machine) or between the brushes and slip rings (synchronous and induction machines). However, in practical, the brush contact loss is neglected for synchronous and induction machines.
4. In rotating electrical machines, when the armature rotates, there are continuous magnetic reversals and power required for their reversals is called _____________
a) Eddy current loss
b) Hysteresis loss
c) Resistance or ohmic losses
d) Mechanical loss
Answer: b
Explanation: Hysteresis loss is directly proportional to the number of magnetic reversals per second or the speed.
5. The usual lamination thickness selected to minimize the eddy current loss in rotor is _____________
a) 0.1 mm to 0.2 mm
b) 0.3 mm to 0.4 mm
c) 0.4 mm to 0.5 mm
d) 0.9 mm to 0.10 mm
Answer: c
Explanation: If lamination thickness is made less than 0.4 mm, the reduction in eddy current losses is achieved, but at the cost of additional labor charges in assembling the rotor.
6. In an induction motor, which of the following is correct?
a) Stator core loss < rotor core loss
b) Stator core loss = rotor core loss
c) Stator core loss > rotor core loss
d) Any of the mentioned
Answer: c
Explanation: The rotor core loss is almost negligible because of reduced frequency of the flux reversals (equal to slip frequency, sf) in the rotor, and Pe∝f2 and Ph∝f. (Pe = eddy current losses, Ph = hysteresis losses).
7. The pole shoes in DC and synchronous machines are laminated to reduce _____________
a) Resistance losses
b) Pulsation or pole-face losses
c) Mechanical losses
d) None of the mentioned
Answer: b
Explanation: The pulsations in flux density wave arising from slot openings cause losses in the field iron, particularly in the pole faces. This loss occurring due to relative motion, between field poles and slotted armature, is referred to as pulsation loss.
8. What percentage of the rated output for DC machine and synchronous machine is taken as stray load losses respectively?
a) 1% and 0.5%
b) 0.5% and 1%
c) 3% and 0.1%
d) 0.1% and 5%
Answer: a
Explanation: Stray load losses cannot be determined accurately. In DC machine, by convention, it is taken as 1% of the rated output for rating above 150 KW. For synchronous and induction machine, it is taken as 0.5% of their rated output.
9. Which of the given losses are directly proportional to square of speed?
a) Windage loss
b) Eddy current loss
c) Both Windage and eddy current loss
d) Hysteresis loss and brush loss
Answer: c
Explanation: The windage loss includes the power required to circulate air through the machine and ventilating ducts, and is approximately proportional to the square of speed. Also, eddy current losses are proportional to square of speed.
10. Which of the following statements regarding efficiency of electrical machines are true?
(i) efficiency should be calculated by measuring output and input (ii) efficiency is maximum when constant losses = variable losses (iii) electrical machines are designed to have maximum efficiency at full load
(iv) electrical machines are designed to have maximum efficiency at near about full load
(v) efficiency should be calculated by measuring their losses
(vi) efficiency is maximum when constant losses = x times of (variable losses)
a) (i), (iii), (vi)
b) (ii), (iv), (v)
c) (i), (ii), (iii)
d) (iv), (v), (vi)
Answer: b
Explanation: The machine efficiency rises with load. but at a particular load, efficiency is maximum and beyond this load, efficiency diminishes. Also, for both motors and generators, machine efficiency is maximum when variable loss = constant loss.
11. The electromechanical energy conversion devices used in power systems are never operated to deliver maximum power output because at maximum power output _____________
a) Efficiency is less than 50%
b) Temperature of the power devices is much more than the specified allowable temperature rise
c) Half of the power input appears as losses
d) Any of the mentioned
Answer: b
Explanation: In practice, devices are operated at a load somewhat less than rated load, at which the efficiency is maximum.
Basic Concepts In Rotating Machines MCQs
12. 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
Answer: b
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.
13. 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
Answer: d
Explanation: The sum of friction and windage loss under no load is called no load rotational losses.
Machine Ratings
1. Refer the diagram; a single-phase supply of 100V, 50Hz is fed to the step-up transformer having primary impedance of 1 ohms and secondary impedance 20 ohms. What is the power consumed by the load of 5 ohms?
a) 20 W
b) 40 W
c) 10 W
d) 100 W
Answer: a
Explanation: The referred resistance to the secondary, Z2=1*(52) = 25 ohms
Total resistance at secondary = 25+20 = 45 ohms
Total resistance seen by source = 45+5 = 50 ohms
Hence, current in secondary = 100/50 = 2 A
Power consumed by the load = I2 *R = 4*5 = 20 W.
2. 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)
Answer: a
Explanation: The no load current of the transformer is directly proportional to the dimension of the transformer.
3. 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
Answer: a
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.
4. 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
Answer: a
Explanation: The power factor angle is, tan(phi)=pu resistive drop/pu reactive drop
= 4/4 = 1.
5. 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%
Answer: a
Explanation: As the maximum rating is specified and operation condition are also same, it cannot be overloaded beyond the maximum.
6. 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
Answer: a
Explanation: Star connection in three phase has no closed path for the flow of the third harmonic current.
7. 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
Answer: a
Explanation: The current in armature, Ia = 255-250/0.25 = 20 A
Power = Eb*Ia = 250*20 = 5000 W.
8. 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
Answer: a
Explanation: The load shared by the alternators is inversely related to their pu reactance.
9. 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
Answer: a
Explanation: Synchronous speed, Ns = 120*f/P = 120*60/4 = 1800 rpm
Rotor speed = (1-slip)*Ns = 0.95*1800 = 1710 rpm.
10. Auto transformer rating is chosen so as to have best performance when the analogous two winding transformer has ___________
a) two voltage levels fairly close to each other
b) very high transformation ratio
c) any voltage levels
d) step down purpose
Answer: a
Explanation: The auto transformer is best suited with the voltage levels of same rating for a two winding transformer.
11. In the lab, two students X and Y conducted the experiments to verify the name plate ratings of the transformer.But student X conducted the same at higher frequency. The kVA reading of student X will be ___________
a) higher than Y’s reading
b) lower than Y’s reading
c) same as Y’s reading
d) none of the mentioned
Answer: a
Explanation: As rating is directly proportional to the frequency of operation.
Cooling (Loss Dissipation)
1. For the given traction application using a dc series motor for a starting time ’t’ is applied. If this method is replaced by a series-parallel control, giving 50% time for each series and parallel. The saving in the starting energy of ____________
a) 25%
b) zero
c) 50%
d) 75%
Answer: a
Explanation: Initially let the energy utilized was 100% for time t.Then for 50% duration for series, energy used is 50%. For parallel operation for the same circuit, the resistance gets halved assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only utilized. Hence total saving is 25%.
2. If two series motors are identically coupled. One is running as motor while other as generator. For this combination, the iron losses and frictional losses are found identical for ____________
a) identical speed and excitation
b) identical speed
c) identical rating and construction
d) none of the mentioned
Answer: a
Explanation: For same iron and frictional losses, same frequency should be there. For that same speed must be run.
3. Hysteresis loss in a dc machine is _____ of rate of flow of air and _____ on frequency of _____
a) independent, dependent, magnetic reversal
b) independent, dependent, operation
c) dependent, independent, magnetic reversal
d) none of the mentioned
Answer: a
Explanation: Hysteresis loss in a dc machine is independent of rate of flow of air and depends on frequency of magnetic reversal.
4. In dc machine iron losses cause ____________
a) heating in core
b) loss in efficiency
c) rise in temperature of ventilating air
d) all of the mentioned
Answer: a
Explanation: The iron losses cause heating of the core which causes reduction inefficiency and cooling air gets heated up.
5. For squirrel cage and slip ring induction motor, cooling methods is efficient in ____________
a) squirrel cage induction motor
b) slip ring induction motor
c) both of the motors
d) none of the mentioned
Answer: a
Explanation: As the squirrel cage induction motor has more ventilation and more space as compares to slip ring induction motor, there will be more efficient ways to cool down the squirrel cage motor.
6. If one of the phases of the supply breaks down, then the connected three phase induction motor ____________
a) continues to run as a single phase induction motor, provided load does does not increase beyond 57.7%
b) stops operating after few seconds
c) continues to run as single phase induction motor
d) continues to run as two phase induction motor
Answer: a
Explanation: When one of the phases breaks down then the other two phases will supply the rated current but the load should be reduced to 57.7%.
7. If one of the phases of RYB supply gets broken, then the temperature rise of the induction motor ____________
a) remains same as before
b) reduces as compared to the normal operating temperature
c) reduces to temperature rise of corresponding single phase induction motor
d) increases
Answer: a
Explanation: The current flowing in the armature will remain same so to maintain the flux requirements of the machine.Hence the temperature rise will also remain same as the current remains same.
8. A three phase induction motor is connected to the infinite bus operating at the normal conditions. There occurs an unbalancing in the supply, leading to ____________
a) unequal heating losses
b) stopping of motor
c) increase in lower
d) none of the mentioned
Answer: a
Explanation: The unbalancing in the supply phase will create an unbalanced distribution of the current in the phases, thereby unequal heating.
9. A peaky voltage supply is given to the 3 phase power transformer of the connection power system. These results into ____________
a) reduction in iron losses
b) reduction in copper losses
c) increase in iron losses
d) reduction in noise
Answer: a
Explanation: The flux wave will be sinusoidal in nature as the emf is peaky in nature. Sine wave has least losses when compared to other wave forms.
10. The iron losses in a saturated three phase alternator is lesser than the non-saturated three phase alternator.
a) True
b) False
Answer: a
Explanation: The non-saturated reactance is greater than saturated reactance. So the reactive losses or iron losses will reduce due lesser contribution of the same.
11. The hydroelectric plants in the industry are best suited to ____________
a) closed circuit air cooling
b) hydrogen gas
c) direct water
d) any of the mentioned
Answer: a
Explanation: The closed circuit is so used to reuse the air in the system so that to save water again in the cooling system.