Rating and Loss Dissipation topics include: Losses and efficiency, machine ratings and loss dissipation. An electrical machine's rating is the maximum capacity it can operate at. The rating plate lists the numerical values of electrical and mechanical quantities, including: Power rating: Specified in apparent power (KVA or MVA) Power factor (PF): The nominal power factor for other ratings Insulation class: For the primary coil Type of enclosure: (IP code) Rated voltage: Can be either line to line (``line voltage'') or phase Electrical machines can experience loss dissipation, which is... Show more

Rating and Loss Dissipation topics include: Losses and efficiency, machine ratings and loss dissipation.

An electrical machine's rating is the maximum capacity it can operate at. The rating plate lists the numerical values of electrical and mechanical quantities, including:
Power rating: Specified in apparent power (KVA or MVA)
Power factor (PF): The nominal power factor for other ratings
Insulation class: For the primary coil
Type of enclosure: (IP code)
Rated voltage: Can be either line to line (``line voltage'') or phase

Electrical machines can experience loss dissipation, which is when part of the input electricity is converted into heat. Power dissipation is a measure of how much power in a circuit is converted into heat. It's a naturally occurring process that occurs in all resistors that are part of a circuit and have a voltage drop across them. 

Here are some other loss mechanisms in electric motors: Mechanical loss, Resistive loss, Magnetic losses. 

Here are some examples of unwanted effects that can cause loss in electrical systems:
Unwanted heating of resistive components
The effect of parasitic elements (resistance, capacitance, ...) 

Tools for calculating losses can be found in two classes:
Those that rely on device models: SPICE is a typical example
Those that rely on mathematical models generated from test results: More sophisticated tools can calculate junction temperature in a specific application 
 

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Electrical Machines Practice Test: Rating and Loss Dissipation

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25 Questions

1. 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 ____________

increase in lowerunequal heating lossesnone of the mentionedstopping of motor

2. If one of the phases of the supply breaks down, then the connected three phase induction motor ____________

continues to run as a single phase induction motor, provided load does does not increase beyond 57.7%continues to run as single phase induction motorstops operating after few secondscontinues to run as two phase induction motor

3. 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?

100 W40 W20 W10 W

4. A peaky voltage supply is given to the 3 phase power transformer of the connection power system. These results in _________

reduction in iron lossesreduction in copper lossesreduction in noiseincrease in iron losses

5. 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 _________

none of the mentionedidentical speedidentical speed and excitationidentical rating and construction

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 _____________

machine can be overloaded after 100 kWmachine can’t be overloaded after 100 kWmachine can be overloaded by 10%machine can be overloaded by 5% for a small time duration

7. In rotating electrical machines, when the armature rotates, there are continuous magnetic reversals and power required for their reversals is called _____________

Resistance or ohmic lossesHysteresis lossEddy current lossMechanical loss

8. For the same rating machines, which of the following statement is correct regarding the efficiency (η)?

η of low speed machine > η of high speed machineη of low speed machine < η of high speed machineη of low speed machine = η of high speed machineAny of the mentioned

9. The iron losses in a saturated three phase alternator is lesser than the non-saturated three phase alternator.

Don't KnowFalseTrueMaybe

10. 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?

30456090

11. 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?

DecreasesRemains constantNone of theseIncreases

12. In an induction motor, which of the following is correct?

Stator core loss = rotor core lossStator core loss < rotor core lossStator core loss > rotor core lossAny of the mentioned

13. To determine the efficiency of the machine, direct load test is not advantageous because of which of the following reasons?

Both cost of providing large inputs and difficulty in dissipating the large outputsCost of providing large inputsDifficulty of dissipating the large outputsNone of these

14. 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 ____________

50%zero75%25%

15. The hydroelectric plants in the industry are best suited to ____________

closed circuit air coolingdirect waterhydrogen gasany of the mentioned

16. 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?

10580.5

17. Which of the following statements are correct regarding brush contact losses?

In induction machine: neglectedAny of the mentionedIn synchronous machine: neglectedIn DC machine: proportional to armature current

18. In dc machine iron losses cause _________

all of the mentionedheating in coreloss in efficiencyrise in temperature of ventilating air

19. No load rotational losses in electrical machine consists of _____________

No load core, friction and windage lossesRotor core, friction and windage lossesStator core, friction and windage lossesFriction and windage losses

20. 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 ___________

lower than Y’s readinghigher than Y’s readingsame as Y’s readingnone of the mentioned

21. 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 _________

50%zero25%75%

22. For squirrel cage and slip ring induction motor, cooling methods is efficient in _________

slip ring induction motorsquirrel cage induction motornone of the mentionedboth of the motors

23. 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?

2.50.41can not be operated in parallel

24. The hydroelectric plants in the industry are best suited with _________

all of the mentioneddirect waterclosed circuit air coolinghydrogen gas

25. A consideration of the power losses in electrical machines is essential for which of the following reasons?

Temperature riseAll of the mentionedVoltage dropsOperating cost