DC Machines topics are: DC machine applications and its circuit models, mmf and flux density waveforms in DC machines, methods of excitation, efficiency and testing of dc machines. DC machines, which can be classified as DC motors and DC generators, convert mechanical energy into electrical energy. They contain AC currents and voltages, but convert AC voltage to DC voltage. DC machines are used in applications where high starting torque and accurate speed control over a wide range are important.  Here are some types of DC machines: DC motors: Use direct current to produce mechanical force.... Show more

DC Machines topics are: DC machine applications and its circuit models, mmf and flux density waveforms in DC machines, methods of excitation, efficiency and testing of dc machines.

DC machines, which can be classified as DC motors and DC generators, convert mechanical energy into electrical energy. They contain AC currents and voltages, but convert AC voltage to DC voltage. DC machines are used in applications where high starting torque and accurate speed control over a wide range are important. 

Here are some types of DC machines:
DC motors: Use direct current to produce mechanical force. DC motors can vary in size and power from small motors in toys and appliances to large mechanisms that power vehicles, pull elevators and hoists, and drive steel rolling mills.
Shunt DC generators: Mainly used for lighting purposes, charging batteries, and supplying field excitation current to alternators.
Compound motors: Used where higher starting torque and fairly constant speed is required. Examples of usage of compound motors are in presses, shears, conveyors, elevators, rolling mills, and heavy planners. 

Other types of DC motors include:
Series DC motor
Permanent magnet DC motor 

DC machines have two main parts:
Stator: Contains parts such as brushes, magnets, and housing for the whole motor
Rotor: Contains the windings, the communicator, and the output shaft 

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Electrical Machines Practice Test: DC Machines

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25 Questions

1. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the supply terminals, then the __________

direction of ration reverses and commutation will be affecteddirection of rotation reverses and commutation will be unaffecteddirection of rotation will be same and commutation will be unaffecteddirection of ration will be same and commutation will be affected

2. Consider a 200V, 25kW, 30A DC machine lap connected with armature resistance of 0.4 ohms. If the machine is later wave wound, then the developed power is?

20 kW25 kW50 kW12.5 kW

3. A series generator, having external characteristics which is a straight line through 0 to 50 V at 200 A is connected as a booster, between a station bus-bar and feeders of 0.5 ohm resistors. The voltage at the far end of feeder at current of 50 A is?

25 V17 V50 V8 V

4. A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.

4.7AT1.2AT5.9AT3.5AT

5. The number of parallel paths in the armature is increased by __________

increasing number of magnetic polesdecreasing number of magnetic poleslap number of magnetic polesusing more brushes

6. The quantities needed to complete retardation test are/is _________

dw/dtany of the mentioneddw/dt and moment of inertiacurrent

7. The compensating winding in dc generator is provided to __________

neutralize demagnetizing fluxmaintain magnetizing mmfneutralize armature mmfneutralize cross-magnetizing flux

8. ______ test is used for determining the efficiency of a traction motor.

FieldRetardationHopkinsonSwineburne’s

9. A d.c. generator has been provided tappings on the armature winding at intervals of 120 degrees from the side of commutator. The connection will be __________

star connected alternatorstar connected induction motordelta connected alternatorany of the mentioned

10. The number of compensating conductors/pole faces is _______ (where Z is the number of active armature conductors/pole; A is number of parallel paths; Ia is the armature current).

2*A*Ia/ZZ/A*IaZ/2A*IaZ*Ia/A

11. The principle contributors to magneto motive force is/are __________

all of the mentionedfield current and armature currentfield excitationelectromagnetic torque

12. Interpoles are used to start the dc motor above the base speed.

FalseDon't KnowTrueMaybe

13. The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.

none of the mentionedforwardall of the mentionedreverse to rotation

14. The circuit depicting the equation V=Ea – I*Ra.

None of these

15. The possible assumption is/are made while doing swineburne’s test.

(i) mechanical loss constant
(ii) Armature reaction neglected
(iii) Increases in flux

(i), (ii), and (iii)(i)(i) and (ii)(iii) and (i)

16. _____________ is responsible for mechanical power output of a DC motor.

Any of the mentionedAir-gap fluxElectrical input powerArmature emf

17. Hopkinson’s test is also called regenerative test due to ____________

energy of one machine is used to drive otherfeedbacklosses are leastextra motor is used

18. The armature mmf affects __________

generated voltagetorquecommutationany of these

19. Retardation test on DC shunt motor is used for finding _______

iron lossescopper lossesfriction lossesstray losses

20. Inspite of heavy initial investments, dc motors are used due to _____________

improved power factor of the systemflexibility and ease of controllower lossesall of the mentioned

21. If the magnetic circuit of the dc machine is in the saturation region, the armature reaction __________

does not affect flux/poleincreases the flux/poledecreases flux/poleaffects the flux/pole only when armature current is small

22. Centrifugal pumps, fans-blowers use __________

only induction motornone of the mentionedonly shunt motorsshunt as well as induction motor

23. If the terminal voltage of 220-V dc generator having armature resistance of 1 ohms. The induced emf produced is 200-V. The armature current for the above machine is?

-10 A10 A20 A-20 A

24. Which is the most appropriate relation to find efficiency of the generator?

output/(output + losses)(output – losses)/(output + losses)(output – losses)/inputany of the mentioned

25. A shunt generator has an induced voltage on open circuit of 127 V. When the machine is on load the terminal voltage is 120 V. The load current if the field resistance be 15 ohm and armature resistance be 15 ohm.

358 A342 A8 A350 A