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1. In a separately excited dc generator the field is connected to:a) In series with an armatureb) Across the armaturec) To the external supply sourced) None of the above

C

2. If we increase the field of the dc generator, its emf output:a) Increases indefinitelyb) Increases till the winding burnsc) Increases till the magnetic saturation takes placed) First increases and then starts reducing

C

3. In a dc generator the residual magnetism is of the order of:a) 2.5%b) 10%c) 15%d) 25%

A

4. The terminal voltage of the dc shunt generator on loading:a) Increases slightlyb) Decreases sharplyc) Decreases slightlyd) Increases sharply

C

5. For parallel operation, the dc shunt generator normally preferred are:a) Shunt generatorb) Series generatorc) Under compound generatord) Both (a) and (c)

D

6. In a rotating electrical machine the torque produced will be maximum when:a) Torque angle is zerob) Torque angle is 90 degc) Two magnetic fields are aligned with each otherd) Field strengths are maximum and the torque angle is zero

B

7. Flashing of field of dc generator means:a) Neutralization of residual magnetismb) Creation of the residual magnetism by a dc sourcec) Increasing the flux density by providing extra ampere-turns in the fieldd) None of the above

B

8. In an arc welding in order to obtain steady arc one should use:a) DC series generatorb) DC shunt generatorc) DC differential compound generatord) DC cumulative compound generator

C

9. With the increase in the speed of a dc motor:a) Both back emf as well as line current increasesb) Both back emf and line current fallsc) Back emf increases but the line current falls d) Back emf falls and line current increases

C

10. The output indicated on the name plate of any motor is always the:a) Gross powerb) Power drawn in volt amperesc) Power drawn in wattsd) Output power at the shaft

D

11. In a dc compound motor field regulator is provided to:a) Control the fluxb) Limit the ampere currentc) Demagnetize the field partiallyd) None of the above

A

12. The direction of rotation of dc shunt motor can be reversed by interchanging:a) The supply terminals

b) The field terminalsc) The armature terminals onlyd) Either field or armature terminals

D

Q: What is the rotating part of a DC motor?

A. Armature B. The field winding C. Any magnetic field D. The magneticfield developed by the field winding

Correct Answer: Option A

Q: What is used to develop the magnetic field in small sized DC motor ?

A. Zinc battery B. Permanent magnet C. Electromagnet D. Option B and C both

Correct Answer: Option B

Q: In DC motor torque is generated by which principle?

A. Ohm law B. Lorentz force C. The fleming’s law D. Edison’s law


Q: In DC series motor

A. Armature winding & field winding are parallel B. Armature winding & field widing are kept in series with source C. Armature winding is kept in series with the source D. The field winding is kept in series with source


Q: What is universal motor?

A. Commutated DC series motor B. Homopolar DC motor C. Compensated Dc motor D. Shunt DC motor


Q: In DC shunt motor

A. Armature and field winding both in series with source B. Armature winding is parallel with source C. The field winding is parallel with source D. Both armature & field winding are connected in parallel with a common source

Correct Answer: Option D

Q: Can you tell that where DC shunt motors are used

A. Where large starting torque is required B. Where the load fluctuations is more C. Where the required torque is high D. Where the speed is required to remain constant from no-load to full load-condition

Correct Answer: Option D

Q: Can you tell where DC series motors are used

A. Where huge starting torque is required & load is subjected to heavy fluctuations B. Where large starting torque is required C. Where load is subjected to heavy fluctuations D. Where constant speed is required


Q: Which of the following applications needs high starting torque ?

A. Lathe B. Centrifugal pump C. Locomotive D. Air blower

Correct Answer: Option C

Q: Select DC motor that is preferred for cranes & hoists

A. Shunt motor B. Series motor C. Cumulative compound motor D. Any of those


Q: Two DC series motors when connected in series, draw a current of I Amp and run at N rpm. When those motors are connected in parallel and if the motors continue to draw current I, then the speed of the motors will be

A. N B. N / 4 C. 4N D. 2N


Q: Two DC series motors are connected in series to produce a torque T. Now if the motors are connected in parallel, the torque produced will be

A. T / 4 B. 2T C. T / 2 D. 4T


Q: Hopkinson’s test is carried out at which condition

A. Part load B. full load C. Low load D. No load


Q: Select the motor that should not be started on no load

A. Series motor B. Shunt motor C. Compound motor D. Brushless DC motor


Q: If the source voltage of a DC motor is increased, which of the following will decrease

A. Starting torque, operating speed, full load current B. Starting torque C. full load current D. operating spped


Electrical Machine - DC Motor Objectives: Part 7

[1] The basic requirement of a dc armature winding is that it must be(a) A closed one(b) A lap winding(c) A wave winding(d) Either b or c(e) None of the above

Ans: A

[2] The sole purpose of a commutator in a dc generator is to(a) Increase output voltage(b) Reduce sparking at brushes(c) Provide smoother output(d) Convert the induced ac into dc(e) None of the above

Ans: D

[3] In small DC machines, armature slots are sometimes not made axial but are skewed, results in(a) Quieter operation(b) Slight decrease in losses(c) Saving of copper(d) Both a and b(e) None of the above

Ans:D

[4] The critical resistance of the dc generator is the resistance of(a) Armature(b) Field(c) Load(d) Brushes(e) None of the above

Ans: B

[5] In a dc generator, the generator, the generated emf is directly proportional to the(a) Field current(b) Pole flux(c) Number of armature parallel paths(d) Number of dummy coils(e) None of the above

Ans: B

[6] The commutation process in a dc generator basically involves(a) Passage of current from moving armature to a stationary load(b) Reversal of current in an armature coil as it crosses MNA(c) Conversion of ac to dc(d) Suppression of reactance voltage(e) None of the above

Ans: B

[7] The essential condition for stable parallel operation of two dc generators having similar characteristics is that they should have(a) Same kilowatt output ratings(b) Dropping voltage characteristics(c) Same percentage regulation(d) Same no load and full load speed(e) None of the above

Ans: B

[8] An ideal dc generator has .......... voltage regulation.(a) Low(b) Zero(c) Positive(d) Negative(e) None of the above

Ans: B

[9] Which generator has poorest voltage regulation(a) Series(b) Shunt(c) Compound(d) High(e) None of the above

Ans:A

[10] The voltage regulation of an over compound dc generator is always(a) Positive(b) Negative(c) Zero(d) HighAns: B

DC Motors: Part-10 Objective Questions

Solved Objective Questions from DC Machines:[1] Voltage equation of a dc motor isA. V = Eb + IaRaB. Eb = V + IaRaC. V = Eb/IaRaD. V = Eb + Ia2RaE. None of the above

Ans: A

[2] Both Hopkinson's test and Field test

A. Require two electrically coupled series motorsB. Need two similar mechanically coupled motorsC. Use negligible powerD. Are regenerative testsE. None of the above

Ans:B

[3] which of the following motor has the constant speed?

A. Series motorB. Shunt motorC. Cumulatively compound motorD. All of the aboveE. None of the above

Ans:B

[4] The usual test to find the efficiency of the traction motor is

A. Field's testB. Retardation testC. Hopkinson's testD. Swinburn's testE. None of the above

Ans: A

http://electricalobjectivequestion.blogspot.in/2012/10/dc-motors-part-10-objective-questions.html

[5] A DC series motor is best for driving

A. LathesB. Cranes and hoistsC. Shears and punchesD. Machine toolsE. None of the above

Ans: B

[6] Retardation test on a dc shunt motor is used for finding

A. Stray lossB. Copper lossC. Friction lossD. Iron lossE. None of the above

Ans: A

[7] In a DC series motor increasing the load current willA. Decrease the speedB. Increase the speedC. Better commutationD. Increase the back emfE. None of the above

Ans: A

[8] One of the main advantage of the swinburn's test isA. It is applicable both shunt and compound motorsB. It needs one running testC. It is very economical and convenientD. It ignores any charge in iron lossE. None of the above

Ans:C

[9] the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that itA. Requires full load powerB. Ignores any change in iron loss

C. Needs one motor and one generatorD. Requires two identical shunt machinesE. None of the above

Ans: D

[10] The most economical method of finding no losses of a large dc shunt motor isA. Hopkinson's testB. Swinburn's testC. Retardation testD. Field's testE. None of the above

Ans: B

1.In a D.C. shunt motor, speed is

independent of armature current

directly proportional to the armature current

proportional to the square of the current

inversely proportional to the armature currentANSWER : : independent of armature current

Explanation : 2.The type of D.C. motor used for shears and punches is

shunt motor

series motor

differential compoutid D.C. motor

cumulative compound D.C. motorANSWER : : cumulative compound D.C. motor

Explanation : 3.If a D.C. motor is connected across the A.C. supply it will

run at normal speed

not run

run at lower speed

burn due to heat produced in the field winding by .eddy currentsANSWER : : burn due to heat produced in the field winding by .eddy currents

Explanation : 4.In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be

almost negligible

rated full-load current

less than full-load current

more than full-load currentANSWER : : more than full-load current

Explanation : 5.Differentially compound D.C. motors can find applications requiring

high starting torque

low starting torque

variable speed

frequent on-off cyclesANSWER : : low starting torque

Multiple choice questions

Try the following questions to test your knowledge of Chapter 23. Once you have completed the questions, click on the 'Submit Answers for Grading' button at the bottom of this page to get your results.

This activity contains 10 questions.

5.25 V

13 V

25 V

33 V

What is normally used to convert the alternating voltage produced by rotating a coil in a magnetic field, into a direct voltage?

A semiconductor rectifier.

A transformer.

A regulator.

A commutator.

The commutator.

The armature.

The field coils.

The armature windings.

In the diagram of the previous question, what is indicated by the label 'Y'?

The commutator.

The field coils.


The armature.

The slip rings.

The field coils.


The brushes.


The slip rings.

The brushes.

The field coils.

A coil consisting of 50 turns of copper wire has an area of 30 cm2. Determine the peak magnitude of the sinusoidal voltage produced across the terminals of the

coil if it rotates within a magnetic field of 700 mT at a rate of 3000 rpm?

Your Answer:

5.25 V

Correct Answer:

33 V

2. What is normally used to convert the alternating voltage produced by rotating a coil in a magnetic field, into a direct voltage?

Your Answer:A regulator.

Correct Answer:

A commutator.

3. In the following diagram of a DC generator, what is indicated by the label 'X'?

Your Answer:The commutator.

Correct Answer:

The field coils.

4. In the diagram of the previous question, what is indicated by the label 'Y'?

Your Answer:The armature.

Correct Answer:

The commutator.

5. In the following diagram of an alternator, what is indicated by the label 'Z'?

Your Answer:The slip rings.

Correct Answer:


6. In the diagram of the previous question, what is indicated by the label 'W'?

Your Answer:The field coils.

Correct Answer:

The slip rings.

7. A six-pole alternator is required to operate at 50 Hz. What is the required rotation speed?

Your Answer:

1000 rpm

8. What is the rotation speed of a six-pole synchronous motor when used with a single-phase 60 Hz supply?

Your Answer:3,600 rpm

Correct Answer:

10,800 rpm

9. What form of motor is shown here?

Your Answer:DC permanent magnet motor.

Correct Answer:

A squirrel-cage induction motor.

10. Power generation is dominated by DC machines, particularly in high-power applications.

Your Answer:True

Correct Answer:

False

Power generation is dominated by synchronous AC machines.

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