1. In a separately excited dc generator the field is ...docshare04.docshare.tips/files/22467/224676955.pdfIn a separately excited dc generator the field is connected to: a) ... If

1. In a separately excited dc generator the field is connected to: a) In series with an armature

b) Across the armature

c) To the external supply source

d) None of the above

C

2. If we increase the field of the dc generator, its emf output: a) Increases indefinitely

b) Increases till the winding burns

c) Increases till the magnetic saturation takes place

d) 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 slightly

b) Decreases sharply

c) Decreases slightly

d) Increases sharply

C

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

D

6. In a rotating electrical machine the torque produced will be maximum when: a) Torque angle is zero

b) Torque angle is 90 deg

c) Two magnetic fields are aligned with each other d) Field strengths are maximum and the torque angle is zero

B

7. Flashing of field of dc generator means: a) Neutralization of residual magnetism

b) Creation of the residual magnetism by a dc source

c) Increasing the flux density by providing extra ampere-turns in the field


B

8. In an arc welding in order to obtain steady arc one should use: a) DC series generator b) DC shunt generator c) DC differential compound generator d) 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 increases

b) Both back emf and line current falls

c) 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 power b) Power drawn in volt amperes

c) Power drawn in watts

d) Output power at the shaft

D

11. In a dc compound motor field regulator is provided to: a) Control the flux

b) Limit the ampere current c) Demagnetize the field partially


A

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

b) The field terminals

c) The armature terminals only

d) 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


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


Ans:D

[4] The critical resistance of the dc generator is the resistance of

(a) Armature

(b) Field

(c) Load

(d) Brushes


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


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


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


Ans: B

[8] An ideal dc generator has .......... voltage regulation.

(a) Low

(b) Zero

(c) Positive

(d) Negative


Ans: B

[9] Which generator has poorest voltage regulation

(a) Series

(b) Shunt

(c) Compound

(d) High


Ans:A

[10] The voltage regulation of an over compound dc generator is always

(a) Positive

(b) Negative

(c) Zero

(d) High

Ans: B

DC Motors: Part-10 Objective Questions

Solved Objective Questions from DC Machines: [1] Voltage equation of a dc motor is

A. V = Eb + IaRa

B. Eb = V + IaRa

C. V = Eb/IaRa

D. V = Eb + Ia2Ra

E. None of the above

Ans: A

[2] Both Hopkinson's test and Field test

A. Require two electrically coupled series motors

B. Need two similar mechanically coupled motors

C. Use negligible power

D. Are regenerative tests


Ans:B

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

A. Series motor

B. Shunt motor

C. Cumulatively compound motor

D. All of the above


Ans:B

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

A. Field's test

B. Retardation test

C. Hopkinson's test

D. Swinburn's test


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. Lathes

B. Cranes and hoists

C. Shears and punches

D. Machine tools


Ans: B

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

A. Stray loss

B. Copper loss

C. Friction loss

D. Iron loss


Ans: A

[7] In a DC series motor increasing the load current will

A. Decrease the speed

B. Increase the speed

C. Better commutation

D. Increase the back emf


Ans: A

[8] One of the main advantage of the swinburn's test is

A. It is applicable both shunt and compound motors

B. It needs one running test

C. It is very economical and convenient

D. It ignores any charge in iron loss


Ans:C

[9] the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that it

A. Requires full load power

B. Ignores any change in iron loss

C. Needs one motor and one generator

D. Requires two identical shunt machines


Ans: D

[10] The most economical method of finding no losses of a large dc shunt motor is

A. Hopkinson's test

B. Swinburn's test

C. Retardation test

D. Field's test


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 current

ANSWER : : 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. motor

ANSWER : : 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 currents

ANSWER : : 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 current

ANSWER : : 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 cycles

ANSWER : : 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.

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?

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.

In the following diagram of a DC generator, what is indicated by

the label 'X'?

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.

In the following diagram of an alternator, what is indicated by the

label 'Z'?

The slip rings.

The field coils.


The brushes.

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


The slip rings.

The brushes.

The field coils.

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

17 rpm

50 rpm

1000 rpm

3000 rpm

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

60 rpm

180 rpm

3,600 rpm

10,800 rpm

What form of motor is shown here?

A shunt-wound DC motor.

A synchronous AC motor.

A squirrel-cage induction motor.

DC permanent magnet motor.

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

True

False

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:

5.2

5

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: The armature windings.

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.

Documents

1. In a separately excited dc generator the field is ...docshare04.docshare.tips/files/22467/224676955.pdfIn a separately excited dc generator the field is connected to: a) ... If