Direct Current Machines

Question 1

All dc generators consist of three basic parts:

Answer 1

• a set of permanent magnets
• an armature winding that is rotated within the field by a mechanical prime mover
• a commutator and brush set

Question 2

Describe reluctance:

Answer 2

The opposition to flux in a magnetic circuit

Question 3

The basic unit of measurement of magnetic flux is the:

Answer 3

Weber (Wb)

Question 4

The unit of flux density is:

Answer 4

tesla (T)

Question 5

What items form the core of an electromagnet circuit?

Answer 5

-The pole pieces (pole core and pole shoe)
- The yoke (frame)
- The armature core (rotor)
- the air gap

Question 6

Whenever a load is connected to the output of the operating generator, there is current in the armature windings of the generator. Current in the armature produces magnetic flux in the armature. This means that in a generator under load, there are two sources of flux:

Answer 6

1. the main field flux produced by the field coils on the poles and
2. the armature flux produced by the current in the armature windings.

Question 7

Describe armature reaction:

Answer 7

Distortion of the flux pattern in the machine caused by the cross magnetizing effect of the armature-current flux

Question 8

The interpolar zone is in what area?

Answer 8

The area between the poles of the machine

Question 9

The polar zone is the area directly under the pole faces of the machine. T or F?

Answer 9

True

Question 10

In the interpolar zone armature reaction causes the neutral plan to rotate the same direct as rotor rotation causing arcing. T or F?

Answer 10

True

Question 11

Name a method to reduce armature reaction in the interpolar zone:

Answer 11

One of these:

-rotate the position of the brushes to new neutral plane ( sometimes impractical)
-pole face design of the main poles (do not eliminate)
-add interpoles with commutating windings to the generator (neutral plane becomes virtually fixed in the no-load position regardless of the load)

Question 12

What effect does armature reaction have in the polar zone?

Answer 12

Distortion of the flux pattern results in a distortion of the waveform of the generated voltage, and a reduction of generated voltage.

Question 13

A more sophisticated and expensive solution to fixing armature reaction in the polar zone is to use compensating windings. T or F?

Answer 13

True

Question 14

The use of commutating windings and compensating windings in a dc generator are an impractical method to eliminate effects of armature reaction. T or F?

Answer 14

False. The usage of both windings virtually eliminate the effects of armature reaction even under varying load conditions.

Question 15

The distortion of the main field causes the neutral plane to shift. In the case of a dc MOTOR, the neutral plane is shitfed in what direction?

Answer 15

A direction opposite to the direction of rotation

Question 16

Three factors affecting the speed of a dc motor are:

Answer 16

-the amount of load on the motor
- the intensity of the magnetic field
- the amount of armature voltage

Question 17

If a dc compound motor is operating with a light load and the shunt field becomes disconnected, the motor increases to high levels. This is known as :

Answer 17

Runaway condition

Question 18

The shunt motor is considered a constant speed motor. T or F?

Answer 18

True

Question 19

When a series motor is started there is high opposition to current. T or F?

Answer 19

False, there is very little opposition to current (because at start there is no cemf)

Question 20

What is the fundamental motor equation?

Answer 20

Va = cemf + (Iarm + Rarm)

Question 21

There series motor can develop as much as 250% starting torque as compared to full-load torque. T or F?

Answer 21

False. It can develop as much as 500% starting torque as compared to full-load torque.

Question 22

The two ways that field windings of a compound motor may be connected are:

Answer 22

-cumulative-compounded or
-differentially compounded

Question 23

What are the two ways the shunt field winding can be connected in a compound dc dynamo?

Answer 23

The long shunt and the short shunt

Question 24

What are the pros of a cumulative-compounded motor?

Answer 24

• considered a stable motor
• can handle a load that suddenly increases or decreases

Question 25

How does the series field affect the shunt field in a differential compounded motor?

Answer 25

The series field opposes the shunt field