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4th Period Electrical Machine Theory SAIT Alberta Electrician > Direct Current Machines > Flashcards

Direct Current Machines Flashcards

1
Q

All dc generators consist of three basic parts:

A
  • a set of permanent magnets
  • an armature winding that is rotated within the field by a mechanical prime mover
  • a commutator and brush set
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2
Q

Describe reluctance:

A

The opposition to flux in a magnetic circuit

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3
Q

The basic unit of measurement of magnetic flux is the:

A

Weber (Wb)

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4
Q

The unit of flux density is:

A

tesla (T)

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5
Q

What items form the core of an electromagnet circuit?

A

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

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6
Q

Name the two sources of flux in a generator under load

A
  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.
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7
Q

Describe armature reaction:

A

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

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8
Q

The interpolar zone is in what area?

A

The area between the poles of the machine

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9
Q

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

A

True

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10
Q

In a motor, the interpolar zone armature reaction causes the neutral plane to shift the same direct as rotor rotation causing arcing. T or F?

A

False, in a motor the neutral plane shifts opposite the direction of the rotor

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11
Q

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

A

One of these:

-rotate the position of the brushes to new neutral plane ( sometimes impractical)
- Compensating Windings (installed in pole shoe in series with armature)
-add interpoles with commutating windings to the generator (neutral plane becomes virtually fixed in the no-load position regardless of the load)

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12
Q

What effect does armature reaction have in the polar zone?

A

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

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13
Q

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

A

True

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14
Q

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?

A

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

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15
Q

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

A

…opposite the direction of armature(rotor) rotation

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16
Q

Three factors affecting the speed of a dc motor are:

A

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

17
Q

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 :

A

Runaway condition

18
Q

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

A

True

19
Q

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

A

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

20
Q

What is the fundamental motor equation?

A

Vapp = cemf + Varm (Ia x Ra)

21
Q

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

A

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

22
Q

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

A

-cumulative-compounded or
-differentially compounded

23
Q

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

A

The long shunt and the short shunt

24
Q

What are the pros of a cumulative-compounded motor?

A
  • considered a stable motor
  • can handle a load that suddenly increases or decreases
25
Q

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

A

The series field opposes the shunt field

26
Q

Which way does the neutral plane shift in a DC motor?

A

Opposite of the direction of the Rotor/Armature rotation.

27
Q

Define Lenz’s Law. Known as…

A
28
Q

Motor action in a generator is…

A
29
Q

Generator action in a motor is…

A
30
Q

What are the effects of Cemf on a DC motor?

A

Ia=Va/Ra

If there’s no Cemf the armature current is super high.

31
Q

In a generator, the interpolar zone armature reaction causes the neutral plane to shift the same direct as rotor rotation causing arcing. T or F?

A

True

32
Q

T or F?
When load is added to a motor, cemf increases

A

False

33
Q

T or F?
When armature voltage of a motor increases, speed increases

A

False

34
Q

True or False?

The Shunt Winding is made of a few turns of fine wire.

A

False.

Many turns of Fine wire.

(Think “Long Shunt”)

4th Period Electrical Machine Theory SAIT Alberta Electrician (6 decks)

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