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DC Machines > Operating Principles Of DC Generators > Flashcards

Operating Principles Of DC Generators Flashcards

1
Q

What are generators?

A

Machines that provide electrical energy

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2
Q

What principle do generators work under?

A

Faradays Law of electric magnetic induction

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3
Q

What does “field excitation” refer to?

A

Various methods of establishing and controlling the magnetic field so that induction occurs

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4
Q

What do small generators provide better than permanent magnets?

A

A much higher and more controllable flux density

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5
Q

How can the current required to excite the field windings or coils be obtained?

A
  • From a DC external source (separately excited)

- From the armature of the generator itself (self excited)

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6
Q

How can field coils be connected to the armature of a generator?

A
  • Series

- Parallel (shunt)

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7
Q

How are series field windings connected to the armature?

A

In series

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8
Q

Series windings must be of sufficient gauge to carry….

A

Full armature current

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9
Q

Series windings are made of….

A

Few turns of large gauge wire

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10
Q

True or False. Shunt field windings do not need to carry high current.

A

True

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11
Q

Shunt field windings are made of…

A

Large number of turns of small gauge wire

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12
Q

What causes the low current in the shunt field?

A

High coil resistance

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13
Q

The low current in the shunt field is sufficient to provide…

A

A magneto-motive force (ampere turns) capable of establishing the necessary magnetic field

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14
Q

How can generator voltage be controlled?

A

A rheostat in the shunt field circuit

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15
Q

What external sources for field current in separately excited generators derive from?

A

Battery, rectifier circuit, separate DC generator

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16
Q

DC generators that derive the field current from the armature of the machine itself are called ____________.

A

Self excited generators

17
Q

In self excited generators, at the moment the generator starts to rotate, ______________ is available for the field current

A

No voltage

18
Q

The buildup of voltage in a self excited generator requires…..

A

Residual magnetism of the field core

19
Q

What is Faradays law of electromagnetic induction?

A

e=Blv

20
Q

What does the voltage induced in a conductor depend on?

A

Rate at which it is cutting flux

21
Q

The emf generated in the voltage is a function of….

A
  • number of poles
  • flux per pole
  • rotational speed of armature
  • number of paths
  • total number of active conductors on the armature
22
Q

For any given machine, what values are fixed?

A

Number of poles, number of paths, active conductors

23
Q

Generated voltage is directly proportional to the product of _________ and _________

A

Flux, speed

24
Q

At the instant of startup, the speed of the generator is…..

A

Zero

25
Q

The absence of residual magnetism can be rectified by _________.

A

flashing the field

26
Q

Before flashing the field, disconnect ___________ from ________

A

shunt field, low resistance armature

27
Q

Voltage buildup requires that the ________ due to residual magnetism be sufficient to _________ the strength of the magnetic field poles.

A

generated voltage, increase

28
Q

The field current is dependent on ____________ and __________

A

voltage across field coils, resistance of the field circuit

29
Q

A critical value of field resistance exists that will __________ and prevent ________

A

limit field current, voltage buildup

30
Q

For build up to occur, the ___________ curve must lie below the ______________ curve.

A

field resistance curve, saturation curve

31
Q

It is common to control the generated voltage with a _______ in the field circuit.

A

rheostat

32
Q

As resistance in the field circuit is increased, the slope or steepness of the field resistance curve ________ and may move to the ______ of the saturation curve.

A

increases, left

33
Q

To ensure voltage buildup during the start up process, set the field rheostat to their _______________

A

minimum value of resistance

34
Q

Generated voltage is proportional to the product of ______

A

flux speed

35
Q

Too ______ speed may also result in failure of voltage buildup

A

low

36
Q

What is a significant factor that influences the terminal voltage of a DC generator?

A

armature reaction

37
Q

What can be impaired by armature reaction?

A

commutation, by severely affecting the physical conditions of the brushes and commutator

38
Q

What happens if the magnetic field surrounding the armature conductor is incorrect?

A

distorts main field established by field poles

DC Machines (10 decks)

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