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Flashcards in Operating Principles Of DC Generators Deck (38):
1

What are generators?

Machines that provide electrical energy

2

What principle do generators work under?

Faradays Law of electric magnetic induction

3

What does "field excitation" refer to?

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

4

What do small generators provide better than permanent magnets?

A much higher and more controllable flux density

5

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

- From a DC external source (separately excited)
- From the armature of the generator itself (self excited)

6

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

- Series
- Parallel (shunt)

7

How are series field windings connected to the armature?

In series

8

Series windings must be of sufficient gauge to carry....

Full armature current

9

Series windings are made of....

Few turns of large gauge wire

10

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

True

11

Shunt field windings are made of...

Large number of turns of small gauge wire

12

What causes the low current in the shunt field?

High coil resistance

13

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

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

14

How can generator voltage be controlled?

A rheostat in the shunt field circuit

15

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

Battery, rectifier circuit, separate DC generator

16

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

Self excited generators

17

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

No voltage

18

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

Residual magnetism of the field core

19

What is Faradays law of electromagnetic induction?

e=Blv

20

What does the voltage induced in a conductor depend on?

Rate at which it is cutting flux

21

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

- number of poles
- flux per pole
- rotational speed of armature
- number of paths
- total number of active conductors on the armature

22

For any given machine, what values are fixed?

Number of poles, number of paths, active conductors

23

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

Flux, speed

24

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

Zero

25

The absence of residual magnetism can be rectified by _________.

flashing the field

26

Before flashing the field, disconnect ___________ from ________

shunt field, low resistance armature

27

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

generated voltage, increase

28

The field current is dependent on ____________ and __________

voltage across field coils, resistance of the field circuit

29

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

limit field current, voltage buildup

30

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

field resistance curve, saturation curve

31

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

rheostat

32

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.

increases, left

33

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

minimum value of resistance

34

Generated voltage is proportional to the product of ______

flux speed

35

Too ______ speed may also result in failure of voltage buildup

low

36

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

armature reaction

37

What can be impaired by armature reaction?

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

38

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

distorts main field established by field poles