Operating Principles Of DC Generators

Question 1

What are generators?

Answer 1

Machines that provide electrical energy

Question 2

What principle do generators work under?

Answer 2

Faradays Law of electric magnetic induction

Question 3

What does “field excitation” refer to?

Answer 3

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

Question 4

What do small generators provide better than permanent magnets?

Answer 4

A much higher and more controllable flux density

Question 5

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

Answer 5

• From a DC external source (separately excited)

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

Question 6

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

Answer 6

• Series

- Parallel (shunt)

Question 7

How are series field windings connected to the armature?

Answer 7

In series

Question 8

Series windings must be of sufficient gauge to carry….

Answer 8

Full armature current

Question 9

Series windings are made of….

Answer 9

Few turns of large gauge wire

Question 10

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

Answer 10

True

Question 11

Shunt field windings are made of…

Answer 11

Large number of turns of small gauge wire

Question 12

What causes the low current in the shunt field?

Answer 12

High coil resistance

Question 13

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

Answer 13

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

Question 14

How can generator voltage be controlled?

Answer 14

A rheostat in the shunt field circuit

Question 15

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

Answer 15

Battery, rectifier circuit, separate DC generator

Question 16

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

Answer 16

Self excited generators

Question 17

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

Answer 17

No voltage

Question 18

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

Answer 18

Residual magnetism of the field core

Question 19

What is Faradays law of electromagnetic induction?

Answer 19

e=Blv

Question 20

What does the voltage induced in a conductor depend on?

Answer 20

Rate at which it is cutting flux

Question 21

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

Answer 21

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

Question 22

For any given machine, what values are fixed?

Answer 22

Number of poles, number of paths, active conductors

Question 23

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

Answer 23

Flux, speed

Question 24

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

Answer 24

Zero

Question 25

The absence of residual magnetism can be rectified by _________.

Answer 25

flashing the field

Question 26

Before flashing the field, disconnect ___________ from ________

Answer 26

shunt field, low resistance armature

Question 27

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

Answer 27

generated voltage, increase

Question 28

The field current is dependent on ____________ and __________

Answer 28

voltage across field coils, resistance of the field circuit

Question 29

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

Answer 29

limit field current, voltage buildup

Question 30

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

Answer 30

field resistance curve, saturation curve

Question 31

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

Answer 31

rheostat

Question 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.

Answer 32

increases, left

Question 33

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

Answer 33

minimum value of resistance

Question 34

Generated voltage is proportional to the product of ______

Answer 34

flux speed

Question 35

Too ______ speed may also result in failure of voltage buildup

Answer 35

low

Question 36

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

Answer 36

armature reaction

Question 37

What can be impaired by armature reaction?

Answer 37

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

Question 38

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

Answer 38

distorts main field established by field poles