Dc Generators Year 2

Front 1

What is the main difference in the construction of a DC Generator and DC Motor?

Back 1

The type of surrounding enclosure

Front 2

The rotating part of a DC Generator is called the....

Back 2

Armature

Front 3

3 basic types of armature windings?

Back 3

Frog Leg

Lap

Wave

Front 4

What is the electrical function of the commutator?

Back 4

To maintain the current in 1 direction to the external circuit

Front 5

Common materials used in making brushes?

Back 5

Carbon, Electro-Graphite, Graphite, Copper Graphite

Front 6

Suitable brush pressure is usually between _____ and _____ psi

Back 6

1 and 2

Front 7

Describe the function of field poles

Back 7

They provide the magnetic field in which the armature rotates

Front 8

Grease nipples in the end shields indicate that the machine contains _______ bearings

Back 8

Open Bearings

Front 9

Small generators that use permanent magnets for field excitation are called...

Back 9

Magnetos

Front 10

The difference between separately excited and self excited fields are...

Back 10

Separately excited gets field current from an external source ie battery whereas Self Excited gets FC from the armature itself

Front 11

2 ways in which field coils are connected in self-excited generators?

Back 11

Series and Parallel(Shunt)

Front 12

Which field windings are made of a low number of turns of large gauge wire?

Back 12

Series field windings

Front 13

What device in the shunt field circuit is used to control generator voltage?

Back 13

Rheostat

Front 14

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

Back 14

Residual Magnetism

Front 15

Voltage buildup occurs until the generator reaches its....

Back 15

No load Voltage Eg

Front 16

Appropriate residual magnetism can be established on the field poles by a process commonly called....

Back 16

Flashing the Field

Front 17

The value of field resistance that will limit field current to a value insufficient for voltage buildup is called....

Back 17

Critical field resistance

Front 18

A generator characteristic curve shows the effect of ______ on ________

Back 18

Connected Load, Terminal Voltage

Front 19

Describe the conditions that must be satisfied to obtain the desired performance when connecting a generator

Back 19

Field coils must be properly connected with respect to the armature and each other

Front 20

List the letters that are used in the NEMA system to identify the armature, shunt field and series field

Back 20

Armature = A

Series = S

Shunt = F

Front 21

The terminal voltage of a shunt generator is controlled by....

Back 21

A rheostat in the field circuit

Front 22

What determines the amount of current that flows through an armature?

Back 22

The impedance of the connected load

Front 23

Factors that cause a reduction in terminal voltage with increasing load?

Back 23

IR drop of the armature,

Armature reaction

Loss of field current due to a reduction in generated voltage

Front 24

Is the series generator a practical machine?

Back 24

No (main use is an arc welder)

Front 25

Why would it not be practical to control the output of a series generator with a field rheostat?

Back 25

Because the rheostat would have to carry the high armature current, resulting in high wattage losses

Front 26

Why is the no-load voltage of a series generator extremely low?

Back 26

In the absence of a connected load (open circuit) no current flows through series field coils

Front 27

In a series generator, why does the terminal voltage rise with increasing load?

Back 27

As armature current increases, strength of the series field increases

Front 28

A compound generator contains both _____ and ______ fields

Back 28

Series, Shunt

Front 29

The condition under which the series field adds to the shunt field is called

Back 29

Cumulative Compound

Front 30

The condition under which the magnetic flux of the series and shunt fields oppose each other is called....

Back 30

Differential Compound

Front 31

Describe the difference between long shunt and short shunt connections

Back 31

Long shunt: shunt field is connected across the line

Short shunt: shunt field is connected across the armature

Front 32

A compound generator that has a relatively constant terminal voltage characteristic is called....

Back 32

Flat Compound

Front 33

Line drop can be compensated for by using a generator that is....

Back 33

Over-Compound

Front 34

With respect to long versus short shunt connections, what is the general rule?

Back 34

Connect the shunt field across the source of emf

Front 35

What is the effect on generated voltage if the speed of the prime mover is decreased by one third?

Back 35

Decreases by one third

Front 36

Define Armature Reaction

Back 36

Armature reaction is a distortion of the main field flux caused by current flowing in armature conductors

Front 37

List 2 effects of armature reaction on the operation of a generator

Back 37

-A reduction in terminal voltage with increasing load

-Impaired Commutation

Front 38

Interpole coils consist of a....

Back 38

Small number of turns of Large Gauge wire

Front 39

Interpole coils are connected...

Back 39

In series with the Armature

Front 40

The effects of armature reaction can also be minimized by placing windings called ______ in the main pole faces

Back 40

Compensating windings

Front 41

List 3 types of power loss in a generator

Back 41

Mechanical loss

Electrical Loss

Core Loss

Front 42

Rotational losses include...

Back 42

Bearing friction, movement of air

Front 43

How is the effect of eddy currents minimized in the construction of DC machines?

Back 43

Laminating core materials

Front 44

Heat produced by current flowing through the armature and field coils is called ____

Back 44

Copper Losses

Front 45

Define efficiency

Back 45

Ratio of Power out to power in

Front 46

DC machines use ____ or ____ bearings

Back 46

Sleeve or Ball Bearings