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DC MACHINES > DC MACHINES > Flashcards

DC MACHINES Flashcards

1
Q

Describe the left hand conductor rule

A

Thumb points to the direction of current flow and fingers show direction of magnetic field

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2
Q

Describe the left hand coil rule

A

Fingers wrap around the coil in the same direction as current flow and thumb points to direction of the north pole

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3
Q

Describe the left hand generator rule

A

1st finger points direction of field flux
Thumb shows direction of conductor
Middle finger indicates direction of current flow

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4
Q

Unlike magnetic poles will?

A

Attract

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5
Q

Like magnetic poles will?

A

Repel

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6
Q

Do flux lines ever cross?

A

No

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7
Q

Flux lines can pass through what type of materials

A

All types

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8
Q

Flux concentrates into highly

A

Permiable paths

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9
Q

T/F flux has elasticity

A

True

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10
Q

Flux loops always pull to the

A

Smallest possible size

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11
Q

Bigger the air gap the ___________ voltage

A

Less

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12
Q

What kind of part is the field yoke
Magnetic
Electric
Mechanical

A

Magnetic

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13
Q

What kind of part is the pole cores
Magnetic
Electric
Mechanical

A

Magnetic

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14
Q

What kind of part are the field windings
Magnetic
Electric
Mechanical

A

Electric

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15
Q

What kind of part are the end Bella
Magnetic
Electric
Mechanical

A

Mechanical

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16
Q

What kind of part is the rotor shaft
Magnetic
Electric
Mechanical

A

Mechanical

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17
Q

What kind of part is the armature core
Magnetic
Electric
Mechanical

A

Magnetic

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18
Q

What kind of part are the armature conductors
Magnetic
Electric
Mechanical

A

Electric

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19
Q

What kind of part is the comutator
Magnetic
Electric
Mechanical

A

Electric

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20
Q

What kind of part are the pigtails
Magnetic
Electric
Mechanical

A

Electric

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21
Q

What kind of part are the brushes and holders
Magnetic
Electric
Mechanical

A

Electrical

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22
Q

Do DC machines generate AC?

A

Generate AC first then converted to DC

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23
Q

How do you change output voltage on a permanent magnet generator

A

Increase RPM

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24
Q

Changes is field current are usually used in varying output _________

A

Voltage

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25
Q

Series generator has _________ turns

A

Very few

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26
Q

Series generator has _______ current

A

High

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27
Q

Series generator has ________ resistance

A

Low

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28
Q

T/F series generator can be excited separately and self excited?

A

False

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29
Q

Shunt generator has ________ turns

A

Many

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30
Q

Series generator has a _______ cm wire size

A

Large

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31
Q

Shunt generator has a _______ cm wire size

A

Small

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32
Q

Shunt generator has _______ current

A

Low

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33
Q

Shunt generators have ________ resistance

A

High

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34
Q

T/F shunt generators can be self excited and separately excited

A

True

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35
Q

In a DC generator line 2 needs to be

A

Negative

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36
Q

Explain flashing the field

A

To apply rated current for 10-30seconsa to re-establish field domain on a dead generator

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37
Q

How do you know a generator is dead

A

By running the prime mover at full speed and not getting desirable voltage output

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38
Q

List the rotating parts of a DC machines

A

Rotor shaft
Armature core
Armature core conductors
Comutator

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39
Q

List the stationary parts of a DC machine

A

Field yoke
Pole cores
Field windings
End Bells
Brushes and holders
Pigtails

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40
Q

What part of a DC generator turns the AC to DC

A

Commutator

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41
Q

What are the four conditions required to be met for a self excited generator to build up rated voltage

A

-resdiual flux
-must be below critical resistance
-correct direction of rotation
-rated rpm

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42
Q

A generator with terminals A1 A2 F2 F1 is called

A

Shunt

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43
Q

A generator with only terminals A1 and A2 is called a

A

Magneto

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44
Q

Which generator can only reverse voltage output polarity by switching the directions of rotation

A

Seperatle excited shunt magneto

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45
Q

Describe armature reaction

A

Distortion of the flux field caused by load current in armature conductors

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46
Q

What is the purpose of a field rheostat

A

To control the field current

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47
Q

What are the different types of brushes used

A

Carbon
Electro-graphite
Graphite
Copper graphite

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48
Q

What is the formula for motor output

A

Force=flux x length x Iarm

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49
Q

HP formula is

A

HP=(ft-lbs x Rpm)÷5252

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50
Q

Describe the right hand motor rule

A

1st finger shows field direction
Middle finger shows direction of current thumb shows direction of thrust

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51
Q

What is the standard rotation of a DC motor

A

Ccw

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52
Q

T/F motors can be self excited

A

False

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53
Q

Generated voltage is dependent on

A

Speed

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54
Q

Slower the motor the ______ Eeff

A

More

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55
Q

To switch direction of a motor you would switch

A

Only the armature terminals

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56
Q

Compound motors have to be wired

A

Cumulative

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57
Q

The shunt motor has the ________ % speedreg

A

Lowest

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58
Q

%speedreg is the

A

Constant speed

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59
Q

What prevents runaway motor

A

Cemf prevents runaway by acting against applied power

60
Q

Field loss causes?

A

Runaway

61
Q

Compound motor has ___________ % speedreg

A

Higher

62
Q

When wiring a compound motor you have to make sure not to?

A

Wire in differential position

63
Q

How do you do a compound motor series polarity test

A

Place jumper parelell to the series coil and if the rpm increases slightly it is wired in the proper polarity

64
Q

What’s a hazard of a series motor

A

Loss of connected load causes runaway

65
Q

Series are a _________ torque motor

A

High

66
Q

Describe the considerations in starting a DC motor

A

Size of motor
Type of controller
Type and size of load
Amount of starting current
Motor and supply voltage

67
Q

What is the correct polarity for an interpole in a motor

A

Same polarity as the main pole

68
Q

A mechanical load increases, the neutral plane shifts, which direction would it move

A

Opposite direction of motor rotation

69
Q

Why does motor current increase as speed goes down

A

Lowering cemf creates greater net voltage

70
Q

What determines how much torque a motor will develop

A

Field flux
Armature flux

71
Q

What does base speed mean

A

The speed with rated current and voltage

72
Q

What is torque

A

A twisting force that acts around an axis

73
Q

How does the operation of the armature differ from a motor to a generator

A

Generator is a source
Motor is a load

74
Q

How do you calculate initial inrush of a motor

A

Source voltage divided by the armature resistance + the series resistance

75
Q

To reduce motor speed field current must be

A

Increased

76
Q

Why does a series motor produce more torque than a shunt

A

Both field fluxes increase at the same time

77
Q

What motor has almost constant speed

A

Shunt

78
Q

What hazard exists with a differential compound motor

A

Accidental reversal under heavy load

79
Q

What protection against runaway motors are used in a shunt

A

Field loss relay

80
Q

How is speed of a series traction motor achieved

A

Voltage controller

81
Q

Where is a field loss relay connected

A

In series with the shunt field

82
Q

Can a compound motor drive loads connected by belts

A

Yes

83
Q

What happens to a traction motor if the field becomes open and what kind of motor is this

A

Stops
Series

84
Q

Can the speed of a compound motor be changed by increased the rheostat resistance

A

Yes

85
Q

How is rotation affected if the line terminals to a shunt motor are switching

A

Rotation remains unchanged

86
Q

Is field loss protection required for a compound motor

A

Yes

87
Q

DC motors can be line started if of what size

A

2hp or less

88
Q

To slightly reduce motor speed field current must be increased or decreased

A

Increased

89
Q

How is speed affected by increasing field resistance
Increased or decreased

A

Increased

90
Q

How are the armature windings physically connected to the commutator segments

A

Soldering

91
Q

What holds the armature windings in the arm core slots

A

Slot wedge

92
Q

What property of magnetic lines is most important

A

Elasticity

93
Q

Why are brushes placed in the neutral plane

A

To prevent arcing

94
Q

Why are pole cores and arm core assembled with insulated laminations of Silica steel

A

To prevent heat loss due to Eddy currents and mag loss

95
Q

What is the mathematical relationship in the reduced heat loss from a solid laminated core

A

(Layers squared)÷power

96
Q

Explain the characteristics of frog leg, lap and wave

A

Frog leg=med current med volts
Wave=low current high voltage
Lap=high current low voltage

97
Q

What kind of brush is used in lower speed, lower current density machines

A

Carbon brushes

98
Q

What range of brush pressure is usually used

A

1-2psi

99
Q

What is the difference between interpole and compensation winding

A

Location

100
Q

As electrical load increases in a generator the neutral plane will shift in what direction

A

Towards rotation

101
Q

Why does output voltage of a self excited generator decrease with an increase in load

A

Arm current causes voltage drop internally

102
Q

What effect is produced by rotating a generator to slowly

A

The output voltage may be too low or may not build at all

103
Q

How does a generator place load on a prime mover

A

Counter torque

104
Q

What generator is the most convenient for changing the output voltage polarity

A

Seperatle excited shunt

105
Q

What is a commutating pole

A

Interpole

106
Q

Difference between I²R losses, rotational loss and mechanical loss

A

I²R=power dissipated by copper of electrical circuits
Rotational loss= combination of magnetic loss and mechanical loss
Mechanical loss is the heat dissipated by wind and bearing friction

107
Q

For a generator the short or long connection is preferred

A

Short

108
Q

Explain the relationship of Enoload and Efullload for flat, under and over compound generators

A

Flat-Enoload=Efullload

Under-Enoload is the greater than Efullload

Over-Enoload is less than Efullload

109
Q

Describe the physical difference between a field rheo and a diverter rheo

A

Field-series with shunt field

Diverter-parellel with series field

110
Q

How do the ratings compare for field rheostat and diverter rheostat

A

Field- high R low P

Diverter- low R high P

111
Q

What is the importance of residual flux

A

Allows voltage to build up in self excited generator

112
Q

Will a series generator with no connected load develop any voltage

A

Yes

113
Q

What does the term critical resistance mean with respect to generators

A

Level of field resistance to high to allow field current to develop

114
Q

Does armature reaction affect brushes and commutator

A

Yes

115
Q

Three measure used to correct for armature reaction

A

Inter pole
Compensating winding
Adjust brushes

116
Q

What DC machine is totally enclosed

A

Motor

117
Q

What is the formula to determine force of a motor

A

Force=flux x length x Iarm

118
Q

As a motor increase speed what happens to its internal voltage

A

Increases

119
Q

During the operation of a motor when is the current the largest

A

When it’s stationary

120
Q

Express formula for speed control in a motor

A

Nrpm=applied voltage/field flux

121
Q

Armature reaction causes what effect in a DC motor

A

Increased speed

122
Q

Which of the following generator losses are influenced mostly by connected load
Friction loss
Core loss
Armature copper loss
Field copper loss

A

Armature copper loss

123
Q

The motor effect causes loading on what in a generator

A

Loading of the prime mover

124
Q

An iron core strengthens an electromagnet because it has ________ reluctance than air

A

Less

125
Q

In order to supply a load with a long distance from the source the best generator would be a

A

Over compound generator

126
Q

When should a series DC motor never be used

A

When a belt drive is used to connect to load

127
Q

When the mechanical load on a series DC motor increases the armature current increases because

A

The speed decreased

128
Q

During acceleration from standstill for DC motors resistance is inserted where

A

In series with armature

129
Q

What can be adjusted to increase output voltage in a permanent magnet generator

A

Speed

130
Q

What can be adjusted to change output voltage on a seperatly excited generator

A

Field rheo to change the flux field
And speed

131
Q

What can be adjusted to change out put voltage of a self excited generator

A

Field threo for flux
Speed

132
Q

What is the most common and most accessible way to change out put voltage

A

Field rheo to adjust flux

133
Q

What is the basic operation of a series generator and what is a real life example

A

Provide steady current output

Arc welding

134
Q

Can you use a manual starter for a large motor

A

No 2hp or less only

135
Q

What HP is considered a large motor

A

Anything over 2hp

136
Q

How much HP is considered a small motor

A

2hp or less

137
Q

What kind of starters can be used for a small motor

A

Electronic manual and magnetic

138
Q

What kind of started can be used for large motors

A

Electronic
Magnetic

139
Q

Advantage of friction braking

A

Failsafe
Provides braking when stopped

140
Q

Disadvantage of friction braking

A

Expensive
Bulky
High maintenance

141
Q

DisAdvantage of Eddy current/magnetic brake

A

Additional equipment needed
Braking effect is reduced with decrease in speed
Has to be used with friction brakes
No failsafe-loss of voltage =loss of brake

142
Q

Advantages of Eddy current/magnetic brakes

A

Opposing torque made by magnetic field means no friction parts
Initial braking is very strong

143
Q

Advantages of dynamic braking

A

Very little additional equipment
Low maintenance

144
Q

Disadvantage of dynamic braking

A

Still need friction brake
Not fail safe

145
Q

Explain the difference between dynamic and regenerative braking

A

Dynamic dissipates energy and regenerative restores its losses

DC MACHINES (1 decks)

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