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Electrical Level 3 and 4 > DC Machines > Flashcards

DC Machines Flashcards

1
Q

A DC machine that is rotated by a prime mover is

A

A Generator

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2
Q

A DC machine that is connected to an electrical supply is

A

A motor

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3
Q

For a DC generator, to generate voltage there must be

A

Flux, a conductor and relative motion between them (E=Blv)

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4
Q

What produces the flux on a generator?

A

Stationary windings when excited

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5
Q

Stationary windings when excited produce what for a generator?

A

The flux (B)

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6
Q

The windings on the rotating part (armature) provide what for a generator?

A

The length of conductors (L)

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7
Q

The length of conductors for a generator are provided by?

A

The windings on the armature

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8
Q

The prime mover for a generator provides?

A

The velocity of relative motion (V)

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9
Q

For a generator what provides the velocity of relative motion?

A

The prime mover

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10
Q

For a generator, what is the armature used for?

A

It is where AC is induced into

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11
Q

Where is AC induced into for a generator?

A

Armature

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12
Q

Often in DC machine, the armature is referred to as

A

The entire rotating assembly

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13
Q

List the components of the armature

A 
Shaft
Laminated stampings
Coil windings (magnetic wire)
Cell insulation (cellulose paper and wedges)
Coil banding
Bearings
Fan
Commutator
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14
Q

What are the laminated stampings for on the armature?

A

Reduce eddy currents

Cheap Construction

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15
Q

What is the commutator used for on the armature?

A

Rectifies the induced AC to DC

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16
Q

What is the commutator constructed of?

A

Insulated copper segments(bars)
Coils are connected to the segments
Mica (or synthetic) insulation between segments

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17
Q

(Generator) what is the purpose of the brushes?

A

To provide electrical contact to the rotating commutator

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18
Q

Brush construction (generator)

A
  • usually rectangular (may be cylindrical)
  • often have a “brush shunt” which is a flexible braided copper lead embedded in the brush to shunt current away from the brush spring
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19
Q

A shunt may also be called

A

Pigtail

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20
Q

What are the 4 main materials modern brushes are made from

A
  1. Carbon
  2. Graphite (softer than carbon and more slippery)
  3. Electrographite - graphatized carbon
  4. Copper or graphite and copper
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21
Q

What factors determine brush selection?

A 
Voltage
Speed
Current
Cost
Duty Cycle
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22
Q

What brush material has a negative temp coefficient?

A

Carbon (semi-conductor) (Resistance goes down when pressure goes up)

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23
Q

What are the 2 types of field windings for generators?

A

Shunt Windings

Series Windings

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24
Q

Explain Shunt windings

A

Have many turns of small wire connected in parallel to the armature
Have values of 10s of ohms (ex. 40, 60, 100 etc.)

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25
Q

Explain Series windings?

A

Have few turns of large wire connected in series with the armature
Have very low values of ohms (ex. 0.08, 0.12 etc)

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26
Q

Why are windings preferred over permanent magnets?

A

The current through the windings can be varied which controls flux and output voltage

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27
Q

What are other windings that can be found on DC machines?

A

Interpoles (commutating windings)

Compensating Windings

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28
Q

Wat does adding more coils do for the windings?

A

Makes for a smoother and greater voltage

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29
Q

What is Armature Reaction?

A

Distortion of the main field flux caused by armature current

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30
Q

What negative effects does armature reaction have on the machine?

A

Sparking of the brushes

Reduction of overall flux in the poles

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31
Q

What is the purpose of interpoles in a generator?

A

to achieve good commutation (no sparks) by inducing a voltage into the coil undergoing commutation that is equal and opposite to the reactance voltage.

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32
Q

Interpole polarity is a …. ?

A

sign of things to come

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33
Q

what does the polarity of the interpole do?

A

Helps to strengthen the leading tip of the main field and reduce the effect of armature reaction

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34
Q

What can be used to counteract field distortion, excessive sparking and perhaps a fire ball around the commutator?

A

Compensating windings can be embedded in the pole faces and connected in series with the armature

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35
Q

What is a compound field?

A

When both shunt and field windings are used on each pole

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36
Q

When a compound field is connected so that their fluxes aid each other

A

Cummulative Compound

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37
Q

When a compound field is connected so that their fluxes oppose eachother?

A

Differential Compound

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38
Q

What are the conditions for build of voltage?

A
  1. Primer mover is turning
  2. Residual magnetism must be present
  3. Complete circuit between armature and field
  4. Resistance in the circuit is not higher than the “critical resistance”
  5. Proper connections are made for the direction of rotation.*
  6. The generator is not started with a big load (low R) connected.
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39
Q

If the prime mover direction is reversed and EMF is induced into the armature with the opposite polarity, this voltage will push a current through the field such that it will neutralize the residual flux and output voltage will be 0, what can this be corrected?

A

Reverse the armature leads and flash the field.

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40
Q

What are the power stages in a generator?

A

Input Power (HP x 746)
Losses (windage, friction) (hysteresis, eddy currents (iron losses))
Generated Power (Egen x Ia)
Losses (I2R armature, I2R fields (copper losses))
Output Power (Vterminal x Iload)

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41
Q

Input Power =

A

Output power + losses

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42
Q

Efficiency =

A

Power out divided by power in

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43
Q

Define percent voltage regulation

A

How good the generator maintains terminal voltage from no load to full load

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44
Q

What is the purpose of a diverter rheostat in a long shunt compound generator?

A
  1. Adjust current though series field
  2. High wattage
  3. Low resistance
  4. Adjusts the level of compounding.
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45
Q

What happens to the prime mover of a generator when electrical load is increased and why?

A

Tends to slow down because as the load current increases, the opposing flux increase and the primer mover has to work harder (motor effect)

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46
Q

What are the forces at work in a motor?

A

Flux and current

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47
Q

Explain the “right hand rule” for motors

A

Thumb - motion
Index Finger - Field/flux
Middle Finger - Current

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48
Q

Another name for “work” is?

A

Torque

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49
Q

Torque =

A

Force x distance in Nm (newton meters) or Ftlbs (pound feet)

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50
Q

The amount of force on the armature conductors is dependent on?

A

the strengths of the main field flux and the armature flux combined

Torque = Flux main field x Flux armature

Torque = Flux field x armature current

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51
Q

Horsepower is

A

the rate of doing work

HP = NT divided by 5252

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52
Q

What controls the amount of armature current in a motor?

A

Counter EMF or CEMF

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53
Q

What is CEMF?

A

an induced EMF that opposes the applied voltage

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54
Q

What can happen if the field is weakened or lost in an unloaded shunt motor

A

it can runaway above base speed (very hazardous)

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55
Q

When may a series motor be used?

A

For traction purposes (trams, trolleys, ski lifts, electric drills)
High Torque applications

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56
Q

For series motors torque is proportional to

A

Ia squared

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57
Q

Series motors have very poor

A

speed regulation

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58
Q

Most compound motors have protection to prevent?

A

Runaway if the shunt field is opened

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59
Q

What are the power stages in a motor?

A 
Input Power - Vt x IL
Losses - I2R fields, I2R armature (Cu losses), Hysteresis, eddy currents (Fe losses)
Mechanical Power - Ecemf x Ia
Losses - windage, friction
Output Power - HP x 746
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60
Q

NEMA Standards - Rotation is viewed from?

A

The opposite drive end (lead end)

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61
Q

NEMA Standards - Generator standard rotation

A

Clockwise (CW)

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62
Q

NEMA Standards - Motor standard rotation

A

Counter Clockwise (CCW)

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63
Q

NEMA Standards - Standard Compounding

A

Cummulative

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64
Q

NEMA Standards - Positive Line Lead

A

L1

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65
Q

NEMA Standards - Current flow is from subscript

A

2 to 1

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66
Q

NEMA Standards - Rotational reversal

A

Interchange A1 and A2 leads

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67
Q

NEMA Standards - Differential compounding

A

Interchange S1 and S2

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68
Q

NEMA Standards - Shunt field current should always be

A

F2 to F1

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69
Q

Define magnetic field

A

the distribution of flux lines around a magnet

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70
Q

define flux density

A

quantity of flux lines per unit area (Tesla)

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71
Q

Define magnetic poles

A

the distinct region where flux lines concentrate

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72
Q

Ferro-magnetic materials

A

`those substances that exhibit the same magnetic properties as iron

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73
Q

Electromagnetism

A

the establishment of flux lines (magnetic field) resulting from the current flowing through the conductor or a coil

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74
Q

Magnetomotive Force

A

the strength which produces electromagnetism or ampere turns

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75
Q

Reluctance

A

the opposition to establishment of magnetic flux lines

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76
Q

Permeability

A

the ease which a material will become magnetized

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77
Q

BH Curve

A

a graph which illustrates the relationship between flux density and magnetizing force for electrical materials

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78
Q

Saturation

A

that max level of flux density where an increase in magnetizing force does not produce a significant increase in magnetic flux density

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79
Q

Residual Magnetism

A

the permanent magnetism retained by an electro magnet after the removal of a magnetizing force

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80
Q

Coercive Force

A

the magnetizing force required to eliminate residual (permanent magnetism)

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81
Q

List three fundamental laws of magnetism

A
  1. Magnetic line form closed loops
  2. Magnetic lines never cross each other
  3. Magnetic line stay short like elastic bands
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82
Q

To prevent permanent magnets from losing their magnetism, some precautions should be taken when storing them, what are they?

A
  1. Avoid mechanical shock
  2. High temperature
  3. AC fields
    4, Use “keepers”
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83
Q

What is the “left hand rule” for electromagnetism for conductors and coils?

A

Conductors - Thumb points in direction of electron flow and fingers indicate field direction
Coils - Fingers indicate direction of electron flow and thumb points in field direction

84
Q

List three factors that affect the field strength of an electromagnet?

A
  1. Magnitude of current flow
  2. Number of coil turns
  3. Reluctance of core material
85
Q

When is an electromagnet said to be saturated?

A

When increasing magnetizing current does not appreciably strengthen field flux

86
Q

What three conditions must be satisfied to induce any voltage?

A
  1. Magnetic field
  2. Length of conductor
  3. Relative motion of conductor

E=Blv

87
Q

“left hand rule” for generators

A

Thumb - conductor motion
Index - field direction
Middle - current flow

88
Q

Define the term “neutral plane”

A

Conductor position relative to the magnetic field where no voltage is induced within the conductor

89
Q

What is a generic term for a generator?

A

Dynamo

90
Q

What is the difference between a “separately-excited” and “self excited” DC generator?

A

Separately excited - field circuit is energized from external DC source
Self excited - induced armature voltage also energizes the field winding

91
Q

Why is it necessary for self-excited DC generators to have “residual magnetism”

A

There will be no induced voltage to provide field strength and allow voltage build up for the machine

92
Q

What is meant by the expression “flashing the field”

A

Application of external DC to the field circuit in order to establish residual magnetism within field iron

93
Q

Generator Construction - End Shields

A

House bearings and support shaft, mechanical protection

94
Q

Generator Construction - Frame

A

To house field structure and complete magnetic circuit

95
Q

Generator Construction - Field Coil

A

Provides excitation

96
Q

Generator Construction - Shaft

A

Allows rotation of armature

97
Q

Generator Construction - Brush Holder

A

Supports and protects brush

98
Q

Generator Construction - Brush

A

Provides armature connection to the load

99
Q

Generator Construction - Field Pole (laminated)

A

Magnetic circuit for armature conductors

100
Q

Generator Construction - Commutator

A

Mechanical rectifier (AC to DC)

101
Q

Generator Construction - Armature Core

A

Magnetic circuit for armature conductors

102
Q

Generator Construction - Armature Winding

A

Active conductor length

103
Q

State one advantage of “self excited” versus “separately-excited” DC generators.

A

Self - no need for separate DC supply, cheaper

Separate - good voltage regulation, output for either direction of rotation, easier start up

104
Q

Why is a “field rheostat” employed with a shunt generator?

A

Permits varying of field current to control terminal output voltage

105
Q

What does a magnetization curve for a DC generator illustrate?

A

The build up of terminal voltage with field current

106
Q

How does the “no load” terminal voltage of a DC shunt generator compare to its “full load” value?

A

Full load voltage is lower than no load due to armature IR drop, armature reaction, resultant field weakening

107
Q

What is the simplest method of controlling output voltage of a shunt generator?

A

Varying the field excitation (field rheostat)

108
Q

Why is it necessary to have the proper direction of rotation for a DC generator?

A

Generated armature voltage, for a given direction of rotation must excite field to aid residual magnetism of field poles

109
Q

Why is a series DC generator very seldom used?

A

Since the field current is dependent on load current, terminal voltage varies with load, resulting in poor voltage regulation

110
Q

With no load connected, what is the terminal output voltage expected from a series DC generator?

A

very low due to residual magnetism

111
Q

What would distinguish if a compound generator was “cummulative” or “differential” as a load was applied?

A

Cummulative terminal voltage should rise (or constant) with load increase
Differential terminal voltage would drop with load increase

112
Q

Explain “over compound” in compound generators

A

Full load voltage is higher than no load voltage

113
Q

Explain “flat compound” in compound generators

A

Full load voltage is the same as no load

114
Q

Explain “under compound” in compound generators

A

Full load voltage is lower than no load voltage

115
Q

what is meant by the term voltage regulation?

A

compares change in terminal voltage between no load and full load

116
Q

How are the affects of armature reaction minimized in DC generators?

A

Interpoles (commutating poles), Compensating windings, Brush shifting

117
Q

What is the general “rule for interpole polarity” in DC generators?

A

is the same as the main field pole AHEAD in the direction of rotation

118
Q

What fundamental principle of magnetism develops rotation for a DC motor?

A

Law of repulsion

119
Q

What function does the commutator serve in a DC motor?

A

switches direction of current through the armature conductor (when under the opposite pole face) to maintain torque in the same direction of rotation

120
Q

What are the two basic factors that determine the amount of torque that a DC motor will develop?

A

Main field strength
Armature current

T is proportionate to FLUX x Armature current

121
Q

Why does increasing the mechanical load on a motors shaft cause an increase in armature current?

A

Armature slows down, less CEMF is developed, armature current increase to develop more torque to keep load turning at a slower r/min

122
Q

What is meant by the term “base speed”?

A

rated motor speed with rated field and armature currents (full load)

123
Q

Another name for a shunt motor?

A

constant speed motor

124
Q

Why is a shunt motor typically referred to as a “constant speed” motor?

A

Very little difference in RPM between full load and no load (typically 5% change)

125
Q

State what happens to the speed of a series motor as load is increase up to its rate value?

A

Motor speed drops drastically with increase in mechanical load

126
Q

How could speed control of a series DC motor be accomplished?

A

Generally the amount of mechanical load control rpm, sometimes a diverter rheostat

127
Q

Compare series and shunt DC motors for speed regulation and torque capability.

A

Shunt - very good speed regulation, moderate torque

Series - very poor speed regulation, extremely good torque

128
Q

List two applications where DC series motors are commonly employed?

A

Hoists, cranes, elevators, ski lifts, drills

129
Q

What hazards are associated with differential compounded DC motors?

A

May suddenly stall on increasing load and reverse direction (suicide machine)

130
Q

When interpoles are used in a DC motor for commutation, what is the rule for their connected polarity?

A

moving in the direction of rotation, the interpoles have the same polarity as the main poles behind them

131
Q

List three possible reasons for excessive brush sparking on DC machine?

A

Excessive load
Wrong interpole polarity
Brush mechanical troubles

132
Q

Describe the appearance generally desirable on the commutator surface?

A

Chocolate brown

133
Q

Why should emery cloth never be used to clean a commutator surface, or seat brushes?

A

Contains conductive particles

134
Q

What should never be used to clean a commutator surface or seat brushes?

A

Emery cloth

135
Q

What is meant by the term “undercutting” a commutator?

A

Cutting mica insulation between commutator bars to depth below the commutator surface

136
Q

List two fundamental requirements of a DC motor starter?

A

Limit starting current
Provide running overload protection
Provide smooth acceleration

137
Q

Why is the starting current for a DC motor so much higher than its full load value?

A

No CEMF, only armature resistance to limit in rush current

138
Q

Should a shunt motor be started wit reduced or full field excitation?

A

Full field, to develop full starting torque

139
Q

What would be the main disadvantage of a manual type DC motor starter?

A

No provision for remote operation, operator error

140
Q

State the function of a motor starter?

A

to start and accelerate a motor

141
Q

Why is it essential that switch and relay contacts be DC-rated?

A

B/C of the arcing that occurs when the current is interupted

142
Q

Why does in rush current persist longer in large DC motors than in small Dc motors?

A

The inertia of the armature causes a slower rate of acceleration and a longer interval

143
Q

Describe the basic construction of a drum controller.

A

Consists of a series of copper contacts mounted on a cylinder that is insulated from a central shaft

144
Q

Name the two basic configurations of manual faceplate starters?

A

Three terminal

Four Terminal

145
Q

Which faceplate starter provides no-field protection?

A

Three terminal

146
Q

What faceplate starter allows speed control?

A

Four terminal

147
Q

Name the six main parts of a DC contactor.

A 
Electromagnetic coil
Armature
Moveable contacts
Fixed contacts
Spring assembly
Blowout coil
148
Q

Why is the holding current of a DC contactor much less then the pull in current?

A

For the pull in current, the instant the coil is first energized there is a large air gap that requires large current, once pulled in current decrease due to a decrease in reluctance

149
Q

Give two methods that are used to prevent overheating of the coil of a DC contactor?

A

Inserting a resistor in series with the coil

Equipping the contactor with dual coils

150
Q

The operation of a “blowout coil” is based upon what principle?

A

a principle of magnetism called the motor effect

151
Q

What device is at the heart of the solid-state DC controller?

A

Silicon-controlled rectifier (SCR)

152
Q

List three unwanted conditions that a large inrush current could cause.

A

Overheating of armature conductors
Excessive starting torque
IR volt drop in supply conductors

153
Q

Define current-limit acceleration

A

Insertion and removal of external resistance to the armature circuit during the acceleration of motor from stand still to normal operating speed

154
Q

As CEMF increases, armature current …. ?

A

decreases

155
Q

In a CEMF controller, where is the coil of the accelerating relay connected?

A

Across armature leads A1 and A2

156
Q

What control device is used to provide definite-time acceleration?

A

Timing relays

157
Q

What type of timer operates by forcing a fluid through the orifice of a piston-cylinder assembly?

A

Dashpot

158
Q

Why is it important that starting resistors not remain in circuit for too long?

A

They are rated for intermittent duty and can over heat

159
Q

What principle does the capacitor-timing started use to provide time delay?

A

R-C constant of a resistor and capacitor

160
Q

Why is field loss protection required?

A

For protection if the shunt field circuit opens it stops the motor from going an unsafe speed

161
Q

Field loss protection is most commonly provided by using what?

A

FLR (Field Loss Relay)

162
Q

The coil of a field loss relay is connected?

A

In series with the shunt field

163
Q

Connecting a rheostat in series with the shunt field will enable a DC motor to operate…

A

Above base speed

164
Q

Why are field windings are sometimes left energized ?

A

To keep machines warm

165
Q

List 5 basic reasons that can cause excessive brush sparking?

A 
Brush troubles - specifically wrong tension
Commutator surface
Open armature circuit
Excessive Load
Sudden load chnages
166
Q

What are sometimes connected across the armature windings to reduce or prevent damage to the armature insulation due to the collapsing field during commutation?

A

Free wheeling diodes

167
Q

When electromechanical braking is used with an electric motor, the braking system is operated by a

A

Solenoid

168
Q

Braking systems used in electric motor applications are designed to

A

Fail-safe

169
Q

How is the fail-safe in braking systems achieved?

A

the mechanical braking system is designed so that the brake is applied when the solenoid is de-energized

170
Q

Define dynamic braking

A

a method of deceleration that involves reconnecting a DC motor to function as a DC generator

171
Q

During dynamic braking of a DC motor, the armature conductors experience a force acting

A

opposite or counter

172
Q

Why does the shunt field remain energized during dynamic braking?

A

Shunt field excitation is necessary for the motor to function as a generator during the braking period

173
Q

Why are mechanical brakes often used in conjunction with dynamic braking?

A

Since counter-torque is proportionate to speed, the braking effect decrease as the motor slows down

174
Q

What device is used to disconnect the shunt field after the dynamic braking operation?

A

Off-delay timing relay

175
Q

What is the main disadvantage of dynamic braking?

A

Loss of heat dissipated by the braking resistor

176
Q

Which braking system stores, or returns to the source, the energy released during the braking of a DC motor?

A

Regenerative Braking

177
Q

What are the two fundamental steps in troubleshooting DC motor controls?

A

Visual Inspection

Circuit testing and measurement

178
Q

The majority of faults in motor control involve ?

A

Loose connections

Failed components

179
Q

What two mechanical conditions can be checked by manually operating a DC contactor?

A

Actuating device travels freely without binding or excessive friction
Travel distance provides proper operation of the contacts

180
Q

What precautions must be taken before manually operating an electromechanical control device?

A

Control and power circuits must be tested to ensure they are both de-energized

181
Q

What are the two main differences between DC and AC electromagnetic control devies?

A

Shading coil is not required in DC

Arcing is more serious in braking DC and requires use of blowout coils and greater air gaps

182
Q

What device is often used to test for shorted coils on an armature?

A

Growler

183
Q

A diverter rheostat, used with a DC generator, is connected

A

Across S1 and S2

184
Q

During acceleration from a standstill, DC magnetic starters insert resistance

A

in series with the armature

185
Q

The motor effect of a DC generator is what….

A

causes loading of the prime mover

186
Q

The generator effect, in a DC motor, is what…

A

Keeps the armature current at a reasonable level

187
Q

Large DC motors are usually started with…

A

solid state controls

188
Q

If the interpole winding of a series DC motor opened while operating at rated load, the motor would?

A

Stop

189
Q

The efficiency of a DC motor at no load is..

A

Less than at full load

190
Q

A field acceleration relay is normally used to

A

limit armature current on inrush

191
Q

What is an important part that must be check on brushes to maintain optimal operation?

A

Check for suitable pressure against the commutator surface (usually between 1 and 2 psi)

192
Q

What are a couple key probable causes when there is excessive sparking at brushes when load is applied?

A

Insufficient brush pressure

Loose brushes

193
Q

The generator effect, in a DC motor, is what?

A

Keeps the armature current at a reasonable level

194
Q

The efficiency of a DC motor at no load is?

A

less than at full load

195
Q

Where is the “field failure relay” normally located in a compound DC motor control circuit?

A

in series with the shunt field

196
Q

In order to obtain “below base speed” control for DC motors, it is standard practice to

A

apply full voltage to the field and reduce voltage to the armature

197
Q

EMF is actually produced in a generator by

A

conductors cutting flux

198
Q

A separately excited shunt generator has a _______ in terminal voltage with an ______ in load.

A

Decrease

Increase

199
Q

Flux distortion due to armature reaction causes the electrical neutral plane in a generator to

A

Move in the direction of rotation

200
Q

Compensating winding are connected in ______ with the armature

A

series

201
Q

A DC series generator has an ______ in voltage with an ______ in load up to normal load

A

Increase

Increase

202
Q

When a given armature coil is being commutated, the brushes _____

A

Short circuit it

203
Q

With an increase in load resistance, the output voltage of a series generator will?

A

Decrease

204
Q

Armature reaction is counteracted by?

A

Compensating windings

205
Q

The effect of DC motors drawing more current is best explained by the reduction of

A

CEMF

206
Q

Below base speed control of a separately excited DC shunt motor is best done by inserting resistance _______

A

in series with the armature

207
Q

Above base speed control of a separately excited DC shunt motor is best done by inserting resistance ______

A

in series with the field

Electrical Level 3 and 4 (10 decks)

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