DC MACHINES Flashcards
(145 cards)
1
Q
Describe the left hand conductor rule
A
Thumb points to the direction of current flow and fingers show direction of magnetic field
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
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
4
Q
Unlike magnetic poles will?
A
Attract
5
Q
Like magnetic poles will?
A
Repel
6
Q
Do flux lines ever cross?
A
No
7
Q
Flux lines can pass through what type of materials
A
All types
8
Q
Flux concentrates into highly
A
Permiable paths
9
Q
T/F flux has elasticity
A
True
10
Q
Flux loops always pull to the
A
Smallest possible size
11
Q
Bigger the air gap the ___________ voltage
A
Less
12
Q
What kind of part is the field yoke
Magnetic
Electric
Mechanical
A
Magnetic
13
Q
What kind of part is the pole cores
Magnetic
Electric
Mechanical
A
Magnetic
14
Q
What kind of part are the field windings
Magnetic
Electric
Mechanical
A
Electric
15
Q
What kind of part are the end Bella
Magnetic
Electric
Mechanical
A
Mechanical
16
Q
What kind of part is the rotor shaft
Magnetic
Electric
Mechanical
A
Mechanical
17
Q
What kind of part is the armature core
Magnetic
Electric
Mechanical
A
Magnetic
18
Q
What kind of part are the armature conductors
Magnetic
Electric
Mechanical
A
Electric
19
Q
What kind of part is the comutator
Magnetic
Electric
Mechanical
A
Electric
20
Q
What kind of part are the pigtails
Magnetic
Electric
Mechanical
A
Electric
21
Q
What kind of part are the brushes and holders
Magnetic
Electric
Mechanical
A
Electrical
22
Q
Do DC machines generate AC?
A
Generate AC first then converted to DC
23
Q
How do you change output voltage on a permanent magnet generator
A
Increase RPM
24
Q
Changes is field current are usually used in varying output _________
A
Voltage
25
Series generator has _________ turns
Very few
26
Series generator has _______ current
High
27
Series generator has ________ resistance
Low
28
T/F series generator can be excited separately and self excited?
False
29
Shunt generator has ________ turns
Many
30
Series generator has a _______ cm wire size
Large
31
Shunt generator has a _______ cm wire size
Small
32
Shunt generator has _______ current
Low
33
Shunt generators have ________ resistance
High
34
T/F shunt generators can be self excited and separately excited
True
35
In a DC generator line 2 needs to be
Negative
36
Explain flashing the field
To apply rated current for 10-30seconsa to re-establish field domain on a dead generator
37
How do you know a generator is dead
By running the prime mover at full speed and not getting desirable voltage output
38
List the rotating parts of a DC machines
Rotor shaft
Armature core
Armature core conductors
Comutator
39
List the stationary parts of a DC machine
Field yoke
Pole cores
Field windings
End Bells
Brushes and holders
Pigtails
40
What part of a DC generator turns the AC to DC
Commutator
41
What are the four conditions required to be met for a self excited generator to build up rated voltage
-resdiual flux
-must be below critical resistance
-correct direction of rotation
-rated rpm
42
A generator with terminals A1 A2 F2 F1 is called
Shunt
43
A generator with only terminals A1 and A2 is called a
Magneto
44
Which generator can only reverse voltage output polarity by switching the directions of rotation
Seperatle excited shunt magneto
45
Describe armature reaction
Distortion of the flux field caused by load current in armature conductors
46
What is the purpose of a field rheostat
To control the field current
47
What are the different types of brushes used
Carbon
Electro-graphite
Graphite
Copper graphite
48
What is the formula for motor output
Force=flux x length x Iarm
49
HP formula is
HP=(ft-lbs x Rpm)÷5252
50
Describe the right hand motor rule
1st finger shows field direction
Middle finger shows direction of current thumb shows direction of thrust
51
What is the standard rotation of a DC motor
Ccw
52
T/F motors can be self excited
False
53
Generated voltage is dependent on
Speed
54
Slower the motor the ______ Eeff
More
55
To switch direction of a motor you would switch
Only the armature terminals
56
Compound motors have to be wired
Cumulative
57
The shunt motor has the ________ % speedreg
Lowest
58
%speedreg is the
Constant speed
59
What prevents runaway motor
Cemf prevents runaway by acting against applied power
60
Field loss causes?
Runaway
61
Compound motor has ___________ % speedreg
Higher
62
When wiring a compound motor you have to make sure not to?
Wire in differential position
63
How do you do a compound motor series polarity test
Place jumper parelell to the series coil and if the rpm increases slightly it is wired in the proper polarity
64
What's a hazard of a series motor
Loss of connected load causes runaway
65
Series are a _________ torque motor
High
66
Describe the considerations in starting a DC motor
Size of motor
Type of controller
Type and size of load
Amount of starting current
Motor and supply voltage
67
What is the correct polarity for an interpole in a motor
Same polarity as the main pole
68
A mechanical load increases, the neutral plane shifts, which direction would it move
Opposite direction of motor rotation
69
Why does motor current increase as speed goes down
Lowering cemf creates greater net voltage
70
What determines how much torque a motor will develop
Field flux
Armature flux
71
What does base speed mean
The speed with rated current and voltage
72
What is torque
A twisting force that acts around an axis
73
How does the operation of the armature differ from a motor to a generator
Generator is a source
Motor is a load
74
How do you calculate initial inrush of a motor
Source voltage divided by the armature resistance + the series resistance
75
To reduce motor speed field current must be
Increased
76
Why does a series motor produce more torque than a shunt
Both field fluxes increase at the same time
77
What motor has almost constant speed
Shunt
78
What hazard exists with a differential compound motor
Accidental reversal under heavy load
79
What protection against runaway motors are used in a shunt
Field loss relay
80
How is speed of a series traction motor achieved
Voltage controller
81
Where is a field loss relay connected
In series with the shunt field
82
Can a compound motor drive loads connected by belts
Yes
83
What happens to a traction motor if the field becomes open and what kind of motor is this
Stops
Series
84
Can the speed of a compound motor be changed by increased the rheostat resistance
Yes
85
How is rotation affected if the line terminals to a shunt motor are switching
Rotation remains unchanged
86
Is field loss protection required for a compound motor
Yes
87
DC motors can be line started if of what size
2hp or less
88
To slightly reduce motor speed field current must be increased or decreased
Increased
89
How is speed affected by increasing field resistance
Increased or decreased
Increased
90
How are the armature windings physically connected to the commutator segments
Soldering
91
What holds the armature windings in the arm core slots
Slot wedge
92
What property of magnetic lines is most important
Elasticity
93
Why are brushes placed in the neutral plane
To prevent arcing
94
Why are pole cores and arm core assembled with insulated laminations of Silica steel
To prevent heat loss due to Eddy currents and mag loss
95
What is the mathematical relationship in the reduced heat loss from a solid laminated core
(Layers squared)÷power
96
Explain the characteristics of frog leg, lap and wave
Frog leg=med current med volts
Wave=low current high voltage
Lap=high current low voltage
97
What kind of brush is used in lower speed, lower current density machines
Carbon brushes
98
What range of brush pressure is usually used
1-2psi
99
What is the difference between interpole and compensation winding
Location
100
As electrical load increases in a generator the neutral plane will shift in what direction
Towards rotation
101
Why does output voltage of a self excited generator decrease with an increase in load
Arm current causes voltage drop internally
102
What effect is produced by rotating a generator to slowly
The output voltage may be too low or may not build at all
103
How does a generator place load on a prime mover
Counter torque
104
What generator is the most convenient for changing the output voltage polarity
Seperatle excited shunt
105
What is a commutating pole
Interpole
106
Difference between I²R losses, rotational loss and mechanical loss
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
For a generator the short or long connection is preferred
Short
108
Explain the relationship of Enoload and Efullload for flat, under and over compound generators
Flat-Enoload=Efullload
Under-Enoload is the greater than Efullload
Over-Enoload is less than Efullload
109
Describe the physical difference between a field rheo and a diverter rheo
Field-series with shunt field
Diverter-parellel with series field
110
How do the ratings compare for field rheostat and diverter rheostat
Field- high R low P
Diverter- low R high P
111
What is the importance of residual flux
Allows voltage to build up in self excited generator
112
Will a series generator with no connected load develop any voltage
Yes
113
What does the term critical resistance mean with respect to generators
Level of field resistance to high to allow field current to develop
114
Does armature reaction affect brushes and commutator
Yes
115
Three measure used to correct for armature reaction
Inter pole
Compensating winding
Adjust brushes
116
What DC machine is totally enclosed
Motor
117
What is the formula to determine force of a motor
Force=flux x length x Iarm
118
As a motor increase speed what happens to its internal voltage
Increases
119
During the operation of a motor when is the current the largest
When it's stationary
120
Express formula for speed control in a motor
Nrpm=applied voltage/field flux
121
Armature reaction causes what effect in a DC motor
Increased speed
122
Which of the following generator losses are influenced mostly by connected load
Friction loss
Core loss
Armature copper loss
Field copper loss
Armature copper loss
123
The motor effect causes loading on what in a generator
Loading of the prime mover
124
An iron core strengthens an electromagnet because it has ________ reluctance than air
Less
125
In order to supply a load with a long distance from the source the best generator would be a
Over compound generator
126
When should a series DC motor never be used
When a belt drive is used to connect to load
127
When the mechanical load on a series DC motor increases the armature current increases because
The speed decreased
128
During acceleration from standstill for DC motors resistance is inserted where
In series with armature
129
What can be adjusted to increase output voltage in a permanent magnet generator
Speed
130
What can be adjusted to change output voltage on a seperatly excited generator
Field rheo to change the flux field
And speed
131
What can be adjusted to change out put voltage of a self excited generator
Field threo for flux
Speed
132
What is the most common and most accessible way to change out put voltage
Field rheo to adjust flux
133
What is the basic operation of a series generator and what is a real life example
Provide steady current output
Arc welding
134
Can you use a manual starter for a large motor
No 2hp or less only
135
What HP is considered a large motor
Anything over 2hp
136
How much HP is considered a small motor
2hp or less
137
What kind of starters can be used for a small motor
Electronic manual and magnetic
138
What kind of started can be used for large motors
Electronic
Magnetic
139
Advantage of friction braking
Failsafe
Provides braking when stopped
140
Disadvantage of friction braking
Expensive
Bulky
High maintenance
141
DisAdvantage of Eddy current/magnetic brake
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
Advantages of Eddy current/magnetic brakes
Opposing torque made by magnetic field means no friction parts
Initial braking is very strong
143
Advantages of dynamic braking
Very little additional equipment
Low maintenance
144
Disadvantage of dynamic braking
Still need friction brake
Not fail safe
145
Explain the difference between dynamic and regenerative braking
Dynamic dissipates energy and regenerative restores its losses
