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dc motors Flashcards

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1
Q

why do you need a load on a generator

A

create counter emf and flow

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2
Q

when a conductor cuts through a field an emf is generated in the conductor.
a)what determines the magnitude of the emf?
b) what determines the polairity of the emf?

A

a) amount of flux being cut/second
b)direction of rotation relative to the magetic field

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3
Q

a) what would the output emf be if the fields were arranged to provide the same flux polarity on both sides of the loop in a simple generator
b) what would happen to the output polairity if the conductor was moved in the opposite direction

A

a)no voltage. One voltage would cancel out the other
b)polarity would flip

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4
Q

A generator converts _________ energy into __________ energy.

A

mechanical into electrical

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5
Q

what ways can you change the output voltage of a generator

A

speed, flux, # of turns, lenth of condictor that is in the magnetic field.

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6
Q

what does a commutater do in a generator

A

converts AC to a DC waveform. (switched positive to negative)

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7
Q

how often do split rings change directions in a generator

A

180 degrees

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8
Q

what is EMF

A

rate of change in flux, amount of lines cutting per second

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9
Q

inducted voltage in a generator will be ________ of the voltage that created it

A

opposite

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10
Q

For faradays right hand rule what does the thumb,pointer finger and middle fingers mean

A

thumb-thrust
pointer- polarity (north and south poles)
middle -directon of current flow

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11
Q

what does a frame in a genrator do

A

supports the field poles and provide part of the low reluctance magnetic cct.
frame is thicker then required because a low reluctance path for the field is important.

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12
Q

what do field poles in a generator do

A

iron pole pieces, laminated with one or more windings. they are permanant megnats for a generator for the south and north poles

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13
Q

For DC machines how many poles do they need to have

A

2-24

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14
Q

what is the armature and what does it consist of. why is it mounted on bearings

A

the rotating portion, consists of the drive shaft, a commutator a slotted iron core and the armature winding.

so it can rotate within the magnetic fiels provided by the field poles

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15
Q

what is a commutator made from

A

wedge shaped copper bars, seperated by sheets of mica insulation

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16
Q

what does a commutator do

A

serves as a connection point for the armature coils within the brushes. also provides sliding contact that allows the armature to turn. acts as a rotating switch that changes the armature connections as the armature coils pass from pole to pole

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17
Q

what does commutate mean

A

reverse the direction of an electrical current

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18
Q

what are armatire windings in a genrator made from and how are they arranged

A

made from a series of coils. connected to the commutator and arranged so they pas under one pole as they loop out of the commutator as the next returnd

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19
Q

where are the brushes in a generator and what are they made from

A

fit into box shaped brush holders. mounters above the surface of the commutator. made from carbon composite

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20
Q

what are commitating poles in a generator

A

(interpoles) they control armature reaction. their windings are connected in the armature cct so their influence on the fiels varies with the load

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21
Q

what are interpoles

A

commutating poles mounted between the main field poles

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22
Q

what is armature reaction

A

effect that the mmf generated by armature current has on the magnectic field.

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23
Q

in a generator what do the armature conductos do

A

are driven through the firld and drag the field flux in the opposite direction.

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24
Q

in a motor what does the armature reaction and what does it do

A

armature reaction is opposite to the direction of rotation. armature mmf pushes against the field flux to create the force that drives the load.

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25
Q

armature reaction only occurs _______________

A

under load

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26
Q

what are compensating windings

A

they do the same as interpoles but on bigger machines. the windings are busbars. they eliminate armature reaction and cancel most of the armature reactance

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27
Q

what are end covers on a generator

A

enclose the ends of the machine and support bushes and bearings. important to put on the correct way. they protect the armature, windings and brush assembly and spports the bearings

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28
Q

what functions does the frame provide other then mechanical support

A

low reluctance path for the magnetic cct and completes the magnetic cct.

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29
Q

in what 2 ways can the main field of a DC machine may be provided?

A

wound poles or permanant magnents

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30
Q

what are the main components of the armature

A

drive shaft
commutator
slotted iron core
armature windings

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31
Q

list the functions of the commutator and brushes.

A

commutator-switch AC to DC
brushes-slide connection coils

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32
Q

whats the cause of armature reacrion and its effect on the main field

A

fies being dragged, distorted fields. caised by mmf generated by armature current

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33
Q

what do commutating poles and compensating windings do

A

eliminate armature reaction

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34
Q

when a DC machine is used as a motor armature current is driven by the supply voltage. what drives the armature current when the same machine is used as a genreator?

A

load-closes the cct loop
no load=no current
load=more voltage=more current
current goes up. so does the voltage.
the more voltage the faster it spins and the more lines of flux cut

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35
Q

on a diagram how many humps does a shunt fiels and series have in a generator

A

shunt field- 4 (f1,f2)
series-3(s1,s2)

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36
Q

what is a shunt field winding designed for

A

parallel connection to a DC source. wound with lots of turns of small conductor.has max V and I rating.current is limited by the resistance od the field and can be controlled by changing the field supply voltage or installing a shunt firls rheostat in series with the field winding. high resistance= very magnetic

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37
Q

what is the saftey warning about shunt fields

A

very inductive. field discharge on shutdown can cause serious arching at the switch and make high voltages. dont open at an energized cct exept at the normal diconecct point.

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38
Q

what are the windings like on a series field

A

few turns of large conductor.series winding is intended to be conncected in series with the armature of the machine and is rated for the FLC of the current machine. resistance of the winding is very low. current is limited by the armatuer cct.

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39
Q

decribe and compare the physical characteristics od shunt and series field windings

A

field= lots of turns on the shunt field of thin wire, high resistance
series= low turns, thick wire, low resistance

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40
Q

why do the individual field pole windings are connected with alernating electrical polarity

A

creates opposite magnetic polarity

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41
Q

what are the standard terminal markings for the shunt and series field

A

series= s1,s2
shunt fiels=f1,f2

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42
Q

why are shunt fields so inductive

A

lots of loops/turns

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43
Q

what would happen if you disconnect one lead from the shunt field of a large DC machine, when the field is energized

A

arc because magnetic field collapses

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44
Q

why are commutating poles and compensating windings not usually considered as field windings

A

in series with armature not field cct

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45
Q

if the shunt field of a generator is connected with F1 positive, F2 negative and the machine is driven in clockwise roatation. (viewed from opposite the driven end) what is the polariry of the output?

A

output will be positive at A2 and negative at A1

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46
Q

what are the 2 common armature winding configuration

A

lap
wave

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47
Q

characteristics of lapwound on the brushes and poles

A

of brushes= # of poles

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48
Q

wave wound characteristics

A

have at least 4 poles. dont need brushes for each pole bc there are only 2 parallel paths in the armature cct. starts on one end and termininates on the other. will alwasy have 2 brushes and 2 parallel paths

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49
Q

machines with less bruh rows then poles are ___________

A

wave wound

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50
Q

if one side of an armature coil is directly under a south field pole, where will the opposite side of the coil be

A

north

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51
Q

if the lead end of coil 1 of a simplex lap winding terminates at commutator segment 1 and the finish end at segment 2, where will the start and finish ends of coil 2 terminate

A

start at 2 end at 3

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52
Q

when a simplex armature winding is completed but the brushes havent been installed, are the coils connected in series or parallel

A

series

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53
Q

how many parallel paths are there in the armature cct of
a) a simplex lap wound 6 pole DC machine
b)a simplex wavewound 6 pole machine

A

a) 6 paths, 6 brushes
b)2 paths, 2 brushes

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54
Q

what is the minimum number of poles for a machine with a:
a)wave wound ammature
b)lap wound armature

A

a)4 poles (will only have 2 brushes)
b) 2 poles (will have 2 brushes)

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55
Q

besides the armature windinfs what parts of a large 8 pole generatir are connected between A1 and A2

A

commuatating poles, interpoles,compensation windings (busbars)

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56
Q

draw tge different coil symbols for shunt field, series field and commutating windings

A

pg 23

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57
Q

F1 in a generator must always be ___________

A

positive

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58
Q

to reverse polarity or roation of a generator what do you reverse

A

only the armature (a1,a2)

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59
Q

what is the standard rotation for a DC generator

A

clockwise (f1 and A2 are positive)

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60
Q

what is standard rotation for DC motors

A

counter clockwise (F1 and A1 are positive)

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61
Q

what determines the polarity of a voltage induced into a conductor passing through a field

A

polarity of field poles, direction of armature conductors

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62
Q

what determines the direction od the force applied to a conductor current carrying current through a magnetic field

A

connections at field terminals

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63
Q

what happens to the mag polarity of the field poles, if the connections to the field windings are reversed

A

reverses the polariry

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64
Q

is it best the reverse the field or armature connections if the outpur of a generator, or rotation of a motor has to be reversed

A

armature

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65
Q

direction of roation depoends on the location of the viewer. relative to the machine. what is the standard viewpoint

A

opposite of drivepoint

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66
Q

the rotation always goes towards ________________

A

A1

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67
Q

in permant magnets of DC machines when is the field polarity set

A

during construction and cant be changed

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68
Q

the output polarity of a permant magnent generator is determined by….

A

direction of rotation and will reverse if the rotation is reversed.

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69
Q

what determines the output voltage of a permant magnent generator

A

speed

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70
Q

self exited generators depend on __________________

A

residual flux

71
Q

what is resitidual flux

A

magnetism retained in the iron poles after the machine is shut down to provide initial exitation

72
Q

does self exited or seperatly exited have better voltage regulation

A

seperatly exited

73
Q

witg a standard connection what are the polarities of A1 and A2

A

A1-, A2+

74
Q

what happens when the polarity if the shunt is reversed on
a) seperatly exited generator
b)self-exited generator

A

a) reverses polarity of the load
b)will fail to build a mag field, output voltage will go low and may cause a flash

75
Q

what is the purpose of a shunt field rheostat

A

current control through field windings and controls magnetism

76
Q

if the self-exited generator fails to build up and the roation and connections are correct
a)whats the problem
b)what should be checked
c)how can the problem be corrected
d)what procedures should be followed

A

a)residual magnetism could be gone
b)shunt field rheostat may be set too high of a resistance
c)decreases resistance to allow current to pass, flash the field
d)check rheostat before flashing the field and leave connected for awhile so the magnetism can build up

77
Q

what makes series generator/motor

A

when a wound field machine is connected with the series field alone

78
Q

why arnt series generators used often

A

output voltage varies with load current. no load voltage is low bc there is no field current. full load voltage is too high bc the field is at a maximum

79
Q

how to change roation in a series generator

A

flip A1 and A2

80
Q

self exited current will be _____________ as the load current

A

the same

81
Q

seperatly exited motor has __________ voltage regulation

A

better

82
Q

the lower the self regulation percentage the __________ the regulation

A

better

83
Q

in seperatly exited motors why is the field current unaffected by the load

A

fed from a seperate source

84
Q

the ___________ is the source for a generatior

A

armature

85
Q

what is a cumatively compounded generator connection

A

series winding has same polarity and they add together

86
Q

what is differentially compounded generator connection

A

series winding has opposite polarity and work against eachother (subtract)

87
Q

what does the degree of compounding depend on

A

mmf generated by the series field relative to the mmf of the shunt field. also determines machine characteristics

88
Q

undercompounded machines have the __________ series field

A

weakest

89
Q

over-compounded machines have the ____________ series field

A

strongest

90
Q

shunt generators characteristics

A

near perfect
good voltage regulation
(voltage drops only a small amt from no load to full load)

91
Q

characteristics of under-compounded machines

A

have better voltage regulations then shunt
voltage still drops slightly as the machine is loaded

92
Q

characteristics of flat compounded machines

A

-near perfect voltage regulation
-voltage is the same at full load as it is at no load. may be slightly higher at partial load

93
Q

characteristics of over-compounded machines

A

-better for long distance
–negative voltage regulation
-voltage rises slightly with increaing load and is the highest at full load.
-can be used to compensate for line loss to provide a near constant voltage at the end of a long supply limne

94
Q

characteristics of differentially-compounded machines

A

-very poor voltage regulation
-used where their natural current limiting characteristics are more important than a constant output voltage

95
Q

In generators, differentially compounding is used to provide ____________. ________ in the load and ____ series field mmf. this _____ the main field and _____ output voltafe. the ____ the load are connected in ___________

A

-used to provide current limiting.
-increase in the load means higher armature current and more series fiels mmf
-this weakens the main field and reduced output voltage
-armature and load are in //

96
Q

shunt field current can be adjusted with a series connected _______________

A

shunt field rheostat

97
Q

series field current can be adjusted with a ___________

A

// connected diverter resistor

98
Q

shunt field rheostats are an _________

A

operating control

99
Q

a motor rheostat is connected with ___________________ resistance fully ___________ which __________motor speed

A

-max resistance fully clockwise which increases motor speed

100
Q

series field diverter allows some of the _________________ to _________________

A

-armature current to bypass the series field windings
-wired in // with s1 and s2
high current= low resistance

101
Q

reducing the resistance of a diverter________________________

A

increases its share of the armature current which reduces the current through the series field
-are not an operating control

102
Q

series field diverters are specialized devices with _______________resistance and __________ current ratings

A

-very low resistance
-high current

103
Q

what is the difference between a long-shunt and short-shunt connection?

A

-long- has series windings and connected across both armature and series field
-short-shunt field connected only across armature

104
Q

what is meant by the term “cumulativeky compounded”

A

series windings same polarity and add together

105
Q

compare no- load and full-load voltages od under-,flat-, and over-compounded generators

A

flat= near perfect, voltage reg is the same at full load and no load (FL=NL)
over=- voltage reg full load is higher then no load
under= better voltage reg, slight volt drop when machine is loaded. no load is higher then full load

106
Q

what is the usual reason for using differential compounding in a generator

A

-current limiting characteristic
-when load is higher voltage is lower

107
Q

how is a series field diverter connected and when is it usually set

A
  • in // with s1 and s2
    -set when machine is commistioned and not changed
108
Q

how would a shunt field rheostat have to be connected to provide an increase in generator voltage with a clockwise roatation of the control?

A

-connected to provide max resistance with the control turned fully counter clockwise

109
Q

for DC machines what is the ambient temperature if its not specified

A

40 degrees celcius

110
Q

how to calculate max rated operating temperature

A

40 degrees+ temperature it says

111
Q

a generator is rated at 10kW, 240V. will it be operating within its ratings if it is supplying 10kW at 200V?

A

-no.
-power ratings based on operation at full voltage
-current will be too high

112
Q

what effect does operating speed have on genreated voltage

A

-speed the armature conductors cut through the field

113
Q

under what conditions should generator output voltage match the nameplate rating

A

if operating at ratyed speed witgh rated shunt field and full load

114
Q

why does generator voltage slightly decline as the machine heats up

A

resistamce of windings increase=reduces shunt field current=weakens the field=increases internal voltage drops and loses=decreases output

115
Q

what is an appropreate ohmmeter range for measuring armature resistance

A

lowest range avaliable.

116
Q

can armature resistance be measured accurately at the ammuture terminals (a1,A2)

A

-no
-bc measurements will include the resistance of commutating windings and brushes

117
Q

what would happen if the series field were accidentally connected as the shunt field

A

short out supply

118
Q

what happens to the output voltage of an operational self-exited shunt generator, if the shunt field connections are reversed

A

fail to build up voltage the next time its brought up to speed.

119
Q

why do you connect a load to obtain a useful voltage from a series genertator

A

-no load=no current
-field flux comes from residual magnetism

120
Q

how to you test for grounds

A

maggometer

121
Q

if a machine is in good working order, the insulation is clean and there are no connections to the terminals, what should the resistance readings be from the terminals to ground?

A

infinate

122
Q

*what can cause leakage to ground in a DC machine?

A

-moisture
-airborne contaminants such as carbon dust from brushes

123
Q

*why would a ground on the armature winding read a low resistance on both A1 and A2?

A

-resistance of armature is very low

124
Q

why would a ground on the shunt field give a much higher resistance reading to ground on one terminal, compared to the other?

A

-ground is near one end of the winding
-winding has significant resistnace

125
Q

A DC generator has a full load output voltage of 248V, and no load output of 265V. what is the percent voltage?

A

use formula= 6.85%

126
Q

draw a connection diagram for a self-exited shunt generator, with a rheostat for voltage contril. generator is to be driven clockwise.

A

pg 54

127
Q

what connection should be changed if the roatation of a generator has to be reversed

A

armature connections

128
Q

the exitation provided by a series winding ___________ as the load is added

A

increases

129
Q

A series generator has ______________ voltage regulation

A

poor

130
Q

A compunded generator has a no load output of 125V and a full load output of 145V
a)what is the percent voltage regulation
b)is the generator under, over, or flat compounded

A

a) -13.8%
b)over

131
Q

draw a connection diagram for a cumulatively compounded generator with counter clockwise roatation

A

pg 58

132
Q

what device could be connected to convert an over compounded generator to flat compounding

A

-series field diverter
-connected in // with the series field

133
Q

if the output of a flat-compounded generator is 120V at no load, what is it at full load?

A

120V

134
Q

what is the percent voltage regulation of a flat-compounded genrator?

A

0%

135
Q

right hand rule is for _____

A

generators

136
Q

left hand rule is for __________

A

motors

137
Q

what was the __________ in a generator is now the ______ for a motor

A

-load in generator
-source for motor

138
Q

how is mmf created in a motor

A

-through current conductors
-interacts with the main field
-creating a force that can be used to move a mechanical load

139
Q

in a motor the __________ creates the _________ that becomes the motors ______________ output

A

-mmf
-force
-mechanical

140
Q

in a motor the direction of motion and the polarity of the field are ______________ so the polarity of the emf is also ________

A

-the same
-the same

141
Q

in a motor it becomes a ____________ that opposes the ___________ and acts to _____motor ______

A

-counter emf
-applied voltage
-regulate motor current

142
Q

generator action occurs when a _______________ is used to drive the _________________ through a field

A

-mechanical force
-conductors

143
Q

motor action occurs when _____________ from an electrical supply is _______________- through a ___________ in the field

A

-current
-driven
-conductor

144
Q

motors demand ____________ to turn it into __________

A

-electrical
-mechanical

145
Q

when a current passes through a conductor in a field a mechanical force is generated that attempts to push the conductor at right angles to the current and the field.motor
a) what determines the magnetiude of th eforce?
b)what determines the direction of the force?

A

a)strenth of the 2 magnetic fields (1 in armature and one in field)
- density of the field
b)polarity of the field and current reralative to eachother

146
Q

a)what would output force be if the fields were arranged to provide the same flux polarity on both sides of the loop in a simple motor
b)what determines the direction of force?
c) what would happen if the polarity of both fields were reversed?

A

a)will sit and buzz bc both forces are going in opposite directions whichb results in no motor action
b) roatation would flip/reverse
c)would still go the same direction bc it would double flip

147
Q

reversing the polarity of the field OR the armature connections in a motor _______________

A

reverses the direction of rotaton in a motor

148
Q

standard rotation for a motor is ________________

A

counter clockwise (f1 and A1 are positive)
-viewed from end opposite the drive

149
Q

what determines the direction of the force applied to a conductor current carrying current through a magnetic field?

A

-polarity of the 2 fields

150
Q

what happens to the magnetic polarity of the field poles, if the connections to the field winding are reversed?

A

-flips

151
Q

is it best to reverse the field armature connections if the roatation of a motor has to be reversed?

A

armature

152
Q

direction of rotation depends on the location of the viewer, relative to the machine. what is the “standard viewpoint”?

A

opposite end of the drive

153
Q

if a motor is connected with F1 and A2 positive. which way will it rotate in a motor

A

clockwise

154
Q

the direction of the rotation for permanant magnet motors can be reversed by _________________________

A

changing the polarity of the supply connections

155
Q

what connections must be changed to reverse the rotations od a permanant magnent motor

A

A1/A2 or supply

156
Q

in what way is a permanat magnent motor unique amoung DC motors

A

-only motor that can change roatation by changing polarity of the supply (armature) connections

157
Q

for more resistance in a motor __________ magnetism=___________ torque=____________ speed

A

less=less=more

158
Q

for less resistance in a motor _________ magnetism=___________ torque=__________speed

A

more=more=less

159
Q

in a motor what will happen if you dont have a high enough starting resistance

A

destroy the armature bc they have low resistance and need to be high resistance

160
Q

where can you find the base speed on a motor

A

name plate

161
Q

can DC motors be started across the line

A

no

162
Q

shunt motors should alwasys be started with ___________ to provide ___________ and ensure a ________of __________

A

-full rated field
-maximum starting torque
-rapid build up of current limiting cemf
(start at max resistance with a armature rherostat)

163
Q

the base speed of a motor is ___________and listed on the _________

A

motor speed at full load
listed on the name plate

164
Q

adjusting the _______provides speed control __________, with ____________ from 0rpm

A

-armature voltage
-below base speed
-full torque

165
Q

adjusting field current provides __________ speeds at ___________

A

-above base speed
-reduced torque

166
Q

a constant motor maintains a ______________speed under ___________

A

constant speed
under changing load

167
Q

the speed of a varying-speed motor ____________ with _________ in the load

A

changes significantly
changes in the load

168
Q

an adjustable speed motor is a __________ that can be speed ___________

A

-constant speed
-speed contrilled over a defined range (you control)

169
Q

an adjustable varying speed motor can be speed _________ but also __________________

A

-controlled
-changes speed with changes in load

170
Q

a DC shunt motor is a _____________ that can be operated as an ________________

A

-constant speed motor
-adjustable speed motor

171
Q

weakening the field of a motor _________ motor speed but ____________ torque

A

increases
lowers

172
Q

what will happen if the motor cant handle the load if the torque has been lowered

A

motor will stall

173
Q
A

Level 2 (5 decks)

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