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Electrician 3rd Year > Transformers > Flashcards

Transformers Flashcards

1
Q

What is the current flowing in the primary winding called?

A

exciting current

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2
Q

The voltage induced in the primary winding ____ the primary voltage

A

opposes

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3
Q

The magnitude of the primary induced voltage is ____ than the primary voltage

A

less

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4
Q

When voltage is applied to the primary winding, the ____ current flows even though there is no load on the secondary windings

A

exciting

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5
Q

The main component that resists current in the primary winding is:

A

the primary induced voltage

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6
Q

The secondary current flux ___ the exciting current flux

A

opposes

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7
Q

The resultant flux produced a primary induced voltage that is ___ than the induced primary voltage operating at no load

A

less

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8
Q

When the induced primary voltage is reduced, the primary current:

A

increases

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9
Q

As the secondary current of a transformer increase, the primary current must _____

A

increase

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10
Q

The voltage drop caused by the total primary current and the impedance of the primary winding ____ the primary induced voltage

A

reduce

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11
Q

When the secondary voltage is reduced, the secondary output voltage is ____

A

reduced

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12
Q

The secondary output voltage at full load is ____ than its value at no load

A

less

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13
Q

What is the formula for voltage regulation?

A

Vno load - V full load/ V full load

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14
Q

What are the two main core losses?

A

hysteresis, eddy current

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15
Q

In what form are losses in the core usually found?

A

heat

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16
Q

What is hysteresis?

A

magnetic domains in the core are forced to flip back and forth due to flux alternations

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17
Q

What are eddy currents?

A

loss due to circulating currents in the iron core itself due to voltage induced by the changing flux

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18
Q

What are copper losses?

A

Line or I squared x R Losses

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19
Q

Are core losses fixed or constant?

A

Yes

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20
Q

Do copper losses change with load?

A

yes

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21
Q

What are copper losses proportionate to?

A

The square of the currents

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22
Q

Formula for copper losses?

A

I^2 x R

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23
Q

Core losses are constant from full load to no load. True or False?

A

True

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24
Q

Copper losses are proportionate to:

A

current

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25
Q

The copper losses can be determined using a _____ test

A

short cct

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26
Q

In a short circuit test, the rated secondary current is obtained at a primary voltage that is considerably ____ than the rated primary voltage

A

less

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27
Q

Formula for efficiency?

A

P output/P Inout

Pout/Pout + losses

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28
Q

Describe impedance voltage

A

it is the primary voltage required to circulate the rated full-load secondary current in a transformer when the secondary winding is short circuited

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29
Q

What is the typical %IZ?

A

2 - 10%

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30
Q

What is the symbol for percent impedance voltage?

A

%IZ

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31
Q

What is the formula for max short circuit current?

A

Secondary rated current/%IZ

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32
Q

What is the danger of paralleling transformers?

A

Very high short cct current

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33
Q

What are three requirements for paralleling transformers?

A

same tap size, same polarity, same secondary voltage

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34
Q

Why should two transformers connected in parallel have the same %IZ?

A

They wouldn’t share the load proportionally if they had different %IZ. The transformer with the lower percent impedance would take more load which could result in overloading

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35
Q

Why must we shut down all transformers in parallel before working on only one of them?

A

Backfeed

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36
Q

Transformers in parallel have to have a percent impedance within ___% of each other

A

5

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37
Q

Why must we focus on proper polarity when connecting transformers in parallel?

A

They could easily short circuit if we hooked it up backwards

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38
Q

What property opposes a change in current?

A

inductance

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39
Q

List three types of transformers

A

core, shell, H Type

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40
Q

Transformer efficiency is between what percentages?

A

95 - 98%

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41
Q

What does RBCN stand for?

A

Reduced capacity below normal

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42
Q

The overall resultant magnetic flux in a transformer from no load to full load will ____

A

stay the same

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43
Q

What type of test is used to determine core losses?

A

open circuit test

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44
Q

As load increases, the core losses will:

A

stay the same

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45
Q

Impedance voltage is obtained using a ___ test

A

short circuit

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46
Q

The efficiency of a transformer will ___ as load is added

A

increase

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47
Q

How can a mutual induction transformer be reconnected to make an autotransformer?

A

connect line to line

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48
Q

The section of winding that has the same voltage as the load in a step down autotransformer is called what?

A

the common winding

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49
Q

If a mutual induction transformer is reconnected as an autotransformer, what would happen to the available KVA?

A

increase

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50
Q

When an autotransformer goes from no load to full load, the secondary output voltage would be ____ than an equivalent induction transformer

A

higher

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51
Q

Can autotransformers be used as isolation transformers?

A

fuck no

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52
Q

Can we use an autotransformer for residential 416-:240 distribution? Why or Why not?

A

No, we can’t because if the common winding were to be open circuited, the people’s houses would receive line voltage (4160)

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53
Q

Do autotransformers have high or low %IZ?

A

low

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54
Q

Do autotransformers have higher or lower short circuit current than regular transformers?

A

higher

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55
Q

True or false? An autotransformer is connected so that the source and the load share a direct connection point

A

true

56
Q

What is the formula to find the series winding voltage in a step down autotransformer?

A

V series = Vsource - Vcommon

57
Q

In a step up autotransformer, the load current is the same as the:

A

series winding current

58
Q

Autotransformers have a lower %IZ than regular transformers. Is this a good thing or a bad thing?

A

It is a bad thing because they have higher short cct current

59
Q

autotransformers have a ___ voltage regulation than normal transformers

A

lower

60
Q

Energy transfer in a transformer uses what principle?

A

Induction

61
Q

List the three main parts of a transformer

A

Core, primary winding, secondary winding

62
Q

List the three main uses of a transformer:

A

step up, step down, isolation

63
Q

The higher voltage terminals are labeled:

A

H

64
Q

Can we use transformers for DC?

A

No, there is no induction in DC

65
Q

A transformer can operate up to ___% of its rated voltage

A

110

66
Q

What does FCAN stand for?

A

Full capacity above normal

67
Q

Turns ratio is directly proportional to voltage ration in a transformer. True or false?

A

true

68
Q

What test is used to determine impedance voltage?

A

short cct

69
Q

What is required to prevent backfeeding when transformers are connected in parallel?

A

Either a transfer switch or shutting the entire transformer bank down

70
Q

Do autotransformers use mutual induction or self induction? Why?

A

Self induction because they are both connected to the same core

71
Q

What are the two windings in an autotransformer called?

A

series winding and common winding

72
Q

KVA in = KVA out. T or F?

A

True

73
Q

What is the total apparent rating of a transformer?

A

its capability to supply the max rated voltage and max rated current without overheating

74
Q

What are autotransformers ideal for?

A

small voltages changes

75
Q

state three advantages of an autotransformer compared to a normal transformer

A

lower cost, better voltage regulation, smaller size

76
Q

state three disadvantages of an autotransformer compared to a normal transformer

A

do not isolate secondary from primary, dangerous if open circuit, higher fault currents

77
Q

Is the volts/turn ratio always the same in primary and secondary windings?

A

yes

78
Q

Meters are either ___ or they are not

A

self contained

79
Q

Metering equipment over 200A and/or 600V requires ___

A

instrument transformers

80
Q

What is the maximum current and voltage for a self contained meter?

A

200A or 600 V

81
Q

What are the most common types of meters?

A

120/240 3 wire
120 V single phase
120/208 3 wire network

82
Q

What is the most common meter socket for single phase circuits?

A

120/240 3 wire

83
Q

How many jaws on a 120/240 3 wire meter?

A

4

84
Q

What is the most common meter socket in residential?

A

120/240 3 wire

85
Q

Where are 120/240 three wire meters designed for?

A

overhead or underground

86
Q

What is the only common 120V single phase meter application?

A

a billboard

87
Q

How many jaws on a single phase, 3 wire 120/208 network meter?

A

5

88
Q

What is the difference between a 120/208 3 wire network and a 120/240 single phase?

A

the neutral is actually a common conductor that can carry the same current is the lines

89
Q

___ phase panelboards are used on a single phase, 3 wire 12 meter

A

single

90
Q

Does a single phase 3 wire 120/208 network meter need a tickler?

A

yes

91
Q

where does the tickler go on a single phase, 120/208 network meter?

A

from the tickler jaw to the common

92
Q

What is the tickler for?

A

to maintain correct volt drop and phase relationships with the current

93
Q

What are the two styles of three phase meter sockets?

A

4 wire wye

4 wire delta

94
Q

What style of meter are most 3 phase 4 wire sockets connected in?

A

Wye

95
Q

What are the three main voltages for a 3 phase 4 wire wye meter?

A

120/208
277/480
347/600

96
Q

How many jaws on a three phase 4 wire wye socket?

A

7

97
Q

Can a wye meter socket be connected to supply both single phase and three phase?

A

yes

98
Q

In a delta meter socket, which conductor has higher voltage than the others and what is this higher phase called?

A

the high leg

99
Q

What phase is the high leg?

A

Always phase A

100
Q

What precaution must be taken with a high leg?

A

a barrier between the high leg and the grounded conductor

101
Q

Which side of the meter socket is the high leg always on?

A

the side closest to the tickler

102
Q

True or False. Sometimes there are two separate 4 wire 3 phase meters, one for single phase operation and one for three phase operation

A

True, but they are not installed anymore, just maintained

103
Q

What is likely the only time you will see a 13 jaw meter?

A

when it is a seven jaw meter that also accomodates data communication (an additional six jaws)

104
Q

When are instrument meters required?

A

when there is more than 200A or 600V

105
Q

What is the standard voltage for a potential transformer?

A

120V

106
Q

What happens if the supply circuit voltage is lower than the rated voltage in a potential transformer?

A

The potential transformer also has a lower voltage

107
Q

Is the secondary of a potential transformer grounded?

A

yes

108
Q

Is the H1 terminal in a potential transformer connected on the supply side or the load side?

A

supply

109
Q

How do current transformers operate? What do they do?

A

they produce a reduced value of current for monitoring and metering purposes which is proportionate to the load

110
Q

Does a CT provide electrical isolation?

A

yes

111
Q

Why do we use CT’s?

A

they provide a safe and manageable value and provide isolation from a high voltage primary. If the current is too high to safely measure, we can use a CT to bump it down

112
Q

Name several applications for CTs

A

installations for current, power and energy measurement

operation of overload relays, OC devices, and GFCI relays

revenue meters for services over 200A

113
Q

What is the standard current for a CT?

A

5A

114
Q

What is the ratio of a CT?

A

the comparison of the primary current to the step down secondary current

115
Q

If a CT shows 5A and the ratio is 30:1, what is the primary current?

A

150A

116
Q

What are the three types of CT’s?

A

wound primary
bar type
toroid type

117
Q

Describe a wound primary CT

A

Both the primary and secondary windings are wrapped around a common core.

The H winding (primary) has a large area because of high current

The secondary (X winding) is smaller and wraps around the core both times

Both are insulated

118
Q

What is the Wound Primary CT best for?

A

metering applications because the wound primary allows better control of turn ration.

119
Q

What is a Wound Primary CT not that good for?

A

circuit protective devices

120
Q

How many primary conductor(s) does a bar type CT have running through its core and why?

A

only one because it has a secondary conductor wrapped many times around the same core

121
Q

are the primary or secondary conductors bigger in a CT and why?

A

the primary because they have to carry the full load current. The secondary only has to carry 5A (Usually)

122
Q

What are bar type CTs best suited for?

A

protection applications because they can handle the high mechanical stress brought on by fault current

123
Q

What is another name for a toroid CT?

A

a doughnut or window

124
Q

Which CT is the most difficult to replace?

A

toroid because we have to remove the conductors passing through it before it can be replaced

125
Q

When do we have to ground the secondaries on CTs?

A

when the primary is connected to 300V or greater

when they are mounted on switchboards

126
Q

Describe the turns in a CT

A

very few around the primary and many around the secondary

127
Q

What must never happen in a CT?

A

an open circuit

128
Q

Why can there never be an open circuit in a CT

A

then we would see voltage across the secondary

129
Q

Do we put an OC in a CT?

A

no because then it could trip and open cct and we cant have that

130
Q

What is the burden?

A

It is the load attached to a CT secondary and includes meters, relays and conductors

131
Q

What is the burden rating?

A

the resistance in ohms and inductance in millihenries that can be connected without causing an error

132
Q

What is the load rating of a CT?

A

the amount of current that can be applied to primary

133
Q

What are the conductor requirements for CTs?

A

10 AWG minimum and not more than 8 m in length

134
Q

What must we do to CTs?

A

short cct them

135
Q

What are the polarities of CTs and PTs?

A

subtractive

136
Q

How can we close circuit the secondarys of CTs?

A

a path through meter

shorted bars

Electrician 3rd Year (6 decks)

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