Transformers Flashcards
1
Q
What is the current flowing in the primary winding called?
A
exciting current
2
Q
The voltage induced in the primary winding ____ the primary voltage
A
opposes
3
Q
The magnitude of the primary induced voltage is ____ than the primary voltage
A
less
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
5
Q
The main component that resists current in the primary winding is:
A
the primary induced voltage
6
Q
The secondary current flux ___ the exciting current flux
A
opposes
7
Q
The resultant flux produced a primary induced voltage that is ___ than the induced primary voltage operating at no load
A
less
8
Q
When the induced primary voltage is reduced, the primary current:
A
increases
9
Q
As the secondary current of a transformer increase, the primary current must _____
A
increase
10
Q
The voltage drop caused by the total primary current and the impedance of the primary winding ____ the primary induced voltage
A
reduce
11
Q
When the secondary voltage is reduced, the secondary output voltage is ____
A
reduced
12
Q
The secondary output voltage at full load is ____ than its value at no load
A
less
13
Q
What is the formula for voltage regulation?
A
Vno load - V full load/ V full load
14
Q
What are the two main core losses?
A
hysteresis, eddy current
15
Q
In what form are losses in the core usually found?
A
heat
16
Q
What is hysteresis?
A
magnetic domains in the core are forced to flip back and forth due to flux alternations
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
18
Q
What are copper losses?
A
Line or I squared x R Losses
19
Q
Are core losses fixed or constant?
A
Yes
20
Q
Do copper losses change with load?
A
yes
21
Q
What are copper losses proportionate to?
A
The square of the currents
22
Q
Formula for copper losses?
A
I^2 x R
23
Q
Core losses are constant from full load to no load. True or False?
A
True
24
Q
Copper losses are proportionate to:
A
current
25
The copper losses can be determined using a _____ test
short cct
26
In a short circuit test, the rated secondary current is obtained at a primary voltage that is considerably ____ than the rated primary voltage
less
27
Formula for efficiency?
P output/P Inout
Pout/Pout + losses
28
Describe impedance voltage
it is the primary voltage required to circulate the rated full-load secondary current in a transformer when the secondary winding is short circuited
29
What is the typical %IZ?
2 - 10%
30
What is the symbol for percent impedance voltage?
%IZ
31
What is the formula for max short circuit current?
Secondary rated current/%IZ
32
What is the danger of paralleling transformers?
Very high short cct current
33
What are three requirements for paralleling transformers?
same tap size, same polarity, same secondary voltage
34
Why should two transformers connected in parallel have the same %IZ?
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
35
Why must we shut down all transformers in parallel before working on only one of them?
Backfeed
36
Transformers in parallel have to have a percent impedance within ___% of each other
5
37
Why must we focus on proper polarity when connecting transformers in parallel?
They could easily short circuit if we hooked it up backwards
38
What property opposes a change in current?
inductance
39
List three types of transformers
core, shell, H Type
40
Transformer efficiency is between what percentages?
95 - 98%
41
What does RBCN stand for?
Reduced capacity below normal
42
The overall resultant magnetic flux in a transformer from no load to full load will ____
stay the same
43
What type of test is used to determine core losses?
open circuit test
44
As load increases, the core losses will:
stay the same
45
Impedance voltage is obtained using a ___ test
short circuit
46
The efficiency of a transformer will ___ as load is added
increase
47
How can a mutual induction transformer be reconnected to make an autotransformer?
connect line to line
48
The section of winding that has the same voltage as the load in a step down autotransformer is called what?
the common winding
49
If a mutual induction transformer is reconnected as an autotransformer, what would happen to the available KVA?
increase
50
When an autotransformer goes from no load to full load, the secondary output voltage would be ____ than an equivalent induction transformer
higher
51
Can autotransformers be used as isolation transformers?
fuck no
52
Can we use an autotransformer for residential 416-:240 distribution? Why or Why not?
No, we can't because if the common winding were to be open circuited, the people's houses would receive line voltage (4160)
53
Do autotransformers have high or low %IZ?
low
54
Do autotransformers have higher or lower short circuit current than regular transformers?
higher
55
True or false? An autotransformer is connected so that the source and the load share a direct connection point
true
56
What is the formula to find the series winding voltage in a step down autotransformer?
V series = Vsource - Vcommon
57
In a step up autotransformer, the load current is the same as the:
series winding current
58
Autotransformers have a lower %IZ than regular transformers. Is this a good thing or a bad thing?
It is a bad thing because they have higher short cct current
59
autotransformers have a ___ voltage regulation than normal transformers
lower
60
Energy transfer in a transformer uses what principle?
Induction
61
List the three main parts of a transformer
Core, primary winding, secondary winding
62
List the three main uses of a transformer:
step up, step down, isolation
63
The higher voltage terminals are labeled:
H
64
Can we use transformers for DC?
No, there is no induction in DC
65
A transformer can operate up to ___% of its rated voltage
110
66
What does FCAN stand for?
Full capacity above normal
67
Turns ratio is directly proportional to voltage ration in a transformer. True or false?
true
68
What test is used to determine impedance voltage?
short cct
69
What is required to prevent backfeeding when transformers are connected in parallel?
Either a transfer switch or shutting the entire transformer bank down
70
Do autotransformers use mutual induction or self induction? Why?
Self induction because they are both connected to the same core
71
What are the two windings in an autotransformer called?
series winding and common winding
72
KVA in = KVA out. T or F?
True
73
What is the total apparent rating of a transformer?
its capability to supply the max rated voltage and max rated current without overheating
74
What are autotransformers ideal for?
small voltages changes
75
state three advantages of an autotransformer compared to a normal transformer
lower cost, better voltage regulation, smaller size
76
state three disadvantages of an autotransformer compared to a normal transformer
do not isolate secondary from primary, dangerous if open circuit, higher fault currents
77
Is the volts/turn ratio always the same in primary and secondary windings?
yes
78
Meters are either ___ or they are not
self contained
79
Metering equipment over 200A and/or 600V requires ___
instrument transformers
80
What is the maximum current and voltage for a self contained meter?
200A or 600 V
81
What are the most common types of meters?
120/240 3 wire
120 V single phase
120/208 3 wire network
82
What is the most common meter socket for single phase circuits?
120/240 3 wire
83
How many jaws on a 120/240 3 wire meter?
4
84
What is the most common meter socket in residential?
120/240 3 wire
85
Where are 120/240 three wire meters designed for?
overhead or underground
86
What is the only common 120V single phase meter application?
a billboard
87
How many jaws on a single phase, 3 wire 120/208 network meter?
5
88
What is the difference between a 120/208 3 wire network and a 120/240 single phase?
the neutral is actually a common conductor that can carry the same current is the lines
89
___ phase panelboards are used on a single phase, 3 wire 12 meter
single
90
Does a single phase 3 wire 120/208 network meter need a tickler?
yes
91
where does the tickler go on a single phase, 120/208 network meter?
from the tickler jaw to the common
92
What is the tickler for?
to maintain correct volt drop and phase relationships with the current
93
What are the two styles of three phase meter sockets?
4 wire wye
| 4 wire delta
94
What style of meter are most 3 phase 4 wire sockets connected in?
Wye
95
What are the three main voltages for a 3 phase 4 wire wye meter?
120/208
277/480
347/600
96
How many jaws on a three phase 4 wire wye socket?
7
97
Can a wye meter socket be connected to supply both single phase and three phase?
yes
98
In a delta meter socket, which conductor has higher voltage than the others and what is this higher phase called?
the high leg
99
What phase is the high leg?
Always phase A
100
What precaution must be taken with a high leg?
a barrier between the high leg and the grounded conductor
101
Which side of the meter socket is the high leg always on?
the side closest to the tickler
102
True or False. Sometimes there are two separate 4 wire 3 phase meters, one for single phase operation and one for three phase operation
True, but they are not installed anymore, just maintained
103
What is likely the only time you will see a 13 jaw meter?
when it is a seven jaw meter that also accomodates data communication (an additional six jaws)
104
When are instrument meters required?
when there is more than 200A or 600V
105
What is the standard voltage for a potential transformer?
120V
106
What happens if the supply circuit voltage is lower than the rated voltage in a potential transformer?
The potential transformer also has a lower voltage
107
Is the secondary of a potential transformer grounded?
yes
108
Is the H1 terminal in a potential transformer connected on the supply side or the load side?
supply
109
How do current transformers operate? What do they do?
they produce a reduced value of current for monitoring and metering purposes which is proportionate to the load
110
Does a CT provide electrical isolation?
yes
111
Why do we use CT's?
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
Name several applications for CTs
installations for current, power and energy measurement
operation of overload relays, OC devices, and GFCI relays
revenue meters for services over 200A
113
What is the standard current for a CT?
5A
114
What is the ratio of a CT?
the comparison of the primary current to the step down secondary current
115
If a CT shows 5A and the ratio is 30:1, what is the primary current?
150A
116
What are the three types of CT's?
wound primary
bar type
toroid type
117
Describe a wound primary CT
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
What is the Wound Primary CT best for?
metering applications because the wound primary allows better control of turn ration.
119
What is a Wound Primary CT not that good for?
circuit protective devices
120
How many primary conductor(s) does a bar type CT have running through its core and why?
only one because it has a secondary conductor wrapped many times around the same core
121
are the primary or secondary conductors bigger in a CT and why?
the primary because they have to carry the full load current. The secondary only has to carry 5A (Usually)
122
What are bar type CTs best suited for?
protection applications because they can handle the high mechanical stress brought on by fault current
123
What is another name for a toroid CT?
a doughnut or window
124
Which CT is the most difficult to replace?
toroid because we have to remove the conductors passing through it before it can be replaced
125
When do we have to ground the secondaries on CTs?
when the primary is connected to 300V or greater
when they are mounted on switchboards
126
Describe the turns in a CT
very few around the primary and many around the secondary
127
What must never happen in a CT?
an open circuit
128
Why can there never be an open circuit in a CT
then we would see voltage across the secondary
129
Do we put an OC in a CT?
no because then it could trip and open cct and we cant have that
130
What is the burden?
It is the load attached to a CT secondary and includes meters, relays and conductors
131
What is the burden rating?
the resistance in ohms and inductance in millihenries that can be connected without causing an error
132
What is the load rating of a CT?
the amount of current that can be applied to primary
133
What are the conductor requirements for CTs?
10 AWG minimum and not more than 8 m in length
134
What must we do to CTs?
short cct them
135
What are the polarities of CTs and PTs?
subtractive
136
How can we close circuit the secondarys of CTs?
a path through meter
shorted bars
