Transformers Flashcards
What is the current flowing in the primary winding called?
exciting current
The voltage induced in the primary winding ____ the primary voltage
opposes
The magnitude of the primary induced voltage is ____ than the primary voltage
less
When voltage is applied to the primary winding, the ____ current flows even though there is no load on the secondary windings
exciting
The main component that resists current in the primary winding is:
the primary induced voltage
The secondary current flux ___ the exciting current flux
opposes
The resultant flux produced a primary induced voltage that is ___ than the induced primary voltage operating at no load
less
When the induced primary voltage is reduced, the primary current:
increases
As the secondary current of a transformer increase, the primary current must _____
increase
The voltage drop caused by the total primary current and the impedance of the primary winding ____ the primary induced voltage
reduce
When the secondary voltage is reduced, the secondary output voltage is ____
reduced
The secondary output voltage at full load is ____ than its value at no load
less
What is the formula for voltage regulation?
Vno load - V full load/ V full load
What are the two main core losses?
hysteresis, eddy current
In what form are losses in the core usually found?
heat
What is hysteresis?
magnetic domains in the core are forced to flip back and forth due to flux alternations
What are eddy currents?
loss due to circulating currents in the iron core itself due to voltage induced by the changing flux
What are copper losses?
Line or I squared x R Losses
Are core losses fixed or constant?
Yes
Do copper losses change with load?
yes
What are copper losses proportionate to?
The square of the currents
Formula for copper losses?
I^2 x R
Core losses are constant from full load to no load. True or False?
True
Copper losses are proportionate to:
current
The copper losses can be determined using a _____ test
short cct
In a short circuit test, the rated secondary current is obtained at a primary voltage that is considerably ____ than the rated primary voltage
less
Formula for efficiency?
P output/P Inout
Pout/Pout + losses
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
What is the typical %IZ?
2 - 10%
What is the symbol for percent impedance voltage?
%IZ
What is the formula for max short circuit current?
Secondary rated current/%IZ
What is the danger of paralleling transformers?
Very high short cct current
What are three requirements for paralleling transformers?
same tap size, same polarity, same secondary voltage
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
Why must we shut down all transformers in parallel before working on only one of them?
Backfeed
Transformers in parallel have to have a percent impedance within ___% of each other
5
Why must we focus on proper polarity when connecting transformers in parallel?
They could easily short circuit if we hooked it up backwards
What property opposes a change in current?
inductance
List three types of transformers
core, shell, H Type
Transformer efficiency is between what percentages?
95 - 98%
What does RBCN stand for?
Reduced capacity below normal
The overall resultant magnetic flux in a transformer from no load to full load will ____
stay the same
What type of test is used to determine core losses?
open circuit test
As load increases, the core losses will:
stay the same
Impedance voltage is obtained using a ___ test
short circuit
The efficiency of a transformer will ___ as load is added
increase
How can a mutual induction transformer be reconnected to make an autotransformer?
connect line to line
The section of winding that has the same voltage as the load in a step down autotransformer is called what?
the common winding
If a mutual induction transformer is reconnected as an autotransformer, what would happen to the available KVA?
increase
When an autotransformer goes from no load to full load, the secondary output voltage would be ____ than an equivalent induction transformer
higher
Can autotransformers be used as isolation transformers?
fuck no
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)
Do autotransformers have high or low %IZ?
low
Do autotransformers have higher or lower short circuit current than regular transformers?
higher
True or false? An autotransformer is connected so that the source and the load share a direct connection point
true
What is the formula to find the series winding voltage in a step down autotransformer?
V series = Vsource - Vcommon
In a step up autotransformer, the load current is the same as the:
series winding current
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
autotransformers have a ___ voltage regulation than normal transformers
lower
Energy transfer in a transformer uses what principle?
Induction
List the three main parts of a transformer
Core, primary winding, secondary winding
List the three main uses of a transformer:
step up, step down, isolation
The higher voltage terminals are labeled:
H
Can we use transformers for DC?
No, there is no induction in DC
A transformer can operate up to ___% of its rated voltage
110
What does FCAN stand for?
Full capacity above normal
Turns ratio is directly proportional to voltage ration in a transformer. True or false?
true
What test is used to determine impedance voltage?
short cct
What is required to prevent backfeeding when transformers are connected in parallel?
Either a transfer switch or shutting the entire transformer bank down
Do autotransformers use mutual induction or self induction? Why?
Self induction because they are both connected to the same core
What are the two windings in an autotransformer called?
series winding and common winding
KVA in = KVA out. T or F?
True
What is the total apparent rating of a transformer?
its capability to supply the max rated voltage and max rated current without overheating
What are autotransformers ideal for?
small voltages changes
state three advantages of an autotransformer compared to a normal transformer
lower cost, better voltage regulation, smaller size
state three disadvantages of an autotransformer compared to a normal transformer
do not isolate secondary from primary, dangerous if open circuit, higher fault currents
Is the volts/turn ratio always the same in primary and secondary windings?
yes
Meters are either ___ or they are not
self contained
Metering equipment over 200A and/or 600V requires ___
instrument transformers
What is the maximum current and voltage for a self contained meter?
200A or 600 V
What are the most common types of meters?
120/240 3 wire
120 V single phase
120/208 3 wire network
What is the most common meter socket for single phase circuits?
120/240 3 wire
How many jaws on a 120/240 3 wire meter?
4
What is the most common meter socket in residential?
120/240 3 wire
Where are 120/240 three wire meters designed for?
overhead or underground
What is the only common 120V single phase meter application?
a billboard
How many jaws on a single phase, 3 wire 120/208 network meter?
5
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
___ phase panelboards are used on a single phase, 3 wire 12 meter
single
Does a single phase 3 wire 120/208 network meter need a tickler?
yes
where does the tickler go on a single phase, 120/208 network meter?
from the tickler jaw to the common
What is the tickler for?
to maintain correct volt drop and phase relationships with the current
What are the two styles of three phase meter sockets?
4 wire wye
4 wire delta
What style of meter are most 3 phase 4 wire sockets connected in?
Wye
What are the three main voltages for a 3 phase 4 wire wye meter?
120/208
277/480
347/600
How many jaws on a three phase 4 wire wye socket?
7
Can a wye meter socket be connected to supply both single phase and three phase?
yes
In a delta meter socket, which conductor has higher voltage than the others and what is this higher phase called?
the high leg
What phase is the high leg?
Always phase A
What precaution must be taken with a high leg?
a barrier between the high leg and the grounded conductor
Which side of the meter socket is the high leg always on?
the side closest to the tickler
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
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)
When are instrument meters required?
when there is more than 200A or 600V
What is the standard voltage for a potential transformer?
120V
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
Is the secondary of a potential transformer grounded?
yes
Is the H1 terminal in a potential transformer connected on the supply side or the load side?
supply
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
Does a CT provide electrical isolation?
yes
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
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
What is the standard current for a CT?
5A
What is the ratio of a CT?
the comparison of the primary current to the step down secondary current
If a CT shows 5A and the ratio is 30:1, what is the primary current?
150A
What are the three types of CT’s?
wound primary
bar type
toroid type
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
What is the Wound Primary CT best for?
metering applications because the wound primary allows better control of turn ration.
What is a Wound Primary CT not that good for?
circuit protective devices
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
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)
What are bar type CTs best suited for?
protection applications because they can handle the high mechanical stress brought on by fault current
What is another name for a toroid CT?
a doughnut or window
Which CT is the most difficult to replace?
toroid because we have to remove the conductors passing through it before it can be replaced
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
Describe the turns in a CT
very few around the primary and many around the secondary
What must never happen in a CT?
an open circuit
Why can there never be an open circuit in a CT
then we would see voltage across the secondary
Do we put an OC in a CT?
no because then it could trip and open cct and we cant have that
What is the burden?
It is the load attached to a CT secondary and includes meters, relays and conductors
What is the burden rating?
the resistance in ohms and inductance in millihenries that can be connected without causing an error
What is the load rating of a CT?
the amount of current that can be applied to primary
What are the conductor requirements for CTs?
10 AWG minimum and not more than 8 m in length
What must we do to CTs?
short cct them
What are the polarities of CTs and PTs?
subtractive
How can we close circuit the secondarys of CTs?
a path through meter
shorted bars
