216 terms
Transformers,Single phase transformer operation, autotransformers, energy measurements
3rd yr electrical, - into to transformers
Induction
Create/produced voltaged
Reluctance
The opposition offered to the magnetic flux in a magnetic circuit
Air has high/low reluctance?
High reluctance
Iron has high/ low reluctance
Low reluctance
What can cause transformer losses
Eddy currents and hysteresis losses
What are eddy currents
EC Occur due to circulating currents in the iron cor itself due to voltage induced by changing flux.
Causes heating in the core which causes the transformer to be less efficient
Causes heating in the core which causes the transformer to be less efficient
What are Hysteresis losses
HL Occur when magnetic domains in the core are forced to flip back and fourth as the flux alternates
What are the three basic types of transformers construction
Core, Shell and H Type
How efficient are Transformers
95-98%
Why are Transformers not 100% efficient
There are copper losses in the winding, as well as eddy currents and hysteresis losses in the core
Eddy currents and
produce?
produce?
Currents which are induced into the metal core material by changing the magnetic field as the ac produces a changing flux.
Heat- which is power loss
A core loss
Heat- which is power loss
A core loss
Hysteresis
Molecular friction caused by the reversal of the direction of current flow,
Constant rubbing together causes heat
Which is power loss
A core loss
Constant rubbing together causes heat
Which is power loss
A core loss
What are Core losses
Eddy currents and Hysteresis
Define Flux leakage
Represents a small amount of lost energy. Does not produce heat,
Power loss
Power loss
What is listed on a Transformer Name Plate
Primary voltage
Secondary Voltage
kVA rating
Secondary Voltage
kVA rating
KVA on transformer nameplate always represents the full load (output/input)
Always the full load output
What info may be listed on a transformer name plate?
Primary Voltage,
Secondary voltage
kVA rating
frequency
temperature rise in Celsius
Percent impedance (%Z or %IZ)
type of insulating oil
serial number
type number
whether transformer is single-phas or three phase
Secondary voltage
kVA rating
frequency
temperature rise in Celsius
Percent impedance (%Z or %IZ)
type of insulating oil
serial number
type number
whether transformer is single-phas or three phase
The transformer winding that is connected to the input voltage is where the magnetic flux originates. What is this winding?
The Primary Winding
The winding connected to the load is what winding?
The Secondary Winding
How is Percent impedance expressed?
%Z or %IZ
How is transformer impedance determined? list the physical construction factors
The amount and type of core material
the winding wire size
The number of winding turns
the degree of magnetic coupling
the winding wire size
The number of winding turns
the degree of magnetic coupling
How do you test for the impedance on a transformer
which a short-circuit test
List the 3 Single-Phase Transformer Fuctions
To step up the voltage of the circuit
to step down the voltage of the circuit
to isolate one circuit for another
to step down the voltage of the circuit
to isolate one circuit for another
What is a Step up transformer?
A Transformer where the secondary transformer voltage is higher than the primary voltage
Define a Step down Transformer
A transformer where the output voltage is lower the the input.
Give an example of how a step down transformer will be used
A distribution transformer that is used to supply to residential consumers is a step down.
Isolation Transformer- define
the secondary voltage does not change, the output voltage to the load equals the input voltage. Isolates the supply voltage
Output voltage = Input voltage
Output voltage = Input voltage
What is the American National Standards Institute (ANSI) standard for marking transformer winding's?
ANSI standards require the highvoltage terminals or leads to be identified byt the capital letter H
and the low voltage terminals or leads to be identified by the capital letter X
and the low voltage terminals or leads to be identified by the capital letter X
Do ANSI standards refer to primary or secondary winding's with H and X
No, Because H and X can be primary or secondary depending if it is a step up or step down transformer.
How are the winding's marked in a step up transformer? Primary , Secondary?
Primary - X1,X2 and Secondary, - H1,H2
A Transformer is an alternating current device that generates electrical energy
(True or False)
(True or False)
False, Transfers
The basic mutual induction transformer has at least two coils or windings of wire wound on a common core
(True or False)
(True or False)
True
A step-up transformer is used where the output voltage is to be ______________ than the input voltage
higher
A step-down transformer is used where the output voltage is to be _____________ than the input voltage
lower
The efficiency of a transformer is )___________ over a wide range of loads
very high
The output winding connected to the load is the _________________ winding
secondary
The terminals of the high voltage winding are marked with the letter ____ and the terminals of the low-voltage windings are marked with ____
H, X
What test is performed on a transformer to determine its impedance?
Short Circuit
What is a transformer
it is an alternating current device that efficiently transfers electrical energy from one AC circuit to another by Electromagnetic Induction.
VA input =
VA output
How do you Calculate KVA, if you have the Watts and power factor?
VA=W/pf
What is the turns Ration Calculation
Nprimary/Nsecondary
example
300 turns/600 turns = 1/2=1:2
example
300 turns/600 turns = 1/2=1:2
Voltage ratio is done by
Vprimary/V secondary
example
120/240 = 1/2=1:2
example
120/240 = 1/2=1:2
Current Ratio
I primary/ I secondary = 6A/3A=2/1=2:1,
Current ration is opposite of the Voltage Ratio
Current ration is opposite of the Voltage Ratio
The current ration is opposite of the Voltage ratio
True because the Apparent Power ratio has to equal 1
Volts per turn ratio
Voltage/Turns
example
120V/240 turns=0.5V/Turn
example
120V/240 turns=0.5V/Turn
Transformer polarity is:
Relative direction of the transformer's primary voltage compared to its induced secondary voltage.
When is transformer polarity important?
When transformers are paralleled to gain additional capacity OR to hook up three single phase transformers to make a three-phase transformer bank
According to the ANSI standards, where is terminal H1?
it will always be on the upper left-hand corner
If a transformer is constructed so that X1 is diagonally opposite of Terminal H1, it has ___________ polarity
additive
if a transformer is constructed so that X1 is directly opposite of Terminal H1 it has ____________ polarity
subtractive Polarity
Why Transformer Voltage Taps?
Since the applied voltage attached to the primary does not always exactly match the nameplate, many transformers have taps arranged in the high voltage windings so that the proper secondary voltage is obtained
Tapped Secondary windings
Some transformers can supply more than one voltage from their secondary windings. Taps are taken off the secondary winding at different points to access the secondary voltages
Tapped Primary Windings
The voltage applied may not always be the same as the nameplate primary voltage. Taps are installed to compensate for slightly lower or higher voltage.
-5% tap is used when the primary supply voltage is 5% lower than the rated transformer primary voltage.
-5% tap is used when the primary supply voltage is 5% lower than the rated transformer primary voltage.
What is the calculation for % Tap needed on a transformer?
% Tap=(actual primary voltage- rated primary voltage)/rated primary voltage
example:
(585V-600V)/600 V
=-15/600
=-.025
=-2.5%
example:
(585V-600V)/600 V
=-15/600
=-.025
=-2.5%
A tap labelled 5% full capacity below normal
Used when the primary supply voltage is 5% lower the rated transformer primary voltage.
FCBN
Full capacity below normal
FCAN
Full capacity above normal
RCBN
Reduced capacity below normal
A tap labelled 5% FCAN means
that tap would be used if the supply voltage is 5% higher than the rated transformer primary voltage
A tap labelled 2.5% RCBN
would be used when the primary supply voltage is 2.5% lower than the rated transformer primary voltage. Because this is a reduced capacity transformer, the rated KVA capacity will also be reduced.
What are three ways the taps can be changed on a transformer?
Manually changing the leads on the transformer,
Moving Connecting bars
Installing a mechanical device such as a rotary switch
Moving Connecting bars
Installing a mechanical device such as a rotary switch
Transformers need to be what? when taps are being
de energized
How does a transformer transfer electrical energy from one circuit to another?
Electromagnetic induction
(Mutual induction)
Transformer action
(Mutual induction)
Transformer action
What are two key components that cause voltage drops on the primary and secondary windings a loaded transformer?
Inductive Reactance
Resistance
Resistance
what is the small amount of current that flows in the primary winding?
Exciting current
What does the Exciting current do?
Creates exciting flux in the transformer core
voltages are induced in bothe the primary and secondary by the ___________ flux
Exciting flux
Exciting flux
.266
What is the current called in th secondary winding which produces a magnetic flux?
Secondary current flux
Secondary current opposes the exciting flux
True/False
True/False
True
What is the difference between the secondary current flux and exciting flux?
The resultant flux in the core
There is a slight reduction/big reduction in the overall resultant magnetic flux at full load operation compared to no-load operation
Slight reduction ( a few percent)
VA in =VAOut + ________
Losses
What is the Percent voltage regulation Formula?
Percent Voltage Regulations =
((VS No load - VS full load)/VS full load)x100
((VS No load - VS full load)/VS full load)x100
Define percent voltage regulation of a transformer
the change of the secondary output voltage from a no-load to full - load condition.
((VS No load - VS full load)/VS full load)x100
((120 V- 117V)/117V)x 100
=2.56%
((VS No load - VS full load)/VS full load)x100
((120 V- 117V)/117V)x 100
=2.56%
The current flowing in the primary winding (Ip) is called the (Exisiting, exciting) current?
Exciting
The voltage induced in the primary winding Ep (Assists, opposes) the primary voltage Vp
opposes
The magnitude of the primary induced voltage (Ep) is (greater, less) than the primary voltage
less
When voltage is applied to the primary winding, the exciting current (Flows,does not flow) even if there is no load on the secondary windings.
Flows
The Main component that resists current in the primary winding is (Ep, Es)
EP
What are Core Losses
Hysteresis and Eddy currents\
Hysteresis and Eddy currents are called?
Core Losses - dissipated as heat
How are Core losses tested?
Open Circuit Test
Core Losses are dependent on?
the applied Voltage
If there is a constant rated voltage applied the core losses remain constant from no load to full load. True or False
True
What is the Heat energy dissipated called?
Copper Losses or I2R losses
Copper losses are proportional to the square of the currents and vary from no load to full load conditions
True or False
True or False
True
What is the calculation of Copper Losses?
Primary rated current = VA/V = Amps
Primary copper losses = I2R=amps2 x R= Watts
Example
1000 VA/600 V = 1.67 A
1.67 squared x 10 = 27.89
Primary copper losses = I2R=amps2 x R= Watts
Example
1000 VA/600 V = 1.67 A
1.67 squared x 10 = 27.89
What is the total copper losses
Total copper losses = Primary copper loss+ Secondary Copper Loss
Example 28 W +130 W = 158 W
Example 28 W +130 W = 158 W
Copper losses are proportional to the _________ of the currents
Square
The total copper losses at half load of a transformer. what would the formula look like?
=5 squared x W (full load copper losses)
the total copper losses at one quarter are calculated how?
= .25 squared x W (full load copper losses
How can copper losses of a transformer be determined?
Short-circuit test
Name two types of Losses that contribute to core losses?
Eddy currents
Hysteresis
Hysteresis
Transformer core losses remain basically constant from no load to full load conditions? True or False
True
If a constant rated voltage is applied to a transformer, its core losses are ______ from no load to full load conditions
constant
Secondary copper losses are equal to the square of the secondary (current, voltage) times the resistance of the secondary winding
Current
The copper losses of a transformer can be determined by a ___________ test
short circuit
In a short circuit test the reated secondary current is obtained at a primary voltage that is considerably (less, more) than the rated primary voltage
less
what is the copper loss calculation
KVA/V = Amps
Amps squared x winding resistance= watts loss
Amps squared x winding resistance= watts loss
Describe the Percent efficiency of a transformer calculation
=( Power output/Power input ) x 100
Define Power input
Comes from Power output and core losses and copper losses
Percent Efficiency
= (Power output / Power input)x 100
example
(100 000 W/ (100 000 +230 W +956))x 100
=98.8%
example
(100 000 W/ (100 000 +230 W +956))x 100
=98.8%
List factors that affect the Transformer efficiency
-Load capacity, as a transformer is decreased, the percent efficiency also decreases
- Power factor, as the power factor increase, the percent efficiency of the transformer also increase
- Losses as the copper and core losses increase, the percent efficiency decreases
- Power factor, as the power factor increase, the percent efficiency of the transformer also increase
- Losses as the copper and core losses increase, the percent efficiency decreases
Define Impedance Voltage
is the primary voltage required to circulate the rated full load secondary current in a transforem
_________ _______ voltage is generally between 2-10%, depending on the size and design of the transformer
the Percent Impedance Voltage
what are the symbol for Percent impedance Voltage?
%IZ or %Z
How is an Impedance Voltage obtained
Short Circuit Test
what is the impedance voltage formula
(Impedance voltage/rated voltage )x 100
Example:
(15/600) x100= 2.5%
Example:
(15/600) x100= 2.5%
Short circuit test is how_________ voltage is found
Impedance
short circuit conditions -
Impedance is typically low and current is high
How do you find maximum available short circuit current
Full load secondary current/%IZ
How do you find full load secondary current?
Full load secondary current= VA/A = Amps
How do you find maximum secondary short circuit current?
Full load secondary current/IZ%
Example 250 A/4%
= 6250
Example 250 A/4%
= 6250
What is a practical way to increase the capacity of an existing system
Paralleling single phase transformer
What do you need to be cautious about when paralleling transformers
The impedance drops and the avail short circuit current can go up considerably.
How do you parallel single transformer
Primaries of the transformers are supplied of the same source, secondaries are connected to the same load
What are precautions on paralleling transformers
-No load secondary voltages must be equal
-Same tap settings must be used on both transformers
-percent impedance within 5% of each other
-windings must be connected in phase with each other
- transformer in parallel need to be disconnected from all circuitry bc it can backfeed
-Same tap settings must be used on both transformers
-percent impedance within 5% of each other
-windings must be connected in phase with each other
- transformer in parallel need to be disconnected from all circuitry bc it can backfeed
What are three precautions that must be observed when paralleling transformers
- %IZ within 5%
- same polarity
- same taps used
- same polarity
- same taps used
Why should two transformers connected in parallel have the same rated percent impedance?
For a balanced load
Why dangerous situation exists when transformer B is disconnected as shown?
Back feed
Percent voltage regulation
((V secondary no load - V secondary full load)/ V secondary full load) x 100
Primary current
VA/ primary V = primary current
Primary copper losses
I Squared R
Copper losses are found by _____ circuit test
Short
Core losses are found by ________ circuit test
Open circuit
Impedance voltage are found by ________ circuit test
Short circuit
What is a auto transformer?
It's an ac device that transfers electrical energy from one ac circuit to another by self induction
Difference between transformers and auto transformers.
It is a physical connection between the two windings in the auto transformer
What is a autotransformer that increAses the voltage from primary to secondary
Step up transformer
What are the two sections of the winding called
Common, and series
Draw a step up auto transformer labeling series winding
Common winding
Common winding
A) series winding
B) common winding
B) common winding
What is the ratio of voltages to the ratio of the number of turns is represented by: use a step up transformer
Describe the Voltage Relationship for a Step down autotransformer, Draw a Step down transformer
V1 = VAB+VBC
Describe the ratio of voltages to the ratio of number of turns
V1/V2=N1/N2
Draw a step down Auto Transformer with all the proper labels
Describe the Current Relationship in a Step down Transformer, Draw a step down transformer, Label Current in Primary IP , Current in Secondary IS
ICB = I secondary - I primary
An Autotransformer transfers energy from one ac circuit to another AC circuit by _______________ induction
Self Induction
The two windings in an autotransformer are identified as the (Common, continuous) winding and the (series,parallel) winding.
Common,
Series
Series
Ignoring losses, a basic equation in autotransformer ( or any transformer) describe the calculation
VA input= VA output
VA input on the Primary side of a step up transformer are?
Primary Side - Terminals B and C
input = V x A
= VA
input = V x A
= VA
VA output on the Secondary side of a step up transformer are calculated how?
Secondary Side Terminals A and C
VA output = VA input
Rated secondary current
Rated IS = Rated VA output/VS
= Secondary I
VA output = VA input
Rated secondary current
Rated IS = Rated VA output/VS
= Secondary I
Do you apply kirchhoffs current law in a step up transformer? Describe how you would get the Common Winding Amperage. Draw a step up transformer and label everything
YES
I BC = IPrimary- I secondary
I BC = IPrimary- I secondary
Do you apply kirchhoffs current law in a step up transformer? Describe how you would get the Series Winding Amperage. Draw a step up transformer and label everything
Yes,
I BA= I secondary
I BA= I secondary
Describe Transformed Apparent Power? Draw a step up transformer , label everything
Transformed apparent power is the product of the common winding voltage times the common winding current
Describe Conducted Apparent Power, Draw a step up transformer and label everything
Conducted Apparent power is the product of the common winding voltage x the series winding current
The total apparent power is the sum of conducted VA and Transformed VA ? True or False?
True
Calculation for Conducted VA rating?
=V common winding I series winding
Calculation for Transformed VA rating
= V common winding x I common Winding
Give the calculation for a step up transformer
Total VA=
Total VA=
Conducted VA rating = Transformed VA rating
Calculation for the Rated I primary of a Auto Transformer
Rated I primary= VA input/Voltage primary
= VA/V
=Primary Amperage
= VA/V
=Primary Amperage
Calculations for the Rated S Primary of an Auto transformer
Rated I secondary= VA output/Voltage secondary
=VA /V
=Secondary Amerage
=VA /V
=Secondary Amerage
List the advantages of Auto Transformers
Ideal for small voltage changes
Better percent voltage regulation
Smaller in size and lower in cost
Better percent voltage regulation
Smaller in size and lower in cost
Disadvantages of an Autotransformer
Do not Isolate the secondary circuit from the primary circuit
- Higher available short-circuit currents
-Can damage the load and have serious shock damage
- Higher available short-circuit currents
-Can damage the load and have serious shock damage
What happens if the common winding in a step down auto transformer becomes open circuited?
the source voltage will be applied directly to the load
Metering equipment for services over 200A/and or over 600 V must include what?
Instrument transformers
What is an instrument transformer
is a device so that the meter measures only a percentage of the actual current or voltage values. then the values are multiplied by the appropriate multiplier
Define Self contained Meters
They do not require instrument transformers and are used on serves with a max 200A and 600 V.
They measure the actual service current or voltage.
They measure the actual service current or voltage.
Self contained meters are available for single phase and three phase services. True or false
True
What are the most common single phase meter sockets designed for
120/240 V, Three Wire service
120 V single-phase service
120/208 V , three-wire network service
120 V single-phase service
120/208 V , three-wire network service
Which is the most common meter socket for single phase circuits?
120/240 V three wire service
Are the connections different for a 120/240 three wire service in a overhead or underground service?
No, The connections they are the same in both
Which other service can have the same connections as the 120/240 three wire service?
240/480 - but this applications is special and rarely used
Draw the 120/240 V three wire service
...
Where are 120/240 V three wire services most commonly used?
Residential
Draw a 120V single-Phase service
...
Where are 120V single Phase services most commonly used
Billboards
What is the Service ampacity limited to? and by who
30 Amps and by most utility companies
Draw a Single phase, three-wire 120/208 V network system
...
Where are single phase three wire 120/208 V network services commonly used?
Apartment buildings, small city center parking lots
This is more correctly known as 2 phase, three wire, 120/208 network service
This is more correctly known as 2 phase, three wire, 120/208 network service
what kind of meter does a single phase, three wire 120/208 network service require?
a Five Jaw Meter Socket
What does a Tickler wire do?
It connects the fifth jaw to the common conductor,
Maintains the correct voltage drop and correct phase relationship with the currents,
If there was no tickler in a 120/208 meter system the voltage read would be 104 instead of 120
Maintains the correct voltage drop and correct phase relationship with the currents,
If there was no tickler in a 120/208 meter system the voltage read would be 104 instead of 120
Can a standard residential meter and meter socket be used for a 2 phase three wire 120/208 V network service
No, it would not indicated the correct voltage and phase voltage
it needs the tickler to correct the power reading
it needs the tickler to correct the power reading
Three phase meter sockets are dependent on what
the connection type of the current carrying conducters, (wye or Delta)
Three phase meter sockets can be used for what 2 types of service?
Four wire wye connected service
Four wire delta connected service
Four wire delta connected service
What ability does a four wire wye service have
to provide power for bothe three phase and single phase loads.
Where can you find a Three phase meter socket
Commercial, industrial, Example
What types of voltage can be found in a 3 phase, 4 wire wye service?
120/208
277/480
347/600
277/480
347/600
What type of meter will use a three phase foure wire wye connected service?
Three phase services of 200 AMPs or less use this type of meter socket.
Short circuit tests on a transformer can be used to obtain
Impedance
Copper losses vary with the square root of the load current
False
As load is added to a transformer, the copper loss -does what?
Increases
From no load to full load the core losses of a transformer will what?
Stay the same
In order to determine the core losses of a transformer it is necessary to perform the open circuit test
True or false
True or false
True
The value of the percent impedance for a transformer can be calculated from the figures obtained from a short circuit test
T/F
T/F
True
The max secondary short circuit current of a transformer is determined by the apparent power size of the transformer, the voltage, and the percent impedance voltage
T/f
T/f
True
Full load secondary circuit/ %IZ
Full load secondary
= KVA/secondary Voltage
Full load secondary circuit/ %IZ
Full load secondary
= KVA/secondary Voltage
Factors that affect the efficiency of a transformer
Load and losses
The percent impedance value can be used to help determine the over current protection required for a transformer
T/F
T/F
True
Two transformers are connected in parallel, overall %IZ of the two transformers now has- what?
Decreases
Before attempting to parallel two single phase transformers, we must observe the polarities are the same and they must - have what three things?
-same secondary no load voltage ratings,
-the same percent impedance
-the same tap Settings
-the same percent impedance
-the same tap Settings
What is a hazard of working on transformers that are paralleles
Back feeding
The fault current rating of protective equipment can't be exceeded by paralleling transformers
T/F
T/F
False
The core losses of a transformer are a result of the magnetizing effect of the exciting current
T\F
T\F
True
Your measured primary currents are slightly higher than your calculated primary currents
Bc of exciting currents
The law of conservation of energy
Accounts for the transformation of the energy from one form to another
The excitation magnetic flux in a transformer is what?
Is constant as the load current changes
As load is added to a transformer the applied primary voltage is unchanged and the induced primary voltage is reduced
T/F
T/F
True
There is primary current , but no secondary current, at 0% load because-
Exciting current is present
The core losses of a transformer are a result of the magnetizing effect of the exciting current
True or false?
True or false?
True, hysteresis
Eddy currents
Eddy currents
What does the exciting current of a transformer supply?
Supplies the iron losses and magnetization current
The excitation magnetic flux in a transformer is what
Constant as the load current changes
The magnetic field is caused by what?
Current
Factors that effect efficiency?
Load and losses
Core losses of a transformer can be obtaine by the ________ circuit trst
Open circuit
Copper losses of a transformer can be obtained by a _________ circuit test
Short circuit
Power loss in heat in the windings do to the flow of. Current through the copper is known as what?
Copper losses
What is the reason for transformers having laminated cores is-
To reduce the core loss
A transformer is a devices that can transfer energy from on circuit to another without any electrical connections between the two circuits
T/F?
T/F?
True
A transformer in which the primary and secondary circuits have a portion of the winding common to both is called?
Auto transformer
240 V
3 phase, 3 wire
3 phase, 3 wire
120/208
3 phase
4 wire
3 phase
4 wire
120/240 volt
3 phase, 4 wire
3 phase, 4 wire
120/208 V
2 phase 3 wire network
2 phase 3 wire network
120 volt
2 wire
1 phase
2 wire
1 phase
120/240 V
1phase ,3 wire
1phase ,3 wire