216 terms

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