09a Transformers - Presentation

8/8/2019 09a Transformers - Presentation

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Transformers

Transformer - an electrical device

constructed of two coils placed in closeproximity to each other so that there is a

mutual inductance between the two coils


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Mutual Inductance

A changing

electromagnetic

field produced

by the current in

one coil will

cause an

induced voltagein the second

coil because of

mutual

inductance.


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Basic Transformer

Input connected to

primary winding

Load connected to

secondary winding

No physical

connection

between input andload

isolation


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Transformer Construction

Wires are coated

with varnish

insulation to preventshorts

Amount of magnetic

coupling between

primary and

secondary is set by: core material

relative positions

of the windings


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TransformerConstruction


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Samples of TypicalTransformers


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Turns Ratio

Important parameter for transformers

Ratio of the number of turns in the primarywinding to the number of turns in the secondary

winding

Expressed as a ratio

4:1 (spoken as 4 to 1)

There are 4 windings in primary for every one turns in

the secondary

Determines the voltage induced into the secondary

sec

priRatioTurns !


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Finding Turns Ratios

A given transformer

has 1000 turns in theprimary and 2000

turns in the secondary.

Find the turns ratio.

A given transformer

has 1000 turns in theprimary and 200 turns

in the secondary. Find

the turns ratio.


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Induced Voltage

The turns ratio determines the voltage induced in

the secondary. It is a direct proportional relationship.

If the turns ratio is known, the secondary voltage

can be determined for any voltage applied to the

primary

S

P

S

P

V

V!


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Step-up Transformer

A step-up transformer

has more turns in thesecondary than in the

primary.

The voltage induced

into the secondary willbe greater than the

voltage applied to the

primary.

Example:

A transformer has aturns ratio of 1:3.

Applying 100 AC to

the primary will result

in a secondary voltage

of 300 AC.

NOTE: Transformers

DO NOT work with

DC voltages.


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Step-up Transformers

A given transformer


in the secondary. If

the primary voltage is

120 AC, what is thesecondary voltage?

A given transformer

has 370 turns in theprimary and 2020

turns in the secondary.

If the primary voltage

is 24 AC, what is thesecondary voltage?


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Step-downTransformers

The opposite of step-up

transformers.

There are more turns in

the primary than in the

secondary.

The voltage induced into

the secondary will be lessthan the voltage applied to

the primary

Example:

A transformer has aturns ratio of 3:1

Applying 100 AC to

the primary will result

in a secondary voltage

of 33.3 AC.


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Step-downTransformers

A given transformer





A given transformer






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Current inTransformers

ince the power in the

secondary mustalways = the power in

the primary, the

current in the primary

and secondary areinverse relationships

Assume a step-down

transformer of 2:1 has 10 volts

applied to the primary and thesecondary load requires 1 amp.

The secondary voltage will be 5

volts.

The power in the secondary will

be 5 watts. That 5 watts must be supplied

by the primary.

The primary has 10 volts

applied, therefore the Ipri must

be 500 ma.


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Current is an inverse proportion of voltage

oltage is a direct proportion of the turnsratio.

Note that current relationship is inverted.

pri

sec

sec

pri

sec

pri

I

I

N

N!!


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Current in Transformers

As shown, how

much current is

flowing through

the load?

If turns ratio is

doubled, what is

Isec? If turns ratio is

halved, what is

Isec?

Ipri = 100 ma


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Primary-Secondary Phase Relationship


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Center-TappedSecondary


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Multiple Secondaries

Multiple secondaries

allow several different

voltages to be inducedby the same input.

Each section is treated

as a separate

transformer with its

own turns ratio. um of the secondary

power cannot exceed

the max specified

primary power


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Multiple Secondaries

Find the

induced

secondary

voltages as

indicated

A-B =

C-D =

C-CT =

D-CT =

E-F =


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Multiple Primaries

Either primary can

be used alone

rimaries can be

connected in series

for less voltage in

the secondary

rimaries can beconnected in

parallel for

increased current


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Autotransformer


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Transformers

Reflected Load

A load connected to the secondary of a transformerappears to have a resistance that is not necessarily equal

to the actual resistance of the load.

The actual load is essentially reflected into the

primary circuit altered by the turns ratio.

This reflected load (Rpri) is what the primary source

effectively sees and it determines the amount of

primary current.


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Derivation of ReflectedResistance Equation

2

sec

pri

sec

pri

sec

pri

sec

pri

pri

sec

sec

pri

sec

sec

pri

pri

sec

pri

sec

secsec

pri

pripri

N

N

N

N

N

N

R

R

:onsubstitutiby

I

I

IIR

R

IR

IR

!

y

!

y

!

z

!

!!


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Reflected ResistanceEquation

Therefore:

sec

pri2

sec

pri

R

R

N

N!

sec

pri

sec

pri!

sps

p

p

s

s

p

s

pPPand

I

I

V

V!!!!


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Reflected Load


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Reflected Load

;!

;!

!;

!;

!

;

25.6

1001616

1

100

4

1

100

4

1

100

pri

pri

pri

2

2pri

2pri

2

sec

pri

sec

pri

N

N

!


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Transformer Uses

= I2R

Calculate the loadpower for each value

of RL 25;

50; 75;

100;

125;


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Maximum PowerTransfer Theorem

When a source is connected to a load,

maximum power is delivered to the loadwhen the load resistance (impedance) is

equal to the source output resistance

(impedance).

Transformers can be used forimpedance

matchingpurposes.


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Impedance Matching

The 1:2 step-up transformer causes the reflected load

impedance to appear to be 75;. Therefore there is

maximum power transferred from the source to the load.


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Determine the primaryimpedance


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Determine the loadimpedance


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Determine the turns

ratio required fortransfer of maximum

power


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Determine the turns

ratio required fortransfer of maximum

power


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A DreadedWord

roblem AboutImpedance Matching

An audio amplifier has an output impedance

of 800 ;. The speakers to be used have animpedance of 8 ;. A coupling transformer

must be used to ensure maximum power

transfer from the amplifier to the speaker.

What must the turns ratio be?


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Transformer Losses

Up to this point, only idealtransformers have been

considered. However, there is no such thing as anideal component.

There are 4 types of transformer losses that must

be considered:

copper losses flux leakage

hysteresis losses

eddy current losses


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Transformer Losses

Losses in a transformer are indicated by the

efficiency specified by the manufacturer. i.e. a transformer that is 95% efficient will

deliver only 95% of the primary power to

the secondary. Most, but not all losses are dissipated in the

form of heat. The transformer will be

warm.


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Copper Loss

Copper loss is caused by the ohmic

resistance of the windings. Heat (I2

R) isdissipated in both the primary and

secondary windings.

Can be overcome by using larger diameter

wire (or lower resistance wire) in the

windings.


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Flux Leakage

aka magnetic leakage

Caused by not all the flux lines produced in theprimary cutting the secondary.

Can be reduced, but not completely eliminated, by

using tighter coupling and/or using a magnetic

core. This type loss appears as a slightly lower

secondary voltage than expected for a given turns

ratio.


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Hysteresis Loss

Apparent in transformers using a ferromagnetic

core.

Caused by the residual magnetism left in the core

material by the magnetizing force of the flux lines

generated.

Appears as heat loss (warm operation). Can be minimized by using a core material that

has low retentivity, such as soft iron or permalloy.


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Eddy CurrentLoss

Appear in transformers using iron cores.

Electron flow is perpendicular to the magneticlines of force.

The core material is an electrical conductor as well

as a magnetic conductor.

Electrical current will flow in the core material ina circular motion


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Eddy CurrentLoss


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Reducing Eddy

Currents byLaminations

09a Transformers - Presentation

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