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ET201 Electrical CircuitsET201 Electrical Circuits

‘‘TRANSFORMER’TRANSFORMER’

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•Transformer BasicsTransformer Basics

•Transformer ConstructionTransformer Construction

•Transformer Operating Transformer Operating PrinciplesPrinciples

•Transformer Symbols And Transformer Symbols And TestingTesting

•Step-up And Step-down Step-up And Step-down TransformerTransformer

TOPICS COVERED:TOPICS COVERED:

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• Transformer- Isolation DeviceTransformer- Isolation Device

• Transformer Current Calculation Transformer Current Calculation and Efficiencyand Efficiency

• Transformer RatingTransformer Rating

• Power LossesPower Losses

• Impedance MatchingImpedance Matching

• Transformer ApplicationTransformer Application

TOPICS COVERED:TOPICS COVERED:

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TRANSFORMER BASICS

When an AC voltage is applied in a coil, the electromagnetic field expands, collapses and reverses as the current increases, decreases and reverses.

When two coils are placed very close to each other, the changing magnetic field produced by the first coil will cause an induced voltage in the second coil because of the mutual inductance between the two coil.

HOW???

When two coils are placed very close to each other, the changing magnetic flux line produced by the first coil will cut through the second coil. The two coils are said to be magnetically linked or coupled. As a result, a voltage is induced.

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• Because there is no electrical connection between the two coils (only magnetic link), so they are electrically isolated.

• The amount of voltage induced in the second coil depends on the mutual inductance, LM which is the inductance between the two coils.

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LM

k

1 2L1 L221LLkLM

• The coefficient of coupling is a measure of how well the coils are linked; it is a number between 0 and 1.

• The coefficient of coupling depends on factors such as the orientation of the coils to each other, their proximity, and if they are on a common core.

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LM

k

1 2L1 L221LLkLM

• The basic transformer is formed from two coils that are usually wound on a common core to provide a path for the magnetic field lines. Schematic symbols indicate the type of core.

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Air core Ferrite core Iron core

Small power transformer

TRANSFORMER BASICS

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THE FUNCTION

Function of a transformer: To transfer current, voltage and power from one

series of windings (coils) to another.

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TRANSFORMER CONSTRUCTION

The transformer consists of core, surrounded by at least two series of coils.

The core is used as to aid in linking the flux from the primary coil to the secondary coil.

Dots are used in diagrams of transformers to indicate the current polarities for the windings.

Dotted terminals have the same polarity.

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(a) Voltage are in phase (b) Voltage are out of phase

Dot Notation

A useful parameter for ideal transformers is the turns ratio defined* as

Turns ratio

sec

pri

Nn

N

Nsec = number of secondary windings

Npri = number of primary windings

Most transformers are not marked with turns ratio, however it is a useful parameter for understanding transformer operation.

A transformer has 800 turns on the primary and a turns ratio of 0.25. How many turns are on the secondary?

TURN RATIO

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OPERATING PRINCIPLES

Transformer action: based on the laws of electromagnetic induction.

There is no electrical connection between the primary and secondary. The AC power is transferred from primary to secondary through magnetic flux.

• If an ac voltage is applied to the primary coil, magnetic flux will be created.

• When the magnitude of the applied flux changed, then the generated flux changed.

• This changing flux will link the primary and secondary coil and induce a voltage across the secondary windings.

There is no change in frequency (output power has the same frequency as the input power).

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TRANSFORMER SYMBOLS AND TESTING

The easiest way to test a transformer is to test the primary and secondary coils for continuity.

Since the primary and secondary of a transformer are coils, both of them should have continuity between each end of the coil.

(a) (b)

STEP-UP AND STEP-DOWN TRANSFORMER

• A transformer that has more turns on the secondary than the primary side of transformer will increase the input voltage. – This is called a step-up transformer.

• A transformer that has more turns on the primary than the secondary side of the transformer will decrease the input voltage.– This is called a step-down transformer.

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TRANSFORMER-ISOLATION DEVICE

• A transformer can also be used as an isolation device.

• The circuit on the primary side of the transformer is electrically isolated from the circuit on the secondary side of the circuit.

• Isolation transformers usually have the same secondary voltage as the primary.

• They are mainly used to isolate a piece of equipment from the AC mains for safety reasons.

• Television set is an example of the need for isolation transformers for diagnostic and repair work.

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• The amount of current in the primary side of a transformer is dependent on the turns ratio of the transformer and the load that is applied to the secondary side of the transformer.

• To calculate the primary current of a transformer, we compare the power-in to the power-out of the transformer.

TRANSFORMER CURRENT CALCULATION and EFFICIENCY

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For an ideal transformer, the PPrimary is equal to the PSecondary, or the power input is equal to the power output.

Pout = Pin

VsIs = VpIp

Is/Ip = Vp/Vs Efficiency of transformer,

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Example 1:A doorbell requires 0.4 A at 6V. It is connected via a transformer whose primary coil contains 2000 turns to a 120V AC line.

Calculate (a) N2 and (b) I1

Reflected load

• A transformer changes both the voltage and current on the primary side to different values on the secondary side. This makes a load resistance appear to have a different value on the primary side.

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and pri secpri L

pri sec

V VR R

I I

2

1 1 1 = pri pri sec

L sec pri

R V I

R V I n n n

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IMPEDANCE MATCHING

• To maximize power transfer, the load resistance must be matched with the source resistance

• Normally, the two resistance are not matched and fixed (e.g.: speaker’s resistance (source) is only a few ohms, but internal resistance of audio power amplifier (load) is several thousand ohms. To connect them, we need a transformer. Or else, more power dissipated at the source.)

• To match the resistance, reflect the load resistance into the primary.

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• An amplifier has an 800 internal resistance. To provide max power transfer, what turns ratio must be used in the matching transformer?

• Calculate the turns ratio of a transformer to match a 100 load to a source with internal impedance of 2.5k. Find the load voltage when the source voltage is 30V

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• Determine the turn ratio n that will cause max power transfer to the load.Calculate the max power to the load

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• Find n for max power transfer. Then determine the power in the load if n=10

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POWER LOSSES

Iron or core lossesThe magnitude of these losses is quite small. This losses due to eddy current and hysteresis loss in it.

• A practical transformer differs from the ideal transformer in many respects.

• The practical transformer has iron or core losses and copper losses.-Iron or core losses- eddy current and hysterisis

losses-copper losses- in the resistance of the windings

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POWER LOSSES

(1) Eddy Current

The magnetic core of a transformer consists of many laminations of a high-grade silicon steel.

When the alternating flux cuts the steel core, an emf is induced in each lamination, causing a current (eddy current) to flow in the closed electrical.

Due to this eddy current and resistance in each lamination, a certain amount of power will be absorbed, producing heat in each lamination and also in the core.

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POWER LOSSES IN A NON-IDEAL TRANSFORMER

2) HysteresisThe alternating flux causes changes in the alignment of the molecules in the magnetic cores.

The change is energy consuming and heat is produced within the core.

The energy loss is referred to as hysteresis loss, the degree of loss being dependent on the material used .

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Copper lossesIs the energy loss in the windings when the transformer is loaded.

Total copper loss, Pc = Ip2Rp + Is

2Rs

Total losses in transformer Is the summation of core/iron losses and copper losses

Total transformer losses,

PT = Ip2Rp + Is

2Rs +core losses

POWER LOSSES IN A NON-IDEAL TRANSFORMER

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Example 3:

A power transmission line has a resistance of 0.2 ohm. Calculate the power loss if 200 kW of power is transmitted from a power generator to a city at:

(a) 240 V and (b) 4.4 kV.

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Example 4:A transformer for home use of a portable radio reduces 120V ac to 30V ac. The secondary contains 50 turns, 10ohm resistance. Given the primary resistance is 5ohm.- The primary turns- Calculate the total transformer losses if the core losses is 50W.

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TRANSFORMER APPLICATION

Step-up or step-down voltage and current.

Isolation deviceelectrical isolation. Electrical isolation can be used to

improve the safety of electrical equipment.DC isolation. Coupling transformers can be used to block

DC signals from reaching the secondary circuit.

Impedance matching for max power transfer

AND MANY MORE

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Special transformer applications

• Autotransformers– Use in low-voltage applications since has only one

winding.• Induction Circuit Breaker

– Use to shut off the current in a circuit, thus prevent fires cause by a short in an electrical device.

• Lighting Ballast– Use to start the lamp by producing the necessary

voltage and limits current thro the lamp.• Coupling Transformer

– Isolates each section of the amplifier – so individual amplifier characteristics will not interfere with others

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