Chapter 18

Transformers

2

• Objectives– After completing this chapter, the student

should be able to:• Describe how a transformer operates.

• Explain how transformers are rated.

• Explain how transformers operate in a circuit.

• Describe the differences between step-up, step-down, and isolation transformers.

3

• Describe how the ratio of the voltage, current, and number of turns are related with a transformer.

• Describe applications of a transformer.

• Identify different types of transformers.

4

• Electromagnetic induction– The action caused when two electrically

isolated coils are placed next to each other and an AC voltage is put across one coil, resulting in a changing magnetic field which induces a voltage into the second coil.

– The device used to create this action is called a transformer.

5

• Transformers– The coil containing the AC voltage is the

primary winding.– The coil in which the voltage is induced is the

secondary winding.

6

– Coefficient of coupling• A number from 0 to 1.

– 1 indicating that all the primary flux lines cut the secondary windings.

– 0 indicating that none of the primary flux lines cut the windings.

7

• The design of a transformer is determined by:– The frequency at which it will be used.

• Low-frequency applications use iron cores.

• High-frequency applications use air cores.

– The power it must handle.– The voltage it must handle.

8

• Transformers are rated in volt-amperes.

9

• Transformers are wound with tapped secondaries.– Center tapped secondary is equal to two

secondary windings.– Used for power supply to convert AC voltages

to DC voltages.

10

• Mutual inductance– The primary induces a voltage into the

secondary and the secondary induces a voltage back into the primary.

11

• Turns ratio– Determines whether a transformer is used to

step up, step down, or pass voltage unchanged.– The number turns in the secondary winding

divided by the number of turns in the primary winding.

– Expressed as: turns ratio =

– where N = number of turns.

P

S

NN

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• A step-up transformer– A transformer with secondary voltage greater

than its primary voltage.– Expressed as:

– The turns ratio is always greater than one.P

S

P

S

NN

EE

13

• A step-up transformer– A transformer that produces a secondary

voltage less than its primary voltage.– The turns ratio is always less than one.

14

• When a transformer steps up the voltage, it steps down the current.– This is expressed as:

PP = PS

(IP)(EP) = (IS)(ES)

15

• The current is inversely proportional to the turns ratio.This is expressed as:

P

S

S

P

NN

II

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• Impedance ratio is equal to the turns ratio squared.– This is expressed as:

2

S

2

P

S

P

NN

ZZ

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• Applications for transformers– Stepping up voltage and current.– Stepping down voltage and current.– Impedance matching.– Phase shifting.– Isolation.– Blocking DC while passing AC.– Producing several signals at various voltage

levels.

18

• Transformers are used for:– Transmitting electrical power to homes and

industry.– Isolating electronic equipment from 120-volts

AC, 60-hertz power while it is being tested.• Does not step up or step down the voltage.

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• Autotransformers– A device used to step up or step down applied

voltage.– Both the primary and secondary windings are

part of the same core.

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• In Summary– Transformers consist of:

• two coils

• a primary winding

• a secondary winding

– Transformers allow an AC signal to be transferred from one circuit to another.

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– Transformers allow:• stepping up the signal.

• stepping down the signal.

• passing the signal unchanged.

– Transformers are designed to operate at certain frequencies.

– Transformers are rated in volt-amperes.

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– Turns ratio determines whether a transformer is used to:

• step up a voltage.

• step down voltage.

• pass voltage unchanged.

– A step-up transformer:• produces a secondary voltage greater than its

primary voltage.

• has a turns ratio that is always greater than one.

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– A step-down transformer:• produces a secondary voltage less than its primary

voltage.

• has a turns ratio that is always less than one.

– The turns ratio determines the amount of voltage that is stepped up or down.

– Transformer applications include:• Impedance matching.

• Phase shifting.

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• Isolation.

• Blocking DC while passing AC.

• Producing several signals at different voltage levels.

– Isolation transformers• Pass the signal unchanged.

• Used to prevent electric shocks.

– Autotransformers• Used to step up or step down voltage.

• Do not provide isolation.