Unit 27

Transformers

• 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.

• 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.

• 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.

• Transformers– The coil containing the AC voltage is the

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

secondary winding.

– 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.

• 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.

• Transformers are rated in volt-amperes.

• 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.

• Mutual inductance– The primary induces a voltage into the

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

• 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

• 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

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

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

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

PP = PS

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

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

P

S

S

P

NN

II

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

2

S

2

P

S

P

NN

ZZ

• 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.

• 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.

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

voltage.– Both the primary and secondary windings are

part of the same core.

• 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.

– 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.

– 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.

– 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.

• 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.