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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, stepdown, 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.
NS
– Expressed as: turns ratio =
NP
– where N = number of turns.
• A step-up transformer
– A transformer with secondary voltage greater
than its primary voltage.
– Expressed as:
ES NS

EP NP
– The turns ratio is always greater than one.
• 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 = P S
(IP)(EP) = (IS)(ES)
• The current is inversely proportional to the
turns ratio.
This is expressed as:
IP NS

IS N P
• Impedance ratio is equal to the turns ratio
squared.
– This is expressed as:
2
ZP N P
 2
ZS N S
• Applications for transformers
–
–
–
–
–
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–
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.