BENE 1113
PRINCIPLES OF ELECTRICAL
AND ELECTRONICS
CHAPTER 3
‘TRANSFORMER’
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TOPICS COVERED:
• Transformer Basics
• Transformer Construction
• Transformer Operating Principles
• Transformer Symbols And
Testing
• Step-up And Step-down
Transformer
2
TOPICS COVERED:
• Transformer- Isolation Device
• Transformer Current Calculation and
Efficiency
• Transformer Rating
• Power Losses
• Impedance Matching
• Transformer Application
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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???
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 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.
LM
LM  k L1L2
L2
L1
k
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LM
LM  k L1L2
L2
L1
k
• 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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TRANSFORMER BASICS
• 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.
Air core
Ferrite core
Iron core
Small power
transformer
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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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Dot Notation
(a) Voltage are in phase
(b) Voltage are out of phase
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Turns
ratio
TURN
RATIO
A useful parameter for ideal transformers is the
turns ratio defined* as
n
N sec
N pri
Nsec = number of secondary windings
Npri = number of secondary 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?
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
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same frequency as the input power).
TRANSFORMER SYMBOLS AND
TESTING
(a)
(b)
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.
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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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TRANSFORMERISOLATION 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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TRANSFORMER CURRENT
CALCULATION and EFFICIENCY
• 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.
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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
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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.
Vpri
V
Rpri 
 V pri

RL  Vsec
R pri
I pri
  I sec
 
  I pri
and RL 
sec
I sec

 1  1  1
 =     2
 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
•
•
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
Iron or core losses
The magnitude of these losses is quite
small. This losses due to eddy current and
hysteresis loss in it.
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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) Hysteresis
The 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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POWER LOSSES IN A NON-IDEAL
TRANSFORMER
Copper losses
Is the energy loss in the windings when the
transformer is loaded.
Total copper loss, Pc = Ip2Rp + Is2Rs
Total losses in transformer
Is the summation of core/iron losses and
copper losses
Total transformer losses,
PT = Ip2Rp + Is2Rs +core losses
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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 with the
current drawn is 10A. Calculate the total
transformer losses if the core losses is 50W.
Given the primary resistance is 5ohm.
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TRANSFORMER APPLICATION
Step-up or step-down voltage and current.
Isolation device
 electrical 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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THANK
YOU
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