Transformer

1. UNIT-4
Single phase Transformer

2. • Transformer is a static device which transfers the electrical energy from
one circuit to another circuit, without changing its frequency.
• The voltage can be raised or lowered in a circuit, but with a
proportional increase or decrease in the current ratings.
Construction of a Transformer:

3. • Basically a transformer consists of two inductive windings and a laminated steel
core.
Windings:
• There are two windings in a transformer. Which winding is connected to supply is
called primary winding and which winding is connected to load is called
secondary winding.
• Windings are made up of copper. The windings are insulated from each other as
well as from the steel core.
Iron Core:
• In all types of transformers, core is constructed by assembling laminated sheets of
steel, with minimum air-gap between them (to achieve continuous magnetic path).
• The steel used is having high silicon content and sometimes heat treated, to
provide high permeability and low hysteresis loss.
• Laminated sheets of steel are used to reduce eddy current loss. The sheets are cut
in the shape as E,I, C and L.

4. Working principle of a Transformer:
• The main principle of operation of a transformer is mutual inductance
between two circuits which is linked by a common magnetic flux.
• A basic transformer consists of two coils that are electrically separate and
inductive, but are magnetically linked through a path of reluctance

5. The main principle of operation of a transformer is mutual inductance between two
circuits which is linked by a common magnetic flux.
• A basic transformer consists of two coils that are electrically separate and
inductive, but are magnetically linked through a path of reluctance

6. • When the supply is given to the primary winding then an alternating
flux produced due to alternating current.
• This flux is linked to the secondary winding through the magnetic core.
An emf will be induced in the secondary winding from the principle of
faradays law of electromagnetic induction.
• This emf is caused to flow current when the circuit is in closed path
A transformer carries the operations shown below:
• Transfer of electric power from one circuit to another.
• Transfer of electric power without any change in frequency.
• Transfer with the principle of electromagnetic induction.
• The two electrical circuits are linked by mutual induction.

7. • E.M.F Equation of a Transformer:
Let,
N1 = Number of turns in the primary winding
N2 = Number of turns in the secondary winding
ϕm = Maximum flux in the core in webers = Bmax * A
f = Frequency of alternating current input in hertz (Hz)
The general equation for induced EMF, e is expressed as

8. A sinusoidal varying flux is represented as
Φ= ϕm sinꞷt
Then

12. Applications of a transformer:
Transformers are used in most electronic circuits.
A transformer has only 3 applications
1. To step up voltage and current.
2. To Step down voltage and current.
3. To prevent DC – transformers can pass only Alternating Currents so they totally
prevent DC from passing to the next circuit.
4. Mobile adapters, substations, TV’s, Computers etc...

13. • Losses in a Transformer:
An electrical transformer is a static device, hence mechanical losses (like windage
or friction losses) are absent in it.
A transformer only consists of electrical losses (iron losses and copper losses).
Transformer losses are similar to losses in a DC machine, except that transformers
do not have mechanical losses.
(i) Core Losses or Iron Losses or Constant losses
Eddy current loss and hysteresis loss depend upon the magnetic properties of the
material used for the construction of core.
Hence these losses are also known as core losses or iron losses.

14. Hysteresis loss in transformer:
• Hysteresis loss is due to reversal of magnetization in the transformer core.
• This loss depends upon the volume and grade of the iron, frequency of magnetic
reversals and value of flux density.

15. • Eddy current loss in transformer:
In transformer, AC current is supplied to the primary winding which sets up
alternating magnetizing flux.
When this flux links with secondary winding, it produces induced emf in it. But
some part of this flux also gets linked with other conducting parts like steel core or
iron body or the transformer, which will result in induced emf in those parts,
causing small circulating current in them.
This current is called as eddy current. Due to these eddy currents, some energy will
be dissipated in the form of heat.

16. • (ii) Copper Losses or I²R losses or Variable losses:
Copper loss is due to ohmic resistance of the transformer windings. Copper loss for
the primary winding is I1²R1 and for secondary winding is I2²R2.
Where,
I1 and I2 are current in primary and secondary winding respectively,
R1 and R2 are the resistances of primary and secondary winding respectively.
It is clear that Cu loss is proportional to square of the current, and current depends
on the load. Hence copper loss in transformer varies with the load.

17. Efficiency of a Transformer:
• Efficiency of a transformer can be defined as the ration of output
power to the input power.
• Efficiency = output / input
• Transformers are the most highly efficient electrical devices.
• Most of the transformers have full load efficiency between 95% to
98.5%.
• Efficiency = (input - losses) / input = 1 - (losses / input).

18. Condition for Maximum Efficiency

20. • Where, X is Load (Full load-1, Half load-1/2…etc.)
• Cosϕ is Power factor
• KVA Rating is rating of the Transformer
• Pi is Iron losses
• Pcu is Copper losses

21. • Voltage Regulation of a Transformer
Definition: The voltage regulation is defined as the change in the magnitude of
receiving and sending voltage of the transformer.
The voltage regulation determines the ability of the transformer to provide the
constant voltage for variable loads.
When the transformer is loaded with continuous supply voltage, the terminal
voltage of the transformer varies.
The variation of voltage depends on the load and its power factor.
Mathematically, the voltage regulation is represented as:

22. where,
E2 – secondary terminal voltage at no load
V2 – secondary terminal voltage at full load
The voltage regulation by considering the primary terminal voltage of the
transformer is expressed as