Download TRANSFORMERS A transformer changes electrical energy of a

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TRANSFORMERS
A transformer changes electrical energy of a given voltage into electrical energy at a different
voltage level. It consists of two coils which are not electrically connected, but which are
arranged in such a way that the magnetic field surrounding one coil cuts through the other coil.
When an alternating voltage is applied to (across) one coil, the varying magnetic field set up
around that coil creates an alternating voltage in the other coil by mutual induction. A
transformer can also be used with pulsating dc, but a pure dc voltage cannot be used, since only a
varying voltage creates the varying magnetic field which is the basis of the mutual induction
process.
A transformer consists of three basic parts, as shown below. These are an iron core which
provides a circuit of low reluctance for magnetic lines of force, a primary winding which
receives the electrical energy from the source of applied voltage, and a secondary winding which
receives electrical energy by induction from the primary coil.
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Types of Transformers
Transformers either Step up or step down the input voltage. The function of the transformer is
determined by the number of windings on the input coil compared to the output coil windings.
The diagram below shows two types of transformers:
The ratio of the transformer input voltage to the output voltage is the same as the turn’s ratio if
the transformer is 100 percent efficient. Thus, when 10 volts are applied to the primary of the
transformer shown in A , two volts are induced in the secondary. If 10 volts are applied to the
primary of the transformer in B of figure, the output voltage across the terminals of the
secondary will be 40 volts.
Turns Ratio (n)
This is the ratio of turns in the input coil compared to the number of turns on the output coil
n =
Input and Output Voltage in transformer
Input and Output Current
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Np = # windings on primary coil
Ns = # windings on Secondary coil
Vp = Input Voltage
Vs = Output Voltage
Is = Input Current
Is = Secondary Current
No transformer can be constructed that is 100 percent efficient, although iron core transformers
can approach this figure. This is because all the magnetic lines of force set up in the primary do
not cut across the turns of the secondary coil. A certain amount of the magnetic flux, called
leakage flux, leaks out of the magnetic circuit.
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Ideal power equation
The ideal transformer as a circuit element
If the secondary coil is attached to a load that allows current to flow, electrical power is
transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly
efficient. All the incoming energy is transformed from the primary circuit to the magnetic field
and into the secondary circuit. If this condition is met, the input electric power must equal the
output power:
Efficiency of a Transformer
No transformer is 100% efficient. Therefore, the efficiency of a transformer is given by the
equation:
Efficiency =
(Vs Is / Vp Ip) x 100
Losses in a transformer
In actual practice output is not equal to input therefore actual transformers are not 100%
efficient. However commercial transformers have very high efficiency in the range of 95% to
99%.
EDDY CURRENTS
Due to variation in magnetic flux eddy currents are induced on the surface of iron core
which in turn produce heating and therefore reduce the amount of power to the secondary
coil.
In order to avoid eddy currents , the core is laminated, made of thin sheets of soft iron.
Each sheet is separated from the next by a layer of insulating varnish
HYSTERESIS LOSS
Each time the direction of magnetization is reversed, some useful energy is wasted in
overcoming internal friction. This is known as "hysteresis loss" and it also produces
heating in the core.
Hysteresis loss is minimized by using special alloys known as "perm alloy" for core material.
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HEATING
Some energy is dissipated as heat in the coil.
This is reduced by using suitably thick wire.
FLUX TRANSPORT FAILURE
Some loss of useful energy occurs because a small amount of the flux associated with the
primary coil fails to pass through the secondary
Questions
1. When the turns ratio of a transformer is 20 and the primary ac voltage is 12 V, what is the
secondary voltage?
2. A certain transformer has 400 turns in the primary winding and 2,000 turns in the
secondary winding. What is the turn ratio for this transformer?
a. If the input current is 5A, what is the Output current?
b. How efficient is this transformer?