TRANSFORMER: is an electrical device which converts alternating current from low voltage to high
voltage or high voltage to low voltage without any change in frequency.
Step up transformer
Step down transformer
Principle of transformer
Basic principle of behind the working of transformer is mutual induction which states that rate of change
of current in one coil induces an emf in the nearby coil.
Construction (labeled diagram for step up and step down transformer)
Transformer consists of two sets of coil wound over a rectangular frame made of soft iron core.
Working: The alternating voltage (Vp) to be stepped up or down is applied in the primary. Magnetic flux
linked with primary changes leading to the production of an induced emf,
𝑑ф
Ep = -Np 𝑑𝑡 …….(1)
where Np is the number of turns in the primary.
Since there is no magnetic flux leakage, the change of magnetic flux in primary is totally linked with
secondary also, leading to the production of emf in secondary,
𝑑ф
Es = -Ns 𝑑𝑡 …….(2) where Ns is the number of turns in the secondary.
From (1) and (2) ES/ EP = NS/NP, since Es = Vs, and Ep = Vp, VS/ VP = NS/NP
If NS > NP , VS > VP then the transformer is step up and if NS < NP , VS < VP then the transformer is step
down.
Efficiency of a transformer (Ƞ) is the ratio of output power to input power
Efficiency in % = (
𝑜𝑢𝑡 𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟
)
𝑖𝑛𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟
x100% or Ƞ =
𝑽𝒔𝑰𝒔
𝑽𝒑𝑰𝒑
.
For an ideal transformer efficiency is 100% (output power = input power, Ƞ =1)
In an ideal step up transformer when voltage is stepped up, current is stepped down by the same factor.
Similarly, in a step down transformer when voltage is stepped down, current is stepped up by the same
factor. So the total input power and total output power remains the same. Even for a non – ideal
transformer the total input power and total output power, which includes the different power losses in
transformer, remains the same. Hence working of transformer is satisfying law of conservation of energy
(do not violate law of conservation of energy)..
Energy losses in a transformer
1. Magnetic flux leakage (complete magnetic flux of the primary may not be reaching the secondary due
to poor design of the coil or air gaps in the core. Can be reduced by winding secondary coil close and
tight over the primary coil
2. Resistance of the windings led to wastage of energy in the form of heat (joule loss). Can be reduced
using thick windings in high current low voltage coils (primary of step up and secondary of step down)
3. Eddy current loss. Alternating current induces eddy currents in the core and causes heating of the
core. This energy loss can be minimized by laminating the core
4. Hysteresis loss. Repeated magnetizations and demagnetizations of the core lead to wastage of energy
in the form of heat called hysteresis loss. This can be reduced by using core with low hysteresis loss like
soft iron