Chapter: 9th Science : Magnetism and Electromagnetism
Transformer
Transformer
Transformer is a device
used for converting low voltage into high voltage and high voltage into low
voltage. It works on the principle of electromagnetic induction. It consists of
primary and secondary coil insulated from each other. In Figure 3.26, the
alternating current owing through the primary coil induces magnetic field in
the iron ring. The magnetic field of the iron ring induces a varying emf in the
secondary coil.
Depending upon the
number of turns in the primary and secondary coils, we can step-up or step-down
the voltage in the secondary coil as shown in Figure 3.27.
Step up transformer: The transformer used
to change a low alternative voltage to a high alternating voltage is called a
step up transformer. ie (Vs > Vp) . In a step up transformer, the number of
turns in the secondary coil is more than the number of turns in the primary coil
(Ns > Np).
Step down transformer: e transformer used to change a high alternating voltage to a low alternating voltage is called a step down transformer (Vs < Vp). In a step down transformer, the number of turns in the secondary coils are less than the number of turns in the primary coil (Ns < Np).
The formulae pertaining
to the transformers are given in the following equations.
A transformer cannot be
used with the direct current (DC) source because of current in primary coil is
constant (ie. DC). Then there will be no change in the number of magnetic field
lines linked with the secondary coil and hence no emf will be induced in the
secondary coil.
Exercise 3
The primary coil of a
transformer has 800 turns and the secondary coil has 8 turns. It is connected
to a 220 V ac supply. What will be the output voltage?
Solution:
In a transformer, Es
/ Ep = Ns / Np
Es = Ns
/ Np x Ep
= 8/800 x 220
= 220/100
Es = 2.2 volt
Exercise 4
A transformer is
designed to give a supply of 8 V to ring a house bell from 240 V ac mains. The
primary coil has 4800 turns. How many turns will be in the secondary coil.
Solution:
In a transformer, Es
/ Ep = Ns / Np
Ns = Es
/ Ep x Np
= 8/240 x 4800
Ns = 4800/30
= 160 turns
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