A transformer is an electrical machine which changes a given
alternating emf into larger or smaller alternating emf.
Transformer is one of the most useful electrical devices
ever invented. It can use for increasing or decreasing the alternating voltage.
It is a device which has large number of uses like electricity generation, transmission
and distribution. In this chapter we shall discuss some of the basic properties
of transformer and designing of transformer.
Principle:
It works on the principle
of mutual induction between the coils. A changes current in the primary coil
induces an emf in secondary coil.
Construction:
It consists of two windings
primary winding to
which source is applied and secondary
winding from which stepped up or stepped down alternating voltage
is obtained and an iron
core
over which primary and secondary windings are wound.
An alternating emf is applied
to the primary, if the flux in the primary is changing at the rate of flux then this alternating flux links both
windings and induces emf E1 and E2. The emf E1 is termed as primary emf and E2
is termed as secondary emf.
The emf induced in the
primary will be back emf which will oppose the applied voltage if resistance of
coil is negligible then back emf is opposite to applied voltage.
V
= -E
According to Faraday’s
law
E = -N (change in flux)
V = - (
V = (N (change in flux))
For
primary coil
Vp = NP(change in flux)
For
secondary coil
Vs
= NS(change in flux)
From
(A) & (B)
Vp /Vs = NP/NS
Vp /Vs = NP/NS
Types
of transformer:
Depending upon number of turns transformer is
divided in to two types:
Step up transformer:
Such a transformer in
which voltage across secondary is greater than primary is called step up
transformer.
NS > NP
VS > VP
Step down transformer:
Such a transformer in
which voltage across primary is greater than secondary is called step down
transformer.
NS < NP
VS < VP
The transformer steps up or steps down but
power remains same.
Primary power (Pp) = Secondary power (Ps)
Vp Ip = Vs Is
Vp / Vs = Is / Ip
Losses in transformer:
Losses that occur in transformer are:
Core losses: Eddy current losses & Hysteresis losses.
Copper losses: Occurs in the winding.
Designing of transformer:
Let us we have to design a transformer of 12V and 3A output.
Vout
= 12 V
I
out = 3A
Power of transformer = V x A
= 12 x 3 =
36VA
Vin = 240 V
Step
1: Area of bobbin.
Size
= =
1.07 inch2
Step 2: Number of turns.
Step 2: Number of turns.
Copper windings = 8V per
turn
Primary side = = 30 turns
Now to decrease losses multiplying turns of both windings by a
constant number.
Primary
side
= 30 x 20 = 600 turns
Secondary
side
= 1.5 x 20 = 30 turns
Step
3: Currents calculation.
Step 4: Calculation of S.W.G (Square wire gauge):
To calculate
S.W.G, first of all calculate diameter which is given by following formula:
d = sqrt((4*A)/pi)
Where A = Area
And area is given by: A = i/ mu
Where A = Area
And area is given by: A = i/ mu
Primary
side:
Ap =
Secondary
side:
As =
Ap =
By using above
formula:
d = = 0.276 mm
d = = 1.23 mm
To convert mm to
S.W.G, you can use standard tables, according to given table:
d = 0.276mm S.W.G = 32 SWG
d = 1.219 S.W.G = 18 SWG
As we have standard wire gauge corresponding to 1.219, so
recalculating As, Is.