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Transcript
DO PHYSICS ONLINE
MOTORS AND GENERATORS
TRANSFORMERS
A transformer is a device for either increasing or decreasing an ac voltage.
Transformers are used everywhere. Our electrical supply from our power points
is 240 Vrms, 50 Hz. Many electrical circuits in home devices operate at much
lower voltages. So, transformers are used to produce smaller ac voltages.
A transformer is made of a two coils called the primary and secondary (figure 1).
They are usually wound onto a soft iron core (iron does not remain magnetised
when current falls to zero). The iron core is laminated to reduce energy losses
due to eddy currents.
The changing magnetic flux produced by the ac current in the primary coil
windings induces a changing magnetic flux at the secondary coil. This changing
magnetic flux the induces an emf in the windings of the secondary coil.
Secondary
Primary
~VP
Np
NS
~VS
output
input
laminated iron core
Fig. 1. A transformer.
Transformer operates only on ac
voltages – DC current in primary
does not produce a changing
magnetic flux.
Fig. 2. When DC voltage is switched
on/off  current induced in secondary
circuit e.g. spark plug in a car: high
voltage created across a gap to produce
a spark  ignition coil switches 12 Vdc
to produce voltage spike ~ 25 kV).
battery
VP
VS
switch closed
switch opened
t
opening & closing switch
t small  induced emf large
t
Conservation of energy (assume zero energy losses in this simple model)
energy input = energy output
power input (primary) = power output (secondary)
Changing magnetic flux in primary = Changing magnetic flux in secondary
d
 d 
VP  N P  B   N P B
dt
 dt  P
VP N P

VS N S
d
 d 
VS  N S  B   N S B
dt
 dt  S
IP NS

IS NP
PP  PS
V represents in rms (root mean square) or peak values of the ac voltages
step-up transformer :
increase in secondary voltage (decrease in secondary current )
NS > NP VS > VP IS < IP
step-down transformer:
decrease in secondary voltage (increase in secondary current)
NS < NP VS < VP IS > IP
The ferromagnetic core used in transformers is laminated to reduce ohmic
heating caused by induced eddy currents.
induced eddy currents
induced eddy currents
Binduced
Bcoil
I
I
current increasing
current
increasing
eddy currents cause a heating effect
insulating layers
Laminations reduce magnitude of eddy currents
 less ohmic heating ( I 2 R )
Fig. 3. Laminations reduce the magnitude of eddy currents. This reduces
the ohmic heating of the metal core.
Do problems
P6998