25/02/2013
Source to Consumer
Transformer Efficiency
Transformers are not 100% efficient. Energy
can be lost due to:
Heat losses in the coils – reduced by using low
resistance wire
Magnetising/demagnetising losses – reduced by
using a soft iron core
Eddy currents in the core – reduced by using a
Source to Consumer
Transformer Efficiency
To calculate the efficiency of a transformer,
the input and output power must be calculated
using:
Pin = Ip x Vp and Pout = Is x Vs
Efficiency is calculated from:
laminated core
Sound losses – reduced by good design and
tightly wound coils
Source to Consumer
Transformer Efficiency
To allow us to carry out calculations about
transformers, sometimes we assume that a
transformer is 100% efficient. (We know that is
not the case.)
For a 100% efficient transformer:
Vs
Vp
=
Ns
Np
=
efficiency =
Pout
Pin
x 100%
Source to Consumer
Transmission Line Efficiency
Energy is lost as heat in a transmission line due
to the resistance of the line. The energy lost
can be calculated using the power, current and
resistance formula. (P = I2 x R)
P
I2
Ip
IpVp = IsVs
Is
R
By reducing the current, the power losses can be
reduced. Thus the voltage is stepped up (by a
transformer) so that the current is reduced.
Transformer Example#1
Transformer Example#2
If a 230V ac supply is connected to the 4800
turn primary coil of a transformer, what voltage
is produced by the 120 turn secondary coil?
A car battery charger charges up a 12V battery,
taking its power from 230V mains. If the
charging current is 4A, what current does the
charger take from the mains?
Ns
Np
=
Vs
Vp
=> Vs =
Vs = 5.75V
Ns x Vp
Np
=
120 x 230
4800
IpVp = IsVs => Ip = Vs x Is =
Vp
12 x 4
230
Ip = 0.21A
1