Download Transformers - Noadswood Science

Noadswood Science, 2013
Tuesday, May 23, 2017

To understand how step-up and step-down transformers
work


Electromagnetic induction is the creation of a potential
difference across a conductor which is experiencing a change
in magnetic field
If the magnet or coil is moved backwards and forwards a
potential difference which keeps swapping direction is
produced (AC current)



Transformers change the voltage of electricity
Transformers can be either: ◦ Step-up – they step the voltage up!
◦ Step-down – they step the voltage down!
Transformers work by electromagnetic induction…



The primary coil produces a magnetic field which stays within
the iron core (nearly all of it passes through the secondary
coil)
The alternating current (AC) in the primary coil causes the
field in the iron core to constantly change direction - it is a
changing magnetic field
This magnetic field is felt by the secondary coil, inducing an
alternating voltage in the secondary coil (with the same
frequency as the AC current in the primary) – this is
electromagnetic induction

The relative number of turns on the two coils determines
whether the voltage induce by the secondary coil is greater or
smaller than the voltage in the primary
 More turns in the primary = step-down
 More turns on the secondary = step-up


If DC current was supplied to the primary there would be
noting out of the secondary (there would still be a magnetic
field in the iron core, but it wouldn’t be constantly changing
so there would be no induction (a changing field is needed to
induce voltage))
The iron core is also purely for transferring the changing
magnetic field from the primary to the secondary (no
electricity flows around the iron core)

Step-up transformers step the voltage up – they have more
turns on the secondary coil than the primary coil…

Step-down transformers step the voltage down – they have
more turns on the primary coil than the secondary coil…

The output voltage from a transformer can be calculated if
you know the input voltage and the number of turns per coil
Primary voltage
=
Secondary voltage
number of turns on primary
number of turns on secondary
Vp = Np
Vs Ns
Vs = Ns
Vp Np
*Works either way up
Transformer Equation

A transformer has 40 turns on the primary and 800 turns on
the secondary – if the input voltage is 1000V, what is the
output voltage?
Vs = Ns
Vp Np
Voltage secondary = 800
1000
40
Voltage secondary = 1000 x (800 ÷ 40)
Voltage secondary = 20’000V
Power Equation

Power in = power out is a useful formula as transformers are
very efficient (although they are not actually 100% efficient…)
Power in = Power out
Power in (voltage x current) = Power out (voltage x current)
VpIp = VsIs
Power Equation

A transformer has an input voltage of 1000V with an output
voltage of 20’000V and an output current of 5A. What is the
input current?
VpIp = VsIs
1000V x input current = 20’000V x 5A
Input current = 20’000V x 5A
1000V
Input current = 100A
Free Power


Using a step-up transformer to increase the voltage does not
give you something for nothing – as the voltage increases the
current decreases by the same proportion (P = VI)
In reality the power output is always less than the power input
because the changing magnetic field in the core creates eddy
currents, heating the core (this heat is wasted energy, as it is
lost to the surrounding environment)
Transformer Cost



So why use transformers (if they actually waste some energy
during the step-up / step-down process)?
Remember, step-up transformers are used at power stations
to produce the very high voltages needed to transmit
electricity through the National Grid power lines
These high voltages are too dangerous to use in the home, so
step down transformers are used locally to reduce the voltage
to safe levels
Resistance


Electricity is transferred from power stations to consumers
through the wires and cables of the National Grid - when a
current flows through a wire some energy is lost as heat - the
higher the current, the more heat is lost
To reduce these losses, the National Grid transmits electricity
at a low current, however this needs a high voltage: ◦ P = V x I so to transmit a lot of power either the voltage or
current must be very high
◦ High current = high resistance (a lot of energy is lost as
heat as Power loss due to resistance is P = I2R (current 10x
greater = losses 100x greater)
◦ It is much cheaper to have a huge voltage and small
current, even though it requires transformers
Transformer Power

Transformer power is worked out for a transformer (assuming
it is 100% efficient) by one of two equations: P=IxV
VpIp = VsIs

P = power; I = current; and V = voltage
 p
= primary coil; s = secondary coil
Switch Mode Transformers


Switch mode transformers operate at a high frequency (50kHz
– 200kHz). They are more efficient using very little power
when switched on without a load
Mobile phone / laptop chargers use switch mode
transformers as they are light and small (as they operate at a
higher frequency)