Noadswood Science, 2013

To understand how step-up and step-down transformers work

Wednesday, April 19, 2023

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 = number of turns on primary Secondary voltage number of turns on secondary

Vp = NpVs Ns

Vs = NsVp Np

*Works either way up

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 = NsVp Np

Voltage secondary = 8001000 40

Voltage secondary = 1000 x (800 ÷ 40)

Voltage secondary = 20’000V

Transformer EquationTransformer 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 EquationPower 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

Power EquationPower Equation

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)

Free PowerFree Power

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

Transformer CostTransformer Cost

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

ResistanceResistance

Transformer power is worked out for a transformer (assuming it is 100% efficient) by one of two equations: -

P = I x V

VpIp = VsIs

P = power; I = current; and V = voltage

p = primary coil; s = secondary coil

Transformer PowerTransformer Power

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)

Switch Mode TransformersSwitch Mode Transformers