1. Transformers
Noadswood Science, 2011

2. Transformers To understand how step-up and step-down transformers work
Friday, April 14, 2017
Transformers
To understand how step-up and step-down transformers work

3. Power Station
Look at the following steps, ordering them as they occur during electricity production: -
Water boils, producing steam
Generators are turned, producing electricity
Transformers alter the voltage
Fuel is burnt, heating water
Turbines turn

4. Power Stations Fuel is burnt, heating water
Water boils, producing steam
Turbines turn
Generators are turned, producing electricity
Transformers alter the voltage

5. Transformers 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…

6. Transformers
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

7. Coils
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

8. Current
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)

9. Step-Up Transformers
Step-up transformers step the voltage up – they have more turns on the secondary coil than the primary coil…

10. Step-Down Transformers
Step-down transformers step the voltage down – they have more turns on the primary coil than the secondary coil…

11. Primary voltage = number of turns on primary
Transformer Equation
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 = Np
Vs Ns
Vs = Ns
Vp Np

12. Voltage secondary = 1000 x (800 ÷ 40)
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
Voltage secondary = 1000 x (800 ÷ 40)
Voltage secondary = 20’000V

13. Power in (voltage x current) = Power out (voltage x current)
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

14. 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

15. 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)

16. 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

17. 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