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Transformers and the transmission of electricity
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Transformers change the size of an alternating voltage. They let power stations send electricity across the country efficiently.
How a transformer works
Two coils of wire, the primary (input) and secondary (output), are wound on a soft iron core.
An alternating current in the primary makes a changing magnetic field in the core. This induces an alternating voltage in the secondary.
Step-up: more turns on the secondary, output voltage is higher. Step-down: fewer turns on the secondary, output voltage is lower.
Transformers only work with a.c.
An alternating current in the primary makes a changing magnetic field in the core. This induces an alternating voltage in the secondary.
Step-up: more turns on the secondary, output voltage is higher. Step-down: fewer turns on the secondary, output voltage is lower.
Transformers only work with a.c.
The equations
V_p ÷ V_s = n_p ÷ n_s (input voltage ÷ output voltage = primary turns ÷ secondary turns)
For a 100% efficient transformer: input power = output power, so V_p × I_p = V_s × I_s
Stepping the voltage up steps the current down by the same factor.
For a 100% efficient transformer: input power = output power, so V_p × I_p = V_s × I_s
Stepping the voltage up steps the current down by the same factor.
Transmitting electrical energy
Power stations use step-up transformers to raise the voltage to a very high value for the transmission cables.
At high voltage, the current is low, so less energy is wasted heating the cables.
Step-down transformers lower the voltage again to a safer value for homes.
At high voltage, the current is low, so less energy is wasted heating the cables.
Step-down transformers lower the voltage again to a safer value for homes.
A transformer has 200 turns on the primary and 50 on the secondary. The input is 240 V. What is the output voltage?
- V_s = V_p × n_s ÷ n_p
- = 240 × 50 ÷ 200
Answer: 60 V (step-down)
A transformer has primary and secondary coils on an iron core and only works with a.c. Vp/Vs = np/ns. For 100% efficiency, VpIp = VsIs. Step-up transformers make the transmission voltage high so the current is low and less energy is lost as heat; step-down transformers make it safe for homes.
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