K323 / 2027
Electromagnetic induction overview

Topic 4 of 5

How transformers change voltage

A transformer uses a changing magnetic field to transfer energy between two separate windings. The turns ratio sets the voltage ratio in the ideal model.

A current in a coil creates a magnetic field. A changing field through another coil can induce an e.m.f. Power is the rate of energy transfer.

Two windings, one linked field

The primary winding connects to the a.c. input. Its changing current creates a changing magnetic field in an iron core. The core strengthens and channels the field so that it links the secondary winding, inducing an e.m.f. there.

Separate circuits linked by a changing core field

Each winding is continuous and insulated from the iron core. The primary supply circuit and secondary load circuit have no ordinary conducting connection.

Two electrically separate transformer circuitsA closed rectangular iron core passes through two continuous windings on opposite legs. The left primary winding closes through an alternating-voltage source; the right secondary winding closes through a load. There is no joining wire between these loops. Hidden half-turns pass behind the opaque iron core. Np and Ns label their numbers of turns, Vp labels the primary supply voltage and Vs labels the voltage across the secondary load. Fewer secondary turns are illustrated schematically; the actual counts for a calculation are supplied in its question. The iron core links the changing magnetic field, not a current of primary electrons into the secondary circuit.NpNsVpVsA.c.LoadPrimarySecondaryClosed iron core

The drawing illustrates fewer secondary turns. Use the stated Np and Ns in a calculation. Continuing a.c. changes the core field and can sustain a secondary output; steady d.c. cannot after switching transients.

The primary and secondary are separate insulated circuits linked by the iron core's changing magnetic field. The input connects to the primary; the load connects to the secondary.

The windings are insulated from the core and from one another. Primary electrons do not travel through the core into the secondary circuit. The coupling is magnetic, while charge flows in each winding's own complete circuit.

A steady primary current gives a steady field and no sustained induced secondary output in this model. Switching on or off can produce a brief transient; continuing transformer action needs a changing field, as provided by an a.c. input.

Use the ideal-transformer relationships

Let N be the number of turns, V the voltage across a winding and I its current. Subscript p means primary and s means secondary.

Vp / Vs = Np / NsPair each voltage with its own turn count. Use matching units for the two voltages.

More secondary turns give a step-up voltage transformer; fewer give a step-down voltage transformer. A useful rearrangement is Vs = Vp x Ns / Np.

VpIp = VsIsInput power equals output power for the stated ideal model. Use volts and amperes to calculate watts.

The examples use the stated operating a.c. values and resistive loads. At the same ideal transferred power, a higher voltage comes with a lower current. The turns ratio alone does not determine current for an unspecified load.

Step down from 240 V to 12 V

Voltage ratio, then power

1000 primary turns and 50 secondary turns

The ideal transformer has Np = 1000, Ns = 50 and Vp = 240 V.

Vs = 240 x 50/1000 = 12.0 V.

The connected load takes Is = 2.0 A, so output power = 12.0 x 2.0 = 24 W.

For the ideal transformer, input power is also 24 W. Therefore Ip = 24/240 = 0.100 A.

The lower-voltage winding carries the larger current. The primary and secondary currents are not equal, because these are separate circuits linked by energy transfer.

Step up from 12 V to 60 V

Keep transferred power fixed

200 primary turns and 1000 secondary turns

Vp = 12 V, so Vs = 12 x 1000/200 = 60 V.

The specified ideal transferred power is 30 W. Primary current Ip = 30/12 = 2.5 A; secondary current Is = 30/60 = 0.50 A.

The voltage rises fivefold and the current falls fivefold at this same power. The transformer has not created additional electrical energy.

Recognise what the ideal model leaves out

Real transformers have losses, including heating in the windings and core. Their useful output power is less than their input power; energy is still conserved because the difference is transferred elsewhere.

An iron core made of insulated laminations reduces circulating induced currents and associated heating in the core. This improves efficiency, but it does not make every practical transformer loss-free.

Optional check An ideal transformer has 1000 primary turns and 50 secondary turns. Its primary voltage is 240 V, and a connected resistive load takes 2.0 A from the secondary. What are the secondary voltage and primary current?
An ideal transformer has 1000 primary turns and 50 secondary turns. Its primary voltage is 240 V, and a connected resistive load takes 2.0 A from the secondary. What are the secondary voltage and primary current?