TRANSFORMERS  A power station produces an AC pd of ~25 kV  This AC pd is stepped up to ~132 kV so that it can transport through cables without losing energy  The AC pd is stepped down to 230 V before it enters your home  Transformers change the size of the voltage of an alternating current.  They all have two coils, the primary and the secondary, joined with an iron core.  There are two types of transformers; Step-up that steps the voltage up and step-down that steps the voltage down.

Iron Core ~ AC supply Primary coil Secondary coil

~ AC supply An AC current passes through the primary coil An AC pd is produced in the secondary coil The AC in the primary coil produces an alternating magnetic field The lines of the magnetic field pass through the secondary coil This creates an alternating pd between the terminals of the secondary coil Electrical energy is transferred even though the coils arenot connected in the same circuit

~ AC supply Step-up transformer Makes the pd across the secondary coil greater than the pd across the primary coil There are more turns in the secondary coil than in the primary coil

~ AC supply Step-down transformer Makes the pd across the secondary coil less than the pd across the primary coil There are fewer turns in the secondary coil than in the primary coil

~ AC supply The Core The core of the transformer ‘guides’ the field lines in a loop through the coils The core is laminated (layered) – this cuts out induced currents in the iron This increases the efficiency of the transformer

~ AC supply The secondary pd of a transformer depends on the primary pd and the number of turns in each coil The transformer equation pd across primary, V p pd across secondary, V s = number of turns on primary, n p number of turns on secondary, n s npnp nsns

~ AC supply The transformer equation VpVp Vs Vs = npnp nsns

~ AC supply For a step-up transformer The number of secondary turns, n s is greater than the number of primary turns, n p. Therefore V s is greater than V p. npnp nsns

~ AC supply For a step-down transformer The number of secondary turns, n s, is less than the number of primary turns, n p. Therefore V s is less than V p npnp nsns

A transformer is used to step a pd of 230 V down to 10 V. The secondary coil has 60 turns. Calculate the number of turns of the primary coil.

VpVp Vs Vs = npnp nsns = npnp 60 n p = 230 x n p = 1380

A transformer with 1200 turns in the primary coil is used to step a pd of 120 V down to 6 V. Calculate the number of turns on the secondary coil VpVp Vs Vs = npnp nsns = 1200 nsns 20 = 1200 nsns 20 x n s = 1200 n s = n s = 60

Transformer efficiency – Transformers are almost 100% efficient – When a device is connected to the secondary coil, almost all of the electrical power supplied to the transformer is delivered to the device. – If we know how much electrical power a device requires to work normally we can calculate how much electrical power must be supplied to the transformer.

Transformer efficiency – Power supplied to the transformer = primary pd, V p x primary current, I p – Power delivered by the transformer = secondary pd, V s x secondary current, I s Power supplied to transformer = Power delivered by transformer V p x I p = V s x I s

Switch mode transformer Frequency Converter Switch mode transformer Low voltage AC to DC converter -+ Mains input 230 V, 50 Hz Direct voltage output at low voltage Operates at frequencies between 50 kHz and 200 kHz Lighter and smaller than traditional transformers Uses little power when no device is connected across the output terminals Contains a ferrite core – lighter than iron and can work at higher frequencies