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P6G Transformers A transformer is a device that changes the voltage. Transformers only work with AC (alternating current) electricity. They do not work with DC electricity, and they do not change AC into DC. There are two types of transformer: step-up transformers increase the voltage step-down transformers decrease the voltage Step-down transformers are used in everyday items such as mobile phone chargers, radios and laptop computers. Inside a transformer A transformer consists of two coils of wire, wound around an iron core. The AC input passes through the primary coil, and the AC output passes through the secondary coil. Note that the two coils are not in electrical contact with each other. Step-up transformers In a step-up transformer, the number of turns on the secondary coil is greater than the number of turns on the primary coil. The ratio of the number of turns determines how much the voltage will be increased. For example, the voltage is doubled if the secondary coil has twice as many turns as the primary coil. Step-down transformers In a step-down transformer, the number of turns on the primary coil is greater than the number of turns on the secondary coil. As with the step-up transformer, the ratio of the number of turns determines how much the voltage will be increased. For example, the voltage is halved if the primary coil has twice as many turns as the secondary coil. Isolating transformers Isolating transformers are designed to improve the safety of some electrical circuits, rather than change the voltage. Different transformers You should be able to recognise and draw the circuit symbol for a transformer. For example, they are used in shaver sockets found in bathrooms. Since there is no electrical contact between the mains supply to the primary coil and the secondary coil, the sockets are not ‘live’. This means that you cannot get electrocuted if you touch them. The number of turns on the primary coil in an isolating transformer is the same as the number of turns on the secondary coil, so there is no change in voltage. Transformers in the National Grid The National Grid is used to distribute electricity around the country from power stations to consumers. Different types of transformers are used: Step-up transformers increase the voltage from the generators at the power station for supply to the transmission cables in the Grid Step-down transformers reduce the voltage from the transmission cables to a safer level for domestic use (in homes) and commercial use (in factories and shops) The National Grid Less power is lost when electricity is transmitted through the National Grid at high voltages. Take a look at the following two sections - the equations and worked examples can help you to understand this. Power loss and current The power loss is related to the current flowing and the resistance of the wire: power loss = current2 × resistance where: power loss is measured in watts, W current is measured in amps, A resistance is measured in ohms, Ω For example, if the current is 3 A and the resistance is 2 Ω, the power loss is: 32 × 2 = 9 × 2 = 18 W On the other hand, if the current is doubled to 6 A, the power loss is: Electricity from a power station goes to: 62 × 2 = 36 × 2 = 72 W Step-up transformers High-voltage transmission lines Step-down transformers Note how much greater the power loss is when the current is increased. This means that it is best to transmit the electricity at low currents so less power is lost. To do this, a high voltage is needed. Consumers Some power is lost as heat by the National Grid. This happens from the transformers and from the cables. Voltage and current Assuming that a transformer is 100% efficient: Transformer calculations This equation links the number of turns to the voltages: VpIp = VsIs voltage across the primary coil/ voltage across the secondary number of turns on the primary coil/ coil = number of turns where: on the secondary coil Vp is the voltage across the primary coil in volts, V For example, a transformer has 20 turns on its primary coil and 100 turns on its secondary coil. If the required output voltage is 60 V, what is the input voltage? Ip is the current flowing through the primary coil in amps, A Vs is the voltage across the secondary coil in volts, V voltage across primary coil =number of turns on primary coil/ number of turns on secondary coil x voltage across secondary coil Is is the current flowing through the secondary coil in amps, A voltage across primary coil = 20/100 x 60 = 1200/100 = 12 V For example, if the voltage across the primary coil is 12 V and the voltage across the secondary coil is 240 V, what is the output current when the input current is 20 A? Is = VpIp / Vs = 12 x 20 / 240 = 240 / 240 = 1A Transformers - Alternating current Transformers only work with AC (alternating current) electricity. The alternating current induces (causes) a continually changing magnetic field around the primary coil. The iron core concentrates this field and transfers it throughout the core. The continually changing magnetic field moving through the secondary coil induces a continually changing voltage there. If the secondary coil is connected to an external circuit, an alternating current flows