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25/02/2013 Source to Consumer Transformer Efficiency Transformers are not 100% efficient. Energy can be lost due to: Heat losses in the coils – reduced by using low resistance wire Magnetising/demagnetising losses – reduced by using a soft iron core Eddy currents in the core – reduced by using a Source to Consumer Transformer Efficiency To calculate the efficiency of a transformer, the input and output power must be calculated using: Pin = Ip x Vp and Pout = Is x Vs Efficiency is calculated from: laminated core Sound losses – reduced by good design and tightly wound coils Source to Consumer Transformer Efficiency To allow us to carry out calculations about transformers, sometimes we assume that a transformer is 100% efficient. (We know that is not the case.) For a 100% efficient transformer: Vs Vp = Ns Np = efficiency = Pout Pin x 100% Source to Consumer Transmission Line Efficiency Energy is lost as heat in a transmission line due to the resistance of the line. The energy lost can be calculated using the power, current and resistance formula. (P = I2 x R) P I2 Ip IpVp = IsVs Is R By reducing the current, the power losses can be reduced. Thus the voltage is stepped up (by a transformer) so that the current is reduced. Transformer Example#1 Transformer Example#2 If a 230V ac supply is connected to the 4800 turn primary coil of a transformer, what voltage is produced by the 120 turn secondary coil? A car battery charger charges up a 12V battery, taking its power from 230V mains. If the charging current is 4A, what current does the charger take from the mains? Ns Np = Vs Vp => Vs = Vs = 5.75V Ns x Vp Np = 120 x 230 4800 IpVp = IsVs => Ip = Vs x Is = Vp 12 x 4 230 Ip = 0.21A 1