Transformers TRANSFORMER THEORYVoltageRatioThe voltage of the windings in a transformer is directly proportional to the number of turns onthe coils. This relationship is expressed in Equation (13-4).(13-4)V_{P}V_{S}N_{P}N_{S}whereV_{P} = voltage on primary coilV_{S} = voltage on secondary coilN_{P} = number of turns on the primary coilN_{S} = number of turns on the secondary coilThe ratio of primary voltage to secondary voltage is known as the voltage ratio (VR). Asmentioned previously, the ratio of primary turns of wire to secondary turns of wire is known asthe turns ratio (TR). By substituting into the Equation (13-4), we find that the voltage ratio isequal to the turns ratio.VR = TRA voltage ratio of 1:5 means that for each volt on the primary, there will be 5 volts on thesecondary. If the secondary voltage of a transformer is greater than the primary voltage, thetransformer is referred to as a "step-up" transformer. A ratio of 5:1 means that for every 5 voltson the primary, there will only be 1 volt on the secondary. When secondary voltage is less thanprimary voltage, the transformer is referred to as a "step-down" transformer.Figure 2 Example 1 TransformerExample 1: A transformer (Figure 2) reducesvoltage from 120 volts in the primaryto 6 volts in the secondary. If theprimary winding has 300 turns and thesecondary has 15 turns, find thevoltage and turns ratio.Solution:VRV_{P}V_{S}1206020120:1TRN_{P}N_{S}3001520120:1Rev. 0 Page 5 ES-13

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