Voltage Ratio - h1011v4

Figure 1 Core-Type Transformer

Voltage Ratio - h1011v4_50

Electrical Science Volume 4 of 4
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Transformers TRANSFORMER THEORY Voltage Ratio The voltage of the windings in a transformer is directly proportional to the number of turns on the coils. This relationship is expressed in Equation (13-4). (13-4) V_P V_S N_P N_S where V_P = voltage on primary coil V_S = voltage on secondary coil N_P = number of turns on the primary coil N_S = number of turns on the secondary coil The ratio of primary voltage to secondary voltage is known as the voltage ratio (VR). As mentioned previously, the ratio of primary turns of wire to secondary turns of wire is known as the turns ratio (TR). By substituting into the Equation (13-4), we find that the voltage ratio is equal to the turns ratio. VR = TR A voltage ratio of 1:5 means that for each volt on the primary, there will be 5 volts on the secondary. If the secondary voltage of a transformer is greater than the primary voltage, the transformer is referred to as a "step-up" transformer. A ratio of 5:1 means that for every 5 volts on the primary, there will only be 1 volt on the secondary. When secondary voltage is less than primary voltage, the transformer is referred to as a "step-down" transformer. Figure 2 Example 1 Transformer Example 1: A transformer (Figure 2) reduces voltage from 120 volts in the primary to 6 volts in the secondary. If the primary winding has 300 turns and the secondary has 15 turns, find the voltage and turns ratio. Solution: VR V_P V_S 120 60 20 1 20:1 TR N_P N_S 300 15 20 1 20:1 Rev. 0 Page 5 ES-13

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