Transformers TRANSFORMER THEORYEfficiencyEfficiency of a transformer is the ratio of the power output to the power input, as illustrated byEquation (13-3).(13-3)EfficiencyPower OutputPower InputPSPPx 100wherePS = power of secondaryPP = power of primaryTheoryofOperationA transformer works on the principle that energy can be transferred by magnetic induction fromone set of coils to another set by means of a varying magnetic flux. The magnetic flux isproduced by an AC source.The coil of a transformer that is energized from an AC source is called the primary winding(coil), and the coil that delivers this AC to the load is called the secondary winding (coil) (Figure1).In Figure 1, the primary and secondary coils are shown on separate legs of the magnetic circuitso that we can easily understand how the transformer works. Actually, half of the primary andsecondary coils are wound on each of the two legs, with sufficient insulation between the twocoils and the core to properly insulate the windings from one another and the core. Atransformer wound, such as in Figure 1, will operate at a greatly reduced efficiency due to themagnetic leakage. Magnetic leakage is the part of the magnetic flux that passes through eitherone of the coils, but not through both. The larger the distance between the primary andsecondary windings, the longer the magnetic circuit and the greater the leakage.Rev. 0 Page 3 ES-13
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