Coil Polarity

Transformer Operation Under No-Load

Figure 7 Polarity of Transformer Coils

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TRANSFORMER THEORY Transformers Example: When the secondary of a 120/440 V transformer is open, primary current is 0.2 amps at a PF of .3. The transformer is a 5 kVA transformer. Find: (a) I_P, (b) I_E, (c) I_H, and (d) I_m. (a) Full load current kVA Rating V_P (b) I_P at no load is equal to I_E I_E = 0.2 amp (c) I_H I_E cos q I_E x PF 0.2 (0.3) I_H 0.06 amps (d) I_M I_E sin q q arccos 0.3 72.5° (0.2) sin 72.5° (0.2) (0.95) I_M 0.19 amps Coil Polarity The symbol for a transformer gives no indication of the phase of the voltage across the secondary. The phase of that voltage depends on the direction of the windings around the core. In order to solve this problem, polarity dots are used to show the phase of primary and secondary signals. The voltages are either in phase (Figure 7a) or 180° out of phase with respect to primary voltage (Figure 7b). ES-13 Page 14 Rev. 0

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