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| Figure 7-5 Connections for impedance test. |

| circuited secondary. The transformer should be allowed to heat to approximately its normal full load operating temperature (of about 75°C). Simultaneous readings of the three meters are taken. The temperature reading should also be taken so that the so-called copper losses (heat losses in the winding) may be adjusted to the standard temperature of 75°C. The value of the ac test voltage applied to create full-load current is the impedance voltage, and the percent impedance of the transformer may be determined as follows: |

| From the test data, the total equivalent resistance, the reactance and impedance, in ohms, may be determined and the data may also be used for calculating the efficiency of the transformer. The value of impedance volts, usually expressed as percent impedance, is usually placed on the nameplate. |

| Similar procedures are employed in measuring the impedance of a three-phase transformer. Three-phase source voltage and three sets of instruments may be used, although the power loss may be measured by two voltmeters, rather than three, properly connected to read total three-phase power. |

| The same procedures are used when obtaining the impedance of an induction voltage regulator. The impedance is measured on the regulator primary with the secondary in the maximum boost position. The secondary is short-circuited on itself, and the ratio current is held on the primary. Both volts and watts are read. |

| Transformer Losses |

| For practical purposes, the major transformer losses, the core loss of a transformer and the copper (or winding) losses may be determined separately by |

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