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| Figure 7-4 Connections for ratio test using two voltmeters. |

| which should be kept to approximate design values to eliminate errors from excessive voltage drop in the primary. |

| For three-phase transformers where three-phase power supply is used for testing, it is important that the voltmeter readings be taken on corresponding windings. For power transformers the ratio usually must be within plus or minus 1 percent. Figure 7-4 shows the test circuit diagram. |

| For induction voltage regulators, the ratio may be tested by applying a voltage on the primary and reading the voltage across the secondary with the armature in the maximum boost position. For tap changing under load transformers, the same procedure is applied with the unit set for maximum boost position; tap position voltages may also be measured. For three-phase regulators, measurements may be taken for maximum lower positions in addition to the maximum boost position. |

| Impedance Tests |

| The impedance of a transformer is important since, to a large extent, it determines the regulation of the transformer. It is made up of the internal resistance and reactance and will create a voltage drop caused by current flowing in the windings. Impedance losses (measured in watts) are a combination of heat (I2R) losses in the conductor, eddy-current loss, and losses in other parts of the transformer caused by stray currents. Impedance tests show whether the coil and core arrangements produce satisfactory voltage drops and losses in the transformer. They also determine whether transformers may be satisfactorily paralleled since the manner in which such transformers divide the load depends on their impedance voltages; that is, the one having the higher impedance will take the smaller part of the load, and vice versa. |

| The impedance of a transformer is measured by short-circuiting one winding (usually the secondary) and with instruments connected (Fig. 7-5). An ac voltage is impressed on the primary winding and adjusted until the transformer is carrying rated current, as shown by the ammeter. When full-load current flows in the primary, full-load current will also flow in the short- |

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