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Page 18

| eddies in a pool of water, and are therefore appropriately named eddy currents. These currents in the core also produce an I2R heat loss. |

| Iron Losses.. |

| The sum of both the hysteresis and eddy current losses is usually referred to as the iron losses in a transformer. Since there is only a relatively small difference in the magnetic field in the iron core at any time, as explained above, these losses will present little variation as the load on the transformer is increased or decreased. |

| No-Load Losses |

| It will be observed that when the secondary of a transformer is open, that is, it has no load connected to it, a small current will nevertheless flow in the primary. As mentioned previously, the large self-induced voltage in the primary practically counterbalances the original voltage in that coil. The current which the primary takes under these circumstances (sometimes referred to as the exciting current) serves to supply the hysteresis and eddy current (or iron) losses as well as a small I2R loss in the primary coil itself. The sum of these losses is usually referred to as the no-load loss in a transformer. |

| Impedance and Regulation of Transformer. |

| In determining the performance of a transformer, the impedance of the transformer caused by its core and coils, must be considered. The effect of this impedance is to cause a drop in the secondary voltage of the transformer, depending on the current flow. Transformers are designed to give a predetermined secondary voltage drop at full load current. The nameplate on the transformer gives its impedance, expressed as a percentage of its rated voltage. For example, a transformer connected for 240-volt output, with a 2.2 percent impedance, will deliver 2.2 percent less than 240 volts or 234.72 volts at full load. |

| The regulation of a transformer is the ratio of the change in secondary terminal voltage from no load to full load (at constant primary impressed terminal voltage) to the rated secondary voltage, usually at some specified power factor. This value is usually small, which allows the ordinary power transformer to be known sometimes as a constant potential transformer. |

| Despite the low values, however, the regulation and impedance of transformers is important when two or more transformers are connected in parallel (as in a network illustrated in Fig. 1-17), as the load currents they supply will be different if the regulation and impedance of the units are different; that is, the transformers will not share the load equitably. |

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