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Page 99

| in which the current is constant; increased load, therefore, means increased resistance in the secondary circuit and consequently increased voltage delivered by the secondary. It is impossible to overload a constant-current transformer. As load is added (secondary resistance increased), the coils come closer and closer together, increasing the output voltage. Maximum voltage will be delivered when the moving coil has reached it limit of travel with respect to the stationary coil. If still more load (resistance) is added, no more voltage is available and the secondary current must decrease. |

| Power and distribution transformers reach their maximum temperature at maximum load, but constant-current transformers reach their maximum temperature at zero load, when the secondary is short-circuited. Since the currents in the windings of these transformers are the same for all conditions of load, the copper conductor losses are the same, and since the applied voltage is the same, the loss in the iron core is somewhat less at full separation than at minimum separation. But the magnetic field also cuts the mechanical parts which help support the transformer and flows over a longer path of the iron core. As the coils separate, these effects become more pronounced so that maximum temperature will result where maximum coil separation is reached. |

| Constant-current transformers may have taps connected in their primary windings and sometimes also in their secondary windings. They are usually constructed for station operation, in which case the coils and mechanism may be exposed for better ventilation, and the whole enclosed in a cage for safety. They are also installed in tanks for both overhead and underground installations, in which case the leads are brought through the case by means of bushings, similar to other distribution transformers. They are rated in kVA, the usual standard sizes being 15 kVA and 25 kVA. |

| Although obsolete, installations will exist for some time into the future. |

| IL And SL Transformers |

| As the voltage at the socket of a series lamp may be high, it is sometimes desirable to feed the lamp through a transformer which will reduce the voltage to safe values. This is particularly applicable to metal street standards. This transformer, small in size, is known as an isolating or individual lamp series transformerusually referred to as an IL transformer (Fig. 6-11 on page 100). The ratio of the transformer in this case is 1:1. Sometimes, these transformers are used to supply larger 15 or 20 ampere lamps from a 6.6 ampere system, in which case the ratios are different. |

| Another type of series transformer works on the same principle, but may feed several lamps connected in series, this is commonly referred to as an SL transformer (Fig. 6-11). |

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