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Power Distribution

load, lamps, coils, voltage, series, transformers and circuit

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POWER DISTRIBUTION The question of power distribution for electric lamps and other appliances is taken up fully in the section on that subject, therefore it will be treated very briefly here. The systems may be divided into: 1. Series distribution systems.

2. Multiple-series or series-multiple systems.

3. Multiple or parallel systems.

They apply to both alternating and direct current.

The Series System.

This is the most simple of the three; the lamps, as the name indicates, are connected in series as shown in Fig. 45. A constant load is necessary if a constant potential is to be used. If the load is variable, a constant-current generator, or a special regulating device is necessary. Such devices are constant current transformers and constant-current regulators as applied to alternating-current circuits.

The series system is used mostly for arc and incandescent lamps when applied to street illumination. Its advantages are simplicity and saving of copper. Its disadvantages are high voltage, fixed by the number of lamps in series; the size of the machines is limited since they cannot be insulated for voltage above about 6,000; a single open circuit shuts down the whole system.

Alternating-current series distribution systems are being used to a very large extent. By the aid of special transformers, or regulators, any number of circuits can be run from one machine or set of bus bars, and apparatus can be built for any voltage and of any size. It is not customary, however, to build transformers of this type having a capac ity greater than one hundred 6.6-ampere lamps because of the high voltage which would have to be induced in the secondary for a larger number of lamps.

Fig. 45 gives a dia gram of the connection of a single-coil trans former in service. The constant-current trans former most in use for lighting purposes is the one manufactured by the General Electric Com pany and commonly known as a tub trans fPrmer. Fig. 46 shows such a transformer (double-coil type) when removed from the case.

Referring to Fig. 46, the fixed coils A form the primaries which are connected across the line; the movable coils B are the secondaries connected to the lamps. There is a repulsion of the coils B by the

coils A when the current flows in both circuits and this force is bal anced by means of the weights at W, so that the coils B take a position such that the normal current will flow in the secondary. On light loads, a low voltage is sufficient, hence the secondary coils are close together near the middle of the machine and there is a heavy magnetic leakage. When all of the lamps are on, the coils take the posi tion shown when the leak age is a minimum and the voltage a maximum. When first starting up, the trans former is short-circuited and the secondary coils brought close together. The short circuit is then removed and the coils take a position corresponding to the load on the line.

These transformers regu late from full load to 4 rated load within -fly ampere of normal current, and can be run on short circuit for several hours without over heating. The efficiency is given as 96% for 100-light transformers and 94.6% for 50-light transformers at full load. The power factor of the system is from 76 to 78% on full load, and, owing to the great amount of magnetic leakage at less than full load—the effect of leakage being the same as the effect of an in ductance in the primary—the power factor is greatly reduced, falling to 62% at / load, 44% at 4 load, and 24% at / load.

Standard sizes are for capacities of 25-, 35-, 50-, 75-, and 100-6.6 ampere enclosed arcs, and they are also made for lower currents in the neighborhood of 3.3 amperes for incandescent lamps. The low power factor of such a system on light loads shows that a transformer should be selected of such a capacity that it will be fully or nearly fully loaded at all times. The primary winding can be constructed for any voltage and the open circuit voltages of the secondaries are as follows: The 50-, 75-, and 100-light transformers are arranged for multiple circuit operation, two circuits used in series, and the vol tages at full load reach 4,100 for each circuit on the 100-light machine.

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