ELECTRIC TOWER.

New Zealand Herald, Volume XLV, Issue 13723, 13 April 1908, Page 3

ELECTRIC TOWER.

A NEW SYSTEM.

ADOPTED IN CHRISTCHURCH. At a recent meeting of the Christchurch City Council a report was adopted recommending the enlargement of the city electric power and lighting plant by changing over from .the present system of distribution by two cables to that known as the three-wire system, with a view' to reducing the present losses in distribution. - ■

The system is a simple one, and is said to effect economy in the distributing cables to a remarkable degree. For the same amount of power and the same consumption the total weight of the cables used for sending out the current (ex-?, elusive of the consumers' wires) may be reduced to 28 per cent, of the total weight of the two-wire mains. The consumers' leads may also be lighter. The threewire system takes advantage of the fact that the higher the pressure of the current the smaller a wire may be to cany the same quantity of energy. The amount of energy in a current.is proportionate to the pressure of the current and to the quantity or bulk of it, so that a low pressure circuit needs, for the same power, a larger wire to accommodate it without overheating, in just the same way as, if the voltage or pressure in a wire rises, the wire is heated, because a larger quantity of electricity is forced through.

On a typical power system to be converted la the three- system it may bo supposed that there is a dynamo supplying current at 200 volts to parallel mains, with lights and motors between, the consumers' wires being, for simplicity of explanation, arranged from one main to the other like rungs of a ladder. The same number of consumers could be supplied by a dynamo at 400 volts, if each cross main supplies two lamps or motors, one after another, or by two dynamos at 200 volts, in series wit!: each other, making a total voltage of 400. In the threewire system two dynamos are connected in series in that way, but a third lighter cable is led from the connection between the two dynamos, and the consumers are connected with one of the outer cables, and the " tiiird wire," in suAh a way that the load is nearly equally divided on each side of the wiring system. If the supply required by one half is in excess of that needed on the other half 'the difference in the amount of current flows along the third wire; if the halves are balanced no current flows into or out of the power station by the third wire, though it may be occupied by local currents in various parts, restoring the balance here and there.

A "balancer' is used on a third-wire system to keep the halves of the service equal as far as the dynamos are concerned. This is a pair of dynamos on the same shaft, coupled to the two sides of the system. When one side demands more current than the other the balancer dynamo on the lighter side becomes a motor, and supplies current from its companion, and increases the current in the more heavily-loaded side. There is a great reduction in the necessary weight of the distributing cables. The voltage being doubled and the current halved the mains need to be only onefourth of the carrying capacity of those in the two-wire system, and the size ot the third wire can be ascertained from the practical differences between the halves of the Supply. Twenty-eight per cent, is quoted as a minimum limit, but in any case a great saving iii weight and cost of the mains is effected for an equal loss due to resistance, while the use of large conductors on a high pressure system reduces the losses very much. _ In the cose of the Christchurch installation the use of the three-wire system will enable a very large increase in the plant to be marie with an outlay much smaller than would be needed if the present system of distribution was continued.