Experimental electrical engineering and manual for electrical testing; by Karapetoff Vladimir

Author:Karapetoff, Vladimir. [from old catalog]
Language: eng
Format: epub
Tags: Electrical engineering
Publisher: New York, J. Wiley & sons; [etc., etc.]
Published: 1908-03-25T05:00:00+00:00

Holat No. 2

Fig. 309. A mechanical analogy illustrating the electrical opposition methods.

reality quite a considerable force is necessary to overcome the friction

in both machines, and to start the movement.

This extra power may be applied either as a driving force on the

shaft, or as an extra weight P added to the load of one of the hoists.

The first solution is analogous to the auxiliary motor shown in Fig.

310, the second solution corresponds to power being supplied from the

line, as in Fig. 311. In either case, efficiency is calculated from the

ratio of the power supplied from the outside, to the load of the machines.

Suppose, for instance, that each of the large weights on the hoists is

100 tons, and that an additional weight of 20 tons is required to move

the system at its rated speed. This corresponds to a friction loss of 10

tons per hoist; the average load is 110 tons, consequently the efficiency

of either hoist is

110-10 01

——— = 91 per cent.

374. Classification of Opposition Methods. — The methods by which two electrical machines may be run in opposition are usually classified according to the way in which the losses are supplied. Copper loss may be supplied electrically or mechanically, and iron loss + friction may also be supplied either electrically or mechanically. This gives the four combinations shown in the following table:

Blondel's method (1) is theoretically the most rational, because electrical loss is supplied electrically, while mechanical losses are supplied mechanically; its disadvantage is that it requires a booster set and an auxiliary motor (Fig. 310).

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