Dynamo-electric machinery : an authoritative treatise on the theory, construction details, calculation, characteristic curves, and design of dynamo-electric machinery by American School of Correspondence Chicago & Crocker Francis Bacon 1861-1921

Author:American School of Correspondence, Chicago & Crocker, Francis Bacon, 1861-1921
Language: eng
Format: epub
Tags: Electric machinery
Publisher: Chicago : American School of Correspondence
Published: 1908-03-25T05:00:00+00:00

Fig. 1 JO. Wave WlDillng.B-l'nlp. S-i ■in-SlnBly KP-rnrrsiit. IS«? WludliiB

as sliown in Fig, 130. These two windings might be furnished with two independent commutators situated at each end of the armature; but usually there is one with the number of its segments doubled, the two sets of bars being alternated between one another. In this case the brushes must be made broad enough to overlap at least two and one-half commutator bars, so as to collect current from both windings simultaneously. Such a winding is known as a duplex singly re-erUrani winding. Triplex-u-ound armatures have three independent windings, with three sets of commutator bars similarly arranged.

The advantage of multiplex windings is that sparking at the brushes may be considerably lessened, for the coils are short-circuited a much shorter length of time and there is a longer brush-resistance path. Hence multiplex windings are much usc<l in machines intended to supply large currents at small voltages, such as generators for electrolytic work.

DYNAMO-ELECTRIC MACHINERY

107

Winding Table for 8-Pole Drum Armature; 202 Conductors; Two-Circuit, Series-Parallel Grouping; Brushes (i) 135** Apart.

B

B

B

B

Paths in Parallel and Conductors in Series between Brushes.

In a simplex lap winding.there will be p paths in parallel l)etween the brushes, and Z^p conductors in series. In a simplex wave winding, there will always he 2 paths in parallel through the armature, and each path will consist of Z-^2 conductors in series.

A multipolar lap winding is often called a parallel-grouped unnding, on account of the numl)er of parallel paths through the armature. Sometimes it is called a muliiplc-circidt winding for the same reason. Similarly, a simplex wave winding is called either a ixco-circuit winding or a series winding.

Combining simplex windings to give a multiplex winding adds to the number of paths in parallel through the armature, but not the generated e. m. f. The latter is increased by multiplying the number of turns per coil, or the number of coils in series; but having a great number of turns per section (that is, element of the

lOS DYNWMO ELECTRIC MACHINERY

winding)aggravates the tendency to spark,as already shown (page 46).

Number of Brush Sets Required. Wliere there are p points around the commutator at which the voltage is zero (see page 35 for method of measuring potential around a commutator), p sets of brushes may be used in any case. In a lap winding, simplex or multiplex, p sets must be used in order to collect the full armature current; while in wave windings, simplex or multiplex, 2 sets of brushes are sufficient, though any number up to p may be used in order to insure sparkless collection of the armature current. For, by referring to Fig. 131, it is seen that if six brushes were placed aroimd the conunutator 60 degrees apart, the extra ones might be considered as cross-connections introduced to reduce the armature PR loss.

Conditions to be Satisfied by a Closed-Coil Drum Winding. A drum winding cannot have an odd number of inductors. Both the front and back pitches must be odd in simplex windings, for the odd-numbered conductors may be regarded as the returns for even-numbered ones.

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