ij

4

:'( 'j 392 ELECTRICAL APPARATUS

"\ i \

I • • While passing under the commutator brushes, the current in

j I the armature coils must change from, —/i, to /%, or by:

*, ! /', = /'4 + /i- (52)

) In the moment of leaving the commutator brushes, the cur-

,f • rent in the armature coils must change from, /'4 to + /i, or by:

:j'-i I* = Ii- /V ' (53)

I The value, /7ff, or the current change in the armature coils

' ; while entering commutation, is of less importance, since during

; ; 1> this change the armature coils are short-circuited by the brushes.

J! ,i Of fundamental importance for the commutation is the value,

, * Tg, of the current change in the armature coils while leaving the

commutator brushes, since this change has to be brought about

by the resistance of the brush contact while the coil approaches

, t ;v the edge of the brush, arid if considerable, can not be completed

' ' thereby, but the current, Ig, passes as arc beyond the edge of the

* brushes.

/. Essential for good commutation, therefore, is that the current,

'' * Ig, should be zero or a minimum, and the study of the commu-

|» , • tation of the single-phase commutator thus resolves itself largely

I i) into an investigation of the commutation currentj I0, or its abso-

- , l lute value, ig.

I The ratio of the commutation current, ig, to the main armature

/ f current, ii, can be called the commutation constant:

* , * = **• ' (54)
I I . ^ tl .

For good commutation, this ratio should be small or zero.

The product of the commutation current, ig, and the speed, S,
is proportional to the voltage induced by the break of this cur-
rent, or the voltage which maintains the arc at the edge of the
commutator brushes, if sufficiently high, and may be called the
commutation voltage:

ec = Si0. (55)

In the repulsion motor, it is, substituting (23) and (51) in (53),
and dropping the term with \4, as of secondary order:
Commutation Current:

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