SINGLE-PHASE COMMUTATOR MOTORS 391

Volt-ampere Input:
Power-factor:

Efficiency:
Apparent Efficiency:

r, =

P

n = p--,

(44)
(45)

(46)
(47)

etc.

216. While excessive values of the short-circuit current under
the commutator brushes, f*, give bad commutation, due to ex-
cessive current densities under the brushes, the best commuta-
tion corresponds not to the minimum value of /4 — as the zero
value at synchronism in the repulsion motor — but to that value
of /4 for which the sudden change of current in the armature
coil is a minimum, at the moment where the coil leaves the com-
mutator brush.

J4 is the short-current in the armature coil during commuta-
tion, reduced to the armature circuit, /i, by the ratio of effective
turns:

short-circuited turns under brushes

total effective armature turns

, ^

The actual current in the short-circuited coils during commuta-
tion then, is:

It = -, (49)

or, if we denote:

X4

(50)

where A* is a fairly large quantity, and substitute (24), it is:

ZK

(51)

Before an armature coil passes under the commutator brushes,
it carries the current, —/i; while under the brushes, it carries
the current, l\] and after leaving the brushes, it carries the cur-
rent,

5; <

In

,.iğ