334

ELECTRICAL APPARATUS

tion, HI, equal to the field excitation, n0, would then give tan
S = i, d = 45°, or 70.7 per cent, power-factor; that is, with an
armature reaction beyond the limits of good motor design, the

power-factor is still too low for use.

The armature, however, also has a
self-inductance; that is, the magnetic
flux produced by the armature cur-
rent as shown diagrammatically in
Fig. 155 generates a reactive e.m.f. in
the armature conductors, which again
lowers the power-factor. While this
armature self-inductance is low with
small number of armature turns, it
becomes considerable when the num-
ber of armature turns, nb is large
compared with the field turns, n0.

Let (Ro = field reluctance, that
is, reluctance of the magnetic

field circuit, and toi =**"—• = the armature reluctance, that is,

b = ~r% = ratio of reluctances of the armature and the field mag-
netic circuit; then, neglecting magnetic saturation, the field flux
is-

FIG. 155.—Distribution of
main field and field of arma-
ture reaction.

the armature flux is:

mi

HI

and the e.m.f. of self-inductance of the armature circuit is:
61 = 27r/n1$110'-8

hence, the total e.m.f. of self-inductance of the motor, or wattless
e.m.f., by (1) and (7) is:

(S)