ALTERNATING-CURRENT MOTORS
Substituting (.109) in (108) gives:

jfrQ =--------~- (^ cos 02 — TI sin #2)

-,/x* *• "'

327

= -r

Z J zi

r

(HO)

and:

(HI)

186. In the exact predetermination of the characteristics of
such a motor, the effect of the short-circuit current under the
brushes has to be taken into consideration, however. When a
commutator is used, by the passage of the brushes from, segment
to segment coils are short-circuited. Therefore, in addition to
the circuits considered above, a closed circuit on the rotor has
to be introduced in the equations for every set of brushes. Re-
duced to the stator circuit by the ratio of turns, the self-inductive
impedance of the short-circuit under the brushes is very high,
the current, therefore, small, but still sufficient to noticeably af-
fect the motor characteristics, at least at certain speeds. Since,
however, this phenomenon will be considered in the chapters on
the single-phase motors, it may be omitted here.

4. POLYPHASE SERIES MOTOR

187. If in a polyphase commutator motor the rotor circuits
are connected in series to the stator circuits, entirely different

Io

FIG. 153.

characteristics result, and the motor no more tends to synchronize
nor approaches a definite speed at no-load, as a shunt motor, but
with decreasing load the speed increases indefinitely. In short,

if :•>?;• 4-
'ft'.f "I1

Jy i

?';'i'i»v

^,^4'.*;

'^li;^

/}/%Li »«t.

f?«i/i

i r. A,

l:wi'

W'l