ALTERNATING-CURRENT MOTORS

329

and the power output:

p __ \Tf" T Y

___________Se* { c (r cos 6 + x sin 6) — r}

[(c*~Z, + Zi) + Z (1 + c2 - 2 c cos (9) + SZ (^"^^ITf2'

(121)

The characteristics of this motor entirely vary with a change
of the brush angle, 6. It is, for 6 = 0: P = ^6 ^^—-> hence

i:
140 130 120 110 100 90 80 70 60 50

D&P &
240 %! 220 110 200 100 180 90 160 80 140 70 120 60 100 50 80 40 60 30 40 20 20 10

*•*——
—.. ___ 1

'

X1

POLYPHASE SERIES MOTOR 640 VOLTS

0
"N.
'N

/^
^""\

X.
•^^^

V

v-^
^.

P

/ \

--.^.-
"^^
— <•

_ ......... _

/
.---
"^x
p^=g" ••
"v
^>^
----- 7J

^
/
\

^N.s
••X^
^-^^
P

/
'/ ,
/

\

^^
***,^->
^^.
-*«*^i^6

f
//

N
Ss

"^^^

/
/

\^

//

\^

//

^^
---~^

V

"^— ^
' ----- ' ---- — .

]l

.2

.8

1.0 1.2 1.4

FIG. 154.

1.6

1.8 2.0

very small, while for 6 = 90°: P =

[K]*

-, hence consider-

able. Some brush angles give positive P: motor, others negative,
P, generator.
In such a motor, by choosing 6 and c appropriately, unity
power-factor or leading current as well as lagging current can be
produced.
That is, by varying c and 6, the power output and therefore
the speed, as well as the phase angle of the supply current or
the power-factor can be varied, and the machine used to produce
lagging as well as leading current, similarly as the polyphase
shunt motor or the synchronous motor. Or, the motor can be
operated at constant unity power-factor at all loads and speeds
(except very low speeds), but in this case requires changing the