326 ELECTRICAL APPARATUS

and a change of phase angle corresponding to a voltage.

cE cos (02 — 0),
= cE sin (0! - 0),

and by choosing then such a position, 0, that the wattless current
produced by the component in phase with 02, is equal and op-
posite to the wattless lagging current of the motor proper, I'0,
the polyphase shunt motor can be made to operate at unity
power-factor at all speeds (except very low speeds) and loads.
This, however, requires shifting the brushes with every change

of load or speed.

When using the polyphase shunt motor as generator of watt-
less current, that is, at no-load and with brush position, 02, it is:

s == 0;

hence, from (78):

or, approximately:

T -

•°

7' _

• ° ~

(105)
(106)

Tf JUQ
I o = ~Fi>

that is, primary exciting current:

./"O — -£/0 77/ /7 _, rj \'

Zi (/ + /0)

or, approximately, neglecting ZQ against Z:
„ _ E05c

/ ° ~ ~z7

(cos 0 + j sin 0)

(107)

jx\

0

{ (r\ cos 6 + Xi sin 6) — j (xi cos 6 — TI sin 0) } ,

(108)

and, since the power component vanishes:

TI cos 0 + Xi sin 0 = 0,
or:

tan 02 =-----•

(109)