184

ELECTRICAL APPARATUS

Of these quantities, Pl and PI are always positive; Po and P
can be positive or negative, according to the value of s. Thus
the apparatus can either produce mechanical power, acting as
a motor, or consume mechanical power; and it can either con-
sume electrical power or produce electrical power, as a generator.

107. At:

s = 0, synchronism, Po = 0, P — 0, Pi = 0.
At 0 < s < 1, between synchronism and standstill.

PI, P and Po are positive; that is, the apparatus consumes
electrical power, P0, in the primary, and produces mechanical
power, P, and electrical power, PI + Pi1, in the secondary, which
is partly, Pi1, consumed by the internal secondary resistance,
partly, PI, available at the secondary terminals.

In this case:

Pl + Pl1 _ 8 .

P

1 -

that is, of the electrical power consumed in the primary circuit,
Po, a part Po1 is consumed by the internal primary resistance,
the remainder transmitted to the secondary, and divides between
electrical power, Pi + Pi1, and mechanical power, P, in the
proportion of the slip, or drop below synchronism, s, to the
speed: 1 — s.
In this range, the apparatus is a motor.
At 5 > 1; or backward driving, P < 0, or negative; that is,
the apparatus requires mechanical power for driving.
Then:
Po -Poi-Pil<Pi;
that is, the secondary electrical power is produced partly by
the primary electrical power, partly by the mechanical power,
and the apparatus acts simultaneously as transformer and as
alternating^current generator, with the secondary as armature.
The ratio of mechanical input to electrical input is the ratio
of speed to synchronism.
In this case, the secondary frequency is higher than the
primary.
At:
s < 0, beyond synchronism,
P < 0; that is, the apparatus has to be driven by mechanical
power.