132 ELECTRICAL APPARATUS
comes very large near synchronism, and from slip, s - 0.025, up
to synchronism the average current remains practically con-
stant, thus at synchronism is very much higher than the current
at constant frequency. The average torque also drops some-
what below the torque corresponding to constant frequency,
as shown in the upper part of Fig, 50.
3. LOAD AND STABILITY
82. At constant voltage and constant frequency the torque
of the polyphase induction motor is a maximum at some definite
speed and decreases with increase of speed over that correspond-
ing to the maximum torque, to zero at synchronism; it also de-
creases with decrease of speed from that at the maximum torque
point, to a minimum at standstill, the starting torque. This
maximum torque point shifts toward lower speed with increase
of the resistance in the secondary circuit, and the starting torque
thereby increases. Without additional resistance inserted in
the secondary circuit the maximum torque point, however, lies
at fairly high speed not very far below synchronism, 10 to 20
per cent, below synchronism with smaller motors of good effi-
ciency. Any value of torque between the starting torque and
the maximum torque is reached at two different speeds. Thus
in a three-phase motor having the following constants: impressed
e.m.f., e0 = 110 volts; exciting admittance, Y = 0.01 — O.lj;
primary impedance, ZQ = 0.1 + 0.3 j, and secondary impedance,
Zi — 0.1 + 0.3 j, the torque of 5.5 synchronous kw. is reached
at 54 per cent, of synchronism and also at the speed of 94 per
cent, of synchronism, as seen in Fig. 51.
When connected to a load requiring a constant torque, irre-
spective of the speed, as when pumping water against a constant
head by reciprocating pumps, the motor thus could carry the
load at two different speeds, the two points of intersection of the
horizontal line, L, in Fig. 51, which represents the torque con-
sumed by the load, and the motor-torque curve, D. Of these
two points, d and c, the lower one, d, represents unstable con-
ditions of operation; that is, the motor can not operate at this
speed, but either stops or runs up to the higher speed point, c,
at which stability is reached. At the lower speed, d, a momen-
tary decrease of speed, as by a small pulsation of voltage, load,
etc., decreases the motor torque, D, below the torque, L, required
by the load, thus causes the motor to slow down, but in doing