28

ELECTRICAL APPARATUS

low induced e.m.f., considerable current is produced, which is
effective in producing torque. Such double squirrel-cage induc-
tion motor thus gives a torque curve, which to some extent is a
superposition of the torque curve of the high-resistance and that
of the low-resistance squirrel cage, has two maxima, one at low
speed, and another near synchronism, therefore gives a fairly
good torque and torque efficiency over the entire speed range
from standstill to full speed, that is, combines the good features
of both types. Where a very high starting torque requires
locating the first torque maximum near standstill, and large size
and high efficiency brings the second torque maximum very close
to synchronism, the drop of torque between the two maxima
may be considerable. This is still more the case, when the motor
is required to reverse at full speed'and full power, that is, a very
high torque is required at full speed backward, or at or near
slip s = 2. In this case, a triple squirrel cage may be used, that
is, three squirrel cages inside of each other: the outermost, of
high resistance and low reactance, gives maximum torque below
standstill, at backward rotation; the second squirrel cage, of
medium resistance and medium reactance, gives its maximum
torque at moderate speed; and the innermost squirrel cage, of
low resistance and high reactance, gives its torque at full speed,
near synchronism.
Mechanically, the rotor iron may be slotted down to the inner-
most squirrel cage, so as to avoid the excessive reactance of a
closed magnetic circuit, that is, have the magnetic leakage flux
or self-inductive flux pass an air gap.
19. In the calculation of the standard induction motor, it is
usual to start with the mutual magnetic flux, 3>, or rather with
the voltage induced by this flux, the mutual inductive voltage
f! = e, as it is most convenient, with the mutual inductive
voltage, e, as starting point, to pass to the secondary current by
the self-inductive impedance, to the primary current and primary
impressed voltage by the primary self-inductive impedance and
exciting admittance.
In the calculation of multiple squirrel-cage induction motors,
it is preferable to introduce the true induced voltage, that is,
the voltage induced by the resultant magnetic flux interlinked
with the various circuits, which is the resultant of the mutual
and the self-inductive magnetic flux of the respective circuit.
This permits starting with the innermost squirrel cage, and