CHAPTER VIII
SYNCHRONIZING INDUCTION MOTORS
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94. Occasionally two or more induction motors are operated
in parallel on the same load, as for instance in three-phase rail-
roading, or when securing several speeds by concatenation.
In this case the secondaries of the induction motors may be
connected in multiple and a single rheostat used for starting
and speed control. Thus, when using two motors in concatena-
tion for speeds from standstill to half synchronism, from half
synchronism to full speed, the motors may also be operated on
a single rheostat by connecting their secondaries in parallel.
As in parallel connection the frequency of the secondaries must
be the same, and the secondary frequency equals the slip, it
follows that the motors in this case must operate at the same slip,
that is, at the same frequency of rotation, or in synchronism with
each other. If the connection of the induction motors to the
load is such that they can not operate in exact step with each
other, obviously separate resistances must be used in the motor
secondaries, so as to allow different slips. When, rigidly connect-
ing the two motors with each other, it is essential to take care
that the motor secondaries have exactly the same relative posi-
tion to their primaries so as to be in phase with each other, just
as would be necessary when operating two alternators in parallel
with each other when rigidly connected -to the same shaft or
when driven by synchronous motors from the same supply.
As in the induction-motor secondary an e.m.f. of definite fre-
quency, that of slip, is generated by its rotation through the
revolving motor field, the induction-motor secondary is an
alternating-current" generator, which is short-circuited at speed
and loaded by the starting rheostat during acceleration, and the
problem of operating two induction motors with their secondaries
connected in parallel on the same external resistance is thus the
same as that of operating two alternators in parallel. In general,
therefore, it is undesirable to rigidly connect induction-motor
secondaries mechanically if they are electrically connected in
parallel, but it is preferable to have their mechanical connection
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