CONCATENATION

41

respective speeds approaches synchronism at no-load; or, still
more generally, any number of concatenated induction motors
run at such speeds that the sum of their speeds approaches
synchronism at no-load.
With mechanical connection between the two motors, con-
catenation thus offers a means of operating two equal motors at
full efficiency at half speed in tandem, as well as at full speed,
in parallel, and thereby gives the same advantage as does series
parallel control with direct-current motors.
With two motors of different number of poles, rigidly con-
nected together, concatenation allows three speeds: that of the
one motor alone, that of the other motor alone, and the speed of
concatenation of both motors. Such concatenation of two motors
of different numbers of poles, has the disadvantage that at the
two highest speeds only one motor is used, the other idle, and the
apparatus economy thus inferior. However, with certain ratios
of the ftumber of poles, it is possible to wind one and the qanae
motor structure so as to give at the same time two different
numbers of poles: For instance, a four-polar and an eight-
polar winding; and in this case, one and the same motor struc-
ture can be used either as four-polar motor, with the one winding,
or as eight-polar motor, with the other winding, or in concatena-
tion of the two windings, corresponding to a twelve-polar speed.
Such "internally concatenated" motors thus give three different
speeds at full apparatus economy. The only limitation is, that
only certain speeds and speed ratios.can economically be produced
by internal concatenation.
29. At half synchronism, the torque of the concatenated couple
of two equal motors becomes zero. Above half synchronism,
the second motor runs beyond its impressed frequency, that is,
becomes a generator. In this case, clue to the reversal of current
in the secondary of the first motor (this current now being out-
flowing or generator current with regards to the second motor)
its torque becomes negative also, that is, the concatenated couple
becomes an induction generator above half synchronism. When
approaching full synchronism, the generator torque of the second
motor, at least if its armature is of low resistance, becomes very
small, as this machine is operating very far above its synchronous
speed. With regards to the first motor, it thus begins to act
merely as an.impedance in the secondary circuit, that is, the first
machine becomes a motor again. Thus, somewhere between