HI& !1"T

42 ELECTRICAL APPARATUS
'«&
half synchronism and synchronism, the torque of the first motor
becomes zero, while the second motor still has a small negative or
generator torque. A little above this speed, the torque of the
concatenated couple becomes zero—about at two-thirds syn-
chronism with a couple of low-resistance motors—and above
this, the concatenated couple again gives a positive or motor
(j '! torque—though the second motor still returns a small negative
'•!' torque—and again approaches zero at full synchronism. Above
•1, !' full synchronism, the concatenated couple once more becomes
J * generator, but practically only the first motor contributes to the
i , generator torque above and the motor torque below full syn-
M chronism. Thus, while a concatenated couple of induction
motors has two operative motor speeds, half synchronism and
]• 'i . full synchronism, the latter is uneconomical, as the second motor
4 holds back, and in the second or full synchronism speed range, it
j is more economical to cut out the second motor altogether, by
1 short-circuiting the secondary terminals of the first motoh
"I t \ With resistance in the secondary of the second motor, the
i maximum torque point of the second motor above half syn-
* '• chronism is shifted to higher speeds, nearer to full synchronism,
and thus the speed between half and full synchronism, at which
the concatenated couple loses its generator torque and again
becomes motor, is shifted closer to full synchronism, and the
motor torque in the second speed range, below full synchronism,
is greatly reduced or even disappears. That is, with high resist-
ance in the secondary of the second motor, the concatenated
couple becomes generator or brake at half synchronism, and
remains so at all higher speeds, merely loses its braking torque
when approaching full synchronism, and regaining it again beyond
full synchronism.
The speed torque curves of the concatenated couple, shown in
Fig. 18, with low-resistance armature, and in Fig. 19, with high
resistance in the armature or secondary of the second motor,
illustrate this.
30. The numerical calculation of a couple of concatenated
induction motors (rigidly connected together on the same shaft
or the equivalent) can be carried out as follows:
Let:
n = number of pairs of poles of the first motor,
n' = number of pairs of poles of the second motor,