462

ELECTRICAL APPARATUS

, I

usually requires excessive amounts of capacity, and gives rather
poor constants. The machine is asynchronous.
Danielson Motor.—41. An induction motor converted to a
synchronous motor by direct-current excitation. (See " Con-
catenation (4).")
Deep-bar Induction Motor.—7. Induction motor with deep
and narrow rotor bars. At the low frequency near synchronism,
the secondary current traverses the entire rotor conductor, and
the secondary resistance thus is low. At high slips, as in start-
ing, unequal current distribution in the rotor bars concentrates
the current in the toŁ of the bars, thus gives a greatly increased
effective resistance, and thereby higher torque. However, the
high reactance of the deep bar somewhat impairs the power-
factor. The effect is very closely the same as in the double
squirrel cage. (See "Double Squirrel-cage Induction Motor.")
Double Squirrel-cage Induction Motor.—II, 18. Induction
motor having a high-resistance low-reactance squirrel cage, close
to the rotor surface, and a low-resistance high-reactance squirrel
cage, embedded in the core. The latter gives torque at good
speed regulation near synchronism, but carries little current at
lower speeds, due to its high reactance. The surface squirrel
cage gives high torque and good torque efficiency at low speeds
and standstill, due to its high resistance, but little torque near
synchronism. The combination thus gives a uniformly high
torque over a wide speed range, but at some sacrifice of power-
factor, due to the high reactance of the lower squirrel cage. To
get close speed regulation near synchronism, together with high
torque over a very wide speed range, for instance, down to full
speed in. reverse direction (motor brake), a triple squirrel cage
may be used, one high resistance low reactance, one medium
resistance and reactance, and one very low resistance and high
reactance (24).
Double Synchronous Machine.—110, 119. An induction ma-
chine, in which the rotor, running at double synchronism, is
connected with the stator, either in series or in parallel, but with
reverse phase rotation of the rotor, so that the two rotating fields
coincide and drop into step at double synchronism. The machine
requires a supply of lagging current for excitation, just like any
induction machine. It may be used as synchronous induction
generator, or as synchronous motor. As generator, the armature
reaction neutralizes at non-inductive, but not at inductive load,