INDUCTION MOTOR

57

For s = 0, Zi* = oo 9 that is, the motor has no power at or near
synchronism.
For:

or

it is:

s =

and the current taken by the motor is a maximum. The power
output thus is a maximum not when approaching synchronism,
as in the typical induction motor, but at a speed depending on the
slip,

and by varying the capacity reactance, x^ various values of reson-
ance slip, §o, thus can be produced, and thereby speed control of
the motor secured. However, for most purposes, this is uneco-
nomical, due to the very large values of capacity required.

Induction Motor Converted to Synchronous

41. If, when an induction motor has reached full speed, a direct
current is sent through its secondary circuit, unless heavily
loaded and of high secondary resistance and thus great slip, it
drops into synchronism and runs as synchronous motor.

The starting operations of such an induction motor in conver-
sion to synchronous motor thus are (Fig. 21):

First step: secondary closed through resistance: A.

Second step: resistance partly cut out: B.

Third step: resistance all cut out: C.

Fourth step: direct current passed through the secondary: D.

In this case, for the last or synchronous-motor step, usually
the direct-current supply will be connected between one phase
and the other two phases, the latter remaining short-circuited
to each other, as shown in Fig. 21, D. This arrangement retains
a short-circuit in the rotor—now the field—In quadrature with
the excitation, which acts as damper against hunting (Danielson
motor).