112

ELECTRICAL APPARATUS

between the motor coils, and thus starting torque can be pro-
duced, especially with a high-resistance armature, that is, a
motor with starting rheostat.

A full discussion and calculation of this device is contained in
the paper on the "Single-phase Induction Motor/7 page 63,
A. I E. E. Transactions, 1898.

FIG. 39.—External' inductive ,
device.

FIG. 40.—Diagram of shading coil.

Internal Inductive Devices
The exciting system of the motor consists of a stationary pri-
mary coil and a stationary secondary coil, short-circuited upon
itself (or closed through an impedance), both acting upon the
revolving secondary.
The stationary secondary can either cover a part of the pole
face excited by the primary coil, and is then called a "shading
coil/7 or it has the same pitch as the primary, but is angularly
displaced therefrom in space, by less than 90° (usually 45° or 60°),
and then, has been called accelerating coil.
The shading coil, as shown diagrammatically in Fig. 40, is
the simplest of all the single-phase induction motor-starting
devices, and therefore very extensively used, though it gives
only a small starting torque, and that at a low apparent starting-
torque efficiency. It is almost exclusively used in very small
motors which require little starting torque, such as fan motors,
and thus industrially constitutes the most important single-
phase induction motor-starting device.
73. Let, all the quantities being reduced to the primary num-
ber of turns and frequency, as customary in induction machines:
Z0 = r0 + jx0 = primary self-inductive impedance,
y = 9 — jb = primary exciting admittance of unshaded poles
(assuming total pole unshaded),