314 ELECTRICAL APPARATUS

The curves show the well-known characteristics of the poly-
phase induction motor: approximate constancy of speed at all
loads, and good efficiency and power-factor within this narrow-
speed range, but poor constants at all other speeds.

1. SINGLE-PHASE INDUCTION MOTOR

178. In the single-phase induction motor one primary circuit
acts upon a system of closed secondary circuits which are dis-
placed from each other in position on the secondary member.

Let the secondary be assumed'as two-phase, that is, containing
or reduced to two circuits closed upon themselves at right angles

FIG. 150.—Single-phase induction motor.
to each other. While it then offers a resultant closed secondary
circuit to the primary circuit in any position, the electrical dis-
position of the secondary is not symmetrical, but the directions
parallel with the primary circuit and at right angles thereto are
to be distinguished. The former may be called the secondary
energy circuit, the latter the secondary magnetizing circuit, since
in the former direction power is transferred from the primary to
the secondary circuit, while in the latter direction the secondary
circuit can act magnetizing only.
Let, in the diagram Fig. 150:
EQ, /o, ZQ = impressed e.m.f., current and self-inductive im-
pedance, respectively, of the primary circuit,
/i, Zi = current and self-inductive impedance, respectively,
of the secondary energy circuit,
/2, Zi = current and self-inductive impedance, respectively,
of the secondary magnetizing circuit,
Z = mutual-inductive impedance,
S = speed,
and let s0 = 1 — S2 (where SQ is not the slip).
It is then, by equation (7):