148

ELECTRICAL APPARATUS

As seen, in this case of the three-phase motor, the third
harmonics have no phase rotation, but are in phase with, each
other, or single-phase voltages. The fifth harmonic gives
backward phase rotation, and thus negative torque, while the
seventh harmonic has the same phase rotation, as the funda-
mental, thus adds its torque up to its synchronous speed, S =
+Jf, and above this gives negative or generator torque. The
ninth harmonic again is single-phase.

Fig. 56 shows the fundamental torque, TI, the higher harmonics

FIG. 56.—Three-phase induction motor, component harmonics and
resultant torque.
of torque, T$ and TV, and the resultant torque, T. As seen, the
distortion of the torque curve is materially less, due to the
absence, in Fig. 56, of the third harmonic torque.
However, while the third harmonic (and its multiples) in the
three-phase system of voltages are in phase, thus give no phase
rotation, they may give torque, as a single-phase induction motor
has torque, at speed, though at standstill the torque is zero.
Fig. 57 B shows cliagrammatically, as T, the development of
the air-gap distribution of a true three-phase winding, such as
used in synchronous converters, etc. Each phase 1, 2, 3, covers
one-third of the pitch of a pair of poles or -^~, of the upper layer,