20

ELECTRICAL APPARATUS

As seen, a very large amount of capacity is required for speed
control. This limits its economic usefulness, and makes the
use of a cheaper form of effective or equivalent capacity desirable.
C. Multispeed Motors
14. The change of speed by changing the number of poles, in
the multispeed induction motor, involves the use of fractional-
pitch windings: a primary turn, which is of full pole pitch for
a given number of motor poles, is fractional pitch for a smaller
number of poles, and more than full pitch for a larger number
of poles. The same then applies to the rotor or secondary, if
containing a definite winding. The usual and most frequently
employed squirrel-cage secondary obviously has no definite
number of poles, and thus is equally adapted to any number of
poles.
As an illustration may be considered a three-speed motor
changing between four, six and eight poles.
Assuming that the primary winding is full-pitch for the six-
polar motor, that is, each primary turn covers one-sixth of the
motor circumference. Then, for the four-polar motor, the
primary winding is % pitch, for the eight-polar motor it is %
pitch—which latter is effectively the same as % pitch.
Suppose now the primary winding is arranged and connected
as a six-polar three-phase winding. Comparing it with the
same primary turns, arranged as a four-polar three-phase wind-
ing, or eight-polar three-phase winding, the turns of each phase
can be grouped in six sections:
Those which remain in the same phase when changing to a
winding for different number of poles.
Those which remain in the same phase, but are reversed when
changing the number of poles.
Those which have to be transferred to the second phase.
Those which have to be transferred to the second phase in the
reverse direction.
Those which have to be transferred to the third phase.
Those which have to be transferred to the third phase in the
reverse direction.
The problem of multispeed motor design then is, so to arrange
the windings, that the change of connection of the six coil groups
of each phase, in changing from one number of poles to another,
is accomplished with the least number of switches.