22

ELECTRICAL APPARATUS

I

poles. Since the tooth densities must be chosen sufficiently low
not to reach saturation at the highest number of poles, and their
core loss is usually small compared with that in the primary core
itself, it can be assumed approximately, that the core loss of
the motor is the same, at the same impressed voltage, regardless
of the number of poles. This means, that the. exciting con-
ductance, g, does not change with the number of poles.

Thus, if in a motor of n poles, we change to ri poles, or by the
ratio

ri

a = —7
n

the motor constants change, approximately:
from: to:

= rl + j
= g -

^0 = ?'0
^1 :=: ?*1
16. However, when changing the number of poles, the pitch
of the winding changes, and allowance has to be made herefore
in the constants: a fractional-pitch winding, due to the partial
neutralization of the turns, obviously has a somewhat higher
exciting admittance, and lower self-inductive impedance, than
a full-pitch winding.
As seen, in a multispeed motor, the motor constants at the
higher number of poles and thus the lower speed, must be
materially inferior than at the higher speed, due to the increase
of the exciting susceptance, and the performance of the motor,
and especially its power-factor and thus the apparent efficiency,
are inferior at the lower speeds.
When retaining series connection of all turns for all speeds,
and using the same impressed voltage, torque in synchronous
watts, and power are essentially the same at all speeds, that is,
are decreased for the lower speed and larger number of poles
only as far as due to the higher exciting admittance. The actual
torque thus would be higher for the lower speeds, and approxi-
mately inverse proportional to the speed.
As a rule, no more torque is required at low speed than at
high speed, and the usual requirement would be, that the multi-
speed motor should carry the same torque at all its running
speeds, that is, give a power proportional to the speed.
This would be accomplished by lowering the impressed voltage