SPEED
The angular velocity of this motor, once
at synchronous speed, is constant and hunting, variations
of angular velocity, is at a minimum. The relative rotor
position or load angle will assume a new position with a
load variation.
The number of poles and line frequency determine the rotor
speed as in the Hysteresis Synchronous motor. The maximum
number of poles, however, is limited by the number of pole
saliencies practical. The salient pole motor is available
in two, four, and six poles. Due to pole saliencies required
for each speed, this type of motor cannot be provided as
a multi-speed unit.
LOAD INERTIA
In the use of Salient Pole Induction motors,
load inertia is an important factor. As the rotor approaches
synchronous speed, there is a critical point at which the
motor must accelerate the load and rotor inertia into synchronous
operation. This transition must take place within the time
the rotor rotates through one-half of a pole pitch. It is
for this reason that a salient pole motor may start a load
which it cannot pull into synchronism. With an inertia load,
the size of the salient pole motor required may be much
larger than a Hysteresis motor since the synchronous pull
in torque must be selected to assure transition from sub-synchronous
to synchronous speed.
STARTING TORQUE
The starting torque, due to the pole saliencies,
is subject to wide variations with rotor position.
NOISE
AND VIBRATION
The salient pole Induction motor operates at an inherently
higher noise and vibration level than the Hysteresis Synchronous
motor. The non-uniform magnetic path caused by the pole
saliencies, accounts for this higher level. It should be
remembered that this is a magnetic, rather than mechanical
phenomena. Schulze designs for minimum noise and vibration.
PHASING
The salient pole Induction motor will phase into
synchronous speed at fixed angular position. On two pole
units, the rotor will phase in at two positions, 0°
and 180° apart; a four pole motor will phase in at four
positions 0°, 90°, 180° and 270° apart.
SUMMARY
The inherent characteristics of the salient pole Induction
motor are:
-
Motor rotates at constant speed, regardless of load and
voltage variations within the rating of the motor.
- Motor
can phase in at definite positions with respect to shaft.
- Although
the Hysteresis motor quickly damps the hunting to a minimum,
there are applications where only minute hunting is permissible.
On such applications, the salient pole motor should be
used.
- Wide
variations in starting torque, dependent on rotor position.
- Capable
of synchronizing a limited load inertia.
- Inherent
noise and vibration level higher than the Hysteresis Synchronous
motor.
- Can
be supplied as single speed motor only.
