This
product has a stator very similar
to that of the AC induction
motor. Windings are placed in
slots throughout the periphery.
The quantity of windings and
slots is determined in part
by the number of phases (usually
3 or 1) and the number of poles
(usually 2 or 4). The stator
produces a rotating magnetic
field that is proportional to
the frequency supplied. The
speed of the rotating field
can be calculated with the following
formula:V=120f / p
Where
p is the number of poles and
f is the frequency. The main
difference between the synchronous
motor and the induction motor
is that the rotor of this motor
travels at the same speed as
the rotating magnetic field.
This is possible because the
magnetic field of the rotor
is no longer induced. The rotor
either has permanent magnets
or dc excited currents, which
are forced to lock into a certain
position when confronted with
another magnetic field. Thus
the problem with slip and speed
variation with varying loads
is solved. However, this introduces
new problems, such as bringing
the motor up to speed when connected
to a 60 Hz. commercial line.
With the extra expense of a
starter mechanism, synchronous
motors overcome this problem.
Applications
where constant speed is necessary
or where two or more motors
need to be in synch are ideal
for the synchronous motor. Besides
direct commercial line power
source, there are other options
to obtain different varieties
of control. Inverters or AC
variable speed drives can be
used to perform excellent speed
control, with out the need for
tach feedback as in the induction
motor. By keeping the load within
the load rating specs, you should
never have a problem.
In
the case where the load gets too high,
the rotor may fall out of synchronization.
This problem has been solved through
self synchronous control. With the
inverter, the frequency is supplied
by the amplifier. With closed loop
self synchronous control, the location
of the shaft position is relayed back
to the amplifier through an encoder
device. Because the amplifier knows
the position of the motor, the rotor
will never fall out of synchronization.
By adding the commutation device,
the synchronous motor can now be called
an electrically commutated motor or
a brushless motor, and the drive that
controls it a brushless amplifier.
