Specifications

124 Control and Automation Solutions Guide
that provides complete control
capability over motor soft-starting,
acceleration, torque, speed maintenance,
deceleration, and holding torque.
Synchronous Motors
A synchronous motor runs
synchronously with the AC excitation
it receives. Various congurations are
possible. One approach applies the
AC line to the stator windings around
the frame while a DC excitation is
applied through slip rings to the
rotor. In many synchronous motors
the rotor has permanent magnets
instead of DC-excited windings. High-
speed synchronous motors are used in
machining applications where the cutter
speed must be maintained at precisely
xed rates or the machined-surface
nish will show signs of speed variation.
When driven mechanically,
synchronous motors will produce
electricity, becoming alternators.
They are used extensively in power
plants to generate grid power.
DC-excited synchronous motors can
also be used in power plants and large
factories to correct the power factor by
being run under no load in parallel with
the large loads. As the DC excitation
of the rotor is modied, it produces
either a leading or lagging power factor
to cancel the nonunity power factor
of the load. In this application they
are called synchronous condensers.
Controllers for AC Synchronous Motors
Various control methods exist for AC
synchronous motors. The motors’
stator windings can be driven with
variable-frequency AC signals from a
VFD, thereby providing soft-starting
and exacting speed control. If a low
frequency is not rst applied to a
stopped synchronous motor, it will not
self-start. It must be given a chance
to “pull in” to synchronization. Some
synchronous motors allow the rotor
windings to be shorted, temporarily
converting it to an induction motor
while it starts. Then once it is close
to synchronous speed, the short is
opened and it becomes synchronous.
If the rotor uses DC excitation, its voltage
can vary with a high-eciency switching
power supply and voltage control.
Linear Motors
Linear motors are eectively motors that
have been unrolled and laid out at.
The moving part is usually called the
forcer and is connected to the external
power source while the rails are lined
with permanent magnets. The opposite
conguration is also used. Everything
from maglev trains (Figure 11) to rail
guns are based on this principle. Very
precise machine positioning systems
use these motors for cutting large
objects with high accuracy. Linear
motors include linear induction motors
(LIMs) and linear synchronous motors
(LSMs). Controllers for linear motors
are quite varied due to the wide range
of applications for them. Nonetheless,
they share similarities with VFDs.
Figure 11. Maglev train driven by a linear motor.