Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 243)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 243
Motor Control Chapter 4
A linear electric motor in concept has rotary electric motor stator cores, unrolled
out over a linear path. The circular stator becomes a linear stator, being defined as
a single-sided linear induction electric motor (SLIM). Likewise, if the circular
stator is cut into two sections and flattened, the electric motor becomes a double-
sided linear induction electric motor (DLIM). The DLIM and SLIM both
require a two or three-phase stator (primary) winding and a flat metallic or
conductive plate-type armature (secondary) instead of a rotor.
The moving member in a linear induction motor is typically a solid conducting
plate or sheet. It does not contain coils or windings. However, a linear electric
motor can also be constructed so the primary moves and the secondary remains
stationary.
Linear induction motors LIMs are increasingly chosen for material-handling
applications and Amusement rides because they are quieter, more reliable, and
less expensive than rotary electric motors. And because linear electric motors do
not drive gearboxes or rotary-to-linear conversion devices, they can be more
efficient.
There are several important differences between linear and rotary electric
induction motors that require understanding. Unlike rotary electric motors, the
linear motor has a beginning and an end to its travel. First, the moving secondary
material enters the primary (stator field) at one end of the electric motor and exits
at the opposite end. Induced currents in the secondary material at the entry edge
resist air-gap flux buildup. And at the exit edge, the material retards the air-gap
flux decay. This results in an uneven air-gap flux distribution which contribute to
challenges in sizing VFDs and optimizing set up of control frequency and
voltage. VFD control uses either fixed V/Hz, or independently controlled
frequency and voltage.
Linear Synchronous Motors (LSMs)
Linear Synchronous Motors (LSMs) are significantly different than Linear
Induction Motors (LIMs) in the way that they produce electromotive forces or
motion.
Linear Synchronous Motors (LSMs) are similar in concept with stator cores,
arranged along a path for motion, rather than contained in a frame for rotary
motion. But the field in a LSM moving secondary element is usually provided by
permanent magnets. There are no significant currents induced. Magnets are
embedded in the moving element. This does allow for more definitive position
control and holding position without excessive heat generation. Generally some
sort of position sensor and feedback are necessary to implement control of LSMs
via VFD are necessary.
At the time of this writing there has been minimal experience applying VFDs to
control Linear Synchronous Motors (LSMs). Only this short description of its
construction is included.