Linear motors

The first linear motor was conceived by Wheatstone more than a hundred years ago. But large air gaps and low efficiencies prevented linear motors from being widely used. Though they still have relatively large air gaps, linear induction motors are increasingly chosen for material-handling applications because they are quieter, more reliable, and less expensive than rotary motors. And because linear motors do not drive gearboxes or rotary-to-linear conversion devices, they can be more efficient.

A linear motor is conceptually a rotary motor whose stator core has been cut and unrolled. The circular stator becomes a linear stator, defining a single-sided linear induction motor (SLIM). Likewise, if the circular stator is cut into two sections and flattened, the motor becomes a double sided linear induction 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). Cutting and unrolling the stator leads to many other possible linear motor configurations. For example, a tubular motor can be conceptually made from the SLIM by rerolling it in the direction of motion. The pole pattern is produced by three-phase windings in alternate clockwise and counter-clockwise directions around the tube. Other designs are also possible, but few of them are used.

There are several important differences between linear and rotary induction motors that bear on selection. Unlike rotary motors, the linear motor has a beginning and an end to its travel.

First, the moving secondary material enters the primary at one end of the 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. Such flux distribution causes little or no thrust under the first few poles at entry and a braking thrust at exit.

At stall and low speeds, the flux distribution is not seriously distorted and is usually ignored. Second, the large air gap which is endemic to linear motors effectively limits linear force. Fortunately, new pole piece designs offset the adverse air-gap effect.

The moving member in a linear motor is typically a solid conducting plate or sheet. It does not contain coils or windings.

A normal force between stator and armature in the SLIM is perpendicular to the direction of travel. The stator and armature are either attracted or repelled by this force. Factors that deter­mine the force direction include armature material composition and thick-ness, stator frequency, air gap, and pole pitch. SLIMs are constructed to minimize the normal force. For DLIMs, rotary, and tubular motors, these forces cancel.

Recent advances in power electronics, microprocessors, and electro-magnetic analysis software are responsible for many new linear-motor designs. Power electronics provide inexpensive pulse-width modulators (PWMs), vector controllers, and variable frequency drives. Microprocessors used for control include 32-bit processors, coprocessors and digital signal processors (DSPs).