AC Electric Motors
AC Linear Electric Motors
The first linear electric motor was conceived by Wheatstone more than 100 years ago. But large air gaps and low efficiencies prevented linear electric motors from being widely used. Though electric motors still have relatively large air gaps, linear induction electric motors are increasingly chosen for material-handling applications 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.
A linear electric motor is conceptually rotary electric motors feature stator cores, which have been cut and unrolled. The circular stator becomes a linear stator, defining 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).
Cutting and unrolling the stator leads to many other possible linear electric motor configurations. For example, tubular electric motors can be conceptually made from the SLIM by rerolling it in the direction of motion. The electric motor pole pattern is produced by three-phase windings in alternate clockwise and counterclockwise directions around the tube. Other electric motors feature designs, which are also possible, but few of them are used.
There are several important differences between linear and rotary induction electric motors that bear on selection. Unlike rotary electric 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 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. Such flux distribution causes little or no thrust under the first few electric motor poles at entry and a braking thrust at exit.
At stall and low speeds, electric motors produce flux distribution, which is not seriously distorted and is usually ignored. But dynamic compensation is required to minimize thrust roll-off at high speeds.
Second, the large air gap which is endemic to linear electric motors effectively limits linear force. Fortunately, new pole piece electric motor designs offset the adverse air-gap effect.
The moving member in a linear electric motor is typically a solid conducting plate or sheet. It does not contain coils or windings. However, a linear electric motor can be constructed so the primary moves and the secondary remains stationary.
Electric Motors: AC Linear Electric Motor Information