AC Electric Motors
AC Linear Electric Motors (cont.)
Linear electric motors also need the secondary to be wider than the primary. The secondary should be wide enough to handle induced current with little resistive losses along the transverse edge. Such losses are known as transverse edge-effects which can reduce the electric motor's useful thrust or force.
An electric motor's normal force between stator and armature in SLIM electric motors is perpendicular to the direction of travel. The electric motor's stator and armature are either attracted or repelled by this force. Factors that determine the electric motor's force direction include armature material composition and thickness, stator frequency, air gap, and pole pitch. SLIM electric motors are constructed to minimize the normal force. For DLIMs, and rotary and tubular electric motors, these forces cancel.
Linear induction electric motors produce up to several thousand pounds of thrust. These electric motors also feature positional accuracies of 1 µin. and velocities of 100 in./sec are possible. Feedback is usually from a linear encoder able to provide high resolution and accuracy. Among the encoder technologies used are optical, magnetostrictive, magnetic, and inductive.
For applications involving high accelerations, the secondary normally moves over a long stationary primary. High force and short stroke applications with low repetition rates call for a moving primary and a long stationary secondary. Conversely, for long strokes and high repetition rates, a moving secondary and long stationary primary are required.
In certain material-handling and coil-processing applications using linear-induction electric motors, the material itself is the secondary. For example, to handle sheet steel, overhead SLIM primaries induce currents in the steel and attract it to the primary. A balanced force is provided by gravity and an air bearing to levitate the sheets in air. The sheets are then propelled into piling zones without touching any surface or other object.
Recent advances in power electronics, microprocessors, and electromagnetic analysis software are responsible for many new linear electric motor designs. Power electronics provide inexpensive pulse-width modulators (PWMs), vector controllers, and variable-frequency drives. Microprocessors used for electric motor control include 32-bit processors, coprocessors, and digital signal processors (DSPs). Precision control is needed to compensate for electric motor magnetic saturation, thrust roll-off, and transverse-edge effects.
Electric Motors: AC Linear Electric Motor Information