A Linear Induction Motor (LIM) is basically a rotating squirrel cage induction motor opened out flat. Instead of producing rotary torque from a cylindrical machine it produces linear force from a flat one.

It is not a new technology but merely design in a different form. Only the shape and the way it produces motion is changed. But there are advantages: no moving parts, silent operation, reduced maintenance, compact size, ease of control and installation.

LIM thrusts vary from just a few to thousands of Newtons, depending mainly on the size and rating. Speeds vary from zero to many metres per second and are determined by winding design and supply frequency. Speed can be controlled by either simple or complex systems. Stopping, starting, reversing, are all easy.

Applications are many and varied, ranging from simple sliding doors to full control of a ten tonne vehicle.

WHAT CAN LIMs DO?

Horizontal travel is only limited by the length of the reaction plate or motor. Long conveyors use multi-motor systems. If the reaction plate is a disc then rotary motion is produced. Motion is not dependent on friction between wheels and rails so LIMs can be used in adverse conditions. Either LIM or reaction plate can move whilst the other is fixed. A non-magnetic stainless steel barrier can even be used between LIM and reaction plate.

WHAT ARE THE ADVANTAGES?

LIMs are simple to control and easy to use. Fast response, high acceleration and braking forces are developed. Speed is not dependent on contact friction. There are no gears or chains to cause backlash, no lubrication, no maintenance. Fewer moving parts mean simpler systems. Long term operating costs are reduced.

HOW DOES IT WORK?

First take a squirrel cage induction motor...

then open it out flat...

then smooth the rotor bars into a conductor sheet...

then apply AC power... and you have a LIM

With two stators you can remove the reaction plate iron

As the diagrams show, the LIM is essentially a circular motor opened out flat. The magnetic field, instead of rotating, now sweeps across the flat motor face.

The stator, usually known as the LIM, consists of a 3 phase winding in a laminated iron core. When energised from an AC supply a travelling wave magnetic field is produced. Travel can be reversed by swapping two phases.

The reaction plate is the equivalent of the rotor. This is usually a conductor sheet of aluminium or copper backed by steel, but any of these may be used alone.

Currents induced in the reaction plate by the stator travelling field create a secondary magnetic field. It is the reaction between these two fields which produces the linear thrust.

WHERE ARE LIMs USED?

Wherever straight line motion or reciprocating forces are needed, or where unusual rotary drives are an advantage. Mechanical transmissions are often eliminated, increasing reliability. The LIM is ideal for applications where space is at premium. Rugged, they can be used in hazardous environments.

POWER SUPPLY

A 3 phase AC supply is required for most LIMs but 1 phase can be used to 600N. All standard voltages are available; 220 / 380 / 400 / 415 / 460 / 480 at either 50 or 60Hz. Any other voltage and frequency can be considered.

SPEED CONTROL

The LIM makes an ideal variable speed drive. It behaves like a magnetic clutch and gives a soft-start action. Maximum speed is determined by winding design and supply frequency but load speed can be controlled in several ways. From on/off switching to closed loop inverter control. All but the crudest of LIM control systems require a speed feedback signal. This is usually derived from an incremental encoder, tachogenerator or pulse sensing system. The type of inverter is limited to one that can be specially configured to drive a LIM. Please ask Force Engineering to assist in this choice.

LIMs can also be operated at stall to produce static thrust.

WHICH LIM TO USE?

When selecting a LIM it is important to understand the interaction between LIM, reaction plate and airgap. The variety of designs suited to different speeds, airgaps and reaction plates is extensive.

The correct choice is not always obvious. Let Force Engineering design the best motor and reaction plate combination for your particular application. Our extensive design and test facilities are available to you.

TYPICAL APPLICATIONS

REACTION PLATES

Aluminium / Steel

Steel - Sheet, tube, or structural

Copper / Steel

Conductor material alone.

This can be shaped to your individual requirements.

MANUFACTURE

Forcepack LIMs are designed for ease of installation and connection. A stringent quality assurance system ensures that all LIMs are built to the highest standards using top grade materials. Motors are vacuum impregnated for complete protection. Each motor is thoroughly tested, including thermal overload, before despatch.

There is a choice of cable or terminal box, and various fixings may be selected. Several cooling methods are available depending on motor rating or environment.

CONSTRUCTION OPTIONS