An induction motor is one in which the current in the rotor is induced by the electromagnetic field in the stator, eliminating the use of magnets. A linear induction motor is a type of linear motor based on a rotary AC induction motor.
Like permanent magnet linear motors (commonly designated as “ironless” and “iron core” types), linear induction motors consist of a primary part (analogous to the stator in a rotary induction motor) and a secondary part (analogous to the rotor in a rotary version). The primary consists of a 3-phase winding assembled on a steel lamination stack and encapsulated in epoxy.
The secondary consists of what is commonly referred to as a “reaction plate” — a conducting aluminum or copper plate, typically with a steel backing. (The steel can be omitted, but the motor’s force will be significantly reduced.)
When power is supplied to the primary, magnetic flux develops and travels across the length of the primary. Eddy currents are induced (hence, the term “induction” motor) in the conducting material of the secondary. The magnetic flux of the primary and the induced currents of the secondary interact to produce a linear force.
Linear bearings are necessary in linear induction motors to maintain the proper air gap between the primary and secondary parts. The linear bearings also support the attractive forces that occur between the primary and secondary when the motor is powered.
A common variation is the double-sided linear induction motor (DSLIM or DLIM), in which the reaction plate passes between two primaries that face each other. For DLIM designs, the reaction plate is only made of conductive material (aluminum or copper) and does not include steel backing. The double-sided design produces higher thrust forces and eliminates the attractive forces between the primary and secondary parts.
Linear induction motors can be connected directly to the 3-phase AC supply if the application will run at a single speed. For variable and precise speed control, a variable frequency drive or vector drive can be used. (Note that single-phase AC supply can also be used for linear induction motors, but efficiency will be decreased.)
The linear speed of the primary is proportional to the frequency of supply voltage and the pole pitch of the primary part laminations.
Vs = velocity of stator (primary) (m/s)
t = pole pitch (m)
fs = frequency of power supply (Hz)
But linear induction motors are asynchronous, which means the secondary travels at a speed slower than the magnetic field of the primary. The difference in speed is referred to as “slip.”
Vr = velocity of rotor (secondary) (m/s)
s = slip
The thrust produced by a linear induction motor is a factor of the supplied voltage, the amount of slip, and the size of the air gap, as well as the influence of end effects. And because linear induction motors have lower efficiencies, cooling can have a measurable effect on the motor’s ability to produce thrust. For this reason, manufacturers typically provide both force-velocity and force-duty cycle charts for selecting a motor.
End effects are caused by the relative motion between the primary and secondary parts and cause an uneven flux distribution in the air gap, with weaker flux at the leading edge of the primary and stronger flux at the trailing edge. This can lead to a braking effect (opposition to motion), especially at low slip.
Like permanent magnet linear motors, either the primary or the secondary can be the moving part in a linear induction motor. But to reduce cabling issues (since power is supplied to the primary), the secondary is often the moving part. These motors can also have either a shorter primary part and longer secondary part, or a longer primary and a shorter secondary. If the required stroke is relatively long, the primary part will typically be shorter to avoid the cost and complexity of manufacturing a long primary part with many windings.
Linear induction motors offer high continuous thrust forces, high speed and acceleration rates, and relatively simple controls. If you’ve ridden on a maglev train or a “launched” roller coaster, you may have experienced the force and speed capabilities of linear induction motors. Industrial applications typically benefit from linear induction motors in conveying and material handling applications.
Feature image credit: Keith Gibbs