A Linear Induction Motor (LIM) is a special form of induction motor in which the electromagnetic field does not rotate in a circular manner, but instead travels linearly along a straight path. Instead of producing rotational motion like a conventional induction motor, a LIM produces direct linear or translational motion. Because of this unique behavior, LIMs are widely used in transportation systems, magnetic launchers, material handling devices, sliding doors, and high-speed automation processes.
The LIM is conceptually obtained by imagining a traditional cylindrical induction motor cut along its axis and opened out into a flat plane. The stator becomes the primary of the LIM, while the rotor becomes the secondary conducting plate. The electromagnetic principle remains the same, but the motion produced is linear instead of rotational.
Definition of Linear Induction Motor
A Linear Induction Motor is an induction machine in which a traveling magnetic field is produced along a straight path, and when this traveling field interacts with a conducting secondary sheet, a linear thrust force is developed instead of torque. The motion generated is translational, and the secondary body moves along the direction of the traveling magnetic field.
In short, the conventional induction motor torque becomes thrust, and angular velocity becomes linear velocity.
Construction and Working of Linear Induction Motor
(a) A conventional induction motor is cut along its axis and then unfolded into a flat structure to form a Linear Induction Motor (LIM).
(b) A LIM configuration where the primary winding is on one side and the secondary consists of a flat metallic sheet.
(c) A LIM in which the secondary metallic sheet is supported by an additional backing layer made of ferromagnetic material to improve the magnetic path.
(d) A double-sided LIM in which the secondary metal sheet is placed between two primary windings, one on each side.
A Linear Induction Motor (LIM) can be visualized by imagining an ordinary induction motor that has been cut open along its length and unfolded into a flat structure. Even though the physical shape changes, the operating principle remains the same as that of a rotating induction motor. However, instead of producing rotational motion, the LIM produces straight-line or translational motion.
In a rotating induction motor, the stator forms the stationary part and the rotor forms the rotating part. But in a LIM, these elements are referred to as primary and secondary. The primary corresponds to the stator and carries the three-phase winding, whereas the secondary corresponds to the rotor. Unlike a conventional rotor that contains embedded conductors, the secondary of a LIM is usually made from a flat metal sheet, commonly aluminum.
If we compare both machines, the angular speed of a rotary induction motor becomes the linear velocity in a LIM, and the torque produced in a normal motor becomes thrust (linear force) in a LIM. While a rotary motor produces continuous rotation, a LIM produces a continuous straight-line force along its length.
In many designs, the primary winding is made shorter than the secondary. In some applications, the opposite arrangement may be used, depending on motion requirements. To improve the magnetic path and reduce reluctance, the secondary metal sheet is often supported by a backing layer of ferromagnetic material such as iron.
A LIM may be constructed with either a single-sided primary or a double-sided primary. In the single-sided type, the primary winding is placed on one side of the secondary sheet with an air-gap between them. In the double-sided type, there are two primary windings placed on opposite sides of the secondary, which helps produce stronger thrust and improved performance.
Overall, a Linear Induction Motor is essentially a modified version of a conventional induction motor that has been transformed from a circular structure into a flat, linear form to generate direct translational motion instead of rotation.
When a three-phase AC supply is applied to the primary windings of a Linear Induction Motor, a travelling linear magnetic field is produced. This field behaves in the same way as the rotating magnetic field in a conventional induction motor, except that here it moves in a straight line instead of rotating. The secondary of the LIM is generally a flat metal sheet, usually made of aluminum, and it is often supported by a layer of ferromagnetic material to improve the magnetic path. As the travelling magnetic field passes over the secondary sheet, eddy currents are induced in it according to Lenz’s Law. These induced currents create their own magnetic field that opposes the original field.
The magnetic field produced in the primary and the opposing field in the secondary interact with each other. Because of this interaction, a repulsive force or thrust is developed. This thrust causes motion as the magnetic field travels along the secondary surface. If the primary remains fixed and only the secondary is free, then the secondary moves in the same direction as the travelling magnetic field. However, if the secondary is fixed and the primary is allowed to move, then the primary will move in the opposite direction to the travelling field.
In simple terms, the movement always occurs due to the reaction between the travelling magnetic field and the induced current field in the metal sheet.
Important Constructional Parts
- Primary (Stator Equivalent): Contains three-phase distributed windings placed in slots similar to a normal stator.
- Secondary (Rotor Equivalent): Usually a flat conducting sheet made of aluminum or copper.
- Ferromagnetic Backing Plate: Often placed behind the secondary to improve flux linkage and reduce reluctance.
- Air-Gap: Separates the primary and secondary parts; in LIMs it is larger compared to rotary machines.
- Mechanical Frame: Provides support, insulation and mounting structure.
In many LIM designs, the primary winding may be located on one side or both sides of the secondary plate, resulting in:
- Single-sided LIM
- Double-sided LIM
Working Principle of Linear Induction Motor
The working principle of a Linear Induction Motor is based on the production of a travelling magnetic field. When a three-phase AC supply is applied to the primary winding, a magnetic field is produced which travels linearly along the length of the primary. This traveling magnetic wave cuts the conducting secondary plate and induces eddy currents in it.
According to Lenz’s Law, the induced currents produce their own magnetic field which opposes the cause producing them. The interaction of the traveling magnetic field and the induced secondary field results in a thrust force, causing linear motion of the secondary or primary depending on which one is free to move.
Performance Characteristics and Discussion
The linear induction motor shown in figure (b) is obtained by modifying the rotary induction motor shown in figure (a), where the cylindrical motor is cut open and converted into a flat linear form.
In a conventional induction motor, when a three-phase AC supply is applied to the stator windings, a rotating magnetic field is produced which rotates at synchronous speed. The same concept applies to a Linear Induction Motor (LIM), but instead of a rotating field, a travelling flux wave is produced along the length of the primary winding.
When the three-phase supply is fed to the primary of a LIM, a travelling magnetic flux density wave moves forward along the motor, as illustrated in the figure. This changing magnetic field cuts the secondary conductor and induces an emf in it. As a result, current begins to flow in the secondary sheet. The interaction between the induced current and the travelling magnetic field produces a linear force or thrust.
If the primary remains stationary and the secondary is free to move, the secondary body will travel in the same direction as the moving magnetic wave. On the other hand, if the secondary is fixed and the primary is allowed to move freely, then the primary will move in the opposite direction of the travelling field. The velocity of the travelling magnetic wave is represented as \(V_s\), while the actual velocity of the moving secondary is denoted as \(V_s\). The difference between these velocities leads to translational slip, which defines the relative motion between the flux wave and the secondary.
Thrust-Speed Characteristics of LIM
At high speeds, thrust reduces and becomes zero when the secondary speed approaches synchronous velocity $V_s$. Thus, maximum thrust occurs at some intermediate slip value.
Translational Slip and Synchronous Velocity in a Linear Induction Motor
In a Linear Induction Motor (LIM), the motion of the secondary conductor is compared with the speed of the travelling magnetic wave produced by the primary winding. The difference between these two speeds is expressed in terms of translational slip. Slip represents how much slower the secondary moves compared to the travelling magnetic field.
The slip \(S\) is mathematically defined as:
$$ S = \frac{V_s – V}{V_s} $$
where \(V_s\) is the synchronous (travelling wave) velocity and \(V\) is the actual velocity of the secondary member.
The synchronous linear velocity of the travelling magnetic wave is given by:
$$ V_s = 2\,T_p\,f $$
Here, \(T_p\) is the pole pitch and \(f\) is the supply frequency. The pole pitch is expressed as:
$$ T_p = \frac{2\pi}{P} $$
where \(P\) represents the number of poles.
For comparison, in a conventional induction motor the synchronous speed in revolutions per minute (rpm) is:
$$ N_s = \frac{120f}{P} $$
and in revolutions per second (rps):
$$ n_s = \frac{2f}{P} $$
Therefore, the corresponding linear synchronous velocity becomes:
$$ V_s = 2\pi n_s = \frac{2\pi}{P} \times 2f = 2fT_p $$
This relationship shows that the synchronous speed of the travelling field in a LIM depends directly on the supply frequency and the pole pitch. The slip determines the thrust-producing interaction between the secondary conductor and the travelling magnetic wave.
Types of Linear Induction Motors
| Type | Description |
|---|---|
| Single-Sided LIM | Primary winding on one side and a single secondary plate. |
| Double-Sided LIM | Primary windings on both sides with the secondary plate in between. |
| Short Primary LIM | Primary length shorter than secondary; commonly used in transportation. |
| Short Secondary LIM | Secondary shorter than primary; used where moving mass must be light. |
Advantages of Linear Induction Motor
- Produces direct linear motion without mechanical gears or screws.
- High starting thrust and fast acceleration.
- Simple construction and low mechanical wear.
- Suitable for automation and motion-control applications.
- Ideal for magnetic levitation and transportation systems.
- Silent operation and smooth movement.
Disadvantages of Linear Induction Motor
- Low efficiency due to large air-gap and high magnetizing current.
- Poor power factor compared to rotary machines.
- High eddy current and copper losses.
- Requires precise alignment and material selection.
- Higher manufacturing cost.
Applications of Linear Induction Motor
- High-speed transportation and maglev trains
- Automated sliding doors and elevators
- Material handling and conveyors
- Airport baggage transport systems
- Theme-park and roller-coaster propulsion
- Robotics and precision actuation
- Stage curtains and automatic gates
- Shuttle launchers and magnetic launch systems
Conclusion
The Linear Induction Motor is a powerful electromechanical system that produces direct translational motion using electromagnetic induction principles. Even though its efficiency and power factor are comparatively lower than traditional rotary motors, its ability to generate thrust without mechanical conversion makes it invaluable in specialized applications such as transportation, automation and motion-control systems. With advancements in materials and power electronics, LIM technology continues to evolve and expand into high-performance industrial and transportation environments.