A Linear Induction Motor (LIM) is a special type of asynchronous motor that produces straight-line (translational) motion instead of rotational motion. Imagine cutting open a conventional induction motor and laying it flat — that's essentially how a LIM is constructed.
In this article, you will learn the construction, working principle, linear synchronous speed formula, thrust-speed characteristics, end effects, and applications of linear induction motors.
Table of Contents
What is a Linear Induction Motor?
A linear induction motor is a special asynchronous motor designed to produce linear (translational) force — called thrust — instead of rotational torque. It works on the same electromagnetic induction principle as a conventional induction motor, but produces straight-line motion directly without gears, belts, or mechanical converters.
The key difference from a rotary motor:
- Rotary motor: Rotating magnetic field → rotational torque
- Linear motor: Travelling magnetic field → linear thrust force
Construction
The construction of a LIM can be understood by "unrolling" a conventional induction motor:
- Primary (equivalent to stator): Cut the cylindrical stator and lay it flat. It carries the three-phase winding and is connected to AC supply.
- Secondary (equivalent to rotor): Cut the rotor and lay it flat. It is typically a flat aluminium or copper plate (conductor) backed by a ferromagnetic core (iron).
Types of LIM
The double-sided LIM has primary windings on both sides of the secondary conductor. This eliminates the net normal (attractive) force on the secondary and provides more uniform flux distribution, resulting in higher thrust.
Working Principle
The working principle is identical to a conventional induction motor, but in linear form:
- Step 1: Three-phase AC supply is connected to the primary winding
- Step 2: Instead of a rotating magnetic field, a travelling magnetic field is produced that moves along the length of the primary
- Step 3: This travelling field cuts the aluminium secondary conductor, inducing an EMF (by Faraday's law)
- Step 4: The induced EMF drives eddy currents in the secondary
- Step 5: Interaction between the travelling field and induced currents produces a linear force (thrust) on the secondary
If the primary is fixed and the secondary is free to move, the secondary moves in the direction of the travelling field. Alternatively, if the secondary (track) is fixed and the primary is free, the primary moves — this is how maglev trains work.
Linear Synchronous Speed
The speed of the travelling magnetic field (linear synchronous speed) is given by:
Where:
- Vs = Linear synchronous speed (m/s)
- f = Supply frequency (Hz)
- τ (tau) = Pole pitch — the linear distance between adjacent poles on the primary (m)
The actual speed of the secondary is less than the synchronous speed (just like a rotary induction motor needs slip):
Where s = slip of the linear induction motor. Slip is necessary for thrust production — at zero slip (Vr = Vs), no relative motion exists between the field and secondary, so no EMF is induced and no thrust is produced.
Thrust-Speed Characteristics
The thrust-speed curve of a LIM is similar to the torque-speed curve of a rotary induction motor:
- At standstill (V = 0): moderate starting thrust
- Thrust increases with speed up to a maximum (breakdown thrust)
- Beyond breakdown point, thrust decreases as speed approaches Vs
- At synchronous speed (V = Vs): thrust is zero (no slip, no induced EMF)
However, due to the end effect (discussed below), the actual thrust-speed curve of a LIM is asymmetric and shifted compared to a rotary motor.
End Effect
The most significant difference between a LIM and a rotary induction motor is the end effect. In a rotary motor, the stator forms a complete magnetic circuit (closed loop). In a LIM, the primary has two open ends — the entry end and the exit end.
Consequences of end effect:
- Entry end: The travelling field suddenly appears at the entry — the secondary conductor entering the field experiences a transient that opposes thrust
- Exit end: The field abruptly ends — induced currents persist briefly in the secondary after it leaves, causing energy loss
- Non-uniform air gap flux: The flux distribution is not uniform along the length of the primary
- Reduced efficiency: End effects increase losses and reduce the effective thrust
End effects become more significant at higher speeds. They are one of the main reasons LIMs have lower efficiency and power factor compared to rotary motors of similar rating.
Advantages and Disadvantages
Advantages
- Direct linear motion: No mechanical conversion (gears, belts, rack-and-pinion) needed
- No contact: No physical contact between primary and secondary — no wear, no friction
- High speed capability: No mechanical speed limit (no rotating parts to balance)
- Simple secondary: Just a flat conductor plate — cheap and maintenance-free
- Silent operation: No gears or mechanical transmission noise
Disadvantages
- Lower efficiency: Large air gap and end effects reduce efficiency (typically 50–60% vs 85–95% for rotary)
- Lower power factor: Large magnetizing current due to bigger air gap
- End effects: Cause non-uniform thrust and additional losses
- Higher cost per unit thrust: Compared to rotary motor + mechanical conversion
- Normal force: Single-sided LIM produces an attractive force between primary and secondary that must be managed
Applications
Despite lower efficiency, LIMs are used where direct linear motion, contactless operation, or very high speed is required:
- Maglev trains: Magnetic levitation transportation (Shanghai Maglev, Japan's Chuo Shinkansen)
- Airport people movers: Automated transit systems (e.g., Bombardier Innovia)
- Electromagnetic launchers: Aircraft catapults on aircraft carriers (EMALS)
- Roller coasters: Launch systems for theme park rides
- Conveyor systems: Metallic belt conveyors in factories
- Automatic sliding doors: Contactless door operation
- Liquid metal pumps: Pumping molten metals in foundries (no moving parts in contact with hot metal)
- Material handling: Sorting and positioning of metallic objects
FAQs
How is a linear induction motor different from a linear synchronous motor?
A linear induction motor has a passive secondary (just a conductor plate) and works on induction principle — it needs slip. A linear synchronous motor has an active secondary (permanent magnets or electromagnets) and runs at exactly the synchronous speed with zero slip. Linear synchronous motors are used in high-speed maglev systems.
Why does a LIM have lower efficiency than a rotary motor?
Three main reasons: (1) larger air gap increases magnetizing current and reactive power, (2) end effects cause additional losses and reduce effective thrust, (3) the secondary is a solid conductor (not laminated), so eddy current losses are higher.
Can a LIM work in reverse (as a brake)?
Yes. If the secondary moves faster than the travelling field (slip becomes negative), the thrust reverses direction — opposing the motion. This is called regenerative braking. LIMs can also brake by reversing the phase sequence of the supply.
What determines the speed of a linear induction motor?
The synchronous speed is determined by supply frequency and pole pitch (Vs = 2fτ). The actual speed depends on the load (slip). By varying the supply frequency (using a VFD), the speed can be controlled precisely.
Why is the air gap larger in a LIM compared to a rotary motor?
In a rotary motor, the rotor is precisely centered inside the stator with a small gap (0.5–2 mm). In a LIM, the primary and secondary are separate flat structures that must accommodate mechanical clearance, vibration, and (in transport applications) suspension movement. Typical LIM air gaps are 10–15 mm.