What is a linear induction motor?
Working principle
A linear induction motor (LIM) is a three-phase induction motor that has been cut along its axis and rolled out flat. The stator winding no longer makes a rotating field but one that travels along the machine at the synchronous speed v_s = 2·τ·f, where τ is the pole pitch and f the supply frequency. Facing it across a small air gap is a conductive plate, usually aluminium or copper on a slab of back iron. The travelling field induces currents in the plate, and these currents drag the plate along with the field.
Slip, thrust and the goodness factor
The plate always lags the field: the slip is s = (v_s − v)/v_s. The thrust is the air-gap power divided by the field speed, F = P_airgap / v_s, so it is zero at synchronous speed and large at standstill. How well the plate turns the field into thrust is measured by the goodness factor G = μ0·ω·τ²·σ·d/(π²·g). A good conductor, a high frequency, a long pole pitch and a small gap raise G, and a high G means less magnetising current, a better power factor and a better efficiency.
Large gap and end effect
Unlike a rotary machine, a LIM has an air gap of centimetres, because the plate and its clearance sit between the stator and the back iron. The magnetising reactance falls as 1/g, so the magnetising current is high and the power factor and efficiency are low, often below 0.5. A short stator also has an entry and an exit edge: the eddy currents in the plate cannot die away while it crosses the stator, which removes flux and thrust at high speed. This is the end effect, and it grows with speed.
Normal force and applications
The stator also pulls on the back iron with a normal force that can be several times the thrust, so wheels or guides must carry it. A LIM needs no gearbox, no wheel traction and no rotating parts, which is why it drives maglev and metro trains, conveyors, sliding doors, roller-coaster launches and aircraft catapults. A frequency converter sets v_s and so the speed; swapping the phase sequence reverses the motion, and so brakes the vehicle.
Frequently asked questions
- Why is the power factor of a linear induction motor so low?
- The air gap is large because the plate and its clearance sit between the stator and the back iron, so the magnetising reactance is small and most of the current only builds the field. Shrink the air gap in the simulator: the current falls and the power factor and efficiency rise. The end effect adds loss on top.
- What is the end effect in a linear induction motor?
- The stator has a finite length, so the plate enters and leaves the field. Eddy currents induced at the entry edge decay slowly and are still flowing when the plate leaves; they weaken the air-gap flux and waste power. Switch between ideal and with end effect in the simulator and compare the thrust curves: the loss grows with speed.
- How do you change the speed or direction of a LIM?
- The synchronous speed is v_s = 2·τ·f, so a frequency converter changes the speed of the field and the vehicle follows it with a slip. Swapping two phases reverses the travelling field and the thrust, which also brakes a moving vehicle. Raise the frequency in the simulator and watch the synchronous speed and the thrust curve move.