LINEAR INDUCTION MOTOR (LIM)

Special type of Induction Motor – translational motion or linear motion.

• Operates on the same principle as that of a conventional

Induction Motor.

• In LIM, the movement of the field is rectilinear & so

the movement of secondary.

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Fig. (a) shows a poly phase rotary induction motor.

Fig. (b) shows the machine cut along the dotted line and spread out flat.

Thus LIM can be considered as a developed version of a cylindrical

Induction Motor.

Constructional Details

In LIM, either the primary or secondary can be made mobile.

The stationary member must be continuous throughout the length

of the intended travel.

The field system has a three phase distributed winding placed in slots.

Same may be a Single Primary system or a Double Primary system.

Secondary normally a conducting plate made of Copper or Aluminium.

Air gap usually of the order of 25 mm.

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Fig.(i) short single primary system (ii) short single primary system with ferromagnetic plate

Fig. (b) short double primary system Fig. (c) short secondary system with ferromagnetic plate

Short primary system – Large operating distanceShort secondary system – Limited operating distance

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Principle of operation

Primary energized with 3 Ø supply.

A traveling flux wave is produced, that traverse along the length of the

primary, at a linear synchronous speed .

This traveling flux induces current in the secondary.

The interaction between primary & secondary fields results in production

of Linear force or Thrust ‘F’.

If the primary is fixed, the secondary is dragged in the direction of the

traveling wave, thus reducing the relative speed of the flux w.r.t

secondary plate.

vs

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LIM – Speed, Thrust and Power expressions

Linear Synchronous Speed of the traveling wave, where = pole pitch in m , = supply frequency in Hz.

or , where = wavelength of traveling field .

Speed of the secondary in LIM , .

Slip of the LIM,

Thrust or Linear force or Tractive effort N.

Secondary Copper loss, .

Mechanical Power Developed, .

2v fs

/m s

f

(1 )v v ss

/m sv vssvs

gPFvs

2cu gW sP

(1 )d gP s P

sv f /m s

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Linear Induction Motor - Characteristics

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Tractive effort can be controlled by varying bothvoltage and frequency simultaneously so that induction density remains constant.

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TRANSVERSE EDGE EFFECT & END EFFECT

Paths of the induced currents are not well defined, as the secondary

is a solid conducting plate.

The current paths perpendicular to the

direction of motion, contribute to the

production of thrust. The current paths

along the direction of motion contribute

towards losses & these paths reduces

the effective thrust & hence known

as Transverse Edge Effect.

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In LIM with short primary, the flux near the ends have different

configuration.

The currents induced in the secondary nearer each end go beyond the field

structure length ‘L’ . These currents are known as end-effect currents and

they produce additional forces causing braking action, especially at low

values of slip. This phenomenon is known as End Effect.

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APPLICATIONS

a) Where the field is stationary and the

conducting plate travels.

(i) Automatic sliding doors

(ii) Belt conveyors

(iii) Shuttle propelling applications

(iv) Mechanical handling equipments

(v) Accelerometers for high velocity projectiles

(vi) Actuators for h.v. circuit breakers

(vii) Impact extruders for metals

b) Where the field is moving and the

conducting plate stationary

(i) High speed traveling crane motor

(ii) High speed rail traction

Use of LIM in (a) Crane (b) Railway

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Advantages:

i. Low maintenance cost, because of the absence of rotating

parts.

ii. No limitation of tractive effort due to adhesion between

wheel & rail.

iii. No limitation to maximum speed.

iv. No overheating.

v. Can be designed to have better power to weight ratio.

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Disadvantages:

i. Poor utilization of motor due to transverse edge effect &

end effect.

ii. Larger air gap. Hence low efficiency and poor p.f.

iii. Very high capital cost of reaction rail fixed along the

centre line of the track.

iv. Complications & high cost involved in providing 3 Ø

collector system along the track.

v. Difficulties encountered in maintaining adequate

clearances at points of crossings.