Transcript LINEAR INDUCTION MOTOR (LIM)

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 distance
Short 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
v
s
.
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.
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LIM – Speed, Thrust and Power expressions
Linear Synchronous Speed of the traveling wave,
where
or vs

 f  m/ s
= pole pitch in m ,
, where

Speed of the secondary in LIM ,
Slip of the LIM,
v v
s s
v
s
= supply frequency in Hz.
= wavelength of traveling field .
v  v (1  s ) m / s.
s
Thrust or Linear force or Tractive effort
Secondary Copper loss,
f
v  2 f m / s
s
F
Pg
v
N.
s
Wcu 2  sPg .
Mechanical Power Developed,
Pd  (1  s ) Pg
.
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Linear Induction Motor - Characteristics
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Tractive effort can be controlled by varying both
voltage 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
Use of LIM in (a) Crane (b) Railway
(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
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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.
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