21-05-2013, 12:36 PM
LINEAR MOTORS
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INTRODUCTION
Linear Motors are old motors which date from the 1970’s.
Were not intensively used because of their difficult
control and low performance.
Advancements in Power Electronics and
Microprocessors have overcome the control difficulties
and have revived the use of the linear motors.
Linear motors are nowadays emerging as new
technologies.
Main Types of Linear Motors
Linear Synchronous Motor
Permanent Magnet
Wound Secondary Type
Linear Induction Motor
Single-Sided:
Long-primary & Short Secondary
Short-primary & Long-Secondary
Double-Sided or Sandwich
Linear Stepper Motor
Linear Synchronous Motor (LSM)
For an LSM to operate properly the control system must
accurately track the position of the moving element in order to
properly synchronize the moving field current in the stationary
frame (stator).
If synchronization is lost or interrupted the motor slips and loss of
propulsion can occur. This situation is likely to be encountered by
an LSM when external sensors are relied upon to handle motor
synchronization.
If a sensor becomes dirty or mis-aligned or if fog, smoke or other
airborne obstructions are present, the system will experience
problems.
LINEAR INDUCTION MOTOR (LIM)
Linear Induction Motors (LIMs) are significantly different than
Linear Synchronous Motors (LSMs) in the way that they produce
electro-motive forces or motion.
In a LIM, the motor stator creates an Alternating Current (AC) field
that induces currents into the reaction plate, which is typically an
aluminum fin. This creates eddy currents in the moving element
which react with the moving field in the stator to produce thrust.
The induced currents in the aluminum plate manifest themselves in
the form of heat. In cases of high duty cycles or in locked rotor
conditions (where the moving element is not permitted to move)
overheating can occur. In several reported cases at amusement
parks, LIM fins have cracked and parts have melted.