Linear induction motor By PRASHANT SAMARIYA ABSTRACT Nowadays

Linear induction motor By: (PRASHANT SAMARIYA)

ABSTRACT Nowadays, linear induction motors are widely used, in many industrial application including transportation, conveyor system, actuators, material handling, pumping of liquid metal and sliding door closers etc. with satisfactory performance. The most obvious advantage of linear motor is that it has no gears and requires no mechanical rotary-tolinear converters. This presentation provides a technical review of a linear induction motor with a rotary motors. Linear motor frequently run on a three phase power supply and can support very high speed. However, there are end effects which reduce the force, and it’s often not possible to fit a gearbox to trade off force and speed. Linear induction motor are thus frequently less energy efficient than normal rotary motors for any given required force output.

CONTANTS What is linear induction motor History Principle Construction Working Forces 1. ) Thrust 2. ) End-effect 3. ) Levitation • Performance • Advantages and Disadvantages • Applications of linear induction motor • • •

What is linear induction motor A linear induction motor (LIM) is an alternating current (AC), asynchronous linear motor that works by the same general principles as other induction motors but is typically designed to directly produce motion in a straight line. Characteristically, linear induction motors have a finite primary or secondary length, which generates end-effects, whereas a conventional induction motor is arranged in an endless loop. Despite their name, not all linear induction motors produce linear motion; some linear induction motors are employed for generating rotations of large diameters where the use of a continuous primary would be very expensive. As with rotary motors, linear motors frequently run on a three-phase power supply and can support very high speeds. However, there are end-effects that reduce the motor's force, and it is often not possible to fit a gearbox to trade off force and speed. Linear induction motors are thus frequently less energy efficient than normal rotary motors for any given required force output. LIMs, unlike their rotary counterparts, can give a levitation effect. They are therefore often used where contactless force is required, where low maintenance is desirable, or where the duty cycle is low. Their practical uses include magnetic levitation, linear propulsion, and linear actuators. They have also been used for pumping liquid metals

A typical 3 phase linear induction motor. The "primary" core (grey) has grooves, and the windings are laid into them on top of each other. An aluminium plate above (not shown) serves as "secondary" and will move relative to the primary if a 3 phase AC is applied

History The history of linear electric motors can be traced back at least as far as the 1840 s to the work of Charles Wheatstone at King's College in London, but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described in US patent 782312 (1905; inventor Alfred Zehden of Frankfurt-am. Main), and is for driving trains or lifts. German engineer Hermann Kemper built a working model in 1935. In the late 1940 s, professor Eric Laithwaite of Imperial College in London developed the first full-size working model. In a single-sided version, the magnetic field can create repulsion forces that push the conductor away from the stator, levitating it and carrying it along the direction of the moving magnetic field. Laithwaite called the later versions a magnetic river. These versions of the linear induction motor use a principle called transverse flux where two opposite poles are placed side by side. This permits very long poles to be used, and thus permits high speed and efficiency.

Principle In this electric motor design, the force is produced by a linearly moving magnetic field acting on conductors in the field. Any conductor, be it a loop, a coil, or simply a piece of plate metal, that is placed in this field will have eddy currents induced in it thus creating an opposing magnetic field in accordance with Lenz's law. The two opposing fields will repel each other, creating motion as the magnetic field sweeps through the metal. Where is supply frequency in Hz, p is the number of poles, and magnetic field in revolutions per second. is the synchronous speed of the The travelling field pattern has a velocity of: Where is velocity of the linear travelling field in m/s, and t is the pole pitch. For a slip of s, the speed of the secondary in a linear motor is given by

Construction A linear electric motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots that are often straight cut[5] with coils laid into the slots, with each phase giving an alternating polarity so that the different phases physically overlap. The secondary is frequently a sheet of aluminum, often with an iron backing plate. Some LIMs are double sided with one primary on each side of the secondary, and, in this case, no iron backing is needed. Two types of linear motor exist: a short primary, where the coils are truncated shorter than the secondary, and a short secondary, where the conductive plate is smaller. Short secondary LIMs are often wound as parallel connections between coils of the same phase, whereas short primaries are usually wound in series. The primaries of transverse flux LIMs have a series of twin poles lying transversely side-by-side with opposite winding directions. These poles are typically made either with a suitably cut laminated backing plate or a series of transverse U-cores

Working • Primary of a LIM is. excited by a balanced three phase power supply. • A travelling flux is induced in the primary instead of rotating three phase flux. • Electric current is induced into the secondary due to the relative motion between the traveling flux and the conductors. • This induced current interacts with the traveling flux wave to produce linear force or thrust force. • If the secondary is fixed and primary is free to move, the force will move the primary in the direction of the force, resulting in the required rectilinear motion.

Forces • Thrust The drive generated by linear induction motors is somewhat similar to conventional induction motors; the drive forces show a roughly similar characteristic shape relative to slip, albeit modulated by end effects. Equations exist for calculating the thrust of a motor. Thrust generated as a function of slip

• Levitation In addition, unlike a rotary motor, an electrodynamic levitation force is shown, this is zero at zero slip, and gives a roughly constant amount of force/gap as slip increases in either direction. This occurs in single sided motors, and levitation will not usually occur when an iron backing plate is used on the secondary, since this causes an attraction that overwhelms the lifting force. Levitation and thrust force curves of a linear motor

End effects Unlike a circular induction motor, a linear induction motor shows 'end effects'. These end effects include losses in performance and efficiency that are believed to be caused by magnetic energy being carried away and lost at the end of the primary by the relative movement of the primary and secondary. With a short secondary, the behavior is almost identical to a rotary machine, provided it is at least two poles long but with a short primary reduction in thrust that occurs at low slip (below about 0. 3) until it is eight poles or longer. However, because of end effects, linear motors cannot 'run light' -- normal induction motors are able to run the motor with a near synchronous field under low load conditions. In contrast, end effects create much more significant losses with linear motors.

Performance Linear induction motors are often less efficient than conventional rotary induction motors; the end effects and the relatively large air gap that is often present will typically reduce the forces produced for the same electrical power. The larger air gap also increases the inductance of the motor which can require larger and more expensive capacitors. However conversely, linear induction motors can avoid the need for gearboxes and similar drivetrains, and these have their own losses; and working knowledge of the importance of the goodness factor can minimise the effects of the larger air gap. In any case power use is not always the most important consideration. For example, in many cases linear induction motors have far fewer moving parts, and have very low maintenance. Also, using linear induction motors instead of rotating motors with rotary-to-linear transmissions in motion control systems, enables higher bandwidth and accuracy of the control system, because rotary-to-linear transmissions introduce backlash, static friction and/or mechanical compliance in the control system.

Advantages • There are no moving parts to go wrong. • As the platform rides above the track on a cushion of air, there is no loss of energy to friction or vibration. • As both acceleration and braking are achieved through electromagnetism, linear motors are much quieter than ordinary motors. Disadvantages Because the air gap is greater üMore power is required üEfficiency is lower

Application of linear induction motor • Automatic sliding doors in electric trains. • Mechanical handling equipment, such as propulsion of a train of tubs along a certain route. • Metallic conveyor belts.

Thank you