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. RASET_DEE_ JRN

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. RASET_DEE_ JRN

Short primary system – Large operating distance 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 RASET_DEE_ JRN

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. RASET_DEE_ JRN

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, . RASET_DEE_ JRN

Linear Induction Motor - Characteristics RASET_DEE_ JRN

Tractive effort can be controlled by varying both voltage and frequency simultaneously so that induction density remains constant. RASET_DEE_ JRN

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

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

APPLICATIONS 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 RASET_DEE_ JRN

Low maintenance cost, because of the absence of rotating parts. Advantages: Low maintenance cost, because of the absence of rotating parts. No limitation of tractive effort due to adhesion between wheel & rail. No limitation to maximum speed. No overheating. Can be designed to have better power to weight ratio. RASET_DEE_ JRN

Poor utilization of motor due to transverse edge effect & end effect. Disadvantages: Poor utilization of motor due to transverse edge effect & end effect. Larger air gap. Hence low efficiency and poor p.f. Very high capital cost of reaction rail fixed along the centre line of the track. Complications & high cost involved in providing 3 Ø collector system along the track. Difficulties encountered in maintaining adequate clearances at points of crossings. RASET_DEE_ JRN