Electrical Engineering ⇒ Topic : Working of linear induction motor

When the 3 Φ primary winding of the motor is energised from a balanced three phase source, a magnetic field moving in a straight line from one end to other at a linear synchronous speed V, is given as

                                                                            ................  (1)

where, τ = pole pitch in metres, and

            f = supply frequency in Hz.

As the flux moves linearly, it drags the rotor plate along with it in the same direction with speed V. Consequently the relative speed of travel of the flux w.r.t. rotor place decreases. (In case rotor plate.speed equals that of magnetic field, the latter would be stationary when viewed from rotor plate. When the rotor plate moves faster than synchronous speed of magnetic field,the direction of force is reversed and a form of regenerative braking, based on the principle of induction generator, will come into existence).

   Slip of the motor(s) is given as :                             .............  (2)

where V_s = linear synchronous speed, and

           V = actual speed of the rotor plate.

Thrust or force or tractive effort (F) is given as

                                                                                                    ............. (3)

where P₂, = actual power supplied to the rotor.

                    Also, copper losses in rotor = sP₂

      and mechanical power developed, P_mech      = (1 - s)P_{2                                                                  ................... (4)}

Fig. (a) shows the thrust or tractive effort-speed characteristics. In a linear induction motor the following peculiar effects are encountered :

• Transverse edge effect;
• End effect.

 Due to the secondary. of this motor being a solid  conducting state, the paths of the induced currents in the secondary are not well defined. The portion of the current paths parallel to the direction of motion of the secondary does not contribute anything towards the production of useful thrust but only contributes towards losses. This effect causes reduction in thrust and increases the losses and is known as transverse edge effect (since the current paths parallel to the direction of motion are more towards the edges of the conducting plate).

                                       Fig. (a)Thrust-speed characteristics