Linear Induction Motor

Linear Induction Motor

The linear induction motor works on the same principle as that of normal induction motor with the difference that instead of rotational movement, the rotor moves linearly. If the stator and rotor of the induction motor are made flat then it forms the linear induction motor. The flux produced by the flat stator moves linearly with the synchronous speed from one end to the other. The synchronous speed is given by,

v _s = 2 w f

where v _s = Linear synchronous speed (m/s)

w = Width of one pole pitch (m)

f = Frequency of supply (Hz)

It can be seen that the synchronous speed is independent of number of poles but depends only on width of pole pitch and supply frequency. The schematic of linear induction motor is shown in the Fig. 9.6.1.

The flux moves linearly and forces the rotor to move in straight line in the same directions. In many of the practical applications the rotor plate is a stationary member whereas stator moves. The analysis of linear machines is nearly same as that of rotating machines. All the angular dimensions and displacements are displaced by linear ones and torque is replaced by the force. The expressions for machine parameters are derived analogously and the results are similar in form. Some of the typical results are as given below,

The linear induction motors are widely used in transportation fields i.e. in electric trains. The stator is mounted on the moving vehicle and a conducting stationary rotor forming the rails. The induced currents in the rail not only force the stator to move but also provide magnetic levitation in which the train floats in air above the track. This mechanism proves better for high speed transportation without the difficulties associated with wheel-rail interactions present in conventional rail transport. Thus the trains may have speed of about 300 km/hr. A powerful electromagnet fixed underneath the train moves across the rails which are conducting. This induces the currents in the rail which provides levitation so that the train is pushed up above the track in the air. The operation of such system is automatic and the system is reliable and safe.

Linear motors also find application in the machine tool industry and in robotics where linear motion is required for positioning and for operation of the manipulators. In addition to this, reciprocating compressors can also be driven by the linear machines.

Example 9.6.1 An overhead crane in a factory is driven horizontally by means of two similar linear induction motors whose rotors are the two steel I beams on which the crane rolls. The 3 phase, 2 pole linear stators which are mounted on opposite sides of the crane have a pole pitch of 5 cm and are energized by variable frequency source. The tests on one of the motors gave following results. 

Stator frequency - 50 Hz, Stator Cu and iron loss - 1 kW, Power to stator - 5 kW

Crane speed - 2.5 m/s

Calculate :

i) Synchronous speed and slip,

ii) Power input to rotor

iii) Cu losses in the rotor

iv) Gross mechanical power developed

v) Thrust.

Solution : Synchronous speed, v _s = 2 wf = 2 × 5 × 10^{- 2} × 50 = 5 m/s

Review Question

1. Explain the principle of operation of linear induction motor. List some applications of it. AU : May-03,16, Dec.-15, Marks 8