How does the induction motor start?

Answer 1

It's self-started. and it has a normal speed less than that of

synchronous. but its speed decreases as the load

increases.

An induction motor has 2 sets of coils in it that are physically displaced from each other. (One is the 'start' winding, connected only during starting, and a 'run' winding, connected whenever the motor is turned on.) The sets of coils have different electrical charactreristics, and when current flows in the coils, they each create their own magnetic fields that are displaced in time by the different values of coil resistance and inductance.

The physical and timing displacements produce a combined magnetic field that rotates through the iron core and copper or aluminum bars of the rotor, inducing a current in the bars ('induction' motor....) creating a magnetic field around the bars that repels the original rotating field. This magnetic repulsion pushes the rotor around, driving the motor (and the load).

The 'start winding' has a high resistance and gets VERY hot if left connected once the motor has started. It is usually turned off (by a centrifugal switch) once the motor is running, and the 'pulsating' field from the 'run' winding keeps the rotor turning.

Some motors use a capacitor (or 'condenser') to enhance the displacement between the magnetic fields (and increase the force of repulsion) to improve starting torque, and a smaller capacitor to improve running torque and smoothness of operation.

Small, low torque motors may not have a 'start' winding, but rather a single turn, short-circuited coil ('shading ring') on part of each of its poles to create a weakly rotating magnetic field - but without a switch or capacitor.

Answer 2

a self starting and three phase ac supply