In order to understand the speed control, theory and methods, It is important that the basic theory of the controlled component i.e. the motor is understood. The Squirrel Cage induction motor is the most commonly used motor in industry. It is low cost and requires low maintenance in comparison with other motor types. It converts electrical energy into mechanical energy and it is used to provide motion. The conversion process is electro-magnetic induction. Hence the name induction motor
The Supply voltage in each Stator winding produces currents to flow and create a magnetic field hence flux. Flux is a phenomenon of current carrying conductors in a magnetic field.
Due to the alternating ( changing direction) nature of the AC supply a rotating magnetic field (flux) is produced. The direction of this rotating magnetic field (flux) can be changed by changing the direction of the stator voltage.
The stator rotating flux induces currents that flow in the rotor bars which produces a rotating magnetic field in the rotor.
The interaction of stator magnetic field and the induced rotor magnetic field produces an electro- magnetic force that causes the rotor to rotate in the same direction as the stator magnetic field.
A Coil carrying current produces a magnetic field around it, these lines of electro-magnetic field are called Flux.
Passing or rotating a coil in a magnetic field results in a current to flow in the coil. This current is induced in the coil by the magnetic field. This is what is known as Induction
The rotor will continue to accelerate until it rotates at the same rate as the stator magnetic field. At this point no more flux cutting takes place, no induced rotor currents flow, no rotor magnetic field is produced and the motor torque falls to zero.
Therefore an induction motor produces no torque at the synchronous speed.
Once the motor is loaded the speed of the rotor will drop to a point where the rate of flux cutting induces rotor currents of sufficient strength to produce enough motor torque to support the load.
This reduction in motor speed with increasing motor load is known as slip.