Basics of Induction Motor

Principle of operation:-

· The basic induction motor consists an rotor usually we call it as an field winding which will rotate, stator(Armature Winding) which is fixed.

· Stator and rotor both are connected with three phase windings.

· Three phase connection of both stator and AC supply is shown below:

· As we had applied the AC supply to it current will start flowing in the stator conductors and in the motor.

· Due to this generated current an rotating magnetic field of constant amplitude and rotating at the constant speed will be generated in the air gap between stair and the rotor.

· We can call that rotating magnetic speed as an synchronous speed Ns and it is given as :

Ns=120f1/P

Where;

f1=stator supply frequency & P=Number of poles of motor

· The stator winding which is still fixed so rotating magnetic field will cut the fixed rotor conductors and due to this and emf will be enduced in the stator winding too.

· The rotor Induced voltage will give rise to the rotor currents and the cause behind the rotor Current is the relative velocity between rotating field and rotor hence the rotor current will flow in such a direction that the rotor will experience the force that accelerates it in the same direction as that of rotating magnetic field.

· Without load ideally, the rotor should rotate at the same speed speed as that of rotating magnetic field speed Ns but Practically is isslightlt less than that of Ns because of friction and windage in the motor.

· When motor is loaded mechanically its actual speed N decreases to produce the required amount of torque.

· The percentage difference between the actual speed(N) and synchronous speed (Ns) is known as Slip which is given as:

S=Ns-N/N

v Direction of rotation:-

· As we already know that the rotor will rotate in same direction as that of the rotating magnetic field.

· We can change the direction of the rotor by changing the the direction of rotating magnetic field to do so we are simply interchanging two of phases that we had applied to the Motor.