Fundamental of Induction Motor

What is an induction motor?

               

Induction Motor

           An induction motor also known as asynchronous motor is commonly used to convert electrical energy into mechanical energy. It is a electrical machine which start to rotate when we give rated 3 phase supply to it's terminal. So from supply to it's rotation many things came into picture this blog is all about that topics let's discuss.

Construction

Any rotating machine has two main part,

1. Stator

2. Rotor

              Stator of an induction machine wound for three phase winding. We need to give 3 phase supply on it's stator terminals.

Stator of Induction motor

                 From Rotor construction, induction motor has two type one is squiral cage motor and other is wound rotor motor. The rotor core is of laminated construction with slots suitably punched for accommodating the rotor winding/rotor bars.

Rotor of Induction motor

When 3 phase voltage given to stator winding then stator generate rotating magnetic field in it. Speed of that magnetic field is synchronous speed which is given by,

This RMF cut (interact magnetically) the rotor bars (rotor is standstill at starting), According to faraday's law of electromagnetic induction,emf induce in rotor bar as rotor bar is shorted by end ring, current will flow through it. It create rotor magnetic field.

            There is a relative speed between rotating magnetic field and steady rotor magnetic field so rotor try to reduce (nullify) this relative speed (Lenz's law) by doing so, rotor start to rotate but it will never reach that synchronous speed of RMF because of slip. If  Nr = Ns the relative speed between the stator field and rotor winding will be zero and therefore the induced emfs and rotor currents will be zero and hence no torque is developed. 

   

                 The rotor thus cannot reach the synchronous speed Ns and hence cannot exceed Ns. With the rotor running at Nr, the relative speed of the stator field with respect to rotor bar is Ns-Nr in the direction of Ns.

Rotating Magnetic Field

Whenever a balanced 3 phase winding with phases distributed in space so that the relative space angle between them is 2𝝿/3 rad, is fed with balanced 3 phase currents with relative time phase difference of 2𝝿/3 rad (elect), the resultant mmf rotates in the air gap at a speed of ⍵ = 2𝝿f elect. rad/s where f is the frequency of current in Hz. Now you are thinking that transformer also has 3 phase winding so RMF produce in that or not?

Types of Induction motor

From rotor construction, 3 phase induction motor can be divided into two type,
1. Squirrel cage IM
2. Wound rotor IM

       

             In Squirral cage IM Rotor is look like cage of squirrel. It's rotor has copper or alluminium bar which is shorted through ring. Their resistance and reactance is fix once it design so you cannot get variable quantity which is related with resistance and reactance. Starting torque of induction motor is depend upon resistance of rotor, once resistance fixed starting torque will also be fixed. One of the disadvantage of squirrel cage IM is it draw very high current during starting( 5-6 times of FLC) and and it's starting torque is also poor.

           In Wound rotor(Slip ring IM) you can change the resistance of rotor by your requirement of torque. Cement industries use this motor in cement mill and crusher because their load already present so to cater that load at starting, Slip ring motor is used. This type of motor's cost is high and also they require more maintanance.

For more information please Watch Video.

https://youtu.be/n_r4VBctBW8