Torque Equation of Induction Motor

     The Torque, T of an induction motor is proportional to the product of stator flux per pole ( φ ), rotor current ( I ) and the power factor of the rotor. The torque of an induction motor is due to the interaction of the rotor and stator fields and is dependent on the strength of the fields and their phase relationship.


T  α  ICos φ2

Since rotor induced e.m.f/phase, E is proportional to the stator flux, φ.

i.e.,     E2  α  φ


T  α  E2 I2 Cos φ2

But rotor current/phase, 


Rotor Power factor,



        K  =  Constant of proportionality

             =     3*60 /  2π Ns

Another Method :


       T  =  Torque developed by an induction motor

We know that

Rotor input,

P2  =  2π Ns T / 60

Therefore Torque,

Now rotor input,

But rotor current / phase,

Therefore, rotor input,

Substituting the expression of 'P2' in equation (1), we get



       K  =  Constant

            =     3*60 /  2π Ns

     The above equation is the torque equation for an induction motor under running condition. So torque developed at any load condition can be obtained, by knowing the slip of the induction motor at that load condition.

Starting Torque :

The torque produced by the motor at start is called as starting torque, Tst.

At start, N = 0 and hence slip, s = 1

Therefore, starting torque can be obtained by substituting s = 1 in the equation (2)

i.e., torque equation under running condition.

Also Check out -

      Condition and Expression for Maximum Torque of Induction Motor ?

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