Induction motor torque slip characteristics.

Since we know the motor running torque is

T = K1 * φ * sE2 * R2/( R2² + X2²)

Where φ  - flux per status pole

I₂ - rotor current

E2 rotor emf per phase.

So if S = 0 then T = 0

Let's start from right hand side of the curve from point 0.

Let's say, motor is running near to synchronous speed. The term sX2 approximately equals to zero negligible.

Therefore, T = K1 * s * φ * E2 /R2

Assuming stator flux per pole, rotor EMF per phase, rotor resistance per phase is constant.

T ⇒ s 

So for low value of sleep torque slip service straight line.

As the load increases sleep increases. As the load further increases it touches the maximum torque point when S = R2/X2 called pull- out torque or breakdown torque. Any further increase is in load that is slip increases R2 becomes negligible.

 So,  T ⇒ 1/s

Hence after pull out torque or breakdown torque the curve will be rectangular hyperbola. Hence we can see that any further increases in load motor slow down and eventually stopped. If we provide some electrical protection for overload it will trip the circuit.

the circuit.

From figure it is also clear that by increasing the value of rotor resistance we can get maximum torque at s= 1 or zero speed.

To start a motor successfully, the load torque always be less than motor torque. If motor start successfully and when due to some reason load torque becomes more than motor torque than motor will stop. So to run motor successfully motor torque must be greater than load torque all the time.