I am working on my final year project on a fault detection system of a 3 phase induction motor. Does anyone know how to extract a transfer function based on let say stator current as the input, torque as the output?

Can this be done? Is the induction motor a linear system?

sorry for the trouble. I do not have a lot of knowledge on induction motors. thanks a lot.

 

Induction motors are not linear systems. Also some parameters varying by rotor positions. Therefore you cannot write transfer function.

You can look to Clark-Parc transformation for analytical operations of induction motors.

 

First of all you can derive the transfer function. Maybe you should refer to the books like Fundamentals of Power Electronics. I do not know whether your professor asked for you to derive the transfer function or not but the induction machine is a coupled highly nonlinear fifth order system (up to speed). There are two orthogonal components of the stator current you can find if you know the rotor flux and these are called the d and q components (In this case if you know the rotor flux the torque and the flux currents decoupled from each other which means that you can control the flux and the torque independent from each other). There is a linear relation between the d current and the torque of the machine for a given constant rotor flux operation in the linear region of operation (max torque region).

First to the derivation you have to find the dynamic model of the induction machine.Those that you can use are the models in alpha-beta plane (stator current axis) and d-q plane (rotor flux axis).

 

hi, you say that you are working on a fault detection process. if there is a fault, you can't assume any symmetry in the machine. So, in this case, you shouldn't use the d-q model of the induction machine (which is based on symmetry assumptions). d-q transformation reduces the number of equations representing the machine, in case of "symmetry". A highly nonlinear (number of rotor bars + 3 for stator + mechanical equation) set of equations must be solved in order to simulate the induction machine if there is an asymmetry. (if there is a fault, there must be an asymmetry.)