Induction Motor Equivalent Circuits

You can model it either way, and get the same (external) results if you adjust the modeling parameters appropriately. It is often either a matter of what someone has already done in terms of modeling, or what is most convenient for the user of the model.

Thank you very much for your reply. Indeed the two equivalent circuits give the same external properties, I just want to find out what are the theories behind the different circuits. For instance, the reason for pi-model has something to do with the eddy current and hysteresis and they are all proportional to terminal voltage and so in this case they are put in parallel. But how to explain T-model?

The T model can be explained in the same way. The input "arm" of the "T" represents losses in the stator winding. The "leg" represents iron losses (or hysteresis as it can be called) and are in parallel with the source (input) because iron losses are almost constant throughout the working range of the motor. The second "arm" of the "T" represents losses in the rotor winding.

The overall equivalent circuit resembles the equivalent T circuit of a transformer having the secondary winding shorted (if the motor is squirrel caged).

In PI circuit the iron loss is modelled separately for stator and rotor. In the T circuit are together.

Hope it helps

Thank you very much.

In T-model, do you mean that the magnetising reactance and core loss resistance are combined together to form the "leg" of "T"? In many textbooks from far east the "leg" of T-model is modelled as a resistance and a reactance in parallel.

Thus, in the two different cases, the value for core loss resistances differ quite a lot. How do explain this?

Yes, the magnetising reactance and core loss resistance are paralelled to form the transversal component in the T circuit.

Assuming that by "two different cases" you reffer to the Π ant T equivalent circuits I have to tell you that comparing core resistances does not take you anywhere in this case. The two values are different because the models are built different. Both models start from the actual motor with listed working parameters. In the end the theory has to fit the fact. The model does not have to necessarily tell you how the motor works.

I see. It really helps. Thank you very much.