Current in a motor equivelent circuit

[mull982]

I've been studying the motor equivalent circuit lately trying to determine how the different motor characteristics lead to different motor quantities.

On thing I noticed was in the equivalent circuit the rotor impedance (Zr) is in parallel with the air gap impedance (Xx) and these two can be combined to form a combined impedance (Zrf) in series with the stator impedance (Z1).

The equivalent circuit now has a voltage source in series with the impedances Z1 and Zrf. To determine the current in the motor the voltage source value can be divided by the Ztotal with Ztotal=Z1 +Zrf.

If the current draw in a motor is determined by these internal impedance values in this equivalent circuit then how do different loads on the motor change the current value? Do different loads on the motor change any of these impedance values in this equivalent circuit or are all of these values fixed?

 

[winnie]

There are different motor equivalent circuits, and I can only guess at which you are looking at.

If you are looking at a basic equivalent circuit of an induction motor, you will generally find two terms for the rotor; an inductor which represents the rotor reflected inductance, and a _variable_ resistance which changes with slip. So at a fixed frequency changing the speed will change the resistance term and thus change the current flowing through the machine.

In these simple models, all of the other terms are treated as though they are constant, but in the real world they are not constant, and the model is just an approximation.

-Jon