Changes in Rotor resistance? 1

[hw123]

Do anyone know how the rotor resistance in a induction machine changes during temperature change, frequency change and change in slip?

I need to know a function that descripes these relationships.

Hope you can help me.

Best regards
Robert

 

[Drivesrock]

hw123

As I understand it the rotor circuit is comprised of the rotor resistence - Rr, resistance of the copper/aluminium bars of the squirrel cage - and rotor inductance - Lr, the inductance properties of the squirrel cage.

Rr will vary with temperture and the temp/resistance coefficients of copper or aluminium are well known.

Lr will create an impedance according to the formula XL=2.Pi.f.Lr where f is the slip-frequency. (Slip frequency being the difference in speed of the rotor to the magnetic field in the stator).

Slip frequency can be found by looking at the rpm of a motor compared to its synchronous speed due to number of pole-pairs.

Now, XL is normally very small as slip frequency will be about 1 - 3% of the applied stator frequency and I believe will be independent of the value of that stator frequency. The predominant component then is the rotor resistence.

In good flux-vector control variable speed drives for optimum torque control, Rotor resistence is tracked by the drive to compensate for the resistence changes due to the temperature variations in the rotor caused (basically) by the load on the motor. As these drives are controlling rotor slip then XL is known.
To obtain these values of Rr and Lr (as well as the stator and magnetic circuits R and L) drives will have a calibration run that is performed once at commissioning.

I hope this helps.

DrivesRock

 

[NydenCorp]

Is the torque resistance wavering consistently?