Cable resistance and autotuning of drives

In several drive systems (f.i. Siemens S120) you can fill in the cable resistance of the motor cable between drive and motor (ohm resistance).
Especially when autotuning the drive (current controller, stator resistance, etc.) this could have an importance.
Up to now I have never filled in this cable resistance, but now I realised that in many cases stator resistance and cable resistance are in the same value range.
What are your experiences with filling in a cable resistance and autotuning. Should you advise it?
Thanks

No ideas?

Not enough info.
In what way does the cable resistance change the response? Are you talking about the cables between the drive and the motor? I know, it changes the RL time constant but most motor auto tuners ignore the RL time constant and assume there is only one time constant that deals with the inertia of the motor and load. In most cases the RL time constant is very fast ( short ) compared to the motor and load time constant so it can be safely ignored. So how fast does the RL time constant need to be to be ignored? It depends on other things like the sample time of the controller and the time constant of the load inertia.

There just isn't enough info to give a good response.

Could it have something to do with the motor model and the temperature influence on the stator resistance? The cable res. is much more stable.
Kalle

Originally posted by Peter Nachtwey:

I know, it changes the RL time constant but most motor auto tuners ignore the RL time constant and assume there is only one time constant that deals with the inertia of the motor and load. In most cases the RL time constant is very fast ( short ) compared to the motor and load time constant so it can be safely ignored. So how fast does the RL time constant need to be to be ignored?

Click to expand...

Any of the "vector" drives I have dealt with in the past several years (AB, Danfoss, Siemens, etc) maintain separate electrical and physical time constants. The electrical time constants allow the drive to correctly calculate the stator and rotor flux vectors and to accutately control stator current. Some of the drives will also support a motor/load inertia value for use in velocity loop auto-tuning. But that is held separately. For example, the AB PowerFlex 70 has separare "motor" and "velocity" auto-tuning functions. As a user you can do either or both.

I suspect KalleOlsen is correct with the motor heating model point, although I wouldn't think you would specifically need it for that. You need the cable resisitance for the RL time constant calculation but you DO NOT want to consider cable resistance for the motor I^2t calculation. However, I think most drives look at I^2t based on a percentage of motor FLA.

Keith

PLC_USER

On an S120- setting P1900 to a 2 (motor ID) should always
be completed on the new startup.
This fills in that info automatically

I've never used it and, even on the most precise applications, haven't seen any problem without it.