openinverter forum

Posted: **Sat Jul 02, 2022 10:02 pm**

I'm preparing a configuration for my small drive unit, and I'm happy that I understand the parameters with two exceptions: fconst and fslipconstmax

Please could someone describe the difference between when fslipmax is used and when fslipconstmax is used?

In all the configurations I've seen, fconst is higher than fweak. The documentation suggests that in this case, fconst will not be used. But then what happens with fslipmax and fslipconstmax?

Thanks, and sorry for my incessant questions this week!

Posted: **Sat Jul 02, 2022 10:35 pm**

I've actually just read the source code and I think what's confusing me here is the documentation:

fconst - Frequency where slip frequency is derated to form a constant power region. Only has an effect when < fweak

Am I right in thinking that in fact fconst has an effect when it's greater then fweak?

Posted: **Sat Jul 02, 2022 11:21 pm**

The relevant code is https://github.com/jsphuebner/stm32-sin ... L105..L114

Code: Select all

         if (fstat > fweak) {
            float fconst = Param::GetFloat(Param::fconst);
            float fslipconstmax = Param::GetFloat(Param::fslipconstmax);
            //Basically, for every Hz above fweak we get a fraction of
            //the difference between fslipconstmax and fslipmax
            //of additional slip
            fslipmax += (fstat - fweak) / (fconst - fweak) * (fslipconstmax - fslipmax);
            fslipmax = MIN(fslipmax, fslipconstmax); //never exceed fslipconstmax!
         }

It seems like weird things would happen if fconst was actually < fweak. Anyway I think this makes sense to me now, but if I'm correct, then the documentation is confusing.

Posted: **Sun Jul 03, 2022 4:40 pm**

See what you mean

Frequency where slip frequency is derated to form a constant power region. Only has an effect when < fweak

So should say
Frequency where slip frequency saturates to form a constant power region. Only has an effect when > fweak

Posted: **Sun Jul 03, 2022 10:24 pm**

johu wrote: ↑Sun Jul 03, 2022 4:40 pm See what you mean
Frequency where slip frequency is derated to form a constant power region. Only has an effect when < fweak
So should say
Frequency where slip frequency saturates to form a constant power region. Only has an effect when > fweak

I've updated the wiki:

fconst - Maximum slip is increased from fslipmax to fslipconstmax as frequency approaches this value. Only effective when greater than fweak.
fslipconstmax - Slip frequency at maximum throttle and fconst. Set equal to fslipmax to disable.

Posted: **Sun Jul 03, 2022 10:37 pm**

On a strongly related note, what is the *purpose* of fslipconstmax? Is this purely to reduce torque at lower RPM?

Posted: **Sun Jul 03, 2022 11:54 pm**

Thanks for that.

catphish wrote: ↑Sun Jul 03, 2022 10:37 pm On a strongly related note, what is the *purpose* of fslipconstmax? Is this purely to reduce torque at lower RPM?

It increases slip past fweak to slow down torque drop off. Running that kind of slip below fweak would over current the inverter (and perhaps the motor)

openinverter forum

fconst and fslipconstmax

fconst and fslipconstmax

Re: fconst and fslipconstmax

Re: fconst and fslipconstmax

Re: fconst and fslipconstmax

Re: fconst and fslipconstmax

Re: fconst and fslipconstmax

Re: fconst and fslipconstmax [SOLVED]