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Old 08-27-2015, 10:06 AM   #1893 (permalink)
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Originally Posted by MPaulHolmes View Post
My thinking is that KP-ID 2000 is too big. Notice the first graph of Kp = 2000, Ki = 0, the error VERY QUICKLY goes to zero, but then zooms past and slowly goes to zero from the negative side. That overshoot of zero means Kp is too aggressive. My understanding is, when you double the voltage with PI loop tuning, that you have to cut Kp and Ki in half to get the same response time. And with a different, very large motor, maybe the current can change very fast due to very low resistance in the 3 stator coils? Try this:

Kp = 100 Ki = 0
Kp = 200 Ki = 0
Kp = 300 Ki = 0
And see if you get ZERO overshoot, meaning the error numbers don't zoom from + to - like this sample:

If you see it go from + to -, that's a sign that your Kp (or Ki or both) number is too big. We may have to increase the resolution of Kp and Ki in the software too. Your motor is a very interesting test case. I've never tried tuning the loop on such a big AC motor.
Good - we're getting to "rules of thumb" and being able to recognize good data and bad data. I like this kind of stuff

Starting at Kp-id=2000 was too high .. OK. Being me, I'll likely do the double/half thing and try 1000, then 500, then 250 until we get no overshoot, then go back up until we get just a bit of overshoot. That's what we do to tune a loop at work ... does that apply here?

So at that point I begin to raise the Ki-id from 0 to converge as quickly as possible, getting rid of the overshoot (perhaps allowing maybe 1% overshoot for the first half cycle?)

My testing will be done with 125VDC, since I can run both the DC motor controller and the AC motor controller at this voltage, off the same pack. When my stepson puts the Siemens motor into his RX8 (in maybe 2 years?) I expect that it will be running with at least 250VDC ... more if he can fit the batteries in. So a complete re-tune would be required, right?
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MPaulHolmes (08-27-2015)