You are here

(Not) Stator saturation

5 posts / 0 new
Last post
Stupidspencer
Stupidspencer's picture
Offline
Last seen: 1 year 3 months ago
Joined: 2019-06-20 00:06
Posts: 32
(Not) Stator saturation

Since I finished building my prototype dual-double scooter motor powerboard about a month ago, I have been super happy with its performance. But I have had a bit of a bug in the system. I believe that bug is caused by stator saturation, but I would like if Frank or Benjamin or any of the more knowledgeable members could give me their opinion of whats going on.

Whenever I start riding, the board works flawlessly, but once I have been riding for a few minutes and the motors start to get a little warm, I begin to experience a slight braking action when I get back on the throttle after coasting. I thought I had the problem worked out at one point after changing the throttle deadband setting to 20%. But a few days later I was having the problem again. I paid close attention to the conditions that were present when it would start doing this, and it seems to be correlated with motor temperature.

I wanted temp sensors in the motors when I was building them, but could not find any. So motor temp compensation features in VESC tool are not going to help me. But I don't know if that would help anyway. It is my understanding that high torque low speed motors such as e-bike hub motors (mine are Ninebot scooter hub motors) are the most susceptible to stator saturation. It is also my understanding that when completely saturated, hall sensors cannot detect rotor position. Please correct me if I am wrong. But if that is correct, then I think what is happening is that the hall sensors are telling the VESCs I am at a standstill since everywhere on the rotor "looks the same" to the hall sensors. Until the stator de-saturates and hall sensors can detect position again, increasing the throttle will only increase the strength of a stationary magnetic field. The VESCs think I am trying to start from a standstill and increase the stationary magnetic field to try to get over a rock or something. The VESCs are not braking, but the effect is the same. I have noticed that when this starts happening when I attempt to accelerate, the action of the brakes also becomes all or none. When I command the tiniest amount of braking, I get 100% braking.

Today, I went for a long ride and this issue is now an official safety issue for me because it caused the wheels to lock up at full speed (22mph) while I was crossing the intersection of two very busy streets. I fishtailed a few times and went down hard. My transmitter slammed to the ground, batteries flew all over the road, and my board coasted into oncoming traffic. Luckily those drivers stopped and the board didn't get run over. But I felt embarrassed as traffic had stopped in all directions as I frantically tried to pick up all 8 of my transmitter batteries, the cover and a knob that came off. I am fine, but not without injury. Missing some skin on my right forearm and knee. Ya play hard, ya pay hard.

But now that this has happened, I need to figure it out and fix the problem. If what I think is happening is in fact the case, then I think this would be a non-issue if I ditched the hall sensors for encoders. 

Please help guys, because honestly, I am going to keep riding, and I am going to get hurt again because this sport is too much fun for me to not do it. Nobody knows more about the VESC hardware than y'all which is why I posted in this forum.

district9prawn
Offline
Last seen: 3 months 1 week ago
VESC Free
Joined: 2018-04-26 12:18
Posts: 124

Yikes good thing the traffic came to a stop. Be safe and take it easy until you have things working nicely!

Stator saturation is typically only an issue at low speed and high motor currents. And in sensorless mode. Its probably not what caused your wheels to lock up at 22 mph as the motor currents won't be high enough at full speed.

For your temperature issue you can try running motor detecting when the motor is hot.

On my bike I had what sounds like a similar problem to yours. As soon as I engaged the throttle while i was coasting I would be in braking until I turned the throttle higher. What fixed it for me was halving the observer gain. I also reduced 'observer gain at minimum duty' from the default 30%. 

Stupidspencer
Stupidspencer's picture
Offline
Last seen: 1 year 3 months ago
Joined: 2019-06-20 00:06
Posts: 32

Oh snap, district9prawn! Your suggestion seems to have fixed the issue!

-----------------------------Solution-----------------------------------

I changed "observer gain (x1M)" from 3.59 to 1.79

and changed "observer gain at minimum duty" from 30% to 15%

--------------------------------------------------------------------------

Despite my injuries, I went for a 5 mile ride (like I said I would) to test those settings. And not once did I experience the problems I was having before. I will add that I was hesitant to ride very fast or hard, but the ride was definitely long enough such that the braking problem would have showed up.  But everything seemed to work as it should. Braking was smooth and predictable throughout the ride, and there was no sudden braking when getting back on the throttle after coasting.  Thank you so much for your advice!

Perhaps the motor setup wizard should apply lower default observer gain values to large outrunner and e-bike hub motors?

 

Note:  I edited the subject title to reflect that the problem was (Not) due to stator saturation.

district9prawn
Offline
Last seen: 3 months 1 week ago
VESC Free
Joined: 2018-04-26 12:18
Posts: 124

Nice to hear that did the trick. What vesc hardware are you running? I've only encountered the problem on my a200s esc which has current filters on the current sensor output. Never on regular 4.12 or 6.0 hardware. Its strange how the vesc can lose position right after coasting. While coasting the rotor position is tracked with the motor bemf which should be reliable. 

You are correct in thinking that all these problems will go away with an encoder. Its made things very smooth on my bike, including strong braking right down to a stop. 

bittes
Offline
Last seen: 2 years 1 month ago
Joined: 2020-03-18 16:45
Posts: 1

Hi district9prawn​, thanks for your hint, reducing observer gain by almost 70% and "observer gain at minimum duty" from 30% to 15% did the trick for me.

My e-scooter was having weird cogging issues at mid rpm after the motor was getting warm and I was desperately looking for an answer. 

Cheers, ​Christoph