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Rolling down-hill faster than my board's electric top speed

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koralle
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Rolling down-hill faster than my board's electric top speed

Hey there,

I am pretty new to all this and hoping you guys can help me understand something. I really tried soaking up as much information about electric skateboards and vescs as possible before I built one but apparently I missed something which left me in a messy accident.

I was gunning down a hill on a pretty empty 10s dual 190kv Vesc 4.12 when I decided to slow down a bit but as soon as I touched the brakes, wheels locked and I went chewing asphalt. At least locked wheels is what it felt like. I thought I must have pulled the throttle too hard while not concentrating but recently I came across this post on the builders forum:

"I have one more problem (regular firmware), braking related. I tested it on 6S which happened to be safer thing to do. When I go downhill freely and speed is higher than highest speed setup can go and try to break slowly it’s rapidly slowing to max speed setup allows. I was lucky that I discovered this while going 30kmh and braking slowed me rapidly to like 26kmh (max with 6S) and I managed to keep myself on a deck. Is this how things work..."

This made me think that the same happened to me because I had not given any thought to the problem of coasting at higher speeds than the electric drive is meant to go. Can someone explain to me what exactly happens when you do that?

Another interesting quote concerning this topic:

"The question about overrunning the ERPM is a good one, you have to be carefull with that. One thing I do is keep my throttle fully engaged when hitting my top speed while going downhill. With the throttle engaged as you reach your top electrical speed you will feel the motors produce drag to keep the speed from increasing any further. If you let off the throttle the motors will free wheel and you could overrun the ERPM"

What does full throttle do exactly at those speeds and is it safe to pull even when you are already too fast?

Is a very smooth auto braking feature which automatically limits your max speed something that could be implemented in the firmware? Otherwise I really think that this is a very important issue and should become common knowledge.

Sorry for all these noob questions and thank you in advance,

Gustav

frank
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THX for posting this. Benjamin and I will debate that issue next week when we meet in Nottingham.

Frank

 

koralle
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Solid. Thanks

Roger Wolff
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Do drive a three phase motor, you need three half bridges. Each half bridge has an element that can pull  the phase wire to ground and one that can pull it to the powerline. In addition, you usually want to have some diodes to prevent the phase from going outside the power rails. For example, when the top-side mosfet turns off, and the current is running in the direction you expect it to, the current in the motor will force the phase to ground-and-beyond if you don't have a diode limiting the voltage there to a little below ground. Without that diode the voltage would go as far as needed to make the top-side FET break down. This loses energy that could've gone into running the motor. Provided it doesn't permanently damage the fet. 

 

In any case, with FETs those diodes are automatic. you can't buy fets that don't have them. 

 

So the voltage on each phase is limited to GND-0.6 to VMOT +0.6 . Once you go faster than the battery voltage allows, the back-EMF will simply use the diodes (even when the FETs are not driven!) to charge the battery. 

 

That uncommanded charging of the batery is a bad situation. You don't want that. The FETs on the VESC are able to dissipate about 3W each. That translates to 50A continuous motor current. But when this uncommanded battery charging happens, they start to carry current while acting as a diode, so when the average current exceeds 5A, that 0.6V*5A=3W of power will be exceeded. 

 

There is a small "gray area". When the back EMF equates to about 95 - 100% of the battery voltage. Now you can chose to let it run freely, and there will be no uncommanded electricity flowing. Or you can command "full throttle" and... the VESC will maintain 95% duty. This 95% means the speed may drop abruptly, and say "50A" is available to do that drop.... 

koralle
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Thank you for your long reply! I can't say that I understand everything but I will do further research on mosfets and how the VESC works in general, based on your explanation. So the summary for me for now - holding full throttle while going downhill is a good idea, as long as youre not already above max speed. When youre faster already, shut off remote for safety and brake on your own.

In case anybody is interested:

my settings for each VESC:

sensored FOC
Max ERPM 60.000
Max Motor A: 60
Max Battery A: 30
Min Motor A: -50
Min Battery A: -8

10s4p 30q 
16t Motor Pulley
44t Wheel Pulley
107mm Wheels
190kv 6355 Motor

Newest firmware