14S VESC ?

General topics and discussions about the VESC and its development.
rew
Posts: 924
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: 14S VESC ?

Postby rew » 08 Jun 2016, 20:06

I've put a LOT of thought into "predicting" what the limits are.

My conclusion is that for the motor controller the MOTOR current is the most important. So if we round 12S to 50V, we get 20A baterry current for 1000W. You say peak 2000W and if I assume that this corresponds "about" to the 35A continuous that overheats, you're still at around 50V. Good.

Now if that corresponds to about 90A phase current, that means that your motor is getting ONLY about 20V!

My theory says that at an even lower RPM say half of what you tested at, but still 90A motor current, stuff will still overheat. And at DOUBLE the RPM, but still 35A battery current, the stuff will NOT overheat.

But, just as you I can't easily modulate my load to provide the right load at a specific RPM.

Hummie
Posts: 107
Joined: 10 May 2016, 04:05

Re: 14S VESC ?

Postby Hummie » 09 Jun 2016, 05:06

Thanks for your thoughts.

Are the motor and battery current apeak limit or continuous?

I got potting resin and want to pot my vescs. These limits we're talking about are all temp dependent right? If I sink my escs in resin by 3m made for potting but left the fets exposed could I have problems with heat from other components? I've heard I'll be fine but am cautious

rew
Posts: 924
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: 14S VESC ?

Postby rew » 09 Jun 2016, 07:58

In theory they are LIMITs the VESC will do its best not to exceed those values at all times.

However, if for example a commutation error occurs the motor current will peak enormously. So if those values are exceeded, it will take a short time before the controller will say: "Whoa, I must be broken, I quit!".

I don't know if the resin would improve or worsen the heat removal. On the one hand, it will conduct heat better than air. Air is one of the best insulators around. But air can be moved around, the resin is supposed to stay put...

joe90
Posts: 49
Joined: 15 Dec 2015, 16:52

Re: 14S VESC ?

Postby joe90 » 09 Jun 2016, 14:07

Since All happen while I am accelerating (no logging) I cant say much more that what I observed.
I get 50A batt while throttle is commanding 90A max phase current. This happen at around 67% of max rpm
I have battery sag when doing this so voltage is around 45V
Maybe motor voltage is around .67*45 = 30 volts

2000-2200W is peak while battery sag to 45V

P.S. I have 2 esc and 2 motors so I have plenty of power for my ebike.

14S would have been quite more powerful/fast but 12S is plenty enough.
Michel Cote
Twin v4.7 + SK3 6374 149KV
Bicycle
2500W peak/unit on 12S

benjamin
Site Admin
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Location: Sweden
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Re: 14S VESC ?

Postby benjamin » 09 Jun 2016, 17:52

What causes heat in both the motor and VESC is the motor current. Neither the motor nor the VESC care about the battery current, only the battery does. Also, the battery current can never be higher than the motor current. So when you set the current limits you should set the motor current to a suitable value for the motor and VESC. For setting the battery current you only have to look at the battery. Most batteries, especially LiPos can provide more discharge current than you need, but the charge current is somewhat limited. 18650 cells might not be able to provide as much current as the motor can take, and if the battery has a BMS (such as the space cell) this can also limit the discharge current. Note that the battery current is directly proportional to the motor current times the speed (actually modulation). On low speed the motor current is much higher than the battery current, and on full speed they are the same. This means that if you have very high top speed the VESC will get warm on low power levels when running on low speed. The faster you go, the less heat the VESC generates per power output because the motor current is what causes heat and the battery current is what defines power.

Regarding the motor getting hot under a given load, this has nothing to do with the kv, as I wrote a bit about here:
http://vedder.se/2014/10/chosing-the-ri ... kateboard/
so if you have a given motor with different kv choises and the same copper fill, all of them will generate exactly the same amount of heat going up a given hill at a given speed. Changing to a higher kv value and running on lower voltage and higher current does not change the heat generation of the motor one bit. The motor only cares about the torque it has to provide, regardless of the kv/current combination.

As the MOSFETs have a square relation between heat generation and current, the motor also has the same square relation between torque and heat generation (as explained in my link), and this is the main problem with hub motors. This means that a given hub motor with a given copper fill, regardless of kv and current, will generate 4 times more heat going up a given hill than the same motor with 1:2 gearing. And 16 (!) times more heat than the same motor with 1:4 gearing. Therefore a hub motor has to be much larger than a geared motor to climb hills, and changing the kv does not affect this at all. The kv only affects the ESC and battery choise, and the VESC will work much better with high voltage and low kv. For example, the gearless hub motor on my ebike weights 6kg and will generate more heat going up a quite steep hill than the 500g geared motor on my longboard. This is not because the ebike hub motor is crap, but because of an inherent limitation of hub motors.

Hummie
Posts: 107
Joined: 10 May 2016, 04:05

Re: 14S VESC ?

Postby Hummie » 09 Jun 2016, 18:42

With the same 60/60 settings on my 2 vescs I can get different speeds up my local hill and different motor temps at the top. The lower 80kv is faster than the 100kv. if I simply increase the motor amp limit it should go just as fast. And the extra heat...I think it will be just as cool because it gets up quicker and less time to heat up.

It's said that a motor will run most efficiently at a higher rpm, how is this fact reconciled with the other fact that there's the same torque to heat relationship in the motor?

benjamin
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Re: 14S VESC ?

Postby benjamin » 09 Jun 2016, 19:31

It's said that a motor will run most efficiently at a higher rpm, how is this fact reconciled with the other fact that there's the same torque to heat relationship in the motor?

If you have gearing the motor will run at lower motor torque, and the same mechanical torque. The torque is what heats up the motor (unless it goes really fast and other losses become significant), so the motor with higher RPM will have lower losses because the torque is lower.

If you have the same gearing and higher RPM, and thus go faster, the motor will hive very similar but a bit higher losses (from the fact that more torque is required to overcome wind resistance and that the speed-dependent losses are slightly higher). When it comes to climbing hills, the faster motor can be cooler when you reach the top, but not because it has lower losses, but because it has had less time to heat up and more wind to cool it down.

Hummie wrote:With the same 60/60 settings on my 2 vescs I can get different speeds up my local hill and different motor temps at the top. The lower 80kv is faster than the 100kv. if I simply increase the motor amp limit it should go just as fast. And the extra heat...I think it will be just as cool because it gets up quicker and less time to heat up.


If the 80kv motor is faster, it is because the 100 kv motor does not have enough torque to reach the higher speed (thus the speed is not voltage limited in this case). As you said, if you increase the current limit for the 100kv motor proportional to the difference in kv it should be as fast as the 80kv motor and more or less exactly as hot at the top of the hill (meaning that kv does not matter for the motor). However, the VESC will be significantly warmer with the 100kv motor as current is what generates heat in the FETs and there is a square relation.

Another situation would be if you can't go any faster with the 80 kv motor because the battery voltage is not enough. Then you can reach higher speed with a 100kv motor, but the VESC will have more losses under a given torque. Therefore, with hub motors where you have no gearing to change as I had in my article, you should design you system such that:

1) use the highest possible voltage to keep the VESC cool
2) use the lowest possible kv such that your top speed on flat ground is what you want it to be

then the VESC will stay as cool as possible. The motor won't care since it always has the same heat generation for a given speed and incline. The only situation you would want to increase the kv is to reach higher top speed, but I don't think you need that with hub motors. For example, if you have 80 kv, 12s and 90 mm wheels, the top speed on a moderate charge and 10% derating for the resistive voltage drop is something like:

80 * 12 * 3.7 * 0.09 * pi / 60 * 3.6 * 0.9 = 54 km/h

for 100 kv it would be something like 68 km/h

if you think that 54 km/h is enough there is no reason to choose 100kv over 80 kv on 12s

rew
Posts: 924
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: 14S VESC ?

Postby rew » 11 Jun 2016, 11:58

joe90 wrote:I get 50A batt while throttle is commanding 90A max phase current. This happen at around 67% of max rpm
I have battery sag when doing this so voltage is around 45V
Maybe motor voltage is around .67*45 = 30 volts
WHOA! you are drawing 50A*45V = 2250W from the battery and putting 30V*90A = 2700W in the motor! That is big news! You need to make a video about this and put it on youtube!

joe90
Posts: 49
Joined: 15 Dec 2015, 16:52

Re: 14S VESC ?

Postby joe90 » 13 Jun 2016, 12:54

My numbers are wrong there cannot be more power in the motor than what comes from the battery
but yes I draw 2250W from battery. That all I can get, if more I run into problems

li-ion 12S at 50% SOC on this video (43-44V)

https://www.youtube.com/watch?v=vEGgKefov0c
Michel Cote
Twin v4.7 + SK3 6374 149KV
Bicycle
2500W peak/unit on 12S

Hummie
Posts: 107
Joined: 10 May 2016, 04:05

Re: 14S VESC ?

Postby Hummie » 14 Jun 2016, 14:24

What kind of problems joe? I'm assuming the problems aren't too bad and maybe a shutdown or something happens based on the temp? How are your vescs kept cool? I'd like to raise my phase current up from 60 if I hear I won't destroy something


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