The "Ankle Wreacher" Stooge v5 SN28 Build - Prague Prep

[Flyboy]

@tuckjohn

Very interesting information, and a promising theory. I can follow the argument that going from 90% to 80% efficiency could be the difference between fine and overheating.
However, efficiency != heat generated, right? Unless I misunderstand the graph, In the condition that the motor is along the bottom of the graph, the current going through it is small, so low efficiency*small number = small amount of heat. Whenever the motor starts to get/output appreciable power, the torque will increase, leading to an increased efficiency.

@tuckjohn

@fessyfoo

maybe you’re running into the iron losses thing @Skyart and @Tony_Stark were informing me of over on my v5#1 thread.

This is really seeming like it could part of the issue, and hope it’s not. I think the best solution would be to get smaller KV motors to reduce the iron losses.

These are 2 different languages for describing the same physical phenomenon, in this case the efficiency would be low because the iron losses are high. I was trying to quantify whether it was happening, and you’re right that I haven’t proven anything other than that the problem isn’t copper losses.

Efficiency IS defined by what fraction of the energy goes to heat, and cruising at 300N and 60MPH you’re looking at over 2kW per motor. 80% efficiency would mean 400W of heat per, but looking at the core losses chart, it actually looks improbable that you’re losing much more than 100W to the core. So maybe idk what I’m talking about lol.

[zero_ads]

[glyphiks]

I have no idea what’s going on here, but i’m almost positive that i’ve seen someone else here say that the 7490 motors run really hot…

[Evwan]

Massive stator has massive losses

[sugandese]

Easy enough to try it at a lower voltage with lipos isn’t it? Give them the amps at 8s?

[Arzamenable]

Awesome build! I’m also jealous of your one post build, forum mic drop.

(Get back in here and revise and tinker and post! you know you want to, the mic is right here… just waiting for v2 )

[Skyart]

@tuckjohn

KV

What motor kv are you running?

[Swol_Moon]

This thing is insane. You’re like me if I had more money and let my intrusive thoughts win🤣. No fr tho, incredible work bro!

[Shadowfax]

Ok, here’s my take. @tuckjohn the TLDR, is that you have unusually high iron losses. Your motor RPM doesn’t seem that high, but your ERPM is very high compared to most builds, because you’re running 10 pole pair motors instead of 7 pole pair.

This build is on 265kv, and at 12s that’s only around 10,500 loaded motor rpm. Not crazy high.

Although iron losses will be higher on 20 pole motors vs 14 pole, because higher ERPM. With 20 pole motors you have 42% more ERPM vs 14 pole, at any speed.

• 7 pole pairs * (265kv * 50.4v * 0.95) = 88,817 ERPM
• 10 pole pairs * (265kv * 50.4v * 0.95) = 126,882 ERPM

126,882 ERPM is technically possible for VESC, but it is higher than most of us run.

Switching losses and iron losses could both be significant factors on your rig’s inefficiency. Important to note that iron losses are not linear. At high speed they square, so they increase more than you would expect. Based on the fact that you said these motors are heating up like crazy, this tracks with my theory.

@tuckjohn

Motor current started at 100A/motor, but I lowered it to 85A/motor after qualifying in an attempt to reduce overheating (to little avail)

My theory also tracks with your observation above. You’re battling iron losses, not resistive losses, so lowering amps doesn’t do very much.

It will be easy to test this / fix this, but it will cost $$$:

If you acquire 4x 6395/6380 motors at 265kv, everything about your build would stay the same, but you’d immediately lower your ERPM by 42% (and thus iron losses would be much less, and you wouldn’t overheat anymore).

It sucks to spend the money, but that’s what I would do. I don’t think you’re going to be happy long term a with a race board that hits thermal throttle at 85c and let’s others pass you.

For shits and giggles, you could take a video screengrab of VESC tool with your motors spinning up to full speed under no load (wheels removed). It would be interesting to see how many watts you’re pushing just to keep the motors at top speed.

I hope that’s all helpful. If I’ve botched something I welcome corrections.

[tuckjohn]

@Shadowfax

If you acquire 4x 6395/6380 motors at 265kv, everything about your build would stay the same, but you’d immediately lower your ERPM by 42% (and thus iron losses would be much less, and you wouldn’t overheat anymore).

Yup, same page with you on everything, it makes sense.
It’d suck to buy a whole set of motors… If I had to do that, I’d probably buy the 7490 179kv or 135kv. Reduce the overheating problem just enough that I can last a full race- and part of me like the absurd motor size

@Shadowfax

For shits and giggles, you could take a video screengrab of VESC tool with your motors spinning up to full speed under no load (wheels removed). It would be interesting to see how many watts you’re pushing just to keep the motors at top speed.

Next time I take this board out to the track I’ll do this.

[tuckjohn]

Also, hit a new inefficiency record on Wednesday, 105 Wh/mi

[Shadowfax]

@tuckjohn

I’d probably buy the 7490 179kv or 135kv. Reduce the overheating problem just enough that I can last a full race-

I don’t think you’re going to have a good experience going low kv with 12s and 20 pole motors. The combination of high kv and 20 pole motors is what’s leading to overheating. You’re on 12s so you’re not getting away from high kv motors, unless you’re ok accepting 34mph top speed.

Assuming you want this board to still go 50mph, with 179kv motors, you would need ridiculously low gear ratio (2.39). Even with a lot of amps I doubt the acceleration would be acceptable.

Current build - 52 mph, problem = high ERPM

Low Kv Scenario - 52 mph, problem = gear ratio needs to be super low with low kv motors if you wanna go fast

Switching to any 63xx motor (14 pole) - same gear ratio as current, but you get acceptable ERPM.

I think @zero_ads has 265kv 14 pole motors he isn’t using if you guys wanted to make a deal.

[Arzamenable]

@tuckjohn

I don’t have an exact count, but adding up just the hardware alone reaches 6k. Probably close to …

Measure in smiles/mile when complete. I would quote “a high mileage, 2010’s Camry” and feel stupid after saying it out loud.

[zero_ads]

225kv 6455 so much smaller than I think what Tuck wants lol. Also, I already sold two of them.

[tuckjohn]

Christmas came early

Time for some science

[tuckjohn]

@Shadowfax

You’re on 12s so you’re not getting away from high kv motors, unless you’re ok accepting 34mph top speed.

I’m not attached to 12s; It was just was was convenient to wire with the battery layout in the chassis.
I could pretty easily buy two new batteries and go 16s; I’d need to get creative with the battery layout though.
I’ll keep this option in the back of my mind.

[JeffyJ]

Geez u must have some patience! I changed out motor pinions recently and took me over an hour to do 4wd. I use retaining compound so have to torch the pinions to get them off.

[fessyfoo]

@JeffyJ

I use retaining compound so have to torch the pinions to get them off.

loctite 680(ish) ? do you have a puller? might be able to skip the torch?

[poastoast]

if you need help with fucky wucky battery layouts

i’m yo cat

[JeffyJ]

I do have a puller but it’s not great. Only has two prongs and need to use a bolt on the shaft to enable puller to work. It’s all a big pain in the arse so slightly easier to torch it and lever it off. Still, u then need to re-loctite 4 bolts per motor to change the motor position as you know. It’s just a big job on a 4wd and something i dont have the patience to muck around with too much haha.