Slash 4x4 Buggy 1717 6s

[Lizard]

Its a Y-Wind. Regarding the bold text: Not sure if the load was too low? Considering that it reached only 165kph instead of 172kph with the same gearing, I'd say it was loaded enough?

[Dr_T]

Yeah, that comment was more in general and not specifically about your motor. The troubling part of that quote of Vedder is that it makes it seem that more powerful motors will just run hot anyway, even at lower loads. Maybe not as hot as an overgeared small motor, but still hot...

Your experience with the TP5640 still puzzles me, I think the difference between loaded RPM and unloaded RPM should indeed, like you said, indicate how 'loaded' the motor is, the bigger the load, the bigger the RPM drop... unless..., there's something else going on and it's not the motor that's the bottleneck, but the batteries that cannot provide the over-dimensioned amounts of juice the over-dimensioned motor needs to rev up to where it wants to go? There's some similarities with 0verkill's 4S mini 8ight that also does not go as fast as expected, based on gearing and motor alone, although his over-dimensioned motor (TP29-something) stays cool.

Re the braking we talked about earlier, I think the difference in acceleration and braking distance can be explained from the fact that all the forces (drag, rolling resistance, driveline friction) the motor has to battle during acceleration, are helping it during braking. Not sure how well it holds in practise, but theoretically, the motor Torque is proportional the the motor Current (x 1/kv). So if the motor shaft really would have to endure higher Torque at braking, that should somehow show in the Currents. My XL2 does not seem to log negative Currents, but my EagleTree does, and I've never seen my braking Currents go much past 40A in the 8ight-T set-up that pulls 200+A at acceleration. Now, I don't exactly understand how braking works, as I think braking Current can be divided between a regenerative part, sent back to the batteries (that part is measured by my logger), and a part that is just dissipated in the motor by the ESC shorting the motor windings (that part cannot be measured by my set-up). I think you and Vedder talked about that once, but I did not pay enough attention then to recall exactly :).

Edit:
Without a way ahead this would be too shit of a post :D... So, would be cool if you could find a way to log some stuff. Not sure if you want to use your Monster X on this, but seeing the Currents, ripple and Voltage sag, would definitely help solving this mystery :). Were you running a low-Voltage alarm that could have indicated major sag? Maybe hard to hear the beeps when high-revving metal gears pass by at over 100 mph though :D

[ruudxd]

Maybe it is a stupid thought of me, but if the batterys where the limiting factor, should it be accelerating more slowly, but once on speed, in will be no issue, to get that current?
I havent seen logs, and I don't know much about it, but isnt accelerating the highest draw part, and once at speed not so much, just like in 1:1 cars or horses pulling a car?
In the beginning you would have to pull hard, but once it rolls?

[Lizard]

Maybe it is a stupid thought of me, but if the batterys where the limiting factor, should it be accelerating more slowly, but once on speed, in will be no issue, to get that current?
I havent seen logs, and I don't know much about it, but isnt accelerating the highest draw part, and once at speed not so much, just like in 1:1 cars or horses pulling a car?
In the beginning you would have to pull hard, but once it rolls?

Yes, that is also my understanding. That's why my theory right now is that the motor must be pulling too much current at high RPMs (or maybe at all RPMs). I went 172kph with the same batteries and gearing with the TP4070. If the TP4070 can do 172 without stressing the batteries too much, why not the TP5840 then.

Your experience with the TP5640 still puzzles me, I think the difference between loaded RPM and unloaded RPM should indeed, like you said, indicate how 'loaded' the motor is, the bigger the load, the bigger the RPM drop... unless..., there's something else going on and it's not the motor that's the bottleneck, but the batteries that cannot provide the over-dimensioned amounts of juice the over-dimensioned motor needs to rev up to where it wants to go?

For me the question is, why does it draw so much current in the first place? Gearing was the same, the car is only about 100g heavier with that motor compared to the TP4070 and traction is limited so it shouldn't draw alot more power if I understand the underlying principles right.

Also I have read on boating forums, that the big 6-poles have much lower RPM drop under high load when compared to 40mm 4-poles. But maybe that comparison doesn't hold true for my case, because of the big length difference (5840s = shortest 58mm motor, 4070=longest 40mm motor).

Re the braking we talked about earlier, I think the difference in acceleration and braking distance can be explained from the fact that all the forces (drag, rolling resistance, driveline friction) the motor has to battle during acceleration, are helping it during braking. Not sure how well it holds in practise

I think in reality, rolling resistance and driveline friction is negligible, and drag becomes negligible below somewhere around 80kph :)

My XL2 does not seem to log negative Currents, but my EagleTree does, and I've never seen my braking Currents go much past 40A in the 8ight-T set-up that pulls 200+A at acceleration. Now, I don't exactly understand how braking works, as I think braking Current can be divided between a regenerative part, sent back to the batteries (that part is measured by my logger), and a part that is just dissipated in the motor by the ESC shorting the motor windings (that part cannot be measured by my set-up). I think you and Vedder talked about that once, but I did not pay enough attention then to recall exactly :).

Like you say, the current is either not being logged or not being sent back to the batteries. But it is there (and the torque proportional to it).
Vedder said as far as he knows, hobby ESCs just sort of short the windings and have zero percent regen, but maybe that's only true for really simple/cheap hobby ESCs.

Regarding logging with the Monster X: No, I'll use it in the Kyosho buggy first, want to make sure it works reliably with a "normal" setup before I take it to the extremes. And I think logging won't help much, I have no baseline to compare with anyway. If everything get's hot and the batteries are drained very fast, I know without logging that it must be high currents. If the performance is not accordingly, it must be low efficiency :)

[Dr_T]

Originally Posted by Lizard

If everything get's hot and the batteries are drained very fast, I know without logging that it must be high currents. If the performance is not accordingly, it must be low efficiency :)

Yeah, crap... seems you guys are right and I'm trying too hard not to have to accept bigger might not always be better. There goes my theory of everything RC... did I say crap already?

[Lizard]

The question for me is, is it really bigger motors, or is it just that I chose kv wrong for 6-poles?

Othello once ran a TP5850 600kv Y-Wind in his Losi 5t on 8s and it seemed to do quite well. Got a little warmer than the 2028, but considering that the Losi weighs 17kg and the TP is smaller than the Castle I guess that's okay. The guy that he got it from uses it on 10s normally and another knowledgeable guy on the offroad cult forum also runs them with those low kv values in large scale cars. 35k RPM with a 4-pole would be 23k RPM with a 6-pole. 600kv on 10s would be 22k RPM ...

I also remember a guy on the Traxxas forum some time ago who said the TPPowers he tried got hot in the XO-1 and he had better luck with the Leopard 5692, IIRC he also chose fairly high kv.

The 5670 also got fairly warm in the 3 minutes or so the Alienpower ESC lasted ...

[Mr E-Maxx]

Hi,

I think the 6Pole need lower kv than a 4Pole to work efficient. The Losi has much lower KV than you Motor. I mean the Castle 540 2Pole has more rpm than the 4Pole Version at the same Voltage. I have the some problem with the Tenshock on 4s. This Motor has a lot of torque but with the Xerun 150A definitively a lower rpm than a identical 4Pole Motor. I tested with three motors in the same car etc. The Tenshock has the best start power but on high rpm it get very hot and I misst 3k rpm.
Based on my experience I like a 4Pole Motor in a 1/8 scale car more.

[Lizard]

Yeah, I have asked myself the same question about the Tenshocks some time ago. What I find weird is, that the Tenshocks are offered with about the same kv values (around 1800-2200) as other 4-pole motors. If they really get inefficient with higher RPMs (and they need the same electrical RPMs as 4-Poles to stay in their efficient RPM range), that would mean one has to use around 1000-1200kv on 6s with 6-pole motors. But Tenshock doesn't even offer motors in that kv range. Really weird, I mean, if it's true, Tenshock must know this (?) Or maybe they know this but offer the "wrong" kv anyway?

Here is a video with a short comparison between a Tenshock 2200kv and a 1515 2200kv both with same gearing in the same car, the Tenshock is 10 degrees hotter after the short run:
[youtube]NoKjoi3nr-o[/youtube]

[Dr_T]

Interesting stuff; if it is really the case that 6 poles need lower RPM to be efficient, I think it should somehow show in the no load Current, which is indicative of the non-copper losses - if those grow disproportionally beyond a certain (E)RPM, so should the no load Current.

Temperatures aside, I think it is still weird though that a 6 pole TP5840 apears to have a bigger RPM drop than a smaller 4 pole TP4070, under same load.

[Lizard]

Ahh, so it can be tested by just spinning it to different rpms and logging the current?

Here is a comparison somebody did, the 6 poles have a quite high no-load current.

[Dr_T]

I think so. I asked Vedder once how much he thinks his 60k ERPM 'rule' can be generalized outside of the ~60mm 14 pole outrunners he has most experience with, but he did not really address that yet. Measuring the Io for different RPMs, should give some insight in beyond what (E)RPM range the part of the non-copper losses caused by the rapid EM switching becomes dominant. I posted some simple test I did on my TP5660 once in my GT2 thread, but without reference, it does not say so much, other than that the amount of no-load losses, compared to my TP4070, did worry me a bit already.

That table you psted is a very nice overview; it does show the motors with higher Io also have higher delta T (which I guess represents temperature increase after certain time running unloaded). It is hard though to translate these figures to loaded behavior, as then the (load dependent) copper losses (indicated by winding resistance) also start playing a role. My Turnigy motor for instance had fried windings, suggesting it was the load dependent losses that fried it, and not the RPM depentent losses.

So big challenge I think is finding the right balance between the copper losses (high at start-up, high load; and higher for low kv motors, because more turns means higher winding resistance) and non-copper losses (high at high (E?)RPM, low-load).

[Lizard]

Yeah, since almost all of that energy is being converted to heat and not mechanical energy (the torque needed to keep the rotor spinning is just the torque to overcome the friction of the bearings), higher no-load current also means more heat.

The test was done by spinning the motors for 60 seconds, then waiting for 90 seconds, then taking the temperature.

[Lizard]

Finally got my TP4070 back. Was a long story, at first they wanted to send me a new special 5mm rotor which is 8mm in the center and 5mm at the ends. But I didn't manage to get the old one out.

Then I sent back the motor to TPPowerEurope to have them change the rotor. Now I got the motor back with a 8mm rotor from end to end. Not sure why that is, didn't get an email or an explanation about that. The exchange took quite long because TPPower China forgot to include the correct bearings. And they wanted me to pay around 35$ for shipping.

So now I waited about 1 month and had to pay 35$ for shipping the rotor from TPPower China to TPPower Europe and around 15$ for shipping the motor from here to TPPower Europe. Not very cool considering that this is almost half the price of the motor.

But atleast, I should (hopefully) have a reliable motor now with that 8mm shaft from end to end.

[ruudxd]

So you had to pay because they forgot something?
That doesnt sound correct to me?

But I'll cross my fingers for you!

[Lizard]

No, they wanted the money for shipping the rotor from china headquarters to TP Power europe in holland. The 2nd shipment for the forgotten bearings they apparently paid for by themselves.

Not my understanding of how warranty should be handled, considering that I bought the motor in europe. Communication was nice and polite from their side, but somehow a little weird and chaotic.