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Another brushless 'myth', lower KV = more torque

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Another brushless 'myth', lower KV = more torque

Old 03-09-2009, 05:22 PM
  #101  
Access
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
Oh with reagrds to your last post....the dyno graphs should be for a full-throttle run. If you use PWM or a chopper drive to control the speed, the motor will draw what it needs to increase torque up to a maximum on the graph. So if you hold the throttle partly on and you get to a hill of ever-increasing steepness, the vehicle will not slow down until the angle of the hill becomes steep enough the motor hits maximum torque that it is capable of for that speed. Does that make sense?
I think the problem I envison would be more evident if he changed the dyno voltage from 5V to 1V and ran it again. At 5V it would seem that the 'efficiency sweet spot' is between 16,000RPM - 17,000RPM. ie. that this is where we should try to keep the RPMs to run efficiently. At the lower RPMs the efficiency seems awful. At 1V everything would be 'squished' to the left, presumably, so it would seem like the 'efficiency sweet spot' would be roughly around 3000RPM. So it would seem if you ran the setup at a lower voltage, it would be more efficient at 3000RPM than if you ran it at a higher voltage.

But I think the reality of the situation is that if we are holding the RPMs low (by 'feathering' the throttle, or not holding it all the way down) the efficiency isn't really so bad. Is it? I think what the dyno is showing is the 'maximum acceleration' efficiency or the efficiency when we exert maximum torque. Now is this a linear relationship or not?

And this is now how I drive my brushless vehicles, I am always modulating the throttle to keep it under control. If I want to end up at half speed, I am not going to full throttle it until it gets to half speed and then throttle back. This is how one might drive a nitro... or any ICE, but for the electric brushless I generally just 'walk' the throttle up until it reaches the speed I want. Do other people do the same thing?

Just thinking out loud here.

"The torque equation I posted works out within 2% error for the two motors in the black-background chart you posted, "
The chart I posted was a simulation, at least it appears to be. I don't think it's real data. I think it came from a similar equation, torque being proportional to 1/KV or something like that.

Mattnin's data looks real, at least the torque and power curves do. It mirrors different datasheets and literature I've seen which discuss motors, or compares BLDC motors to internal combustion engines. But one shouldn't mis-interpet the efficiency data, if the efficiency data was really 'that bad' we would see the real-life electric cars having gearing similar to an ICE car.

"For one of these graphs, at a moment in time, you can use current as constant for torque or power being constant......as if you were using a brake dyno vs an intertial one. Or, for example, the constant current at stalled-rotor"
As soon as you have an inductor, you have to bring in the time factor. An inductor is not at all like a resistor. I can have 5V across an inductor and the current draw at that instant will be nothing. Say I have an inductor with a static DC resistance of 1 ohm. The instant I put 5V across it, the current is zero. The current will slowly 'ramp up' to 5A over time. It's not like a resistor where the instant I put 5V across that resistor, 5A or current will flow. Even if the commutation (or ESC) is creating a square wave at each pole, the current ends up looking like this:
http://webpages.charter.net/dawill/t..._Inductor.html
As the frequency of the square wave gets higher (pulses get shorter), the relative effect of the inductor becomes larger and larger.

I say the inductance is dominant in determining how much current actually flows b'cos of the actual numbers.
If you look
http://www.castlecreations.com/produ...rs/nm1400.html
the resistance of a Neu 1415/1Y is 0.008 ohms. The resistance of a mamba max ESC is 0.0003 ohms. This is a 2200KV motor, designed to run off 6s or so, if you had a 'perfect' battery it would draw 25 / 0.0083 = 3012.05A in theory (ie. by treating the whole thing as a DC system). Maybe if you hold the rotor and you had an ESC that could handle it, this would be the case. But 3000A is not reality for this kind of setup. The way I see it, the only time you can ignore inductance is when the rotor is locked in place (ie. pushing the truck into a wall or immobile object). Or at very very low RPMs, where the effect of the inductance might be minimal.
Old 03-09-2009, 09:29 PM
  #102  
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Default RE: Another brushless 'myth', lower KV = more torque

Access, I'll try that out and see what happens. I can measure efficiency now, I have the current sensor. The only motors I have are a hacker 13.5, novak 13.5, and a novak 7.5. I have a sensorless tekin 468 motor and a mamba 5600kv motor too, but I can't measure rpms on them. I am gonna go around the track this wednesday and see if I can borrow a 10.5 Novak to test too. I borrowed another Novak 13.5 and I am going to compare it against mine.

I currently have a few dyno tests of the hacker 13.5 right now, but we are in the final stages of tweaking that spreadsheet, then we will post it for download in case anyone else wants to do this themselves.

I also wanted to say that the efficiency curve comes in slightly after the power curve. I have an efficiency curve for the Hacker, and it comes at around 15k rpms I believe. I'll be posting those graphs within the next few days.

I'll try to do what you ask access, and feather the throttle to keep it at a constant RPM, and yea I agree, I bet it has a good efficiency.

Here is a more recent graph I made using the newer flywheel, no efficiency yet, this was taken I believe last Thurs.



And here is a link to the data.

http://arcadechamp.net/radio/hackerlarge2.pdf
Old 03-09-2009, 11:03 PM
  #103  
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Default RE: Another brushless 'myth', lower KV = more torque

I wonder if the ESC is already kinda doing something like what I mentioned, limiting the power under 3000RPM as a safety measure to prevent large currents. There are 4 datapoints 3000RPM or less, some motor controllers or converters automatically have a 'soft start' feature. All of castle's have a 'starting power' parameter you can adjust, the novaks have something similar I believe.

It otherwise makes no sense, the shape of the torque curve under 3000RPM.
Old 03-10-2009, 03:37 PM
  #104  
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Default RE: Another brushless 'myth', lower KV = more torque

Access, there is a very good reason why that torque curve is like that, it is because it is definitely inaccurate. See, I am having a problem with the dyno that I am not able to solve just yet. I have spoken with Novak tech support and awaiting a response from the tech that built the thing. Anyway, at low RPMs with no power, the Sentry will record a spike, and at initial throttle, the Sentry will show a RPM spike. After about 3000 RPM and until I release the throttle, I don't get a spike. So for instance at time 0.0, it may show the the rotor speed as high as 100k RPM for example. So I have to delete this reading and basically it is just one big headache because I don't have any data in that time slot. It always does it on initial throttle pull and can take over the first three readings. Novak tech said it could be due to static because their own drivers see the same thing. So what I did was connect the Sentry up to its own receiver pack and ran it. I figure this would eliminate any static in the system, well nope.

The way the Sentry hooks up to the brushless motor is via a Y-harness. It plugs inline between the speed control and motor. Then it comes off on a small PCB with a blue LED, and into a small harness with a Red and Black wire and a signal wire. I suppose I can try hooking up a capacitor to stop this noise the Sentry is receiving. Before I start soldering onto stuff, I am going to wait on their tech support.

Also, I am beginning to think that the Sentry would actually be compatible with an optical IR sensor as it would give the same kind of pulse as a hall sensor, something else the Sentry uses to read RPMs. So at this point right now, i am stuck, or I need to figure out a way to just get rid of this crazy RPM spike and the loss of data in that spot mathematically, or scrap it all and just go back to using my fantom dyno but with no efficiency data.
Old 03-12-2009, 12:37 AM
  #105  
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Default RE: Another brushless 'myth', lower KV = more torque

Here you guys go, big preview of the dyno data spreadsheet. First posted here

http://arcadechamp.net/radio/Hacker,...2%20x%202.html
Old 03-12-2009, 03:46 AM
  #106  
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Default RE: Another brushless 'myth', lower KV = more torque

That looks good, what are you using for a flywheel now? If it's taking 6-7 seconds to get 'up to speed', should be able to test the novak 7.5 with the same setup assuming that higher RPM is safe for the flywheel.

You can see the battery 'voltage sag' effect, at 104.33A it is maximized (6.89V) vs. a high of 8.12V or 8.21V at the very end.
Old 03-12-2009, 08:07 AM
  #107  
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Default RE: Another brushless 'myth', lower KV = more torque

Matnin: Nice graph. And I was going to suggest you check to see if your flywheel wasn't tight to the shaft. Glad you figured it out. I see it compares to the brushed motors in one way......speed at max power = 1/2 max speed. It will be interesting when you can plot a motor of different turns to see if the torque curve changes with N, the same as brushed motors.

Access: You had mentioned mentioned about plotting the current for one of these motors so we can see what an ESC puts out. I think the question I have is, what does the ESC do during the time a a pole shold be energized....say for 120 Deg of rotation. Can you describe it with out going to too much trouble? Is it just "ON", or does it pulse the pole?
Old 03-12-2009, 08:56 AM
  #108  
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
Access: You had mentioned mentioned about plotting the current for one of these motors so we can see what an ESC puts out. I think the question I have is, what does the ESC do during the time a a pole shold be energized....say for 120 Deg of rotation. Can you describe it with out going to too much trouble? Is it just "ON", or does it pulse the pole?
Once you get near holding full throttle (ie. what's happening with that dyno), it's just holding the pole on for that phase. Then the next 120 degrees, it switches to a new pole, and so on. That's partly why, as the RPMs increase, the current drawn decreases. From ~106A to ~10A with the dyno test link above. If full throttle is not being held, then it is pulsing to limit power. When the coil isn't 'in phase', it's being switched not to an open circuit (this was a mistake in the schematic above), but to ground.

I'd say this is why the efficiency at the lower RPMs is so bad. I think you could get almost as much current, hence torque, at the lower RPMs if you turn that switch on at only, say, 25% duty cycle or 50% duty cycle. I don't know if this is going to make sense to anyone other than me, but, if you normalize the phase: At low RPM, when voltage is applied, the rate of current increase is high, and when voltage is no longer applied, the rate of current decrease is low. At high RPM, when voltage is applied, the rate of current increase is low, when voltage is no longer applied, the rate of current decrease is high. This means that even with 50% duty cycle at the lower RPM, you can still get a good deal of torque (more than 50%). The rate of current increase (and decrease) is based on both the impedance of the coil, and the back-EMF from the spinning rotor.
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Old 03-12-2009, 10:26 AM
  #109  
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Default RE: Another brushless 'myth', lower KV = more torque

I suspect, that in the end, the efficiency will always be mainly attributable to the I^2R losses (heat). Torque losses (Rotor inertia), core losses, hysterisis, windage, etc. are there, but small in comaprison I think. Biggest performance advantage to brushless I see so far, is losing the brushes. Friction and poor conductivity are eliminated with the brushless system.

From your two first graphs, it appears that the ESC works the way I think it does, and that we can expect the same kind of chart as for brushed motors. The current rise time (due to inductance) becomes more significant at either high rpms, or when you pulse the current for throttle control. So I think I can see why you are thinking that if you are at part throttle, the current will still be rising, not heating up the windings as much and the motor should be more effecient.

Well, it will be more effecient, but that's becausee you've artifically shifted the curves to the left....same as if you used a battery of lower voltage. I think the confusion is likely from the fact that even though you are at part throttle, and using the winding inductance to limit the current, and therefore being more effecient with less heating losses, you are still at part throttle, and not on the full-throttle graph that Matnin is plotting. So.....less throttle = less power = more efficient.

Oh my....I responded to this too quick.....I need to think about it some more.....LOL.

I must admit, I'm keen to see Matnin come up with a graph for another motor. I got a feeling it will match the brushed motor comparison...but I"m not ready to bet on it yet......chuckle.
Old 03-12-2009, 11:58 AM
  #110  
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
I suspect, that in the end, the efficiency will always be mainly attributable to the I^2R losses (heat). Torque losses (Rotor inertia), core losses, hysterisis, windage, etc. are there, but small in comaprison I think. Biggest performance advantage to brushless I see so far, is losing the brushes. Friction and poor conductivity are eliminated with the brushless system.
Don't forget about the case where you're pushing into an immobile object (locked rotor). The motor still draws a lot of current, a lot of electrical power is consumed. But, no actual work is down, no mechanical power is produced, etc. Is this concept similar to voltage being 'wasted' at lower RPMs if you run with 100% duty cycle?

Another big performance advantage is variable forward advance. At any given RPM, a brushless can operate with the optimal forward advanced (done by the ESC). The brushed is stuck operating with a fixed amount, it cannot vary the forward advance. Lots of forward advance is going to favor the higher RPMs, less forward advance is going to favor the lower RPMs.

Also brushless the rotor is a relatively narrow shaft. With brushed the coils are on the rotor, which means more rotating mass, and making it somewhat harder to dissipate the heat that forms on the coils.

"So.....less throttle = less power = more efficient."
If it's a nonlinear relationship at those lower RPMs. If the ratio of electrical power expenditure to torque*rpm produced is constant, the horrible efficiency would stay the same. If half the electrical power will still give you 90% of the torque, only in a case similar to this is it true.
Old 03-12-2009, 01:13 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Access, yes I am using a larger and heavier flywheel than the fantom. I will eventually dyno the 7.5 motor once over I get over my fears using this thing hehe. This thing spins up and I can tell it holds a lot of power. I have been using locktite on the threads holding the flywheel. I am not taking any chances anymore after the 2nd dyno run I did where the flywheel came loose, although it didn't fly off thank god, but where the flywheel could have ended up, who knows.

I will be comparing the Hacker vs the Novak today though when I get back from work, but I believe the spreadsheet is finally completed. We have been working very hard on it getting the mathematics just right, and I think everyone will be pleased. I am also going to be doing a write up to explain how the spreadsheet works, and how to input data.

Everything is linked, so when data is entered, the regression is automatically figured, along with max power, efficiency, and RPM and is linked directly to the graph. So it should be plug and play shortly, or only small modifications will have to be made.

And once again, another first since this is where it all started, the download of the spreadsheet, well before rctech gets it :-) Knock yourself out guys.

Excel Format
http://arcadechamp.net/radio/Novak%2...0Beta%20v3.xls

Open Office Format
http://arcadechamp.net/radio/Novak%2...0Beta%20v3.ods
Old 03-12-2009, 02:00 PM
  #112  
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Default RE: Another brushless 'myth', lower KV = more torque

Good points about the brushless benifits Access.


ORIGINAL: Access

Don't forget about the case where you're pushing into an immobile object (locked rotor). The motor still draws a lot of current, a lot of electrical power is consumed. But, no actual work is down, no mechanical power is produced, etc. Is this concept similar to voltage being 'wasted' at lower RPMs if you run with 100% duty cycle?
Locked rotor.....all input power is dissipated as heat, I^2R losses. Not sure what you mean by your last sentence...you can't run at lower RPM's with 100% duty cycyle....unless you are going up a very steep hill. What I was getting at, was that operating at part throttle with PWM, is the same as operating at a lower battery voltage without PWM. It shifts all the plots on the graph to the left, including efficiency. So at part throttle, you have the same effeciency at that speed as you would at full throttle at its related speed.


ORIGINAL: Access

"So.....less throttle = less power = more efficient."
If it's a nonlinear relationship at those lower RPMs. If the ratio of electrical power expenditure to torque*rpm produced is constant, the horrible efficiency would stay the same. If half the electrical power will still give you 90% of the torque, only in a case similar to this is it true.
Well.....primarily, the Effeciency = Pout / (Pout + (I^2 x R)) (there are other factors for losses, but I^2 R should be the big one)

OR Efficiency = wT / (wT + (I^2 x R))

So efficiency can be seen to rise as w gets larger and I gets smaller. Even putting in I as a function of w, you can't take the derivative to find peak effiency without coming up with the answer R has to equal zero. In otherwords, a steadly climbing line or curve. But it does drop back at higher speeds.

So, why does the effeciency drop back off at very high rpms (w)? Not I^2 x R losses. I think it's because the motor will always draw some current at top speed, but as the change of speed is zero (at top speed), there is no output power and therefore the efficiency drops. Input power is consumed in keeping the motor running, and no shaft output power.

Matnin: is this other motor you are getting, the same as the one presently tested, but with different turns? I don't have Excel, but for graphs, can you put results up as pictures please?
Old 03-12-2009, 04:00 PM
  #113  
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
Locked rotor.....all input power is dissipated as heat, I^2R losses. Not sure what you mean by your last sentence...you can't run at lower RPM's with 100% duty cycyle....unless you are going up a very steep hill. What I was getting at, was that operating at part throttle with PWM, is the same as
You can 'for an instant', ie. on the dyno, for the first few seconds. Or in a 'real' vehicle if you are creeping along at a low RPM and suddenly floor the throttle, without any kind of 'punch control' or such. Depends on the controller, perhaps. It will eventually work its way out of lower RPM (to higher RPM) but while the RPMs are down there, it will still be 100% duty cycle.

ORIGINAL: Argess
So, why does the effeciency drop back off at very high rpms (w)? Not I^2 x R losses. I think it's because the motor will always draw some current at top speed, but as the change of speed is zero (at top speed), there is no output power and therefore the efficiency drops. Input power is consumed in keeping the motor running, and no shaft output power.
Turn off any kind of automatic braking action on the ESC (aka. 'drag brake')
Run the dyno flywheel up to about 20,000RPM and then release the throttle. You can then calculate the loss in Kinetic Energy of the flywheel every second due to 'drag' of all sorts (bearing drag, vibration, etc.) and probably come up with an equation to model it. I don't know how significant it will be.

Mattnin, if you are scared of the flywheel 'coming loose' try the 7.5 motor off the 5V first and see how that goes.
Old 03-12-2009, 04:47 PM
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Default RE: Another brushless 'myth', lower KV = more torque

I could actually quite easily come up with an equation for the graph coming after the throttle is released (friction). I am getting good at it as that is what we had to do to predict angular acceleration at rpm end points and not mid points.
Old 03-12-2009, 05:05 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Here ya go guys, ran the Novak 13.5 vs. the Hacker 13.5. I'll let you draw your own conclusions.


Hacker 13.5
http://arcadechamp.net/radio/Hacker,...try%20Data.pdf

Novak 13.5
http://arcadechamp.net/radio/Novak13...try%20Data.pdf
Old 03-12-2009, 05:23 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Very Cool mattnin.

I'de love to bring my E-Revo to you to see what she puts down.
Old 03-12-2009, 06:16 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Mattnin, which of the Novak tuning rotors are you using in your Novak motor? Novak does say that using a larger tuning rotor means more back-EMF (some loss of KV) but also improves torque.

Your results kinda parallel the local track 'lore' at my track, that the Hacker has the edge in raw torque while the Novak has the edge in efficiency.

Do you find your results are consistent if you run the exact same test multiple times?
Old 03-12-2009, 09:23 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Argess, I only have another 7.5 Novak bonded older motor, a hacker 13.5, my novak 13.5, and a friends Novak 13.5 right now, but I plan on getting motors of all different winds to test. I will post pictures and pdf's.

srt10, I wouldn't mind dyno'ing your motor, but I can only work with the smaller shafts, not the newer Hv-maxx pro 5mm type shafts. You live close enough! I plan on racing at mhor next time I'm staying in Denver.

Access, I'm not sure which rotor is in this motor. This is actually my friends motor, but I assume it is a bone stock Novak SS 13.5, older motor too but ribbed can and sintered. I really hope to get my hands on some tuning rotors soon, I will eventually. I just need to get a bunch of motors to test. What I plan on doing this saturday is fixing up my ESC with alligator clips so I can make testing motors a quick and easy task, and dynoing every motor I come across at the track.

I find that the results are very consistent. For one, the graph curves come out nearly the same every time, although the RPMs at certain time intervals are not the same. However, everything is calculated off of the curve itself, so unless a major change happens like changing rotors, different voltage, the graph curve is going to be the same every time. One thing to keep an eye on are the 4th order poly coefficients. If those numbers are the same for each run, the graph and calculated numbers will be the same, and they should be, because it is this equation that is used to approximate the final graph. Look how close the predicted V #'s are to the actual V #'s seen at the Sentry. Keep in mind the Sentry isn't actually that accurate of a logger. Think of it this way, half the time the Sentry will record a higher rpm reading, and half the time it will record a lower reading. This is where the linear regression curve comes in. It approximates the actual rpm reading based on that deviation that is known. If actual rpm data is plotted against the regression, a very funny thing is seen. The actual data points line up in a 'line' that pass almost perpendicular through the tangent of the regression curve. I will show an example of this at another time.

The final Excel spreadsheet is finished, just waiting on the last word from John. The open office version will be finished tomorrow.
Old 03-13-2009, 07:29 AM
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Default RE: Another brushless 'myth', lower KV = more torque


ORIGINAL: mattnin

Argess, I only have another 7.5 Novak bonded older motor, a hacker 13.5, my novak 13.5, and a friends Novak 13.5 right now, but I plan on getting motors of all different winds to test. I will post pictures and pdf's.
Fine job Mattnin, but I see from your last two graphs, different results for two motors with the same turns. Not surprising as there are many design vaiables. However, I'm still interested in the "myth" and to resolve it, we need two identical motors except for the number of turns. If and when you get them, is it possible to overlay the results on a single graph?... like that black-backgrounded graph posted a while back for brushed motors?
Old 03-13-2009, 08:02 AM
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
design vaiables. However, I'm still interested in the "myth" and to resolve it, we need two identical motors except for the number of turns. If and when you get them, is it possible to overlay the results on a single graph?... like that black-backgrounded graph posted a while back for brushed motors?
Novak's rotors are supposed to be interchangeable, one should be able to remove the rotor from the 13.5 motor and put it into the 7.5 motor without any problems. But I would test the 7.5 with the bonded rotor first just to observe the difference.
Old 03-13-2009, 10:44 AM
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Default RE: Another brushless 'myth', lower KV = more torque





Old 03-13-2009, 01:45 PM
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Default RE: Another brushless 'myth', lower KV = more torque

As requested, Novak 7.5 w/ sintered rotor (not bonded like said previously), and a Novak 13.5SS w/ sintered rotor.



Old 03-13-2009, 03:34 PM
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Access
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Default RE: Another brushless 'myth', lower KV = more torque

Looks good.
Is 30,000rpm the max for your flywheel / dyno?

BTW I'd modify the starting post for this thread, but the 'edit' button doesn't show up, maybe b'cos the post is so far up the thread, or was made so long ago...
Old 03-13-2009, 04:05 PM
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Default RE: Another brushless 'myth', lower KV = more torque

30,000 is the max I'm willing to go more like it :P It gets pretty scary at those high RPMs. We'll just have to extrapolate past that for predicted RPM readings. The 7.5 is the lowest wind I feel like testing.
Old 03-13-2009, 08:32 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Access, you were wanting to know if the device takes consistent dyno readings, well here you go. This should eliminate all doubts :-) I ran a previous dyno test involving the hacker vs some more recent data. Even that weird hump that shows up at the end of the run happened with the older data.

Here is the original:
[IMaGe]http://arcadechamp.net/radio/DYNO/Hacker1350boost1.jpg[/IMaGe]

Here is a previous test:
[IMaGe]http://arcadechamp.net/radio/DYNO/Hackerprectest1reduced.jpg[/IMaGe]

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