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Old 03-06-2009, 01:08 PM   #31
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I attach a sketch of torque-speed line and max power point. Note, to get power in watts you want torque in Newton-meter and speed in radian per second.

To see my approach, which helps confirm the Dyno results, start at the maximum power point.

Double the torque at this point Ts = 2*638 = 1,276 {gf-cm}
Apply the unit conversion factor Ts = 1276*9.81E-05 = 0.125{N-m}
Double the speed at the point wMax = 2*13515*(pi/30) = 2831{rad/s}

Pmax = (0.5*0.125)*(0.5*2831) = 88.5{W}

The Xtreme RC data publishes Tmax at some nonzero shaft speed, and it does not publish torque at maximum power, so I could not isolate the error. The starting torque should be higher as published by XRC, in my opinion, and the power appears to be correct from the Fantom Dyno.

Note: Nominal voltage is typically Vnom = 5{V} on the Fantom. To estimate torque-speed line with a battery, a first approximation is to scale up or down both torque and speed using source voltage Vs:

T'max = (Vs/Vnom)*Tmax
w'max = (Vs/Vnom)*wmax

This ignores the effect of series resistance in the battery and ESC as a first case approximation.
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Novak Sentry as Brushless Dyno-colorts2.jpg  

Last edited by SystemTheory; 03-06-2009 at 01:18 PM. Reason: Voltage Scaling
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Old 03-06-2009, 02:29 PM   #32
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I know that motor theory exist to completely describe all the motor curves from just the Kv ratings of the motor. I went through a long discussion with Batt-Man on a thread here. All that is promised by him or these formulas is 10% accuracy. I have seen plots from brushless early days exagerating power by 30% or more. I see more realistic numbers posted by Novak.
The purpose of the dyno is to avoid error and laxes in theory by taking actual data from the test motor.

the method to calculate power at half the torque and half the angular velocity would be great if we had a straight line relationship. Below is a plot from the Fantom data on a 19 turn that we have been considering. It is clear that the line is kind of straight. There is some curvature especially as the motor starts up.

We don't have a handle on all the factors that create motor power otherwise you could do as you suggest just measure some resistances and dimensions and do the complete plot sans dyno.

Fred-when we add the armature Inertia to our spreadsheet, (I think Mattnin has already done so) we can compare our result to Fantom. As of now we are but a few watts different.

Cetainly if we had the raw data from dyno rather than RPM we could do some regression there and smooth things out. We do not.
John
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Old 03-06-2009, 03:41 PM   #33
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I got a math problem for you guys. My new flywheel weighs 365g and it is 3.427" in diameter, what is the moment of inertia. Answer has to be in kg*m^2

John is not allowed to answer :P
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Old 03-06-2009, 04:20 PM   #34
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dbl post!
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Old 03-06-2009, 04:21 PM   #35
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Hi there, very interesting thread.
You should check this out, a simple way to make a dyno with a mouse! He has the software available for free, and it calculates the moment of inertia of your flywheel for you!
Have been meaning to get around to making one, hopefully in the next couple of weeks I will.

http://www.sci-spot.com/Mechanical/dyno.htm
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Old 03-06-2009, 05:18 PM   #36
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John, I agree a consistent Dyno test procedure is better than theoretical model for brush motor tuning, with spring changes, timing changes, etc., although measurement error could exceed 10%. Timing advance works up to a point by reducing arcing in the brushes, as I understand it, and that is hard to model with any equation! I recall reading how the old big DC machine operators in large factories would simply change the timing by inspecting the arc on the brushes, an sliding an adjustable ring, while the giant DC machines were turning under load.

Xtreme RC Cars publishes Dyno data for two 12T machine wound motors, where motor A beats motor B in everything, power, torque, top speed, but does worse on the track in the same exact car. Motor A is almost as good at the start but falls off in power faster than motor B. Then the writer can't explain, why?

For me, the only way to understand it is to run a system model. Then it makes sense that B is better due to lower rotor inertia and better bearings. It accelerates better per average ampere, and gets better fuel economy, so it stays high on the battery curve longer. Thus the Dyno can be misleading if one does not know how to adjust for factors on the track, intuitively or better yet through a model everyone could follow.

Flywheel 365g at 3.4":

J = 1.38E-03{kg-m^2}

Last edited by SystemTheory; 03-06-2009 at 06:54 PM. Reason: correct mistakes
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Old 03-06-2009, 06:07 PM   #37
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FANTOM DYNO

I once read a specification of the Fantom having an internal 30 amp current limit. I'm not sure if this is correct. If true, the Fantom must correct for the limited start current and torque in the software. Anyone know about it?
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Old 03-06-2009, 06:35 PM   #38
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System, isn't that inertia calc off? I calculated 345.8121 E-6 kg*m^2. The diameter in metric is 87.06mm. After balance it still registers the same weight.

I have the flywheel all balanced up. It was actually easier than I thought it was going be to balance it. If you have ever balanced an RC tire, then you can balance a flywheel.

I hooked up my flywheel to my dubro prop balancer, however the dubro prop balancer shaft is 0.015 mm smaller than a motor output shaft. So I had to use a small strip of aluminum foil wrapped tightly around the prop balancer shaft and the flywheel fit nice and snug. I marked the heaviest area, and went to my drill press, and clamped it down in the vice, and drilled in only about maybe 3mm. It took me only 2 attempts to get it balanced. I had to go back to the drill press and I put in a smaller bit in the same hole and drilled out another 2mm.

I also made a crude box which has a T4 motor plate that I am affixing to it to hold the motor. It will look nicer once I put contact paper around it and a few stickers. But the Sentry dyno is almost all finished up.

I am going to try hooking the Fantom up to the computer however and see if I can get the raw data using hyperterminal.
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Old 03-06-2009, 06:37 PM   #39
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FLYWHEEL *CAUTION HIGH CURRENT* (See Correction Below!):

Matt:

I apply 1kg = 1000g, and can't find any mistakes (found it!):

J = 0.5*m*r^2
m = 0.365{kg}
r = 0.5*3.427{in}*(1/12)*(1/3.281) = 0.0435{m}
r^2 = 7.569E-03

J = 0.5*0.365*7.569E-03 = 1.38E-03{kg-m^2}

I ran a basic model.

John's 19T motor with a direct flywheel load JL = 1.38E-3. The spin-up to peak power takes 20 seconds, meaning heavy current in the motor coils the whole time. If you hang too big a flywheel on a motor, it slows down the system response to the point where heat can damage the coils.

My numbers may be wrong. However, I estimate the reflected inertia in a 1/10Touring Car back to the motor through the gearbox is on the order JL = x.xxE-04 and spin-up typically takes 5-8 seconds (this is consistent with about 200 feet rollout during a full throttle acceleration run as published in Xtreme RC Cars acceleration curves).

Last edited by SystemTheory; 03-07-2009 at 08:05 AM.
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Old 03-06-2009, 09:47 PM   #40
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I posted a set of Fantom dyno data on the previous page. It is actually readable if you click on it after the first enlarging. The first amp reading is about 73 amps. There is no amp limiter in the Fantom dyno. The battery you use for powering the dyno may limit amps. a Lawn and Garden tractor battery is recomended.

A dyno is best used to differentiate between motors intended to be identical by the manufacturer (thus the need for some accuracy and reproducibility in the measure) or to tune a specific can to best power or efficiency. Here again we are looking for small gains.

the motor that tests better and laps worse is a common problem as laps are affected by cooling, power drain, etc. and also power band.

It has always been trouble to equate track performance when things like the number of plates on the stack are changing. Short stack motors tended to dyno very well but would not pull a stock truck out of a corner well. I wrote software called partial average power to improve the dynos performance in this area. It was included on the Robitronic dyno but some horrible data spikes in the Robitronic output limited its utility. We never traced down the source of these spikes. If you have a huge upward spike in a power curve the integral of the curve is pretty useless as it will be high due to the spike.

In my mod truck however, the motor with the most peak power always jumped farther and was the one to use. It was an 11 turn tripple. These were considerably more powerfull than 9 turn motors.

In a more recent case rotor diameter in a 13.5 was a dilemma for me until I dynoed the beast. The small rotor had more power. Who would have thought.

Here is another use for the dyno.
Now if Joe can tell me how they get away with that huge gearing on a 17.5 on the banked oval. I would be enlightened. I would think that gearing the motor to produce max power at max speed would be the way to go. It was for my flat oval 13.5 car. I have a 17.5 in hand now.
John

Last edited by John Stranahan; 03-06-2009 at 10:13 PM.
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Old 03-07-2009, 08:14 AM   #41
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Matt, my mistake was not dividing diameter by two. I'm still two orders of magnitude above your inertia estimate:

J = 0.5*m*r^2
m = 0.365{kg}
r = 0.5*3.427{in}*(1/12)*(1/3.281) = 0.0435{m}
r^2 = 1.892E-03

J = 0.5*0.365*1.892E-03 = 3.45E-04{kg-m^2}

John, I appreciate your comments and Dyno experience based on data.

I am working on models based on racing theory and system theory that should explain why one driveline/chassis combo beats another exiting a given corner into a given straight. I would need a software developer to make the presentation simpler for the avarage racer to gain some insight, as most guys probably don't want to run an engineering system simulator (free version).
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Old 03-07-2009, 08:23 AM   #42
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Quote:
Originally Posted by SystemTheory View Post
Matt, my mistake was not dividing diameter by two. I'm still two orders of magnitude above your inertia estimate:

J = 0.5*0.365*1.892E-03 = 3.45E-04{kg-m^2}
345.8121 E-6 kg*m^2

They are the same answer for the most part, except you dropped off more accuracy.
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Old 03-07-2009, 08:30 AM   #43
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Right. I missed your decimal point. I figure the mass of a 1/10 TC reflected back through the gearbox at around J = 5.0E-05 up to 1.0E-04, so your flywheel is a bit on the large load side. Hope it works out.
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Old 03-07-2009, 11:41 AM   #44
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Here is a new graph I made this morning of the Hacker 13.5 with 10 deg boost and using the new flywheel. The graph is smoothed, but there were many more data points this time due to the larger moment of inertia, so it is more accurate.



And here is a link to the data

http://arcadechamp.net/radio/hackerlarge2.pdf
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Old 03-07-2009, 12:52 PM   #45
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I would expect brushless (BLDC) torque to either start at a peak value, such as for a PMDC motor, or at a constant/rated value (see attached sketch).

The web site for the Tesla Roadster shows a curve starting at constant rated torque up to rated speed, then sloping, as in the attached chart. The Tesla's 3-Phase AC motor is not unlike its littler cousin, the BLDC machine.

Matt, I would check the Dyno functionality against some machines with a known torque-speed curve, to eliminate the drive variations in the BLDC ESC, if I were shaking-down a Dyno.

Looks good in principle!
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Last edited by SystemTheory; 03-07-2009 at 01:00 PM. Reason: add attachment!
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