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Old 03-04-2009, 09:03 AM   #16
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The RS series are fine with the red wire in. I've run it both switch on and switch off with no issues, but I won't go so far as to say nothing bad will happen if you turn the switch on while a rx pack is there with the red wire in. Nothing bad should happen, but I still won't say to go ahead and turn the switch on with an rx pack plugged in.

With the switch in the off position, I can say that you can't hurt the RS. Tekin recommends running it with the red wire IN and the switch OFF to anyone racing an RS with an RX pack.
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Old 03-04-2009, 09:24 AM   #17
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Just a reminder for everyone to keep track of units. Moment of inertia is Kg and Meters for instance.

If I get a chance I'll make a new spreadsheet up over a few pints this evening.
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Old 03-04-2009, 10:05 AM   #18
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Here is the data I got from the 13.5 @ 10 deg boost.



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Old 03-04-2009, 11:51 AM   #19
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Doesn't look like you have enough resolution to measure performance very well with such a light flywheel.

At one second the run with more timing has higher RPM (made more power) and that's what I would expect. The resolution at low RPM doesn't show the higher power because it missed the "peak".

Needs either a heavier flywheel or faster data (like 50 Hz samples or so).
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Old 03-04-2009, 12:00 PM   #20
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Yea I know. I hope to get a heavier flywheel soon.
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Old 03-04-2009, 12:13 PM   #21
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Might be ok for comparison if you use a half second or full second to compare motors. Things will make more sense once you have a current measurement to look at.
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Old 03-04-2009, 12:51 PM   #22
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I'm on my way to the machine shop to get a heavier flywheel made, wish me luck!
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Old 03-04-2009, 01:38 PM   #23
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Good news. The flywheel I am having made won't even cost $30. So the people that want a brushless dyno, an affordable one may be obtained by using a Novak Sentry and a flywheel from your local machine shop and using the equipment you already have.
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Old 03-04-2009, 05:08 PM   #24
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Bad news. My previous calculations for torque are wrong. Stranahan is working with me to develop a better method for calculating angular acceleration as the method I was using is only good for a constant acceleration. I owe him one big time for making me dive into my calculus textbook once again
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Old 03-04-2009, 05:42 PM   #25
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Calculus won't help since it can't make up for the lack of sampling data at the limited logger rate. For practical purposes your equation is correct but the digital sampling rate must increase to get an approximation of the instantaneous acceleration curve.

(2nd thought) In sampling theory if the sampling frequency is too low you get a bad sequence of data points. This is a "wagon wheel effect" in old movies. The spokes on the wagon wheels must be moving forward, but at, say, 32 frames per second, at certain speeds, the wheels appear to be rotating backwards! The curves you plot may include this effect (I have a mental block on the technical term for this).

Commercial Dynos are an expensive "black box" so I am working on a cheaper alternative, where you do a few measurements to specify motor and battery parameters, then run the theoretical model in software to produce a "Dyno" curve. The drive circuitry in most controllers assumes such models to operate ...

The term "aliasing" applies to undersampling such as with "the wagon-wheel effect." If I recall my DSP terms, the Nyquist Sampling Criteria defines the minimum sampling rate in a given application.

(3rd thought) NOVAK speed sensor should sample speed properly and log at .1s intervals. Torque should never increase at greater speed b/c more speed generates more back-emf voltage in the air gap, this reduces input current, and torque is proportional to current. Power should never increase after 1/2 torque-speed (Max Power) point.

Last edited by SystemTheory; 03-05-2009 at 02:17 PM. Reason: Added 2nd Thought, Aliasing, Nyquist Criteria (3rd)
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Old 03-05-2009, 03:57 PM   #26
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System theory-What we have done to avoid some of the sampling problem is to do a linear regression fit of a model to the angular velocity data. A cubic polynomial fits very well. The derivative of this model gives a more accurate angular acceleration at each data point than can be calculated manually. This is used to calculate power. The data is smoothed in the regression. I think accuracy is improved 2-3 % at each individual point. Since the initial points are so few this may be important to distinguish two motors. It's all done automatically in a spreadsheet which is mostly complete. We are adding an input for flywheel and armature inertia. The robitronic has similar imputs for inertia. The spreadsheet will be provided free soon. Here is a copy of the output at present. The data is from a Fantom Dyno run on a 19 turn.

Note these two graphs were used for spreadsheet construction. The first shows the good fit of the cubic trendline to the angular velocity data. The second graphs just has connect the dot lines but is generated from the derivative of the first cubic model shown.
More relevant output graphs will be in the final form.

I know that sampling problems may exist in the RPM data. We will just have to trust that they (Novak, Fantom) did an equally fine job of getting that as accurate as possible.

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Novak Sentry as Brushless Dyno-sentry-dyno-calc002.jpg  

Last edited by John Stranahan; 03-05-2009 at 04:35 PM.
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Old 03-06-2009, 11:50 AM   #27
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NOVAK RPM sensor accurate from 500 to 99,999 RPM. If this data is accurate at .1s intervals, interpolation to curve fit is reasonable. If the sample data is inaccurate you're only fitting bad data so nothing is gained.

Xtreme RC Cars used to publish FANTOM DYNO data for brush motors, but I find problems with the published numbers. Peak power is always too high in terms of listed torque and speed. There is an error somewhere ...

Example is posted online in archive for the Xray T2 007 with 19T motor:

Max Power: 73.2{W} at 10479{rpm}
Max Torque: 0.1 {N-m}
Peak RPM: 18852{rpm} at 2{s} spin-up with flywheel

I assume rpm is the most accurate Dyno measurement. Best case power out would occur for an ideal torque-speed line, so I work with that.

wMax = 2*10479*(pi/30) = 2195{rad/s}

Pmax = 1/2*Tmax*1/2*wMax = 55{W} at 5{V} test voltage

No matter how many FANTOM DYNO curves I study in Xtreme RC there is not a match of power, max torque, and max speed which complies with the linear PMDC model!

Another approach is to measure motor parameters in a bench test, the way an engineer characterizes a system, and predict the response at any applied voltage. This gives more insight into on track performance with different batteries, once one invests time in understanding the dynamic system model.
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Old 03-06-2009, 12:03 PM   #28
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The max torque does not occur at the same RPM as max power, what Fantom Data are you referring to? Attached is data taken from the Fantom for a 19t motor.
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Novak Sentry as Brushless Dyno-fantomcalc.jpg  
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Old 03-06-2009, 12:17 PM   #29
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I know. For a PMDC motor maximum torque occurs at zero rpm so it is called the start/stall torque. If you draw the torque-speed line in the T-w plane, the maximimum torque is the intercept on the T-axis and the maximum speed is the intercept on the w-axis.

The maximum power always occurs at 1/2*Tmax times 1/2*wmax for the PMDC machine. The small brushless (BLDC) machine when driven aggressively as in R/C racing appears to have a very similar torque-speed characteristic to the PMDC motor, but I have little objective data to go on there.

I'll look at your chart and crunch a couple numbers to see how they square with the PMDC brush motor theory.
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Old 03-06-2009, 01:06 PM   #30
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I think that there are some things buried in the calculations for the Fantom that are causing issues. My personal guess is that the armature/rotor inertia is buried in the calculation and is not adjustable.

Most BL motors have unreasonable efficiency on a Fantom dyno (like 95% for some motors).
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