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Old 12-06-2005, 03:54 PM   #76
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Batt-man-Here are some points of interest from your reply. The drill press method in the tutorial should be able to determine RPM per Volts or Kv as it is the same method that you just mentioned. We can use this to calculate Kt with formulas you provide. Thus with a few other simple measures, Motor Resistance, No load RPM, we should be able to calculate power. The only trouble is that the calculated power is not believable in two cases that I know of. This reminds me sorely of the Tekin dyno which is a good tool to measure no load current draw and RPM, but pretty useless to measure actual power as it does not agree with anybody elses dyno's that have more straightforward approaches to measuring power. The other case is this Aveox that I saw run right in front of me. Impressive numbers but ordinary on the track. I might give this drill press dyno a try on my 11 turn tripple. 240 Watts at 7.0 V and 45 amps.

Sadly I just threw away my antique Sanwa motor checker that Jorge Tabush gave me. It measured no load RPM and amp draw. I have plenty of drill presses. Could of had me a dyno.

General Motor Tuning Observations

Here are some general rules to intrepret either the Power vs Amp draw graph or the Power vs Rear wheel torque graph which is very similar.

As you increase the spring tension the RPMs go up the torque goes down, the power peak shifts toward the right on these two graphs. This makes the motor more suitable for heavy trucks, tight courses, road courses.

As you decrease the spring tension the RPM's go down,the torque goes up and this shifts the power peak left to lower amp ranges on these two graphs. This makes the motor more suitable to light cars like pan cars or high speed tracks like ovals or open Road Courses.

Note that this effect on RPM and Torque is the opposite of what you would expect.

If you have two motors that have the same power peak(on either the power vs torque or power vs amp draw graphs) and thus the same general shape then obviously the one that has the higher graph is the better motor. This is not true, if you just look at the motor torque without the effect of the geartrains torque multiplication.

If you have two motors that have the different power peaks, like Yankees two motors, but have the same can, a good approach might be to try to tune them using springs until they have the same or similar power peaks. Then you might be able to tell what the better motor is. When the power peaks are way different like these two motors it is hard to judge the better motor, becase it depends a lot on what type of car and course that it is used in.

Good dyno work
Let each motor cool the same time after soldering on leads or let it cool 5 minutes.

don't do repeat test more often than 15 minutes air cooled, or 5- 10 minutes fan cooled and expect to get the same result or a meaningfull result.

Do your test at close to the same room temperature.

Test motors in good condition, cut the comm, seat the brushes, measure spring tension, otherwise your results will be meaningless.
Just my observations.

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Old 12-06-2005, 06:25 PM   #77
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Motor curves:

POWER CURVES: Power is measured in WATTS and the curve is always a PARABOLA with MAXIMUM POWER (PEAK POWER) being at 1/2 stall toque and 1/2 PEAK RPM. This holds true 95% of the time.

This is one of the reasons why we plot torque in the X-AXIS and al other variables in theY-AXIS. It is very easy to see the motor relationships and how they interact with each other. (torque, RPM, POWER, CURRENT, EFFICIENCY)

If two motors have the exact same peak power but one curve happens to be wider than the other one, the wider curve will always have a WIDER POWER BAND. The tighter the parabola the tighter the power curve. This means that if the power curve is wider across the X-AXIS the motor will work along a wider torque range. This is very imortant to understand..

Just remember that power is generaly at 1/2 stall torque and 1/2 max no load RPMs... so if you look at a power graph from a reliable dyno you can make the assumption that stall torque will be where the curve crosses the X-Axis on the RIGHT side of the AXIS. That should be close to the stall torque for our work.

If you look at the power curve, yo can determine lots of things by its shape. Remember, it wil ALWAYs be a parabola and the wider the curve the wider power band. The outputtorque at the wheels is completely dependent upon the motor. Also, having a smooth transmission is critical. Power can be waisted, so what good does it make to have an awesome motor if the transmission is inefficient, bearings are dirty, etc. Yu need to look at everything involved with the power train to gain power...

Maximum efficiency generaly happens at around 8-15% if the stall torque. Also, as RPMs go up, torque goes down. They are inversely proportional to each other.

Maximum RPM will always hapen at no torque and 0 RPM will be at stall torque. You can plot that and get your STC (SPEED-TORQUE CURVE)

I am not showin a grapg because I would like for you to imagine and figure out the relatonships. If you need help I wil be glad to post one as an example.

Quote:
Originally Posted by John
The only trouble is that the calculated power is not believable in two cases that I know of. This reminds me sorely of the Tekin dyno which is a good tool to measure no load current draw and RPM, but pretty useless to measure actual power as it does not agree with anybody elses dyno's that have more straightforward approaches to measuring power.
Regarding Johns two exceptions, I think we can rule them out... Two instances are not a statistical figure to dictate an exception. It also happens to be that the the TEKIN dyno is very inaccurate. I would not call the TEKIN a DYNO. As john said their power readings are WAY OFF... He is right about that. I have not used it, so I cannot make a good judgement call on how he is calculating the motors power...However, POWER is POWER and as long as it is calculated the right way it should be accurate. What you have to take into consideration along with power is torque, they go hand by hand...It is the same as HP and TORQUE. Depending on the torque trhe motor develops, the power will be developed at different points on a curve. This will help you select a motor with a wider or narrower power band. That is why you may not be getting the performance from your motor. YOu have the power, but at what torque is it being developed. That is where motor selection and knowing how to interpret the data helps...numbers alone do not mean anything

If anyone uses the right formulas used to calculate motor loads, torque, rpm, and all important values, you will find that 90% of the time you will be within 10% of the motors parameters and that is acceptable.

There are no QC indexes in the r/c industry, but in the motor manufacturing world we are satisfied if a motor falls within 10% of the catalog or face plate values.

I can only report that I have taken raw data from a Robi dyno, used just a few reliable numbers and reversed engineered the motor. I have done that with friends and clients many times. They just give me the values that I ask for and I can usualy be within 10-15% of the operating values that the dyno reports. As long as they give me a few values, for example Kv, Ke, and current at two torque points I can draw a motor graph that wil represent the mode of operation extremely close. Then based on that information we work on the motors to get maximum performance.

There is a value that most people ignore and that is the back EMF. BACK EMF is a very important value and you can work miracles with a motor if you understand the behavior.

In electronics we have a law called Kirchoffs law, which states that the summation of all voltages in a circuit must equal to 0. Thus, if the battery puts out 7V, the ESC drops 0.6 Volts, the wires drop 0.09 Volts we have a total of 6.31 volts at the motor terminals. Ignoring the resistance drop across the brushes and armature (just to keep it simple) it means that the motor has to generate -6.31 BACK EMF volts to keep the circuit in balance.

There is another very important factor in motor design/analysis and that has to do with the thermal calculations. Without taking the thermal calculations into consideration you wil never get accurate numbers and performance parameters. NO DYNO takes this into consideration and then racers wonder why the motor behaves diferently from what the curves show. Power losses due to thermal conditions MUST be taken into consideration. Thermal resistance values have to be calculated, also , the winding to case thermal resistance and the case to ambient temperature resistance has to be calculated. This is measured in degrees/watt. By that we mean how many degrees are transfered per increase per watt.

As you can se there are many things that play a role in motor performance...and I have not touched the brushes yet...I can spend hours just explaining how brushes operate and what to look for in a brush.

Knowing all this will bring anyone interested in motor testing/tunning to the next level.

I have been designing and or specifying motors for all kinds of applications for over 15 years. Each time I get involved with the design of a motor I find something new and better. Also, like I said in one of my first postings in this thread, the motor geomnetry plays a HUGE role, and we are limited with that as we cannot change it. Thus we need to understand how the armature operates, its moment of inertia, and how the nagnetic flux lines are developed. This is very impoortant and the air gap plays a huge role too. Motors look very simple in principle, but they can get very complicated. But by understanding how they operate you should be able to work with your motors to extract al the power and torque the motor has to offer.

Onething is motor tunning and another is motor selection. I believe that we must first select the right motor for the desired application and then tune that motor. Trying to get two motors to operate the same way is a waist of time, and I doubt that you will ever get two motors to behave the same way.

By keeping good and reliable data on all your motors, the motor selection process will be much easier.

Quote:
Originally Posted by John S
We don't have a dyno for these brushless motors available at present as far as I know.
I have a dyno that will work with brushless. I use the exact same dyno, the only thing tha changes is the power source to the motor. You need to commutate the voltage to the motor in order to operate, that is the only difference. I am still perfecting the close loop circuitry so that the tests can be conducted in less than a few seconds and no noise and current instability develop. Once that is finished it will be fully operational

Later,

Isaac
Axxis Racing

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Old 12-06-2005, 08:33 PM   #78
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Hello again,

Isaac, I am a product design engineer. I currently develop seating for trains and buses. I do lots of Pro/E, FEA, static and dynamic testing, etc. In grad school I wrote CAD software utilizing C++ and MFC.

I am trying to sift through all of the info as it applies to me and my dyno...

So far I can obtain Kt by finding the slope of a best fit line through my torque vs current data from my dyno. I can find Kv = 1352.4/Kt.

I see I need to find the max torque from the car using a data logger. This will tell me the range that I need to tune in.

From there I am feeling a bit lost. How do I use Kt, Kv and the other constants to tune the motor and compute rollout? That is probably the million dollar question! Anyway, thanks for the help and I will send you a PM with some additional info. Thanks!

Yankey
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Old 12-06-2005, 09:11 PM   #79
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HI Yankee,

There are several theories on how to find the best roll out for a car.

One way of finding this and it is very accurate is by finding the operating torque when you enter the longest straight part of the track, as you are trying to maximize speed. Once you know the torque at this point you can figure out the on load RPM that the motor will generate. Since you know the length of the straightaway and the time you want to make it from start to end, and you know your REAL RPMs that the motor is producing, youu can calculate the rollout from that data. Just figure the distance divided by your tire circumference, and then compute the gear ratio that will give you the roll out at the working RPMs. This is a very good staring point and then you can adjust from there.

YOu can get as complicated as you want, but this seems to work just fine...I can usually dial my cars right on...

You can also work this out by figuring the required output power in watts. Once you know the desired power needed to get you the RPMs necessary, you need to multiply the power output level by a factor of 1.5 to 2 in order to find a suitable motor.

If you need the formulas to figure outh this I will be glad to help...

Remember the formula that I mentioned for output power, you can start by using that and then fiogure out the on load rpms to make your adjustment for the right motor selection.

You also want to consider acceleration time, that is very important.. By knowing the weight of the load, the translation speed difference and the translation force you can calculate the time. (i can give you the equations but this gets a bit more complicated)

Some poeple like to know the horsepower of their motors (although for electrics we use watts) so if you want to convert WATTS to HORSEPOWER just do this... HP=Watts * .00134

All this factors will help you get the right rollout, but the first suggestion I gave you is the best and most accurate.


Well, I have to rest...

I hope this will help you. If you need me to elaborate more I will.
BTW, what dyno do you have? I assume the CE right?

Set your dyno to work with torque not current. Current is OK for comparison purposes but not as accurate when you want to do real load calculations.

In reality you will find that your car will operate within a torque range, you do not need to test every time, just get data per track type and keep good notes. Also, you should keep data sheets on all your motors.
Just remember that whenever yo change brushes and or springs, all yoour constants will change, but the fastest way to get them is to use the back emf and work from there, unless you can get your data from the dyno.

You will see with time that as yu start working with the Ke yu will discover all kinds of ways to optimize brushes, get the highest torque, rpms...everything relates to the Ke and it is very easy to measure. I will be selling a motor tester by Q2-2006 that will give all that data...

We are trying to provide racers with real and accurate equipment. I am sure you have seen that there are lots of products out there that do not give you any data worth a dime to work on motors. I see people buying this so called motor dynos that only give you rpms and no load current. They have a purpose, but very limited...and there is NO WAY you can use that to tune a motor. Unfortunately many people think that higher RPMs and current means BETTER, but it is not the case. Too much Io and you will oversturate (magnetic field will colapse) the motor under load...you loose torque...

Later,

Isaac

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Old 12-07-2005, 11:06 AM   #80
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John,

How are you calculating rear wheel torque. Are you taking the trany efficiency, tire slip (friction) and other variables into consideration?

I'm just wondering and would like to know what approach you are taking. I'm sure you will not mind sharing that information.

I just want to see how accurate the calculations are in comparison to ontrack performance.

Isaac
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Old 12-07-2005, 01:18 PM   #81
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Batt-man- As you know from reading my post I am not a fan of calculated values to judge a motor. The aveox is high. The 15 turn is a little low almost like a stock. The Tekin Smoke and Mirrors Calculated Power number is somewhat useful for oval, but not even close to anybody elses number. Calculated power values are just that. Estimates. A dyno actually measures power from measured acceleration of a flywheel or torque, that's why I don't put much value in using a drill press and a Sanwa Motor Dresser and an ohm meter to determine these motor constants and somehow think that they are going to give me power accurately. Even measuring power has its drawbacks. The ability of the motor to handle the heat of a five minute run being the main drawback. Measuring Power does have some value though as two identical cans will handle heat the same way.

If you use torque to judge your motor, I think you should consider rear wheel torque as it tends to equalize the motors. No I don't mind sharing this information. That's why the spreadsheet is available for free by e-mail. I don't send out Junk mail when I get your address. I just delete it. This spreadsheet is a simple beast. All it does is multiply your torques by the cars overall gear ratio and plots the result. I even put in pretty colors. It also has a box where you can input any overall gear ratio. I used this manually entered gear ratio to select equal torque values at the maximum amp draw. Incidentally Robert Shachuber the designer and builder of the Robtronic dyno uses this same method to calculate an estimated gear ratio given dyno data and the first motors gear ratio. Torque at the rear wheel does not include tire slip as tire slip is something that happens after there is torque at the rear wheel. If you would like to put in a 2-5% reduction for pinion spur efficiency loss and differential pinion and spur loss you can do this with the manually entered overall gear ratio. Just reduce it by 4 to 10 %. Just do it for both motors. Any calculated value or estimate just using the long straight are going to be in error. Races are won in the corners and the short section of track just past the corners, not on the straights. I always use the lap times to get a good gear ratio. Making very fine changes when I get close and taking the 3 best laps from a race heat to observe the change if any. It takes a couple of races to get dead on.

This spreadsheet is not the one that I discussed with you by private message. This other spreadsheet calculates partial average power for the CE dyno. In discussion that I have had with Robert, he decided that this might be of some value and included it in the software that comes with the Robitronic dyno. Trial versions of this software are available for free. This spreadsheet is available free as well. It is easy to use. All you do is punch in your CE dyno numbers to replace the ones that are there and the amp range of interest. Everything else is automatic. All that it does is integrate the power curve over a selected range and calculates partial average power.

The simplest way, though is usually the best. I plot power vs amps because the dynos readily put out these numbers and they show the differences between motors the best. If all motors look alike, on your graph, then that is not much help in picking a motor can for your particular course and car.

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Old 12-07-2005, 01:43 PM   #82
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John,
I used to own a robi dyno and noticed it used the RPM at Torque step 30 for the gear calculator.

Could you tell me what this step represented and how it was determined. You mentioned the robi gear calculator in your last post so I thought u might be able to shed some light on it.

I would think for different motors stock vs 19T vs mod motors that you would want a different torque step, but the robi seemed to use this constant step no matter the motor.
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Old 12-07-2005, 02:26 PM   #83
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erock-I loaded up a couple of mod motors into the Robi software. Torque step 30 means that the RPM's for the calculation were taken from measures where the motors were both producing the same torque (30 Newton-millimeters) I checked the amperage at this torque and it is about 21 for one of the motors so its near the average amp draw. I have an earlier version of the software where you can pick what torque value to make the gear calculation at. The calculation, I believe, was simple at this point and a ratio of the RPM's was used to multiply by the old ratio to get the gear for the new motor. This earlier version gave you the flexibiltity you desire, but then it required a little more guesswork from the user as he had to know what torques were of interest.
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Old 12-07-2005, 02:43 PM   #84
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Thanks John,

I know from measuring a bunch of stock motors at 7.5v on the Robi, the RPM at Torque step 30 usually was in the 25000 to 26800 range On Monsters and Epic Stock.

I also noticed on the Monsters and Epics this torque 30 RPM was very close to the RPM's in amp steps ranging from 17A to 20A.

So What does that tell me for 4 cell stock oval ?

Say
Motor 1
The RPM at Torque step 30 equals the RPM at the 17 amp step

Motor 2
The RPM at Torque step 30 equals the RPM at the 20 amp step.

Would one motor run better on the track than the other?
Or does that not tell me anything ?
Do I want the Torque Step 30 RPM to equal the RPM in a higher or lower amp step?
Sorry for all the questions, just trying to make heads or tails out of this number and how it relates to the gear calculator
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Old 12-07-2005, 03:02 PM   #85
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erock-According to the group that likes to compare motors at equal torque steps the motor with more RPM at 30 Newton millimeter would be the better motor period if this was a relevant torque value. From the amps you specify it probably is a relevant torque value.

now me, I would like to load the motor files into the Robi software, use the partial average power feature (built into the software), find the average power say between 17-22 amps if this is near the average amp draw. Then I would pick the motor with the higher partial average power here. (This would work with rear wheel torque range as well). I would use the data you posted only for gearing. Then I would adjust the gearing in small increments until I got the best lap time. At the same time I would be tuning the good motor with springs and brushes for the highest partial average power in this range consistent with good power at the end of a run. And then the next motor of the month comes out and you start this all over. I enjoy this kind of tinkering. The only problem with this technique is those darn power spikes in the Robi Data. I don't know if this problem has been solved. The CE dyno averages over 4 seconds to avoid any spikes. The Fantom does not have spikes as far as I know but does not have this software feature built in either as fas as I know. It does have a few user selectable power bands, in the latest software the last time I checked, similar to the Robi, but they pick the ranges. Now both of these dynos and myself could have a bad procedure, but nobody has said anything to convince me otherwise yet.

The gearing calculation only works if you have a pretty good gear already selected for the first motor.

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Old 12-07-2005, 03:36 PM   #86
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ok makes sense. I am like you, for my local track we tend to use something like 18-26 amps or some use 16 to 24 average watts to pick the better motor.

I felt the 20 amp RPM was a good all around rpm number to gear from. Just recently my buddy has found that for 19T motors the robi seems to really hit the nail on the head when it comes to their gearing calculator.

For stockers I think it gives too much range. well I should rephrase that, if you use the same type of motor it is useful, but say your benchmark is an Epic roar stock and you are trying to use the robi to gear a Monster its all over the board. Like it once told me to gear a Monster lower and I know you always need to gear a Monster higher maybe only a tooth in some cases but it always needs more gear no matter what the track.
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Old 12-07-2005, 04:12 PM   #87
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erock-Sounds like you would do better with the early version of the software. Then you could pick a different torque value for stocks. I think that over in Europe they don't do stock motors, only mod. Thats probably why it does not work well with stocks. I can send you this early version by e-mail if you like. I have permission from Robert to distribute his software.

This version of the Robi Software does not work on this computer, I had to start it on a computer a year older.
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Old 12-07-2005, 05:48 PM   #88
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Quote:
Originally Posted by John
I would like to load the motor files into the Robi software, use the partial average power feature (built into the software), find the average power say between 17-22 amps if this is near the average amp draw. Then I would pick the motor with the higher partial average power here.
Attached you will find a sample motor curve that shows ALL the relationships between CURRENT, TORQUE, RPMS, and EFFICIENCY. There is one constant to this graph and that is voltage. (this is an actual graph from a small servo motor...but everything is the same for our r/c motors (brushed and BL, just the numbers change)


As you can see, this will support your motor selection method. If you were to overlap two or more motor curves you can see side by side the behavior (power band) and characteristics of each motor. It is very easy to find the motor with the highest power at any desired current and or torque. Finding the average power at any setting (torque or current) can be done very easily mathematically if you have all the data, but you can also do so by just looking at this graph... It is so simple!!!

I hope that by looking at the sample graph racers can see the relationships between torque, power, current, rpms, and efficiency.

As you can see, POWER PEAKS at 1/2 stall torque. This works 99% of the times +/- 5%. Max efficiency usually falls at around 8-20% of stall torque. Why do I mention stall torque so much, because it is a critical number that defines a motor... When I test motors Irun them from no load to stall, and torque is measured throughout that process at 1200 samples per second. This way I get accurate readings. I can also test at any desired torque or current to do comparisons, but in reality just by looking at the graphs I get accurate information.

One of the biggest problems with dynos like the Fantom is that since they measure the torque based on the flywheel weight (mass), they derive the Kt constant from that. So thereafter, they measure torque by recording the current flowing through the armature. So this in turn can give erroneous readings when the currents are out of operating range. The dyno will report a high torque yet the motor can have a much lower torque due to oversaturation (they are calculating the torque based on current flow times Kt...can get out of range at times and give false readings).


Also, there was a comment made about dynos ... Well, the Robi and Fantom are in reality accelerometers that by collecting data can calculate torque. This is a very easy and inexpensive method to build a dyno. You use a known fixed rotational mass, accelerate it, and just by plain physics (dynamics) you can calculate torque. The rest just comes from data points collected and mathematical calculations. The problem with this type of dyno is that you can not apply a constant load to the motor.
The best way to build a dyno is by using a load cell. That way you can measure real torque at the cell and you can apply any torque you may want to the motor. There are many ways to achieve this, from the simplest to either extremely accurate and complicated brakes using hysteresis magnetic brakes, magnetic particle brakes, or hydraulic brakes. Our motors develop small amounts of torque so there is no need to go to an expensive hydraulic brake. EddieO mentioned in this thread thathe was going to get a dyno from MAGTROL. MAGTROL makes some of the best brakes and clutches in the industry!

I hope that by looking at the attached file (graph) it will be easier to understand what I have been writting about. This graph is a very accuraterepresentation of how our motors behave. Pay close attention at the slope of thelines and how they relate to each other.

Isaac
Attached Thumbnails
CE Dyno Question-axx_tq-sp_curve_1-842-x-515-.jpg  

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Old 12-07-2005, 08:17 PM   #89
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So here is a similar graph from The Robitronic Dyno. The partial average feature I refer to is at the bottom left corner. You access this feature by setting the amp range in the setup. Then load two motor files. It is calculated automatically. This is a good power curve of a stock motor. There are no visible spikes. The feature should work good for it. Now don't go looking at the green torque graph as say aha it is not straight. The reason that it is not straight is that there is a voltage ramp used by this dyno. This causes the curvature. The torque line is thus a quadratic and power line a cubic empirically.

Torque is readily available from the flywheel acceleration. Torque = Flywheel inertia x angular acceleration. The only sensor monitors flywheel position, everything else arises from this and the flywheel inertia. Data points at each pass by the sensor are collected.

The Robitronic does collect a lot of other bits of information like motor friction from the coast down, motor resistance using the electronics powering the motor, Motor EMF (Kv) I'll use some of these and try plot a calculated curve at constant voltage and see if it looks reasonable.
Attached Thumbnails
CE Dyno Question-robitronic-dyno-ouput.jpg  

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Old 12-07-2005, 10:13 PM   #90
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Hi John,

Thanks for the Robi data and graphs.

Later on (when I have some free time) I will take some of the Robi data and do my own calculations....we will see how close I get to what they report based on the constants.
I have found the Robi to be very accurate most of the times. I trust their data...something that I cannot say about most r/c dynos.

They (Robi) provide more usefull information than any other commercial dyno (that I have seen). Too bad many persons do not understand what all the numbers mean. It is not used to its peak potential by most racers. They just use it as a comparison tool

Well, I have to take a break and rest.

I hope that the readers will benefit from all the stuff we have been posting.

Later,

Isaac
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Motor dyno question jon123 Electric On-Road 0 10-09-2006 07:28 AM
dyno related question pinggoy Electric On-Road 5 04-26-2006 12:25 PM
RC DYNO QUESTION browncolin Electric On-Road 7 01-04-2006 09:13 AM
Question for Dyno owners.... Geek Electric On-Road 6 01-23-2004 12:35 PM



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