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Old 12-02-2005, 12:25 PM   #46
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I bet I know who's making it! $15G's yea I believe it.Only problem is it won't be the only one out there.
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Old 12-02-2005, 12:56 PM   #47
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theisgroup -Well there is actually something left from the first semester of physics. The formula that describes the torque produced by N number of coils in a magnetic field B (an electric motor) looks like this.

Torque = NIabBsin(theta)

N is the number of coils
I is the current in the coil
ab are the dimensions of the coil
B is the strength of the magnetic field
theta is the angle of the coil to the magnetic field

you will note that Torque and Current (I) are in the numerator.
They are directly proportional. As current increases torque increases linearly. It also starts at 0 so it is a direct proportion. This is the causality that you seem to be ignoring.

In our motors there is some friction so the line does not start at 0. It takes a certain number of amps before the motor will start and overcome this friction. This friction seems to be fairly constant in our motors at the amp ranges that we normally use so it just causes a negative offset to our straight line. I'll try not to discuss this further.

No need to talk about Storks to try and confound the issue.

My apologies for the technical discussion.

Last edited by John Stranahan; 12-07-2005 at 06:04 PM.
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Old 12-02-2005, 01:15 PM   #48
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Quote:
Originally Posted by John Stranahan
theisgroup -Well there is actually something left from the first semester of physics. The formula that describes the torque produced by N number of coils in a magnetic field B (an electric motor) looks like this.

Torque = IabBsin(theta)

I is the current in the coil
ab are the dimensions of the coil
B is the strength of the magnetic field
theta is the angle of the coil to the magnetic field

you will note that Torque and Current (I) are in the numerator.
They are directly proportional. As current increases torque increases linearly. It also starts at 0 so it is a direct proportion. This is the causality that you seem to be ignoring.

In our motors there is some friction so the line does not start at 0. It takes a certain number of amps before the motor will start and overcome this friction. This friction seems to be fairly constant in our motors at the amp ranges that we normally use so it just causes a negative offset to our straight line. It's apparent that you don't know a straigt line relation when you see one nor the physics involved, so I'll try not to discuss this further.

No need to talk about Storks to try and confound the issue.
did not miss it. but like you said first year physical. you are looking at a perfect machine equations.

Torque = Horse power * 5252 / RPM
and
horse power = voltage * current * efficiency / 746

the big variable is efficiency this changes and the change is not linear.

agree to disagree

blue can motors are faster because you turn blue when you get cold
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Old 12-02-2005, 01:28 PM   #49
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Magtrol is making it......and according to them, its the only unit they have ever produced for R/C use......though its possible someone took an older unit and converted it to work....

Later EddieO
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Old 12-02-2005, 04:41 PM   #50
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thanks for all the discussion fellas....the problem is that i nearly failed
physics and don't think I'll ever pass.
For now, I'll try to read over these posts with great interest and try to digest
some of the magical info in them. thanks again, jon
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Old 12-02-2005, 04:47 PM   #51
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Problem is, I never took physics and now I know why,I was too busy out meeting girls, LOL
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Old 12-02-2005, 05:00 PM   #52
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from john's interpretation a motor checker can be used to extrapolate the motor's performance over the different amp load that the motor would go though during a race. so you should be able to get a good idea weather the motor will perform for you as well as your controled motor.

what you may need to do is get a few dyno sample of the same type of motor and graph them. then you should have a line and the r^2 value he used. this will give you a typical amp to torque relationship. then you can just move the line up or down to match what your motor checker puts out in torque and amp load. basicaly slide the line until the value of amp and torque are on the line.

if this work, it may be a great way to use a much more or indi checker and know if the motor will be fast or slow. i don't know what output those checks give you though
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Last edited by theisgroup; 12-02-2005 at 05:15 PM.
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Old 12-02-2005, 11:17 PM   #53
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Quote:
Originally Posted by John Stranahan
Mike D- You could run your same analysis with amp steps by looking at the RPM at equal amp steps. Torque is directly proportional to amps on these motors. Just thought I would throw in my 2 cents. I always go for a motor with more power at least somewhere in a broad run on the dyno. It does not really matter if you use torque steps or amp steps. Then you gear the motor on the track to take advantage of this extra power.
John - Actually, you missed my point... Why compare stock motors on AMP loads if motor amp draw means very little with today's batteries? It is very difficult to gear a motor from a CE dyno sheet if AMP loads are used. It's like camparing apples to oranges across the dyno steps when the rpm and torque values are variables.

Also, the equation you stated relating torque and current is not much use when tuning stock motors. Sure the theory is sound, but using that equation is like building motors with blinders on. Torque is rarely proportional to current in stock motors we use. It ignores many variables (brush bounce, vibration, etc) in the brush commutator interface that are not proportional at different current/torque/rpm ranges.

No apologies are needed for the technical discussion... we are talking about dynos, not paint jobs - people can leave if their brains hurt.
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Old 12-02-2005, 11:22 PM   #54
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Mike,

thanks for your thoughts about using torque steps. since you posted I have been using that method the last couple of weeks on my motors and it does give me a better idea of how the car will perform on the car with a given motor.
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Old 12-02-2005, 11:35 PM   #55
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EddiO
Quote:
Magtrol is making it......and according to them, its the only unit they have ever produced for R/C use......though its possible someone took an older unit and converted it to work....

Later EddieO
CONGRATS!!!! Magtrol makes the best magnetic particle brakes and torque sensors in the WORLD.
Just make sure they do not overdesign it...and you are right , this will be the first one they design because I have had them make the brakes for my dynos, but I designed the electronics and mechanical interface...
Magtrols existing brakes do not work at the torque/rpm levels we test motors...They have torque v RPM limitations that will prevent you from using any off the shelf torque sensor they make...If they are going to make you a custom brake, be carefull that you do not get overcharged...I have gone through that route with them many times...

They are experts at what they do, but they are used to working with big motors and brakes, or smaller servo type motors...not r/c stuff. R/C motors have a very unique RPM v torque relationship...most other motors do not come even close!!!
Make sure you get a magnetic particle break in the right dynamic range that your motors will operate. They will try to over sell and overdesign... Also, their controllers are not the right stuff for r/c motors, just the brakes and sensors.
YOu will have a GREAT dyno if you use their sensors and or brakes... I recommend you get the brake from them and get a good ME-EE to design the rest of the dyno. It will save you thousands and you will have it much sooner. I have worked with Magtrol... both at their NY office and their main office out of Switzerland. They have provided me with dynamic brakes, hysteresis clutches, and torque sensors for years...and for many applications.

NOW....
Regarding the torque equation describerd above...well that takes ALL The compnents in a motor into consideration, but all fixed magnet motors have a linear torque constant called Kt. Kt is directly proprtional to current as long as you stay within the operating parameters of the motor. If the magnetic field gets oversaturated by excessive current draw then the Kt constant will not work. As long as the motor operates within its parameters the Kt constant will be linear. ALWAYS!!!!!

I dont have thetime to elaborate at the moment on the motor equations and behavior, but I will do so within a few days.

The bestway to analyze a motors performance is to plot current v torque and speed v torque.

The graphs that I saw are OK, but if you want to parametrize a motors performance yo need to plot against torque, not current, thus torque should be in the X axis and all other variables in the Y axis (RPM, CURRENT, POWER, EFFICIENCY). Try ploting this way based on the data that he provided and you will see the relationship of the two motors in front of your eyes... and how they compare to each other....

Like I said, I will elaborate more on this at a later time...I am tired and I must SLEEP

I am glad some of you started a very good thread. Hopefully together we can show most racers and motor lovers how to analyze and parametrize a motor the right way.

Just remember, there are four constants that define a motor (Kt, Ke, Kv, Km) and once you know what they are and how to use them you can reverse engineer any motor and then find the best motor that will work for your specific application.

Later...

Isaac K

Last edited by BATT_MAN; 12-03-2005 at 12:52 AM.
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Old 12-03-2005, 08:22 AM   #56
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Mike-I have not misintrepreted nor critizized your method. If it works use it. I did critizised the lack of ability to pick a torque value of interest as we have no handle on this measure except to run a dyno sheet find the amp value of interest and then see what torque is produced. At this point running torque steps is fine in my book. It hurts me to hear you say this "Torque is rarely proportional to current in stock motors we use" Read the post above and my previous posts. Have you actually plotted torque vs current on a regular basis.

Here are two power graphs on Yankees motor. (Incidentally this type of graph is what I use to tune stock motors). One has Amps on the X axis. I think that we should not forget that our finger is on the amp trigger of the transmitter. The other has torque on the X axis.

I look at one graph and I see that one motor is better on the straight, it should be good for oval. The curve is typical of the old green machines with the tri rotor armatures, but may be the result of tuning or weak springs. The other motor has more punch out of the corners. It should be better on a road course than motor I. Note that I have extrapolated the trendline a little bit, but there is a theoretical basis for the quadratic that I used to fit the data so I can do this if I don't go too far.

I look at the other graph and think that Motor II kicks ass under all conditions.

I'll let you decide which conclusions go to which graph.

I am not sold on any particular method. I am always eager to learn new stuff.

I don't know which of my two conclusions is true as I have not personally run the two motors. I have run quite a few motors that I looked at this power graph though. Yankee if you would provide us with more details on the motor I and II we could see whether to strike one of these analysis. What does the armature look like 0, 1 or 2 slots in the armature blank. What springs are your running on both. Are the two motors Idendical except for springs. I'll be happy to prepare other types of graphs as it does not take me too long.
Attached Thumbnails
CE Dyno Question-yankeys-power-vs-amp-draw-65%25.jpg   CE Dyno Question-yankeys-power-vs-torque-65%25.jpg  

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Old 12-03-2005, 10:24 AM   #57
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In response to what is happening here are some facts.

The best way to find the a motor that will work and perform the best is to KNOW the torque area of operation of that motor at the desired track. Once yo know that torque you test your existing motors UNDER that torque level and find the one that produces the MOST POWER and draws the least ammount of current to develop the desired torque. You also need to take into consideration the motors time constant...how fast it accelerates (spool up) and the RPM under LOAD. Once you know the Kt you can find ANY torque at each motors operating current.

The way to find the desired torque is actually simple if you have the right equipment and you know how to analyze a motor.

STEP 1.

YOu use a motor that you know that performs the way you want at a desired track.
You then test that motor to find its Kt (Torque constant.... also known as Torque sensitivity), Kv, and Ke... I will explain later how to test a motor to obtain this motor constants. These constants DEFINE the way the motor will operate.
Once you know a motors Kt you can find its torque, POWER, RPM and efficiency under ANY current.

STEP 2.

Once youu know the motors Kt you run the motor on your car with a current shunt and a portable data logger on your car for a few laps. The data loger will record current v time as you drive.

STEP 3.

YOu remove your data logger and analyze the current levels at which the motor operated. If done the right way, you can actually correlate time v track position and determine the current drawn at different areas of the track. The highest current level recorded is the max torque that the motor developed (as long as you did not crash and or braked too hard) that is why you log the time too, to see what is happening.

Once you know the operating current that this motor drwas you can calculate the ACTUAL torque that the motor produces to make the car move.

STEP 4.

Now that you know the torque that the car develops and the motor needs to supply to get good corner exit and straight speeds you can start evaluating different motors.

YOu should get the Kt of every motor you want to test, then evaluate each motor for the desired operating torque.

The motors that will develop the highest torque (or the one that you are looking for) at the lowest current draw and also has the right RPM UNDER THAT LOAD to give you the speed you need to make the straightaway at the desired time will be the best motor.

This sounds complicated, but once yo understand and know how to do it it takes less than a few minutes to find the best motor. Thereafter, you can tune and optimize that motor to give you even better performance.

Yes, it does take some know how to do this, but once you understand it and perform it once it is a repetitive and simple way to find the best motor(s). I can do this in less than 15 minutes. If you know how to interpret a dynos data you can derive the Kt from there....once you knoe the Kt you can solve for all other constants mathematicaly.

This is the way we select motors for ANY application. We always need to define the operating torque, voltage, and RPMs for the desired application. Once you know that the motor search becomes much easier and accurate.

If any of you want to know more about this I am willing to write a WHITE paper on this subject. With all instructions, formulas, and motor theory to understand this.

I find it very hard for racers to just look at dyno numbers if they do not understand how a motor actually operats and how all factors that affect a motors performance relate. By that I mean the Kv (rpms/volt) Kt (torque/amp) Ke (back EMF)...and dont forget Km ( the real motor constant) These are the three basic constants trhat will help you find the right motor. YOu can find this constants FOR every motor you own. Just remember that these will change as you uchange brushes and the motors magnets wear out...However. finding these constants only takes a few minutes with the right know how and equipment. For brushless motors you only need to find the constants ONCE....it rarely changes...

So basically, the BEST way to find the best motor for any desired application is by TESTING for the OPERATING TORQUE LOAD, not for CURRENT LOADS...as current loads vary from motor to motor.

The torque is a given physical condition that the car needs in order to move....

Isaac
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Old 12-03-2005, 10:34 AM   #58
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NICE INFO WHEN IS THIS WEBSITE GOING TO BE READY!
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Old 12-03-2005, 10:43 AM   #59
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eddieo


chime in please, let us know what seetings you use on your ce dyno and what valus do you look for. do you use amps still like big jim used or torque steps for stock motors i know you prefer the robi. but shed some light.

thanx guys your starting to make me remember high school
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Old 12-03-2005, 10:50 AM   #60
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18, 22, 25, 28, 30 for stock or 19t....7 volts with automatic amp steps...

I just look for the power levels at 28 amps.....

To be honest though....I didn't dyno a single one of Peter's motor at the indoor champs....and they were all rockets on the track....fastest lap in the 19t and stock touring mains....

What a lot of people don't realize about stock motors is that 95% of making them go fast is GEARING......if the motor is built right and geared right, it will be a minimal difference if any between motors.....where you go wrong is that the motor is not built right (arm spaced wrong, bushings not seated correctly, brush hoods not align, brushes not broke in, etc) or its geared wrong....or both....

Later EddieO
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