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Old 04-07-2009, 07:50 PM   #121
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JevUK posted an intersting output curve from the ROBI dyno:

http://www.rctech.net/forum/4096497-post19.html

where I think this is an 8.5 brushless with the ROBI current limit set at 70 amperes.

Note power is a straight line in the region where current is constant and torque is constant, as one would expect from applying Newton's Second Law in rotational analog form to an ideal flywheel (no friction).

This result is consistent with my model of a smaller current source (30 amps) driving the motor for 4 seconds, and with John's graphic for the constant current dyno. However when driven at 70 amperes a powerful motor will accelerate the flywheel much more rapidly and show greater peak power.

I mention these things because I've seen efforts to compare a constant current dyno output to a voltage regulated dyno output, which is comparing apples and oranges. Both can be used to tune a motor but power figures are not going to match.

Last edited by SystemTheory; 04-07-2009 at 07:57 PM. Reason: Removed Sentence
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Old 04-07-2009, 08:28 PM   #122
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Note also when the current limit no longer applies that the Robi uses a voltage ramp to simulate output of a 6 cell pack. The output should correspond well to the output from the Sentry dyno on 6 cell. The Fantom uses a constant 5 V if you run from the internal power supply.
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Old 04-08-2009, 12:46 PM   #123
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Okay, everyone on this board is clearly smarter than I... Regardless, I appreciate this thread. It would be useful if you could explain how to use your methods to compare different motors and predict their performance on the track. I don't think it's important to have an accurate power number as we only need power numbers for comparison to other motors. With the introduction of the LRP SPX and SP Gran Turismo 2.0, I'm curious as to how their very radical timing profiles affect performance as well. I think it was John that said that there isn't much difference between the top controllers; does this apply to these later releases?

Additionally, what equipment do I need to replicate your testing? Instead of using a flywheel, would it be possible to just install motors in a buggy with a set gearing and pair of tires and roughly approximate the moment of inertia? Would that be close enough for comparison testing?

You guys are doing some great work here, I'd just like to figure out the application to help me pick the right motor, controller, and gearing for my cars.
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Old 04-08-2009, 02:16 PM   #124
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Richard, the beauty with this custom dyno is that even ESCs can be dynoed if one ESC is compared to another using the same motor. Please refer to my earlier post showing a Novak 13.5 dynoed at 0 degrees boost and then 30 degrees boost using a Tekin RS Pro. At 30 degrees, the motor showed 8000 more RPMs, however there was a torque and power loss.

If you would like to replicate the testing, you must either purchase or build your own flywheel. Often, you will find flywheels for sale here or on ebay, or you can also do like I did and head to your nearest local machine shop and have them make a flywheel for you. The flywheel I made is 3.427" in diameter, and weighs 365g. I came home and balanced it myself using a dubro prop balancer and my drill press. After you have the flywheel made, then you have to make a motor mount. I made a simple box out of pine and affixed a team associated motor mount to it. I then just screwed my test motor into the motor mount using regular motor screws. Add the Novak Sentry and you are in business. Please note that when using the Excel or Open Office spreadsheets, that we are using a mathematical model to calculate torque, power, and efficiency. If you need help using it, I will help you out and I am sure John would help you out as well.

Concerning building a dyno just using a transmission and tires, it can be done, but I feel it would be inconsistent, and consistency is the key with these dyno tests. You will see that even when you have a nice and balanced up flywheel, that even the smallest detail matters, such as timing the runs consistent (i.e. 15 second dyno runs for a 13.5 motor), temperature, and topping off the battery each time. If any of these variables are off, you will see a comparison difference.
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Old 04-08-2009, 05:17 PM   #125
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Richard and All

Here are the plans. Matt is going to mail me the Sentry and a steel flywheel or two. I am going to work on procedures to get reproducible results with a LIPO battery. The plan is to make a run only when I get the first green blinking light on my FMA direct Scorpion charger. This will insure that at least the starting voltage of the test will always be the same. This happens at about 90% full. This is a very stable region of the battery discharge curve. I will test only with a cool motor. I going to use a readily available thunderpower gold 5400 mA-h pack. The pack chemistry (another brand) will change the results slightly. These batteries are slow to change internally over months if charge rates are kept to a modest 1C. Thats what I use. At least test results at the beginning and end of the month should be consistent.

I plan to use an LRP TC edition speed control. We will likely see an improvement in precision from better more controlled acceleration in the first .4 s. Can you note this on the track. Well yes you can. I'll post some sensorless motor test results from the track in the new thread.

We are going to leave the dyno and motor theory behind with a reference to this thread as RC guys just need to know the type of question Richard just asked.

I will make a plot of Peak power with motor limited amps vs voltage. This graph may indeed be different than my Competition Electronics Dyno. Well see. The only purpose here is to see if there is an accurate voltage conversion. The Fantom dyno makers suggest that a 5 V test is predictive of all voltages.

One striking thing about boost is the efficiency goes down. Here in Houston we are always up against thermal shutdown issues. Less efficiency means more heat.

When using plastic geared transmissions, you will find enough wear from run to run, and in addition all the vibrations and bearing friction will create havoc with your numbers. It is not like a full size car with all precision roller bearings and balanced tires on a full size chassis dyno. I say go with a flywheel unless your goal is to investigate chassis losses. Matts motor mount sounds like a good solution.

I'll review Richards post when I am up and running here in Houston with the Sentry.

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Old 04-08-2009, 08:44 PM   #126
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Gents, I'll stay tuned and see if I can't pick up a Sentry myself...
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Old 04-08-2009, 09:51 PM   #127
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John, you have an email. Looking forward to seeing your results.
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Old 04-08-2009, 10:05 PM   #128
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Richard. Exiting.

I am getting a servo tester from Towerhobbies.com


http://www.astroflight.com/index.php...products_id=19

It will eliminate the need for your radio, receiver, and possibly the receiver pack for the Novak sentry. It has a knob that you can rotate from off to full speed on the speed control. This will simplify the dyno. Steel Flywheels are available from Fantom until they find out what we are doing with them. I suggest a sensored speed control. It should be set to a straight up profile. No sneaky soft power tricks desired.

John

Matt thanks. Got your note. Sentry on the way. Maybe Friday.
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Old 04-09-2009, 09:07 PM   #129
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For those looking for a NEW Novak Sentry for a great price...check out WINDTUNNEL's site... they have one for 100.00

http://www.windtunnelracingproducts....oducts_id=1190
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Old 04-09-2009, 09:53 PM   #130
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Damn you Joe (cost me $100) . They don't list the amp sensor which will also be necessary. It is available at Novak.

Please Join us on this new thread. I hope to discuss gearing a 17.5 on the oval soon.

http://www.rctech.net/forum/electric...al-thread.html
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Old 04-10-2009, 07:15 PM   #131
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I'm glad to see the project moving over to a new thread with more practical focus. I'm looking forward to seeing the results.

I'll keep my comments to a minimum over there so others can focus on the practical applications of the Dyno without distraction.
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Old 04-10-2009, 08:09 PM   #132
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System Theory-Thanks

Please Join us on this new thread. I hope to discuss gearing a 17.5 on the oval soon.

Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.
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Old 01-29-2010, 05:53 PM   #133
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Has anyone gotten the software to work with Windows 7? I've tried on the 64-bit version with no luck. I want to get a netbook for my RC stuff which will be 32-bit and I wondered if it would work there?
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