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Old 05-02-2011, 08:28 PM   #256
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I think what he is saying, he doesn't mind if its 200 rpm off actual RPM, or 1% off in efficiency,
I would understand that to be absolute accuracy.

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as long as its the same for all motors.....that way, he knows the baseline of what a good motor is on the dyno.
I would understand that to be repeatability.

What is considered precision? Is resolution and stability not a concern?

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All the speed controllers have inherent losses........not anything crazy, but its still there.
Later EddieO
Agree, everything from the battery to the wire to the motor has losses. I assume some, like the wire may vary with temperature. I am not sure about the controllers you use. I would assume that the switching losses are small, but I have no idea.

Power factor is a different question. With it being a switched system, it sure seems like this could be a major source of error. I guess it could be ignored as well.

The idea of having a variable load is good. For an electric motor, I would think it would allow you do use one size dyno for several motors. Your comments about the different currents effecting the performance is interesting. Do you normally try and find a sweet spot to operate the motors in?
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Old 05-02-2011, 10:00 PM   #257
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The different amp loads were often used to compare motors for different classes or for different track conditions.

Stock motors, (like the green one you have), we typically looked at the 20 amp and 28 amp numbers on the turbodyno. Motors that produced the best power numbers at those two usually performed best on the track, though as battery power went up, we started to look more at 22 and 30 amps......

In modified, it was very helpful in setting the timing of the motors to get good power, while getting the best efficiency....this was a big deal in 1/12th scale....

It was fairly common to see motors test great at the lower amp loads, but flatten out at the higher amp loads....sometimes motors like this made great 4 cell motors where amp draw was less.

Later EddieO
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Old 05-02-2011, 10:41 PM   #258
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Thanks for the post guys.

With brushed motors I used to look at power bands. We could tune the motor to a different power band by tweaking the brush springs.

Accuracy is how close your result is to the true value.

Let's say I run a motor 10 times and the average is 145 Watts with the slave motor dyno. I look at the raw numbers and the highest vs the lowest (the spread) is 10 Watts. If the true value is 146 Watts then we have good accuracy. A spread of 10 Watts, though, is poor precision. If you only run the motor one time it might be off the true value as high as 5 Watts, you might have two adjacent runs differ by 10 Watts.

Certainly stability is important. If the RPM is varying after lets say a 1 minute test, then the output will have less precision. This is the kind of test I envision for the slave motor. Heat the puppy up for a known time period and then measure power. Resolution has to do with precision. If our output is + or - 1 (or maybe 2) Watt then we will have satisfactory Resolution. I will do a multiple test just like I did with the flywheel dyno and then I can report the Relative Standard Deviation (or + or - number). The flywheel dyno has good precision if you follow certain steps I printed above with it.

When you compare two motors with this slave motor setup and resistors what you want is to compare two of your motors one after the other and have confidence the higher number is the stronger motor. Then go racing. You don't want to do 10 tests. Instead you want good repeatability. If you run the same motor again you want the same (or close to the same) power number. If your Watt number is 2-3 Watts off the true value it does not matter so much. If you want more accuracy run the dyno on flywheel with its more complicated spreadsheet and output.

It would be nice to be able to vary load in discreet steps. Then you would have the Competition Electronics turbo dyno for brushless. With the Mcpappy dyno you could switch in another bank of resistors. I will know more when I actually make a run. I want the load to run up the motor to near max power. The one cell motors are probably raced that way.

Published on resistance for 1 phase of a good controller are very tiny. Smaller than your lead wire. Use short runs of 12 gauge for your lead wire.

I made my first RPM with the dyno tonight. Tests to come. I set my power supply at 4.20 V. It runs the 12 V fans but mighty slow. I need a dammed receiver pack to make the GTB work.

I have a 3.0 in the 1/12 pan. Eventually I will want to run it on the dyno. Those extra peak amps might come in handy.
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Old 05-03-2011, 06:04 AM   #259
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Thanks for taking the time to write this up John. It was helpful.

For a simple "step" system, you could rectify the absorber's output. Then switch in the resistors in parallel. Use R2R rather than the same values. Like a DAC. Three resistors gives you 2^3 or 8 steps. 4 would give you 16. Very stable. Resistors could be switched mechanically if you wanted.

Eddie, I would never have guessed people would dig into this level of detail for their motor testing. Is 300 Watts of output power the high end for you? What is the lowest you need to test to?
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Old 05-03-2011, 09:16 AM   #260
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Some motors now make power in excess of 500 watts.......while we do whats called 1s racing, which testing the motor around 4 volts or so will have motors producing less than 100 at the peak amp levels........

Power levels are all over the map.....

Later EddieO
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Old 05-03-2011, 11:41 AM   #261
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Here is a pic of the dyno complete.
From the left
on top, 8.4 V Receiver Pack (large)-this is plugged into the x port of the Sentry to power the GTB electronics.
Astroflight Servo Tester-This powers the motor by controlling the speed control. It also prevents motor stuttering on the flywheel by having a quick manual ramp to full on rather than instant on.
Sentry- on four cell the 10 data point per second aquisition rate should be sufficient.
GTB controller- for my purposes a straight up controller without boost is better for calibration of the dyno.
Digikey amp sensor-We upgraded to a 200 amp sensor up above in the thread. For 1 cell testing this is probablynot needed.
On switch to the GTB speed control-leave this off
On the McPappy kit
test motor on left-a 13.5
Slave motor on right-an LRP X12 21.5 with black rotor shaft.
I shorted each pole with the supplied .1 ohm resistors.
I have the fan on the power supply, but it could run on the Receiver pack for a little more air.
John
Attached Thumbnails
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread.-mcpappy-dyno-complete-002.jpg  
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Old 05-03-2011, 03:15 PM   #262
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Eddie, is the 500 Watts limited by the rules you have? What is a 1s race? I take there is no interest in testing low power motors.


John, what is a 10 data point aquisition rate?
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Old 05-03-2011, 05:00 PM   #263
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Well, we have different classes....

Stock is usually a spec motor, typically a 21.5 or 17.5...sometimes 13.5 turns....

Modified is whatever ya want, as long as its an approved 540 size motor. So as long as it meets the size dimensions and has gone through the approval process, its legal to race with, so if a 700 watt motor exists, then you can run it....honestly there is more than enough power for any mod class really at this point, most of the testing would center around spec motors (10.5, 13.5, 17.5,21.5)...though using the dyno for mod stuff to help with gearing and stuff is helpful.

1s racing is 1 cell lipo racing..........while most other classes are 2s (2 cell). So 3.7 volts 1s, and 7.4 volts 2s. 1s is only really used in a few classes for onroad and oval......

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Old 05-03-2011, 06:30 PM   #264
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10 data point per second aquisition rate I left out some important units.

I have one older Novak 3.5 that tests at over 800 Watts on 2 cell LiPo. That is a bit more than one horsepower.
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Old 05-03-2011, 09:21 PM   #265
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Do they make you pull your motor for inspection, or is it more a gentlemen's race? Can I package 6 cells into that 1 cell look alike case?


Ok, I follow the sample rate. Can your logger sample faster? What sort of vertical resolution does it have?
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Old 05-03-2011, 10:52 PM   #266
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I checked the Eagle tree site again. Here are some notes.

High speed logging at up to 50Hz (increased from 10Hz).

This means and increase from 10 data points a second to 50 points a second. Very Nice, but it is version 4 which you may or may not have. The car version has an adjustable data rate.


Compatible with excell

This means you can cut and paste data from the logger output into excell. This means you could use it instead of the Sentry.
Note I am using the Sentry from Novak
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Old 05-04-2011, 07:03 AM   #267
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I had searched for the Novak Sentry but could not find any details about it and figured you had dug into it. I'll give them a call.
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Old 05-04-2011, 10:31 AM   #268
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www.teamnovak.com has all the details. You can get the whole thing for $70 now, as its been discontinued. I would imagine they are gonna run out fairly soon on them though with the mcpappy thing making a run on them.

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Old 05-04-2011, 12:01 PM   #269
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I could not find any details on their site beyond some basic info. Waiting to hear back from them.

Good to know it was discontinued.
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Old 05-09-2011, 08:54 AM   #270
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Sorry but I never received a responce from Novak. If the product is discontinued, they may no longer support it.

Their resolution should be pretty easy to determine by looking at the data.

Hard to believe your going to see much sampling so slow. Looking forward to seeing some data off your setup.
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