Real Motor Analysis
 

Dear XMOD Community,

I have been working for some time on something everyone has wanted a way to answer the question – “What is the best motor for my XMOD?” I built a home-made motor dyno to systematically test each motor by placing them under a constant load and measuring the pertinent characteristic parameters. The dyno is constructed by using a load absorbing motor coupled to the drive motor. The drive motor is supplied power by a constant voltage (5V) power supply capable of handling the current draw without a large voltage drop (a battery pack will not work due to the voltage drop under high current loads). The necessary instrumentation is wired to the system so that all information is collected at the same time. This requires the use of two multi-meters and a laser tachometer. The first multi-meter is placed in series with the drive motor to measure the current draw. The second multi-meter is connected to the motor tabs to measure the drive voltage. The laser tachometer is set-up to read the reflection signal off of the motor coupling to give a real-time reading of motor speed. Using the supply voltage, current draw and motor speed at various conditions the motor characteristics can be found (explanation of motor characteristics would be too lengthy for this post – can be found on many websites like www.micromo.com/library/docs/notes&tutorials/ Motor%20Calculations.pdf ). By using DC motor theory and the experimentally determined characteristics I was able to calculate the speed vs. torque, current draw vs. torque, power vs. torque and efficiency vs. torque curves for some of the motors I currently have. I have attached a zip file that contains the results of these tests as well as a spreadsheet file for everyone to use. The functions are already in place and only require two data points to complete the calculations. The first point is the no-load speed and current (measured without the motor coupled to the absorbing motor), and the second point can be at any appreciable constant load placed on the motor. When testing I typically triple check the results by measuring four or five data points and making sure the resultant characteristics are consistent. The two data points are used to find the characteristics and then the DC motor model uses these characteristics and a desired supply voltage to generate the motor curves. You might be saying to yourself, “This is a load of garbage, there is no way this model can be right.” Well I wouldn’t put it out here for everyone to see if I didn’t verify the results by some method. I started by applying the DC motor model to the specifications given by Mabuchi on their website. Using the no-load condition and the maximum efficiency point I was able to accurately determine the motor characteristics. I could then use the model to predict the performance at a different voltage – what do you know – It worked! The one thing I noticed is Mabuchi has a small correction factor in some of their data (the voltage constant is not equal to the torque constant) to make the results match the theory. In most cases this correction factor is around 0.9. I have included this factor into the calculations. I hope this will be a way for everyone to compare motor A to motor B without being so subjective about it. As soon as I get any more data I will make it available as well.

Edit: The Results and Analysis file has been updated 02/04/04

Excellent, top notch work!!! I have thought of making a dyno with a mouse wheel and using current draw and power output in wattage to arrive at torque and micro hp , to help rate the motors i have. i have been working lately thru the manufacturer's specs on the motors, and i would like to have a diagram of your tach. thanks,

bob256

Thank you Bob256,

I first thought of some way to actually measure power output and then realized it would take a lot more instrumentation than I could afford. I couldn’t justify either building or buying a torque cell to measure power output so I had to come up with another method. One way would be to measure the power generated by the absorbing motor and then back calculate the input power from the drive motor. The only problem is that you would have to know the generator efficiency of the absorbing motor, so you are back to having to use some means of directly measuring torque. As for a schematic of the tachometer I use, it is actually a handheld unit I bought from www.web-tronics.com (Thanks again TypeZero). It works very well and can be used to measure many things. It is very useful and only $50. Anyway, Thank you again.

that's very true about being back where you began with the power measurement.thanks for the link to the tach, and i found a link that might be helpful

http://www.simcotech.com/formulas/hp_electronic.htm

seems as though they have quite a few formulas, i'm not sure if this is redudant with micromo's website, but it could help. right now i am experiemnting with the 180 size motor and different winds, i'll keep you updated on how it goes

Hey bob256,

I have also been playing with the 180 motors for some time now (check out a few of my posts in the "180 motor" thread in XMOD Science). I tried to test one of them on the dyno and ended up throwing a wind (a wire came off of the comm. inside the motor, OUCH). Anyway, I should have a rewound motor tonight to test. As soon as I finish the testing I will include it in the results file. As for the equations used they are more along the lines of the ones one the micromo's website (DC motor theory). The ones included on simcotech's site are more fundamental equations. I will keep everyone posted as to any results I generate.

This is truely awesome. Thanks for the hard work, Backtrack. This is a great tool for us. May I suggest locking the all the cells but the ones that the user needs to input? Could prevent people from accidentally jacking up their spread sheet.

Updated Results and Analysis Spreadsheet
 

Ok everyone, here are the latest results from my testing. The new "Results" file now contains information on three new motors. I was able to add a SpinBrush - Stage 1 motor, a FF180SH (taken from a Colgate Motion Brush), and a FK180SH (from a HPI Micro RS4). I have modified both of the 180 motors by doing some rewinding and retiming. I have also included an updated version of the "Analysis" spreadsheet. I have made a seperate Input sheet which can be updated and lock all of the remaining calculation pages to help prevent someone from accidentally messing up some of the cell references (Thank you Rez for this suggestion). As soon as I get some more data I will present it to everyone. This weekend I hope to get my hands on some 260 motors. We will see just how much better they are than the other motors. Until then - happy XMODING.

hmmmmm
 

This is some good stuff...makes me wanna try a plasma after a HUGE fet upgrade, maybe the plasma is best left to the Zs

Re: Updated Results and Analysis Spreadsheet
 

Great job on this so far. The easiest 260 to get a hold of is the Hoppin' Hydros Big Bad Johnson motor. Speaking of which, I forgot to go to Hobbytown tonight, as they should have restocked that motor today. ;)

Anyway, you'll need a FET mod with that motor. It's pretty current-hungry, even stock. Then once you rewind it, fughetaboutit... :cool: