Oct 24, 2004, 08:51 PM

## Motor Constants & Static Tests For Micro Motors

I've been meaning to finish this up, put it on my web site, and do the related tests. But, I've been too busy with other things, most notably small children. There have been enough questions about micro motors in a few different threads that I thought I should post these two work-in-progress (not much progress lately) tables. Some motors need to be double checked, and others are not done yet, and weights and resistances need to be filled in. However, many motors are done and the data is useful.

These tables apply the method in Jochen Bergmeyer's Feb 2003 Inside Story article. To simplify the presentation of lots of motors in one table I give the max efficiency rpm, max power rpm, and amp draw and power and efficiency for these two points. Note that it is output power that is given.

To see how to use these values, I would suggest reading Jochen's excellent article. To see how to simply apply the max power and max rpm points, I suggest reading my March 2004 Inside Story article on pager motors, where I illustrate how to use this information specifically for the Didel 4.5 ohm pager. The concept is the same for other motors.

What do you need to have to make use of this information? You need a tachometer (cheap at about \$30) and an amp meter. Eventually I'll get around to doing static tests for some of the motors in these tables, and writing it all up as an article. But for now if you want to take the trouble to measure your propulsion setup you can answer basic questions about how close to max power or efficiency you are with that gearing and prop. All the caviets about static tests apply. To see exactly which prop will work best in flying conditions, you need to fly it.

There are two tables. The 3.5V corresponds to one-cell applications, generally in the first minute or two of discharge. The 7.0V corresponds to two-cell applications. Only motors that won't quickly burn out on two cells are included in the 7.0V table, although with a sufficiently high gearing some of the lower volt motors might be able to be used at 7.0V.

Enjoy,
Gordon

# Images

Last edited by Gordon Johnson; Oct 25, 2004 at 12:45 PM.
 Oct 27, 2004, 09:32 PM Registered User Thanks Gordon! This is great information. I have been looking for this data on the Superslicks motors - my current motor of choice.
 Oct 27, 2004, 11:26 PM in persuit of low wing loading Glad you like it. I have 7mm gearboxes Carl Martin made for me to test the Super Slicks in all the ratios. It's just a matter of time now. Gordon
 Oct 28, 2004, 01:13 AM Registered User Thanks Gordon, The M20-mv looks good for my 2 cell application. I want to gear it so that it gives the power I need right down to about 6,25volts, .5amps and 40k rpm. At full charge I would need to use part throttle. I'm not used to running a motor that fast even though the current is low. Do you think the brushes can handle runs of 10 min at part throttle? Are they carbon? Thanks, GB
 Oct 28, 2004, 10:38 AM Frequent Poster Hey Gordon- Where can I get GB's for the 7mm without going to Didel?
 Oct 28, 2004, 11:06 AM in persuit of low wing loading Other than Didel you will have to make them. There might be some other source like Larry Park, but I'm not aware of anyone listing them for sale. Since the SS 7mm motors can't be bought at wholesale, there is little reason for someone to develop a gearbox so they can sell a complete propulsion system. It could happen, but hasn't yet. Gordon
 Nov 03, 2004, 09:56 PM My member is not Hi All, Pete, MPS ( http://www.microplanesolution.com/mps_00000d.htm ) apears to have a 2 stage gearbox for the 7mm ( I think ? I am no good at French ) Gordon, eflightdesigns ( http://www.eflightdesigns.com/cgi-bi...ucts.cgi?CAT=3 ) have the Kenway 4.2:1 gearbox with ball bearings . Unfortunately only with the M-20 HV. I've spent some money with them and BSD. Will let you know if it makes any difference to thrust. Cheers BTW MPS now has a combo receiver with a built in 125kHz ESC
 Nov 04, 2004, 08:10 AM in persuit of low wing loading Zlatko, The Super Slicks motors are much lower resistance than the 7mm pager that Coural developed his two stage gearbox for. So, unless you are going to drive a really huge prop, for these hotter wind 7mm motors the gear ratio is probably too high. I guess I was referring to single stage gear boxes in appropriate gear ratios for these. Didel does have 5:1, 6.7:1, and 9:1 single stage gear boxes for the 7mm motors, and several 2-stage gear ratios as well. It will be interesting what your results say in terms of brass versus ball bearings. When I've tested the same pager motors in gearboxes using the same gears but different bearings (ball, brass, nylon) I got virtually the same static thrust. But, it could change with a more powerful motor on more cells. Gordon Last edited by Gordon Johnson; Nov 04, 2004 at 08:19 AM.
 Dec 14, 2004, 11:09 PM Frequent Poster Gordon- I dont know if this is the correct place to post but SS came out with a new 'blue' super-hot motor, the nice little graph shows 10% more RPM than the purple! I bet it would make a nice ducted fan!! If I can get one, I'll send it over for testing, so you dont blow \$20 on it.
 Dec 15, 2004, 07:42 AM in persuit of low wing loading Pete, Sure, I'll test it, and return it when I'm done. I'll just do the motor constants part, not static tests. Wow, hotter than the purple, that's hot. I was in Toys R Us over the weekend but didn't look at the SS cars and accessories as I already have plenty of the motors, so didn't notice the new one. Gordon Last edited by Gordon Johnson; Feb 22, 2005 at 08:50 AM.
 Feb 04, 2005, 09:02 PM in persuit of low wing loading I've finally gotten around to putting some of my more recent static test results on my site. Here is the link directly to the motor page. There are tests for SS7mm, all the major M20's, and the N20-LV. Some motors have more tests depending on what I was interested in at the time. As I do new tests I'll update them on my site. Most of what is required to interpret the results is in this thread. Hope these prove helpful. Gordon
 Feb 07, 2005, 05:27 PM Registered User Gordon, thank you for posting all this information about real motors. After writing the article I haven't taken the time to apply it to motors very often, instead I have followed the suggestions for good motors that others (including Gordon of course) have found out. But I would like to add a few formulas here that are helpful to compare and make good use of motors. These are not new but fit nicely in here. Firstly, a good rule of thumb for maximum input power of a motor is: as many watts as its own weight in grams. Look into the above lists and you will find that this is true for some of the motors that have proven to be working well, e.g. the M20-LV, the Didel 4.5 Ohm and the SuperSlicks red. Secondly, since the maximum output power point is near half of the stall current, it is easy to calculate a good first guess for working voltage of any motor. You need the motor resistance Ri which is easy to measure at the motor terminals. Stall current: Istall = Ubatt / Ri Working point current: ~Iwork = Ubatt / 2 / Ri Input power: Pin,work = Ubatt * Iwork = Ubatt^2 / 2 / Ri = motor weight!!! ==> Working voltage: Ubatt = sqrt( motor weight * 2 * Ri ) ==> Working current: Iwork = sqrt( motor weight / 2 / Ri ) So, just by weighing a motor and measuring the inner resistance at the motor terminals, you get a good first guess for the working voltage and current. Of course you can refine this by applying the formulas from my article and get a better guess for the working point. To do so use the spreadsheet and vary the supply voltage until the motor takes as many watts as its own weight just at the maximum power point. The formulas will deliver the efficiency in this point. This way you can compare output power per gram of motor weight at the "natural" working point and are more fair to your motor. Reards, Jochen Last edited by jberg; Feb 07, 2005 at 05:32 PM.
 Jan 03, 2009, 03:43 PM Trying To Takeoff Hello! I need constants for Parkzone Cessna motor/gearbox. Has anybody measured them? Thank you, Kostya
Jan 03, 2009, 06:24 PM
Easy Glider
Quote:
 Originally Posted by Gordon Johnson Other than Didel you will have to make them. There might be some other source like Larry Park, but I'm not aware of anyone listing them for sale. Since the SS 7mm motors can't be bought at wholesale, there is little reason for someone to develop a gearbox so they can sell a complete propulsion system. It could happen, but hasn't yet. Gordon
What about Plantraco? They have a 7mm gearbox/motor combo and seperate 7mm SS motors.