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Old Oct 24, 2004, 07:51 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Last edited by Gordon Johnson; Oct 25, 2004 at 11:45 AM.
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Old Oct 27, 2004, 07:21 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,428 Posts
Since the questions that prompted this thread primarily concerned M20 motors, I've taken some 5:1 M20-LV results I had from a while ago and quickly done some additional tests for the M20-LV and M20-MV at the 4.2:1 gearing in the commonly used Kenway gear drive.

All tests were done with the BSD prop adapters to adapt GWS props to the 1.2mm shaft of the Kenway gearbox or the 1mm shafts in my Carl Martin gearboxes for other ratios.

Remember that props with pitch/diameter (P/D) ratios over about 0.65 to 0.70 are stalled in static tests and the results misstate what might be achieved in unloaded flight. For slow flying indoor planes or 3D type of planes the high P/D props will likely be stalled or partially stalled even in actual flight. The higher P/D ratio props will be best suited to planes that have higher flying speeds for at least part of your normal flying routine. Pretty much actual flight testing is the best way to confirm which P/D prop works better for you.

M20-LV:
I tested motors from E-Chargers, which are virtually identical to what currently comes in KP00's. The max power RPM is given at the top of the table and is ~23,000 RPM. With the Kenway 4.2:1 gearing the GWS 5x3 prop is a near perfect match. The actual amp draw also matches the predicted amp draw closely. The 5x4.3 prop would seem to be over propping the motor and the motor RPM of ~18,000 RPM is considerably below the max power RPM. It could unload in flight and motor RPM might be closer to the max power RPM. I have not tested the blue 10cm prop at this gearing yet.

I did the 5:1 gearing tests some time ago, and for a wider range of props. I don't intend to test the KP00 4.75:1 gearing as mine is the older style without a shaft for pressing props on. The 5:1 results with Didel gears and a Carl Martin gearbox should be close enough. At this higher gear ratio the 5x4.3 performs better and develops a bit more thrust than the 5x3 prop. This would seem to indicate that if the higher P/D prop is desired, the higher gear ratio is probably the way to go. But, it's hard to know for sure without wind tunnel tests to assess the effect of prop unloading.

M20-MV:
I tested the Toytronics M20-MV (also available from BSD) at 4.2:1 gearing and 7.0 volts to approximate a 2-cell pack. The 5x4.3 appears to be over propping the motor as the motor RPM of 22,000 is substantially below the max power RPM of 26,000. The GWS 5x3 is a near perfect match with this gearing and motor RPM of 27,846 is moderately higher than the max power rpm. This is a setup that is not likely to burn out. Other gear ratios and props will take time to do. But, these few should illustrate some choices.

Edit 11/16/04 I just added a few new tests to the M20-LV and MV and also a very limited set of tests of the N20-LV (which is really more like a MV) at 7.0 volts.

Edit 1/5/05 I deleted the static test tables as they have been superseded by more recent results on my web site
Gordon
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Old Oct 27, 2004, 08:32 PM
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Fairfax Station, VA
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Thanks Gordon! This is great information. I have been looking for this data on the Superslicks motors - my current motor of choice.
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Old Oct 27, 2004, 10:26 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Old Oct 28, 2004, 12:13 AM
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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
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Old Oct 28, 2004, 09:38 AM
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Pete P.'s Avatar
South of Boston
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Hey Gordon- Where can I get GB's for the 7mm without going to Didel?
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Old Oct 28, 2004, 10:06 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Old Nov 03, 2004, 08:56 PM
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Zlatko's Avatar
Melbourne City Hlpt, Australia
Joined Jun 2004
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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
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Old Nov 04, 2004, 07:10 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Last edited by Gordon Johnson; Nov 04, 2004 at 07:19 AM.
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Old Dec 14, 2004, 10:09 PM
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Pete P.'s Avatar
South of Boston
Joined Mar 2004
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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.
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Old Dec 15, 2004, 06:42 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Old Feb 04, 2005, 08:02 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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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
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Old Feb 07, 2005, 04:27 PM
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MSC Rödinghausen, Germany
Joined Feb 2001
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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
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Last edited by jberg; Feb 07, 2005 at 04:32 PM.
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Old Jan 03, 2009, 02:43 PM
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salat's Avatar
Novosibirsk, Siberia
Joined Dec 2006
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Hello!
I need constants for Parkzone Cessna motor/gearbox. Has anybody measured them?
Thank you,
Kostya
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Old Jan 03, 2009, 05:24 PM
igi
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Hildesheim, Germany
Joined Nov 2002
130 Posts
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.
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