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Motor tests

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Geoff S04/07/2015 22:17:22
4034 forum posts
68 photos

There was an advert for OS motors on the back of the June RCME. It was one of the better adverts in that it actually told the reader some factual information about the products (most ads tell you zero IMO). I was quite impressed and resolved to consider an OS motor for my next build.

I was particularly interested in theOMA-3825-750w which has a kv of 750 and claims up to 1.327 kw on a 4 S LiPo. I thought it's suggested retail price of £68 not miles out of the way for a quality product so I investigated on Ripmax's website.

For a start the initial spec there says 590 watts and a current of 30 to 40 amps (450 to 600 watts) and suggests looking at a pdf for further information. The pdf is half in Japanese and half in English which is, I guess fair enough for a Japanese product but somehow I expected more from a quality manufacturer. The spec says that the rated current is 40 amps but the maximum is 75 amps for 5 seconds. However another chart which gives current and power for different props goes as far as 15x7 which causes the motor to draw 79 amps (presumably for fewer than 5 seconds!) with a power output of the 1.327 watts in the advert.

I'm now no longer sure about using an OS motor if the data supplied by them is so conflicted. Is it really OK to fit a 15x7 prop or will the smoke pour out of the motor if I do?

It's quite common for there to be quite comprehensive tests of liquid fuel engines in the magazine but how about a few on electric motors? It would be nice to know just what the claims in what I thought initially an excellent advert really mean in practice.

A similar thing happened with the Foxy motor in my Fanatsy. The suggested prop on 4S is a 14x7 but a prop that size draws about70 amps - 20 more than the 50 amps specified as the maximum - albeit for a more reasonable 30. seconds.

I hope the new editor considers electric motor tests so we don't have to find out the expensive way


PeterF05/07/2015 10:08:18
602 forum posts
743 photos

I am not sure the information is conflicting, the advert has really used the marketing version of information stating the maximum possible rather than the continuous rating. The data behind the motor as supplied states clearly that 40 amps is the continuous limit. The information on the RCME states the weight as 195g and a rule of thumb for the continuous power out of a motor of this size is 3W per gram of motor which gives 600W, but you have to know the rule of thumb. The other clue is that the OS site lists the slightly smaller 3820 as equivalent to a 25 sized glow engine, so the 3825 is going to be a 30, you would not expect a 30 size engine to run a 15x7, the only difference in a glow engine, the revs just sag badly when over proposed, the fuel source is limited, a motor will just keep trying to turn a larger and larger prop regardless and the fuel source is not unlimited until the smoke arrives.

Geoff S05/07/2015 11:56:34
4034 forum posts
68 photos

You're righ that the weight versus power rule of thumb suggests that 40 amps and 600 watts is all that can be expected from that particular motor. However, there are many electric flyers at my club who just take a recommended prop, fit it and fly ... and some of those are flying F3A with very expensive set ups.

It seems to be at best disingenuous and at worst downright deception for a manufacturer to publish figures that are potentially fatal to its product. Especially a manufacturer like OS which has a high reputation for glow engines and is trusted implicitly by those making the move from liquid to electric powered models.

I, and seems you, too, am well aware of how electric motors try to behave under load but I'm sure newcomers aren't and could be decieved by the numbers through ignorance (not their fault,, of course). I've been involved in electrical and electronic engineering literally all my life (we had a radio & TV shop when I was a child) but others haven't and deserve better and clearer information. I think magazine articles directly testing specific motors would help. I suppose that would be mainly the medium to high end rather than the inexpensive ones for a start. But it would be interesting to compare cheap and expensive motors that have nominally the same specification.


Bill_B05/07/2015 13:06:09
1146 forum posts
11 photos

I seem to remember Dave Jones's (RIP) QFI magazine did the odd motor review many years ago. The Keller 35/5 seems to stick in my mind. I also think it would be a very useful column for newcomers to electric power as there are so many aspects to understand about brushless motors. Motor diameter, weight, length of rotor, magnet patterns, Kv rating and current rating, not to mention bearings, shafts, or whether to choose an inrunner or outrunner etc.

PeterF05/07/2015 20:18:52
602 forum posts
743 photos


You are right, which is why I called them marketing values. I like the Dilbert cartoon where before he could go into the marketing department he had to remove his soul. I too am an engineer (not electrical) and apologise if some of the reply was egg sucking instructions. More caution is definitely required in electrics, and any newcomers are advised to consider something like ecalc or motocalc to investigate power options.


Geoff S08/08/2015 19:53:12
4034 forum posts
68 photos

I'm converting a DB 58" Tiger Moth to electric. Actually it's a part completed Moth I was given a few years ago that will be elctric on its first flight.

I was looking for a motor which would consume around 500/600 watts on 4S with a 12x6 or 13x6 prop which I think would provide a reasonable scale performance with moderate aerobatics - nothing special.

The motor I chose is claimed to draw 37 amps on a 12x6 and 43 amps on a 13x6.5 when on 4S LiPos so it seemed ideal. The figures are on the company web site.

However, on test it draws over 70 amps on a 13x6.5 and over 60 amps on a 12x6! Totally unsuitable for my needs! As you may imagine the tests were very brief (about 1sec) so there was little time to take accurate readings. It's hardly surprising many find using electric motors somewhat confusing. I'm reasonably competent in electrical matters having earned a living in the field until I retired and I despair, so how people who are totally new manage I can't think.

Unforunately I'm away all next week so I won't be able to chase this up but I have sent a complaint via email so I'll see what happens. No names until I've had it out with the supplier.


John Privett09/08/2015 12:30:59
6131 forum posts
243 photos
Posted by Geoff Sleath on 08/08/2015 19:53:12:

The motor I chose is claimed to draw 37 amps on a 12x6 and 43 amps on a 13x6.5 when on 4S LiPos so it seemed ideal. The figures are on the company web site.

However, on test it draws over 70 amps on a 13x6.5 and over 60 amps on a 12x6!

I wonder what the current draw using a 3S LiPo would be on those props? I'd guess quite close to the figures the supplier claimed for 4S. So I wonder if they've just got their column headings wrong? Not very good though if they have!

Geoff S09/08/2015 12:43:26
4034 forum posts
68 photos

That did occur to me, John, but I was so cheesed off yesterday I didn't bother to check. In both the introductory outline and the performance table 4S is quoted so if there is a mistake it was carried right through the information. Now I've cooled down a bit I might check that out of curiosity

There are electric flyers at my club who never bother to check current but rely on supplier information ... and that includes some F3a pilots who are using very expensive motors on 10S. Consdering how much attention to detail and setting up their airframes for competition that really surprised me.


After you reminded me, I just tried out with the 12x6 still fitted to the motor and it was near to the current specified (about 38 amps) so perhaps you're right.  Still not what I wanted, of course, and the quoted power fits for 4S as well.  I need a big heavy battery to get the CoG somewhere near and my 4S packs are bigger and heavier.  Plus I don't have any 3S pack bigger than 2200.

Edited By Geoff Sleath on 09/08/2015 12:59:17

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