Spitfire fan Posted January 1, 2016 Share Posted January 1, 2016 I do not understand the details relating to the overall electric motor power train. Please can someone help me understand my motor specification and propeller data chart. The motor in question is the OS OMA-3815-1000. The motor specification gives Rated current 35A and Max current 45A, but no mention of the number of seconds allowed at 45A. On the same document is a chart detailing 16 different propellers from a 9x6E drawing 25A and producing 315W up to a 12x6SF drawing 64A and producing 806W. How can it possibly draw 65A when the spec says rated current is 35A ? I would appreciate any help to understand this. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted January 1, 2016 Share Posted January 1, 2016 Hi Peter, the 3815 is rated at 388W. As it uses a 3s battery at 11.1V nominal that would suggest an equivalent current of 35A. Which is entirely in line with what you say above. So we can take 35A as the maximum continuous current the motor can take. Now having said that, it will take more - how much more and for how long? Well a bit more for quite a long time. But while it might take 45A it would only do so for a very short time - literally a burst 5-10secs. Generally you should not plan for "burst levels" of power. You should ensure that wide open throttle equates to 35A or very close. I think the document that is confusing you is this one. The thing about these motors is that they are all basically designed to a common pattern and so its possible to list common propellers across the range and predict the performance of the motor - but they are not suggesting you actually do this! I know, its misleading! You should pick a prop that will give you a current of 35A (approx) - so a 10x7 or a 11x5.5 for example. BEB PS in my experience with OS motors their figures are not tremendously accurate! You definitely need to check with a watt meter! Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted January 2, 2016 Author Share Posted January 2, 2016 Hi. Thanks for the explanation. You are spot on about the document causing the issue. Fortunately I have chosen a 10x7E prop, as I wanted something over 400W, and this should give me 428W at 34A. I will definitely do a check with a watt meter though. Thanks again. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted January 2, 2016 Share Posted January 2, 2016 hello peter--i bought an OSmotor also...and the watts on a given prop as they state is no where near the published figures that you get on the paperwork with them...100 watts less...that is using exactly what they say in the write up........i read on the internet of people complaining about the same thing. ken anderson...ne....1 trade descriptions act dept. Edited By ken anderson. on 02/01/2016 09:58:43 Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted January 2, 2016 Author Share Posted January 2, 2016 Hello Ken. Many thanks for that info. I will carry out a watt meter test asap incase I need to return the motor for a refund. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
Phil Green Posted January 2, 2016 Share Posted January 2, 2016 ...bearing in mind that a cheap wattmeter might only be accurate to within 10%, try two or more and you'll see what I mean! Cheers Phil Edited By Phil Green on 02/01/2016 14:56:49 Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted January 2, 2016 Author Share Posted January 2, 2016 Point taken Phil thanks.. I will be sure to use a couple of watt meters, however an error of 100 watts is likely to be a lot more than 10%. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted January 15, 2016 Author Share Posted January 15, 2016 I've not had time to do for a watt meter test yet on my OS motor but I wanted to ask how the current drawn during a static watt meter test will compare with the current drawn when the plane is flying and hence the prop is cutting through the air. Does anyone have any knowledge how the current might change ? Thanks. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted January 15, 2016 Share Posted January 15, 2016 You can expect the current in the air to be slightly less than in a static test - as the prop unloads as it moves through the air. Now your next question is going to be "how much less?" isn't it! And that is really difficult (read impossible!) to answer accurately. It will depend on the prop and the flying speed and half a dozen other parameters. If I had to put a nominal figure on it I would suggest somewhere around 10% less would be a conservative guess, it might be slighly more - but it might be less! Some folks may have some telemetry data that would tell them - but as you are not using their prop, their aeroplane flying at their speed I don't think it would tell you much of real value! TBH, I never select components on the basis that the static test may be over spec but "it will be alright in the air" - it's too dodgy! I select for permitted max power in the static test and accept that in the air I'll probably get a bit less than that - but at least everything will be running well within spec. BEB Quote Link to comment Share on other sites More sharing options...
Geoff S Posted January 15, 2016 Share Posted January 15, 2016 Posted by Phil Green on 02/01/2016 14:56:01: ...bearing in mind that a cheap wattmeter might only be accurate to within 10%, try two or more and you'll see what I mean! Cheers Phil Edited By Phil Green on 02/01/2016 14:56:49 I have an original Astroflight Wattmeter which cost £50 about 15 years ago and an inexpensive HK one which also has a LiPo checking facity like most do now. I find they correlate quite closely and also incidentally with my Frsky current telemetry read out. I was quite surprised. In any case I think 10% isn't too big an error because readings can vary that much quite quickly as a freshly charged battery voltage drops. Though IIRC my readings were no worse than 5%. I've commented before on the poor data supplied by motor manufacturers (even one as respected as OS - though I suspect they're riding on their glow engine reputation) as regards current draw with a given propeller. They are often wildly out and usually in the wrong direction which makes a method of measuring current, particularly, just about essential. It's amazing how many electric fliers don't bother. In our club that includes F3a pilots who are running some very expensive power trains without a clue as to the current draw. Geoff Quote Link to comment Share on other sites More sharing options...
PatMc Posted January 15, 2016 Share Posted January 15, 2016 A while ago I flew my Druine Akrobat with an Eagle Tree eLogger fitted over a few weeks. A number of results were recorded on the ground and in the air. Most of the flights were normal take-off, aeros, cruise around at reduced throttle etc. However I also tried a few where I kept the throttle full open from T/O, climbing steeply, fast passes up & down the strip, aeros etc not reducing power until ready to land. The difference to current consumption compared with static was very little. In fact it's hard, if not impossible, to tell whether it's just the reducing battery voltage or the prop being unloaded that's causes the current to drop. Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted January 15, 2016 Author Share Posted January 15, 2016 Thanks for the replies. I had hoped that the current drawn in flight would be less the static test but i was not sure, so now I'm happy. In any event I don't want to be flying drawing at or near the max current of 35A, that would not be good. If I can't get the watts I need then I will change motor. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted January 15, 2016 Share Posted January 15, 2016 Posted by Geoff Sleath on 15/01/2016 20:06:40: . . . I've commented before on the poor data supplied by motor manufacturers (even one as respected as OS - though I suspect they're riding on their glow engine reputation) as regards current draw with a given propeller. They are often wildly out and usually in the wrong direction which makes a method of measuring current, particularly, just about essential. It's amazing how many electric fliers don't bother. In our club that includes F3a pilots who are running some very expensive power trains without a clue as to the current draw. Geoff All manufacturers' data regarding amps draw with various props should be treated as a guide only. Same applies to data produced by on-line calculators. Manufacturing variations in the motors and the props, as well as different brands and styles of prop, where that info is not given by the motor manufacturer, can make an enormous difference to the amps draw. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted January 15, 2016 Share Posted January 15, 2016 Yes, I don't find that surprising. Even if there was 10% unloading it would be hard to distinguish from simple voltage reduction due to relatively rapid battery discharge at WOT. I think the best we can say is - if its going change any way its very much more likely to go down - but you shouldn't base component select on the assumption that it will! BEB Quote Link to comment Share on other sites More sharing options...
Andy48 Posted January 15, 2016 Share Posted January 15, 2016 Posted by PatMc on 15/01/2016 20:15:08: A while ago I flew my Druine Akrobat with an Eagle Tree eLogger fitted over a few weeks. A number of results were recorded on the ground and in the air. Most of the flights were normal take-off, aeros, cruise around at reduced throttle etc. However I also tried a few where I kept the throttle full open from T/O, climbing steeply, fast passes up & down the strip, aeros etc not reducing power until ready to land. The difference to current consumption compared with static was very little. In fact it's hard, if not impossible, to tell whether it's just the reducing battery voltage or the prop being unloaded that's causes the current to drop. I have been doing similar tests recently. Exactly as you say, there was very little difference between static loading and in flight loading, and I found exactly the same, with the voltage drop it was impossible to see any difference at all. Using a pitot tube airspeed indicator I was flying at a max of 55mph with a low wing jobby on 4S. It may be different with a very high speed model though. The conclusion from this is that therefore also measuring static thrust must be a good indicator of actual performance, which does seem to have been borne out in practice. Quote Link to comment Share on other sites More sharing options...
Dickw Posted January 16, 2016 Share Posted January 16, 2016 My own data logging experience with high speed models using high pitch props is that the current drops off considerably as the model speed increases (by high speed I am talking about 100 -150mph). For more normal setups, I agree that static measurements are often not far out. Dick Edited By Dickw on 16/01/2016 11:11:16 Quote Link to comment Share on other sites More sharing options...
Spitfire fan Posted February 24, 2016 Author Share Posted February 24, 2016 I have finally carried out watt meter tests on my motor using multiple watt meters, three to be precise, as it was earlier suggested I should do. As it happens I have to report that i got very similar results with all three meters and the overall result was that the motor gave me a lot less power than specified. So I am grateful to Ken and others for their suggestions/comments. This is a great forum. I have returned the motor and without question I was offered a full refund including the return postage, so they could not have been fairer. Regards, Peter. Quote Link to comment Share on other sites More sharing options...
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