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Thread OP
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Help!
E flight BL15 840kv vs 950kv vs PZ 480 960kv
I did some swapping with a couple of motors I had laying around trying to get a little more performance out of my PZ T-28.
I took the stock .480 960kv motor off and replacing it with an Eflite power 15. I'm confused because I have 2 Eflite 15 size motors. One from an apprentice S15 E which is supposed to be 840Kv and one that's from an E flite P-47D that is supposed to be a 15 size 950Kv. Everything about them appears to be identical. My end goal is to up the ESC to 40amp (I already have one ready) and run it on 4S, just not sure which motor would have the best chance of survival. From all the reading I'm guessing the 840kv from the apprentice, but then would it be a higher torque motor meaning a prop with more pitch would show more benefit? I've flown both motors on 4S with 30amp ESC and stock prop (9.5X7.5)keeping WOT runs short and cruising in between. Motor was cool on landing and ESC wasn't only slightly warm. I'm just lost on where I should go from here, found lots of threads but nothing that mentions both motors. Can't quite put all the info together that I'm finding before getting lost in it. |
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Thread OP
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Well, maybe I can break things down to what I think might be right.
More winds on each post means more Kv. Given the same dimensions to starting motor, the 840kv motor would likely have thinker wire than the 950kv so the 840kv could handle more current. The 840lv would turn slower than the 950kv with no load but under load the 840kv would have more torque allowing it to swing a larger or higher pitch prop. Does any of that sound correct? (I realize I'm making generalities that may not be applicable to these specific motors) |
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The Kv motor constant is inversely proportional to number of winds: more winds result in a lower (not less) Kv, and vice versa.
Motors only have one Kv. Rule of thumb for a half-decent motor: 3watt/gram.
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Last edited by Ron van Sommeren; Nov 10, 2015 at 04:54 PM.
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Thread OP
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Thank you for the links. You've answered my question in that I now realize I don't have anywhere near the information needed to compare these motors.
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For starters there is the 3W per gram rule. If you take the weight of the motor in grams and multiply it by 3 that is the input power in Watts that the motor should be able to handle when loaded to run at it's continuous rating. The continuous rating varies with any given prop (i.e., load). But it is the input power at the highest throttle setting that gives you the maximum safe temperature in the windings. As you can imagine, the heavier the load is the lower that throttle setting is and vice versa. I refuse to go to Horizon Hobby and to try to find the weights on those two motors because those people are not smart enough to make useful info like that readily available and easy to find. And, besides, they have motors that are the same as the average quality motors that can be bought in a number of other places, the only difference is that the motor are over priced at HH. If your motor is a 150g motor consider it to be a 450W motor. So on 3S you can run it at about 450W/12V or with a prop that draws about 38A. For 4S the numbers would be 450W/16V or 28A. You mention a 9.5X7.5 prop and I go look for that at flybrushless.com's Find a Prop database: http://www.flybrushless.com/prop/search I don't find that prop in the listing there so I choose a similar sized prop: APC E 10x7 - www.flybrushless.com/prop/view/42 And I see that the input power that prop needs can be as high as 420W. That is within the capabilities of your motors so there is a pretty good chance that the APC 10 x 7 Thin Electric prop might not be a bad choice for that 450W motor. The motor's no load RPM will vary with the input voltage. 3S = 12.3V x 840 = 10,332 3S = 12.3V x 960 = 11,808 4S = 16.4V x 840 = 13,776 4S = 16.4V x 960 = 15,744 Typical quality motors will run at about 75% of their no load RPM when running at the their continuous rating and about 90% of the no load RPM at full throttle. And full throttle might have to be time limited to prevent overheating. So we can take those percentages and the no load RPM values and guesstimate what you might get for 75% and 90% RPM with that prop: 3S = 12.3V x 840 = 10,332 x 75% = 7,749 x 90% = 9,299 3S = 12.3V x 960 = 11,808 x 75% = 8,856 x 90% = 10,627 4S = 16.4V x 840 = 13,776 x 75% = 10,332 x 90% = 12,398 4S = 16.4V x 960 = 15,744 x 75% = 11,808 x 90% = 14,170 And now you can take those theoretical RPM numbers and look for similar values in the test data there at flybrushless.com. The thrust will be about the same with any motor at a given propeller RPM as long as the Kv rating and battery voltage are not too badly mismatched. 3S = 75% throttle = 7,749 RPM If you look for values close to 7,749 in the test data you can get an estimate of the thrust you'll get from the prop. It looks like it will be somewhere between these two entries: 7710 RPM = 961g (33.90oz) 7800 RPM = 1154g (40.71oz) And for 4S the numbers would be: 4S = 75% throttle = 10,332 RPM and the thrust would be a little more than the highest value seen in the test data: 10,200 RPM = 1658g (58.48oz) The fact that there is not much test data for higher RPM tells me that the full throttle RPM results for 4S, in our calculations above, might be excessive as far as the recommended RPM limits for the prop and also it raises the input power over that 450W we got with the 3W rule so some careful static testing would be in order to make sure that we do not overload that motor on the 4S battery. As far as the RPM limits, APC's recommendation for Thin Electric props is 145,000 RPM / Diameter (inches) so you should be OK at up to 14,500 RPM with that prop. It is not to say that it will perform well at that RPM, only that it is within the recommended limit. So if you run down the weights on those motors and play around with the numbers, you can get to the point where you are reasonably sure it is safe to try it in flight. Let heat in the motor be your guide for avoiding damage to the motors. When the metal base plate on a motor is too hot to keep your finger on it after a test run up or a short flight, you are at or approaching a practical limit. If the base plates are plastic (as some of the HH motors are) you can't use that rule of thumb though. The heat felt on the magnet housing after a test will be much lower than the temp in the windings. Jack |
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Thread OP
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Thanks for the explanation. Starting to come together.
I'll work on getting a watt meter for a better idea of what's going on. Don't have a scale that will give a very accurate weight for the motor, best I could do is weight it at 5oz which puts it in the ballpark of the 150g motor. Converting from oz give me 142g, of course I suspect it could be off up to .5oz in either direction. Luckily, these 2 motors are just extras and if I lose one in my learning I'm not out much. The lack of info I find on them makes me hesitant to keep them for any serious project. And if I burn one up it will be a good project to learn how to wind a motor! Thanks for y'all's help and related links. I'm think I'm starting to get a grasp on how everything works together. |
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Do your RC equipment, wallet, ego, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding a good setup. Some reading for rainy/windy days. The three links will save you, and us , a lot of questions. They will also prevent you from burning up several controllers and/or motors and/or battery. E-flight 101 by RCG member Ken Myers, will at least save you a ruined LiPo (or worse!), a burnt motor and a fried ESC: The Ampeer → Electric Power Basics followed by The Ampeer → Everything youw wanted to know about e-flight (Ed Anderson) Table of Contents
Calculators, multiple choice winding diagrams, charger selection tools, instructive demo's ... www.scriptasylum.com → RC calculations → electrical → system wiring respectively ... → LiPo pack wiring respectively → chargers (2 menu items) The other menu items and demos are also veeeeery handy/instructive for e-folk, add site to your favourites? Vriendelijke groeten Ron |
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Last edited by Ron van Sommeren; Nov 10, 2015 at 08:20 PM.
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