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Old Feb 24, 2012, 05:11 AM
bryansifsof44's Avatar
United States, AK, Anchorage
Joined Oct 2011
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Quote:
Originally Posted by JetPlaneFlyer View Post
Roger,
I dont own any 5s batteries so yes I'm sure I've repeated the tests several times and even recorded with a data logger so i'm sure of the numbers I'm getting. ESC used did make a significant difference but even with an underrated ESC (Castle 50A) I was pulling approx 980W and 64A with a Xoar 14x7... The data logger info is burried in this thread somewhere.

Bryan,
I did measure the kv of my motor at 748kv but the inacuracies of the method used (Castle data logger) would easily mean I could have been out by 25kv or so in either direction. Apparently the ESC used can also skew the kv results a bit. Most accurate method for measuring kv is to spin the motor with a drill or similar and measure voltage generated but I've never had the energy or time to do that. I'm quite happy to accept that my motor is 'approx' 720kv.

Regarding over-amping. I'm mainly concerned about the motor. With the Xoar 14x7 and the ZTW ESC and Gens Ace 30c battery I'd be well over 70A. Ken at Subsonic told me that 60A shouldnt hurt the motor but I suspect 70A is getting into the territory where something could give. In any case I've already got way more power than I need. Even with the APC I'm well over 300W/lb which is tons of power in anyone's book. No point risking the motor in search of extra power that I dont need.
Thanks for the reply, I'm getting excited to install mine it should be here tomorrow. I'll try to run a test or two tomorrow night. I have the ice 75 coming too, which I am kinda interested for the inflight readings vs static testing. Also, props are more durable and less then half the cost of the Xoar.
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Old Feb 24, 2012, 07:51 AM
Just flitting about!!
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Thanks Steve.
Bryan- looking forward to your results.

I have the local Motrofly agent ordering some fresh motors direct from the factory and I've asked him this time to run it up and test the kV himself before sending it to me.

Like I said, I'm sure I've got something closer to 630kV from some old stock as when I asked about the batch/record keeping etc regarding the motor they sent me they went a little vague and then suggested a fresh one from new stock.

They say they have a test motor on their bench rig to test that apparently has actual 728kV but they haven't run it on 4s and 14x7.
On 5s and 13x6.5 it had over 800W and on 15x6 and 4s it was less than 600W, which would mean the motor was overloaded by that prop size.

I'd rather just get the kV you have Steve, throw in a 60-70A esc and manage the throttle to get best performance/ flight time out of the setup.
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Old Feb 24, 2012, 07:57 AM
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Aberdeen
Joined Mar 2006
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Originally Posted by bigroger View Post
on 15x6 and 4s it was less than 600W, which would mean the motor was overloaded by that prop size.
Electric props dont 'overload' (a.k.a. 'bog down') and produce less power in the same way as an i.c. engine does.. If you put an oversize prop on an electric motor it will just pull more and more amps. Amps will keep increasing as you increase the prop size right to the point where the motor burns out.
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Old Feb 24, 2012, 08:21 AM
They Call him Dead!
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United States, SC, Pawleys Island
Joined Jul 2003
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Quote:
Originally Posted by JetPlaneFlyer View Post
Electric props dont 'overload' (a.k.a. 'bog down') and produce less power in the same way as an i.c. engine does.. If you put an oversize prop on an electric motor it will just pull more and more amps. Amps will keep increasing as you increase the prop size right to the point where the motor burns out.
YES. I'm beginning to suspect some other problem here??
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Old Feb 24, 2012, 08:24 AM
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United States, OH, Groveport
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Originally Posted by dead View Post
YES. I'm beginning to suspect some other problem here??
solder joints?
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Old Feb 24, 2012, 08:31 AM
Team Hillbilly (UK)
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United Kingdom, England, Cambs
Joined Oct 2011
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Quote:
Originally Posted by JetPlaneFlyer View Post
Electric props dont 'overload' (a.k.a. 'bog down') and produce less power in the same way as an i.c. engine does.. If you put an oversize prop on an electric motor it will just pull more and more amps. Amps will keep increasing as you increase the prop size right to the point where the motor burns out.
I have found that there is a fine balance when on the ragged edge of a motors capability - if you pull more current as you prop up - what happens is that you will start to exceed the thermal efficiencies of the motor and its winding that as well as the ESC.

When this happens you start to generate more heat than mechanical movement...also the magnetic flux of the field starts to collapse causing this bog down effect...when you overload an electric motor...

Remember a motor has a finite limit of where it can effectively operate based on a number of factors - such as wind - right down to the grade of copper you use....etc when this is reached the motor starts to not become a motor
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Old Feb 24, 2012, 08:36 AM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
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Steve and roger, care to post pictures of your windings? I would be curious to see how they compare to mine.
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Old Feb 24, 2012, 08:42 AM
Just flitting about!!
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Originally Posted by stgdz View Post
Steve and roger, care to post pictures of your windings? I would be curious to see how they compare to mine.
Sure mate, as soon as the sun is up I'll get right on it.

Last thing I want is the new motor the guy is going to send me to be tested by him with a 730+ actual kV and me run it up with 14x7 4s and still get crapy numbers.
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Old Feb 24, 2012, 08:47 AM
Grumpy old git.. Who me?
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Aberdeen
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Originally Posted by Space Bat View Post
I have found that there is a fine balance when on the ragged edge of a motors capability -
I'm no expert..
What I've seen is yes, the efficiency does go 'down the pan' as a motor overloads, and the mechanical power you get out does almost plateao.. However the electrical power input does continue to increase as you load the motor up, the wasted energy comes out as heat and eventually the motor burns out. I've never observed or heard of a motor where putting on a larger prop and loading up the motor more produced a fall in input power?

I stand to be corrected if it can occur....

Steve
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Old Feb 24, 2012, 08:59 AM
Not as Good as The Kid
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South Pasadena, FL
Joined Sep 2009
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I suggest that we all need to start taking RPM readings and reporting them with the rest of the data – all the time. If you want to measure kinetic power, you need to know the relationship between your prop pitch and diameter, and the RPMs. By doing this, we will all know what is happening at the motor shaft and discuss things in a relative way. JPF's motorcalc image shows that he is getting 712 Watts-out at 8730 RPMs. If you do the math, you’ll see that the motor is a 720Kv operating at a 79.35% efficiency on a 14x7. My calculations say about 724W, but that’s close enough because I’m estimating 3.82v per cell.

The 980W is a representation of the Watts-in, not the kinetic power. By tell us what your RPMs are, the make of the prop, the pitch, and the diameter, we can all rate motors in a relative way. Believe me... it will make all this data more valuable to everyone. So far, I have read nothing here to suggest major differences in motor performance, but we really need to see Amps, volts, RPMs, along with the detailed motor and prop specs and everything else will fall in line.
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Old Feb 24, 2012, 09:04 AM
Team Hillbilly (UK)
Space Bat's Avatar
United Kingdom, England, Cambs
Joined Oct 2011
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Quote:
Originally Posted by JetPlaneFlyer View Post
I'm no expert..
What I've seen is yes, the efficiency does go 'down the pan' as a motor overloads, and the mechanical power you get out does almost plateao.. However the electrical power input does continue to increase as you load the motor up, the wasted energy comes out as heat and eventually the motor burns out. I've never observed or heard of a motor where putting on a larger prop and loading up the motor more produced a fall in input power?

I stand to be corrected if it can occur....

Steve

You are quite right on the motor side of things

Sorry wasn't clear as was only speaking really about the possibility of the motor bogging down...

However the ESC may bog down with same rules regarding heat - if the motor is hot the ESC wants to dump more power in, until itself runs hot then the voltage will drop dramatically from the ESC output thus will trigger a cut off - or ESC burnout - possibly in the instance of dropping the input power maybe the ESC operated via a safety circuit and dropped the timing Cant think of a credible answer why input power would drop when switching to the bigger prop in Rogers instance where mfr tested the motor...other than the ESC was clipping the power via a protection method
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Old Feb 24, 2012, 09:06 AM
Not as Good as The Kid
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South Pasadena, FL
Joined Sep 2009
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Quote:
Originally Posted by JetPlaneFlyer View Post
I'm no expert..
What I've seen is yes, the efficiency does go 'down the pan' as a motor overloads, and the mechanical power you get out does almost plateao.. However the electrical power input does continue to increase as you load the motor up, the wasted energy comes out as heat and eventually the motor burns out. I've never observed or heard of a motor where putting on a larger prop and loading up the motor more produced a fall in input power?

I stand to be corrected if it can occur....

Steve
And this is good point, because we can talk about this better when we see RPMs. I have seen evidence where going from a 14x7, to a 14x8.5, to a 15x6, reduced the RPMs to the point where the pitch speed, and static thrust values were reduced, and the Watts-out was as well. I have also seen where too large of a prop increased the heat to the point where the Watts-out sagged considerably to the point where the prop lost its significance for this reason. Watts-in and Watts-out is something we need to address and JPF's motorcalc images show how wide the difference can be... 260W difference between in and out.
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Old Feb 24, 2012, 07:44 PM
Just flitting about!!
bigroger's Avatar
Joined Aug 2011
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Guys, photos of the windings of my DM2820-750 motor.

I did some more runs today playing with timing and PWM settings with three difference speedies and still only changed the rpm readings buy 100-200rpm at wot, so very little noticeable difference.

Still only peaking at 695-700W wot on fresh lipo and settling to around 650W after 15 seconds on 4s and vox 14x7 prop.
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Old Feb 24, 2012, 07:59 PM
Team Hillbilly (UK)
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United Kingdom, England, Cambs
Joined Oct 2011
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Hi roger...

Silly things can niggle at times...but in reality should it matter?

If the plane flies great - gives you the power on demand as you want and does all the things you desire, should the missing 40kV or whatever it is really be an issue?

Perhaps enjoy the plane and forget about it or you will just end up pulling hair - until you either replace the motor with something else or get a Motorfly replacement of the desired kV - but will it change the dynamics of the flight envelope you are capable to fly in?
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Old Feb 24, 2012, 08:10 PM
VOLTS > AMPS
stgdz's Avatar
United States, MN, Buffalo
Joined Jul 2011
3,866 Posts
yeah thats a little different than mine, wonder what JPF and AP's winds look like


these are mine but its custom wound to 600
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