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Old May 07, 2013, 09:26 PM
Dir. of Innovation at Alert 5
Demon 6's Avatar
United States, FL, Fort Lauderdale
Joined Jul 2011
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Discussion
To Toss or Not to Toss?

I scorched a Rimfire .10 after many a great flight.
The winding is burnt, not all of them, but enough to kill the motor.
Question:
Can this motor be repaired?
Is it worth it?
Thanks in advance for any guidance!
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Old May 07, 2013, 10:02 PM
Wake up, feel pulse, be happy!
C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
12,544 Posts
Quote:
Can this motor be repaired?
Yes.

Quote:
Is it worth it?
Depends. How much disposable time do you have? It can be repaired for relatively little cash input, but the process is rather technical and time-consuming until you get the hang of it. If all you want is a functional motor, then you'd be better off buying a new one. If you'd like to learn some new skills and end up with a motor that has superior performance, then rewinding is probably for you.

In any case, though, don't toss it. Even if you're not interested in rewinding it, someone else might be.
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Old May 07, 2013, 10:05 PM
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United States, FL, Pompano Beach
Joined Oct 2011
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Do you know the cause of failure? I'm not directly familiar with that motor but I'm thinking the windings may be gooped down pretty good. If the winding's are glued in place that is typically the biggest challenge to the project, getting a clean, undamaged stator out of the factory ~x~x~x~x wire. Take the bell off and see what you see. As long as the magnets have not been overheated and and the rest of the motor is mechanically sound it's worth rewinding, IMO.
I have resolutions to all manner idiosyncrasies of getting these baby's apart with minimal to no damage. But no sense in going into detail until it's known what has to be dealt with.
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Old May 07, 2013, 10:22 PM
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flydiver's Avatar
United States, WA, Seattle
Joined Jan 2007
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Rimfire seriously over rate their motors. Dr. Kiwi has done a bunch of testing on them. If you use the recommended prop sizes....well, mostly not such a good idea.
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Old May 07, 2013, 11:10 PM
Dir. of Innovation at Alert 5
Demon 6's Avatar
United States, FL, Fort Lauderdale
Joined Jul 2011
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Thanks all... The Rimfire over rated issue is, not that it's over rated, it's just over priced.
The Specs are pretty average, but the weight is very nice... I've looked around for a replacement (non Rimfire), but based on weight/kV, have come up short.
Granted, I have only searched summarily through eBay and such... Anyone with a comparable motor suggestion I'm welcome to it.
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Old May 07, 2013, 11:22 PM
2012 NZ Speedcup - 231 MPH
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New Zealand, Tasman, Richmond
Joined Mar 2006
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This motor is the same weight, same KV, and is rated well for the price.

http://www.hobbyking.com/hobbyking/s...strSearch=2836

Not brilliant but can achieve 80% efficiency. Even has a 4mm shaft like your old motor! I've used quite a few of the turnigy 2836 motors and they work really well.

Edit: And they are ~1/3 the price of your Rimfire(d)
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Old May 07, 2013, 11:38 PM
Dir. of Innovation at Alert 5
Demon 6's Avatar
United States, FL, Fort Lauderdale
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Quote:
Originally Posted by DownUnderPilot View Post
This motor is the same weight, same KV, and is rated well for the price.

http://www.hobbyking.com/hobbyking/s...strSearch=2836

Not brilliant but can achieve 80% efficiency. Even has a 4mm shaft like your old motor! I've used quite a few of the turnigy 2836 motors and they work really well.

Edit: And they are ~1/3 the price of your Rimfire(d)
Thank you good sir! This is why I love this hobby!
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Old May 08, 2013, 01:36 AM
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United States, WA, Seattle
Joined Jan 2007
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Generally accepted output for an outrunner - 3W/gm
71gm motor x 3 = 213W
Claimed (see below) 333 cont.

From Tower Hobbies.
Max. Constant Current: 30A
Max. Surge Current: 35A
Max. Constant Watts: 333W
Max. Burst Watts: 390W
No Load Current: 1.2A
Input Voltage: 7.4 - 11.1V (2 - 3S LiPo)
RPM/V (kV Rating): 1250
Weight: 2.5oz (71g)
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Old May 08, 2013, 08:11 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..The Rimfire over rated issue is, not that it's over rated, it's just over priced..."

Actually it is both over-rated and over-priced. That is what you get when you pay for a house branded motor from a larger company. They get their motors from the same motor makers, hopefully enforce some QC standards to the manufacture, stick their labels and paint schemes on them, import them and support them, and that is what you are paying for.

If you are willing to shoulder the responsibility of finding the limits and using the motor without burning it up, you can usually buy three or four motors of the same weight and Kv that are not supported by a national brand or major hobby house.

The 3 Watts per gram rule is a good starting point on the do it yourself approach, and as far as I am concerned, Great Planes fails the test for accuracy and/or honesty in their advertised specifications. Do the math on most of the motors here to see what I mean:

http://www.electrifly.com/motors/gpmg4505.html

Jack
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Old May 08, 2013, 11:32 AM
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Do the math on Electrifly ESC LVC too. After that you probably won't buy them anymore either.
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Old May 08, 2013, 03:41 PM
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Demon 6's Avatar
United States, FL, Fort Lauderdale
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Thanks for the metrics (3 Watt/gr.) I'm going to take this great advise and try to be a tad more responsible. It was my fault for burning the motor, it was spec'd to run an 8x8 (factory recommend) but, I broke a prop and the only sub I had was a 9x9 and the result, as predicted, was letting the magic smoke out.
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Old May 08, 2013, 04:02 PM
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Factory recommended prop on Rimfire is too big.
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Old May 09, 2013, 01:39 AM
Dir. of Innovation at Alert 5
Demon 6's Avatar
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Quote:
Originally Posted by flydiver View Post
Factory recommended prop on Rimfire is too big.
Figures...
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Old May 09, 2013, 11:27 AM
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You seem like you probably understand KV.
In general most motors are appropriately propped when the working RPM at WOT is ~75% of unloaded RPM. You can use whatever "load/prop" you want, which will change the plane performance, but the motor only cares about the load.

As the load goes up it can be measured in amps/watts.The temperature goes up with the load. How well the motor sheds this excess heat is a function of a lot of variables, external cooling being one, wire resistance being another. People rewind to bring the wire resistance down which decreases heat buildup and decreases resistance. This can increase the useable power noticeably.
You can over prop certainly. This is done on a routine basis by fliers thinking they are getting something for nothing. Indeed the power does go up, but so does the heat and the efficiency starts to drop. Much past the 70% mark and your seeming increase in power is actually mostly waste heat. This does show up on a wattmeter as 'power' but a space heater puts out watts too, but doesn't fly very well.

The heat accumulates in the motor core. If not allowed to cool the resistance in the wire starts to climb with the temp increase. At some point it becomes a runaway reaction: heat > higher resistance > more heat > yet higher resistance > fried motor.

So, you can put on a bigger prop. You can wattmeter it and think you are getting more power. But you also need to pay attention to the thrust output vs. the heat gain to see if you are getting what you want.

A good rule of thumb is to run electronics at 80% of claimed specification unless proven otherwise. If you need more power than that you need to consider increasing the capability of the complete power system, or decreasing the load (drag, weight, etc).
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Last edited by flydiver; May 09, 2013 at 11:57 AM.
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