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Analyzing the failure of GWS 350

Old 02-25-2004, 12:30 AM
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zagibond
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Default Analyzing the failure of GWS 350

Was wondering what type of failure you were experiencing with your GWS350 motors.

Every one of my many burned up motors has been caused by one of the brush arms breaking off. It is usually pretty near the start of a flight. My theory is that they get hot and when they cool down, become brittle. I have thought about some how trying to dissipate the heat on the arm. Perhaps a drop of CPU grease would work?

Why don't the manufacturers build a slightly more robust motor in the same can size? I don't need the extra power or efficiency from an expensive brushless setup, just better reliability.
Old 02-25-2004, 07:05 AM
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Peter Khor
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Default RE: Analyzing the failure of GWS 350

CPU grease does nothing to cool down an item; it's a thermal interface medium. As for robustness, in E-flite we're pushing the motors way beyond the limits of their original intention. Want robust? Go brushless So anyways, within the limits of what these motors can give us, we're getting a pretty good deal overall. fwiw, in the S400 arena, I've found that the S400 LongCan's are a cheap and nice upgrade in terms of durability and power upgrade.

fwiw, here's my take on EPS300/350 failure mode:

http://www.rcgroups.com/forums/showt...34#post1635534
Old 02-25-2004, 06:49 PM
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clively
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Default RE: Analyzing the failure of GWS 350

Great dissertation on the motors!
Old 02-25-2004, 11:29 PM
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zagibond
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Default RE: Analyzing the failure of GWS 350

Thanks for the thread which seems to indicate that the dominant cause of failure was the brush wipe breaking off. I realize that we are pushing these little motors pretty hard, but since the known weakness is the brush arm why not attempt to improve it? My thought on the grease was that since it is such a good thermal conductor and since it has substantially more volume than the arm, it might be able to dissipate some of the heat away from the hotspots.
Old 02-26-2004, 01:28 AM
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clively
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Default RE: Analyzing the failure of GWS 350

Hi Zagibond,
Generally speaking brush wear is caused by two factors, the first is normal friction wear. The second, and more important, is wear due to the electricity arcing between the brushes and the commutator. Arcing causes pits to develop in the brushes. The more pits you have, the more arcing, the more wear, ad infitum until total brush failure. Basically, the electricity just tears the brushes apart. This is why the brush arm snapped. The brush wore to the point that the arm was in contact with the commutator. Skipping, etc, caused it to finally bust.

In order to reduce arcing most people use some method to break in their motor which forces the brushes to wear evenly at a low voltage. The lower voltage during breakin reduces arcing until the brushes are seated. By seating the brushes with few or no pits then brush life can be greatly extended. The best way to break in a motor is to hook up a second motor to it to turn the shaft without directly applying voltage (no chance for arcing); the second best is to use just enough voltage to turn the motor and cause friction wear.

The heat grease will have at best case no effect, at worse case cause other problems. Although it is a good heat conductor, if there is nothing on the other side it will simply store heat quickly and be another source of heat for the brushes/can. In addition, the grease may interfere with the electrical conductivity between the brush and commutator which could cause either additional arcing or simply being an insultator between the brush and commutator. Either way you'll see decreased motor life or a major performance drop.

The heat problem with brushed motors is that the heat has nowhere to go. It's stored in the copper which is rotating in the center of everything. It has a couple of exit points, the air around of the windings (as it spins), and the iron core laminations. Both of these eventually lead to the can itself. By simply attaching a common heatsink to the motor you can help reduce the heat. Most hobby shops carry heat sinks. To help reduce heat make sure the motor area has plenty of airflow.
Old 02-26-2004, 01:22 PM
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zagibond
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Default RE: Analyzing the failure of GWS 350

Thanks again for the very informative explanation. Instead of re-inventing the wheel, I will follow your lead since you aparently have done your homework. Incidently, my brush arms break off way before they touch the commutator.
So for a 3DX what is the best available brushless? I can't get my hands on a razor, nor a Himax 2015-4100.
Do brushless motors wear out? I have read that the magnets eventually loose their polarity?
Old 02-26-2004, 02:01 PM
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clively
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Default RE: Analyzing the failure of GWS 350

As to the best brushless for that system, I'll defer to someone else. To get you started, you need to know the weight of the plane and the type of flying you do. This info will help narrow the field considerably.

As to the magnet question,
magnetic material like Neodymium and Samarium Cobalt have temperature points at which the magnetic strength drops or entirely dissapears. For Neo this is usually around 80 Celcius; for Cobalt it is around 250 C+ (some as high as 350C). Bear in mind most wire insulation fails around 80C. This is the permanent strength loss. All magnets have a temporary strength loss as heat rises. Neo and Cobalt are very resistant to temporary loss. By comparison brushed motors typically have ferrite magnets with an 300C temp ceiling, but they lose more power per degree of temperature than the others.

Impact can cause a magnet to lose strength as well, but it usually takes a pretty big force and would probably destroy the can or commutator first.

The only wearable parts will be the the point where the shaft makes contact with the can. This is through bearings or bushings. Keep any bushings properly oiled (very very lightly, just a drop) and follow the manufacturers directions for bearings. Some bearings require oiling, some are sealed in a way that oil could destroy them. A new bearing runs anywhere from $6 to $12. I think most of the brushless motor manufacturers use bearings anyway.

With all of this in mind, brushless motors can last a very very long time if: 1. Cooled with proper ventilation and a heat sink; 2. not kept running in a stalled condition in which heat rises dramatically; These conditions apply to brushed motors as well.


Hope that helps.
Old 02-26-2004, 07:30 PM
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zagibond
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Default RE: Analyzing the failure of GWS 350

Thanks again Chris,
Have you ever looked at the Astro 280BB motor?
It looks like the brushes can be replaced. It looks quite heavy.
Old 02-26-2004, 08:00 PM
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clively
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Default RE: Analyzing the failure of GWS 350

I think that motor also goes under the names of Multiplex 280BB, Simprop 280BB, Permax 280BB, Acro 280BB, and even as the Great Planes S-280BB.

Prices look to be anywhere from $22 to $40 depending on the name. I couldn't locate anyone actually selling the brushes (google in 30 seconds or less..), but I hear they are around $7.00 a set.

Personally, I'd either go brushless or simply buy the Graupner Speed 300 aka GWS 350. The motors are so cheap that it would take burning 3 motors to make up the difference in brush prices.

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