Motor de-mag?

[Rumdog]

What causes a motor to de-magmetize? I recently had this happen and am trying to figure out why. Setup- sprintcat 34" Feigao 580 8L. 5s2p 25c 5000mah lipos. Seaking 180. m445 prop. COG at 33 percent. Strut was 1/2 inch up from sponson bottoms and neutral. This was on my maiden voyage. All electronics were new. Ran for about 2 mins and lost 90 percent of the power. Motor came in super hot, batts at 90, esc at 94.

[Diegoboy]

motors use a variety of permanent-magnet materials. Early designs employed ceramic or ferrite and AlNiCo magnets. These materials are still widely applied, however, in automobiles and other areas where low cost as well as reliability is important. Newer designs use rare-earth samarium-cobalt and neodymium magnets.

Most magnets have stable magnetic properties within the normal operating temperature range of the motor. But some magnets have a higher temperature coefficient than others. High temperature-coefficient magnets may become too weak if operated at high temperatures for extended periods. Depending on the magnetic material and slope of the motor's magnetic circuit, torque degradation may result over a wide temperature range.

Ceramic or ferrite magnets lose about 0.13% / °C of their remanence above 25°C, while rare earth and AlNiCo may lose only 0.03% / °C. But this loss is generally reversible if the temperature is kept within the motor rating. Colder temperatures are seldom a problem. Since the coefficient curve is linear, magnets are stronger at lower temperatures.

Some grades of rare-earth magnets are more sensitive to temperature than others. Magnets in the neodymium family may have irreversible magnetic losses under wide temperature changes. These magnets have the highest maximum-energy product (MEP), a figure of merit, of any commercial magnet now available. High MEP comes at a premium and should not be lost to temperature extremes. Neodymium magnets are continually being improved with lower temperature coefficients to make them as stable as other rare-earth grades.

Peak loads applied to AlNiCo and ceramic dc motors can degrade their magnetic properties. AlNiCo motors have a peak current rating which usually corresponds to a point above the knee of the B/H curve. Current exceeding this rating, caused by either a current spike or a constant dc input, are over the knee and cause permanent demagnetization. A demagnetized AlNiCo motor may only provide 50 to 60% of its original torque. Fortunately, rare-earth magnets are not as sensitive to demagnetization as AlNiCo and ceramic.

Hopefully this solves the riddle for you.

Source

[Rumdog]

Thanks Diegoboy

[Diegoboy]

Basically... it got too hot.

[Rumdog]

Yeah, now I just have to figure out why.

[sailr]

You want bacon with your eggs? Probably could've fried an egg on that thing!

[Rumdog]

Oh yeah! It was suuuuper hot!! Even the outside of the water jacket was hot enough to burn!

[Jeepers]

Yeah, now I just have to figure out why.

I had a motor that came back like that, a feiago 6L ,when I took it apart the rotor was cracked down the center.