Thanks all; yes I believe that the magnets have lost power.

>From =

http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdeee3_=
5.aspx

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/*"Magnet properties:*/ Dc 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% / =B0C of their remanence =

above 25=B0C, while rare earth and AlNiCo may lose only 0.03% / =B0C. 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."

*******************************************

Mark


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