Check the earth / cable connections also!

RP

On 21/06/07, Mark E. Skeels <meskeels@earthlink.net> wrote:
> Thanks all; yes I believe that the magnets have lost power.
>
> >From
> http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdeee=
3_5.aspx
>
> **************************************
>
> /*"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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