....been there; done that. Richard Prosser wrote: > Check the earth / cable connections also! > > RP > > On 21/06/07, Mark E. Skeels wrote: > = >> Thanks all; yes I believe that the magnets have lost power. >> >> = >>> From >>> = >> http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee3/bdee= e3_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 >> >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> >> = > > = -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist