Dennis Miles via EV wrote:
David, and others, the most fragile part, of a series DC motor, is the commutator. And that is, because the copper segments are glued to an insulating core. When the current is flowing, the segments heat up and soften the glue, then they are subjecting to "lifting," away from that insulating core, and causing problems with the brushes, resulting a need for an extensive repair. But, with no (or very low) current, the glue maintains its strength, and the motor can be spun to a higher RPM, without damage.
Relatively new DC motors are built with commutator bars that are glued in place. Older and heavy-duty motors instead have "steel" commutators that don't depend on glue to to hold the bars. Instead, there are steel rings at each end hold in the bars. They use mica or other materials to insulate the bars. This type of commutator is much stronger, and better able to withstand high temperatures and high RPMs.
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