On Sat, 15 Aug 2020 10:20:08 -0700, Paul Drahn
wrote:

I suspect your motor has a single capacitor and is a split-phase motor.
Cap start, cap run requires two capacitors and the mechanism to switch
between the two.

A grinder has no reason the have a heavy load when it starts, which is
the reason for motors with two caps.

Paul

Agreed. However, the official name is a "Permanent-Split Capacitor
Motor" which has one permanently connected run capacitor. The "Split
Phase" motor has no capacitors. See:

Two-winding Machines
https://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html

Permanent-Split Capacitor Motor
If run efficiency and vibration are important, but start
torque can be compromised, the capacitor can be left in
the auxiliary circuit at all speeds. Sizing the capacitor
to provide balance at a particular load point, the
backwards field can be eliminated, improving efficiency
and eliminating torque pulsations. Eliminating the
centrifugal switch can reduce the manufacturing cost
significantly. The trade-off is lower starting torque,
since the capacitor is not sized to provide balance at
starting, but for run conditions

The design has the disadvantage of having low starting torque, which
is the cause of the slow start. At 80+ years old, I would guess(tm)
that the capacitor is as dead as the bearings that were replaced. As
it gets old, the ESR starts to climb which I presume was the cause of
the observed heating.

Drivel: My experience with motors was mostly with industrial sewing
machines at my father's lingerie factory. At the time some of the
older motors would exhibit the symptoms of a shorted winding. What
was happening was that some motors were not designed to handle
sustained high temperatures. The copper motor wire would expand
slightly, cracking the old enamel insulation. New enamel was quite
flexible, but old enamel became brittle. My guess(tm) is it took
about 40 years to cause problems. Insulation failures were mostly
around sharp wire bends. I think varnish insulation did the same
thing, but at the time, I couldn't tell the difference between varnish
or enamel insulated motor wire. Therefore, when working with really
old motors, I always look for loose insulation flakes, which might be
an indication of impending shorts.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558