On Tue, 07 Feb 2017 17:35:22 -0600, Jon Elson
wrote:

whit3rd wrote:

On Friday, February 3, 2017 at 3:28:30 PM UTC-8, wrote:

I have an old Baldor bench grinder. Probably 70 years old or older.
The motor is the type that uses a capacitor

I decided to run some numbers. The moment of inertia of the 8 inch
grinder wheels is approximately 3.16 times the moment for the 6 inch
grinder wheels.

The rotor is probably denser than the wheels, it might dominate.
One quick way to test a 400uF 330VAC capacitor, would be
to put a smaller known-good capacitor in parallel with it. If that
makes a big difference, the old capacitor needs replacement.

Look for a date on the capacitor (and if you replace it, write a date on
the new one); tracking the history sometimes simplifies things.
I seriously doubt the run capacitor is anywhere NEAR 400 uF. It would be as
big as a toaster in 1950 technology. Maybe 40 uF, but even that sounds big
for a run cap.

Jon
Greetings Jon,
I have a box full of oil filled capacitors. They are all the oval
shaped type and fairly modern. None are more than 50 mfd. I was
getting suspicious of what I wrote and your post convinced me to take
another look at the old cap in the motor base. You are correct of
course, 470 mfd would have been bigger than the grinder. Maybe bigger
than ten grinders.The cap is 4.0 mfd. What I took to be the number 7
was just corrosion on the old can. How could I have expected the old
cap to be 470 mfd? What was I thinking? Just goes to show,, "If you
can't think too good you shouldn't think too much".
Thanks,
Eric