Re: Sizing a Bench Grinder Capacitor



James Sweet <jamessweet@xxxxxxxxxxx> writes:

me wrote:
Sam Goldwasser <sam@xxxxxxxxxxxxxxxxxxx> wrote in
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"leoJ" <JoelHSmith@xxxxxxxxx> wrote in message
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Homer J Simpson wrote:

"leoJ" <JoelHSmith@xxxxxxxxx> wrote in message
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Have a bench grinder that was given to me and runs fine if you
spin it up by hand - the capacitor is missing. Is there a way to
determine what size/part no. capacitor will work with a 115V
60Hz Single Phase motor?

HP?

3/4 HP, 3600 RPM


Likely a motor start capacitor.
http://www.cde.com/new/catalog/motorstart/

CDE motor start capacitor catalog page (download this - right mouse
click - Adobe Acrobat)
http://www.cde.com/catalogs/PSU.pdf

I am looking at a NOS Mallory motor start capacitor on my bench --
it is rated at: 108-130 mfd at 250 VAC

Application Guide
http://www.cde.com/catalogs/AEappGUIDE.pdf

If it's used only for starting.

Easiest test would be to spin it up by hand and then use a multimeter
to check for voltage across the wires where the cap would go. If you
see full line voltage, it's a run cap.

snip waste text

Most bench grinders I've seen use a motor run capacitor.

But if he has to spin it to start it, that would imply it's just a
starting capacitor. Can you hear an audible click as it picks up speed
and again just as it slows to a stop?

No, a split phase induction motor with an open or missing cap will not
start regardless of whether it is a start or run cap. Spinning by hand
will get it going (assuming minimal load).

An induction motor designed to have a run cap will still run without one
if started manually but won't have rated torque.

An induction motor designed to have a start cap will still run without
one if started manually and will have rated torque.

The main functional difference is that the start cap is not rated for
continuous operation and is removed from the circuit once the motor
reaches about 75 percent of operating speed.

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