What size capacitor do I need? Formula?

[Dudley]

I have a bench grinder with a two(2) Horsepower AC electric motor. It
is a single phase, 60Hz, 110 volts/14amps, or 220volts/7amps, 1720rpm.
It appears to have only one capacitor (approx. 3 1/2" long and 1 1/2"
diameter)but their is no information on the capacitor. I believe this
capacitor is bad. I would like to know what size/value capacitor I
should get to replace the bad one. I can wire it for 110 volts, or for
220 volts which ever would be better. If someone has the formula for
figuring this I would appreciate your posting it.
Thanks

 

[Dudley]

John Popelish <jpopelish@rica.net> wrote in message news:<40678370.12D25F1@rica.net>...

Dudley wrote:

I have a bench grinder with a two(2) Horsepower AC electric motor. It
is a single phase, 60Hz, 110 volts/14amps, or 220volts/7amps, 1720rpm.
It appears to have only one capacitor (approx. 3 1/2" long and 1 1/2"
diameter)but their is no information on the capacitor. I believe this
capacitor is bad. I would like to know what size/value capacitor I
should get to replace the bad one. I can wire it for 110 volts, or for
220 volts which ever would be better. If someone has the formula for
figuring this I would appreciate your posting it.
Thanks

At that size, the capacitor is almost certainly a starting capacitor
(nonpolarized electrolytic), not a running capacitor (oil filled paper
or film capacitor) (also implying the motor has a centrifugal switch,
inside, that opens the capacitor circuit after the motor gets up to
speed). Are there no markings on your capacitor that might be useful
to determine its replacement?

Have you checked the condition of the switch contacts to make sure the
existing capacitor is being connected?

Thanks for the information John. No there are no markings on the
capacitor, the outer cover has deteriorated. I haven't checked the
condition of the switch contacts. I suppose I can find the switch by
tracing the wires from the capacitor?
Dudley

 

[John Popelish]

Dudley wrote:

Thanks for the information John. No there are no markings on the
capacitor, the outer cover has deteriorated. I haven't checked the
condition of the switch contacts. I suppose I can find the switch by
tracing the wires from the capacitor?
Dudley

For a single voltage motor, I would think so, but dual voltage motors
are a lot stranger, using motor windings as transformer allow a single
voltage capacitor to work with two line voltages. So, who knows? The
switch would be located in one of the motor end bells, with some sort
of centrifugal weight fly ball mechanism and usually some sort of
thrust bearing to convert rotary motion to an axial switch motion.

As to guessing the original capacitance, I would take the old one to
the dealer and ask what they have that is about the same size,
physically (the Farm Bureau sells a good assortment of them, around
here). The value does not have to be precisely right to start a
motor. You could also experimentally find out what is just enough
capacitance to get the motor turning by connecting various oil filled
capacitors in parallel and then purchasing an electrolytic motor start
cap that was a little bigger than what worked. I have used this
method, but I have a box full of oil filled AC capacitors laying
around.

--
John Popelish