Need a new start capacitor for a bench grinder

[larryq]

Hi all,

I was wondering if someone could help me.  I have an old Black and Decker bench grinder I'm restoring, and as part of the project I'm replacing the start capacitor.
The current one is rated 72-86 MFD and 125V.  For the life of me, the only 72-86 MFD units I've been able to find (that are made in the USA) are 250v. 
I purchased one of those based on a recommendation on another board-- was told the voltage was unimportant-- but it's obviously incompatible.  When I powered on the grinder it just hummed.  Putting in a different start cap (125 v as well, but with something like 160 MFD) it started right up.

So I was wondering what is a good, safe replacement for the one I have in the 125v department?  What MFD range is 'good enough' for what I had before?  Also interested in solid brand names and suppliers I can order from.

Thanks much,

Larry

[Seekonk]

Start capacitors operate off a centrifugal switch for only a shory period of time.  It will not be in the circuit long enough to matter.  The higher voltage shouldn't have made a difference.  You likely got one out of spec.  If what you have fits, just use it.

[larryq]

Thanks for the info.  I thought it was strange, that the 250v start cap didn't work, so it probably is defective or out of spec in some way.

[Seekonk]

Centrifugal switches often have issues with contacts and fiber cracking.  If it is working now I would go back and test that cap again.  You can hook up the one in question in series with a 60W lamp.  I just tried this with  35uF and it was plenty bright.  I would assume if it was at least 35uF the motor would do more than just hum.

This test will likely leave the capacitor with a charge.  Short out terminals before handling.

[sarepairman2]

have you considered a pull string?

[Zero999]

The voltage rating on a capacitor is the maximum allowed voltage. Normally this is given as a steady DC value but in the case of motor start capacitors, it's AC. In short a 250VAC capacitor can be used in any application for 250VAC or less. If a 250VAC capacitor with the same marked capacitance didn't work, then the capacitor is faulty.

[TerraHertz]

Is it _actually_ a start capacitor, with a centrifugal disconnect switch? Or is the motor the type that runs with a capacitor permanently connected to one of the windings?

In the first case, the value won't be too critical. In the latter, better try and get it about right.
Also, are you sure the old cap was original? Sure someone didn't swap in a cap that was whatever they had?

Oh, and air conditioning spare parts outlets tend to carry a lot of motor start capacitors.

[albert22]

Most bench grinders that I have seen do not have a centrifugal switch. I you hear a clicking sound especially when the motor slows down to a stop, that is the centrifugal switch. But AFAIK, you only find CF in motors that require more torque than a bench grinder.
There are tables that tell you the right capacitor once you know the motors HP.
For example http://www.electricneutron.com/electric-motor/single-phase-capacitor-sizing/

EDIT
please discard the linked url it is very bad. Sorry

[larryq]

Thanks for the replies everyone.  In no particular order; yes, it is a start and not a run capacitor-- it's the cylindrical size typical of start caps and has the similar tangs you see on them. 

It's almost certainly original to the grinder, as it was wrapped in a cardboard covering and looked as if it was from the 1950s, both in appearance and condition.

As for the centrifugal switch, it's a bit unusual in that there's a spring and brass bell on one of the rotor arms that expands and touches a copper ring attached to the bell housing that has two wires leading to the capacitor.  I'm not an electronics expert and can't say with authority how it all works but there's obviously a centrifugal component to it all.  I'll try to post some pics later tonight as my description is probably more confusing than enlightening.

The grinder itself is rated at 1/3 HP and 4.5 amps, all cast-iron.  This was Black and Decker in the old days when they built quality products that could last decades.

[station240]

These motor capacitors are +/- 10% or sometimes even 20%, so slight differences aren't such an issue.
Not sure why you need double the capacitance though.
Try it with the 160 MFD and see if there is any sign of overheating that could indicate a shorted winding in the motor.

Somewhere I still have an old Black and Decker drill of the same vintage. It's practically indestructible, so I can see why you would want to fix your grinder.

[profany]

Have u consider pull string

[cimmo]

Just give it a flick/spin with your fingers before you turn it on.
My father had a bench grinder that must have once been used by Noah. I don't think it even had a start capacitor.
When turned on it wouldn't rotate, it would just quiver and hum while all the lights dimmed.

Give it a flick and it spins up and all is good.