Problems with old motor cap?

[et...@whidbey.com]

Hey All,
I have an old Baldor bench grinder. Probably 70 years old or older.
The motor is the type that uses a capacitor for starting and running.
There is no centrifugal starting switch. I know this type of phase
making scheme provides lower torque for starting so that the motor
takes longer to come up to speed. But this grinder takes 5 seconds to
reach operating speed, 3400 rpm. I replaced the bearings in the motor
and it now spins very easily. There is a 3/4 wide 6 inch grinding
wheel on one side and a light weight 6 inch diamond wheel on the other
side. The cap in the thing might be the original cap. It is a large
round tinned metal can with solder lugs. 330 volts and 400mfd. Could
it be that it needs a new cap? I don't know what kind of cap it is.
Oil filled I imagine but from the high mfd campared to all my other
run caps could it be an electrolytic cap? There is no sign of leaking
and there are two places on the top of the cap where it looks like
holes were sealed with solder.
Thanks,
Eric

[pf...@aol.com]

On Friday, February 3, 2017 at 1:00:51 PM UTC-5, et...@whidbey.com wrote:

http://store.eurtonelectric.com/capacitorcd60-850uf300vac-1-1-1-1-1-1-1-1.aspx


You will need an AC-rated motor-start capacitor. But, typically, such a capacitor provides the initial *OOMPH* to get the initial turn on the motor, after which it more-or-less goes away. If the motor starts without difficultly, even if it takes a while to spool up, the cap's work is done in the first few second or so.

If you do replace it, DO NOT use a conventional DC cap, even two caps back-to-back. Non-Polarized does not equal AC rated.

This may also be a Motor-Run capacitor, but in that case, there should be two, a START and a RUN. Runs are almost never electrolytic types. Again, it must be AC rated.

Note: If a start cap, the motor will not start without it. If a run cap, as it weakens, the motor will run improperly or not make speed at all. And if not there, the motor will not start at all as the cap is required to "rotate the field".

Peter Wieck
Melrose Park, PA

[et...@whidbey.com]

On Fri, 3 Feb 2017 10:17:20 -0800 (PST), "pf...@aol.com" <pf...@aol.com>
wrote:

The cap is a start AND run cap. It always stays in circuit and is a
compromise between the proper phase shift for starting and for
running. If it gets replaced it will be with an AC rated cap. I'll
make sure to get a motor run cap too.
Thanks,
Eric

[Massoud]

et...@whidbey.com wrote in news:qdg99cd6t1dchfcdv...@4ax.com:

You have a good start up, so no need to replace the cap.

[et...@whidbey.com]

On Fri, 3 Feb 2017 19:34:45 +0000 (UTC), Massoud <mas...@yahoo.com>
wrote:

5 seconds seems like too long. My other two Baldor grinders spin up to
full speed in 2 seconds. These grinders don't have a centrifugal
starting switch either so I assume they are wired just like my older
grinder. I'm concerned the long starting time might lead to
overheating the windings after many starts.
Eric

[Massoud]

et...@whidbey.com wrote in
news:hip99c55ea73p0cso...@4ax.com:

First concern should be loosing the torque, meaning slows down when
loaded. They are AC cap and no polarities. Generally, oil filed can which
if they don't leak, survive many many years.

The start spin also is related to inertia and load of the rotor, so if
they are not exactly the same, don't expect the same time.

Mass

[et...@whidbey.com]

On Fri, 3 Feb 2017 23:05:58 +0000 (UTC), Massoud <mas...@yahoo.com>

Greetings Massoud,
Thanks for your replies.
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 grinders with 8 inch wheels have twice the HP of 6
inch wheel grinder, but must accelerate 3 times the inertia. And they
still accelerate more than twice as fast as the 6 inch machine. Hmmm.
It seems to me that the smaller grinder is taking too long to spin up.
Am I wrong? Did I goof the numbers? I do that kind of thing.
Eric

[whit3rd]

On Friday, February 3, 2017 at 3:28:30 PM UTC-8, et...@whidbey.com 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.

[Jon Elson]

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

[et...@whidbey.com]

On Tue, 07 Feb 2017 17:35:22 -0600, Jon Elson <jme...@wustl.edu>
wrote:

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

[Baron]

et...@whidbey.com prodded the keyboard with:

4 and 8 Uf capa are common in flourecent light fittings.

--
Best Regards:
Baron.

[micky]

In sci.electronics.repair, on Fri, 03 Feb 2017 10:01:43 -0800,

et...@whidbey.com wrote:

>Hey All,
>I have an old Baldor bench grinder. Probably 70 years old or older.
>The motor is the type that uses a capacitor for starting and running.
>There is no centrifugal starting switch. I know this type of phase
>making scheme provides lower torque for starting so that the motor
>takes longer to come up to speed. But this grinder takes 5 seconds to
>reach operating speed, 3400 rpm. I replaced the bearings in the motor
>and it now spins very easily. There is a 3/4 wide 6 inch grinding
>wheel on one side and a light weight 6 inch diamond wheel on the other

This sounds so cool. I want one.

I found this at a modest price, $17, but maybe I'm blind but I can't
figure out the diameter of the wheel, the diameter of the hole, or even
the width of the disk. I'll bet the thickness of the disk is under a
quarter inch, and that the hole is either standard or I should have an
adapter, but what is the diamter of the wheel? (I have to go to the
basement to remind myself what I use. 6"?)

BTW, what do you use it for?

>side. The cap in the thing might be the original cap. It is a large
>round tinned metal can with solder lugs. 330 volts and 400mfd. Could
>it be that it needs a new cap? I don't know what kind of cap it is.
>Oil filled I imagine but from the high mfd campared to all my other
>run caps could it be an electrolytic cap? There is no sign of leaking

They can fail without leaking. The one in my Hallicrafters failed after
60 years or so, which is a long time but it still failed. I coudln't
easily find a replacement so I replaced it with 10 smaller ones of a
different sor in parallel, but this was a power supply filter. I don't
think that kind of substittuiion works for starting a motor.

>and there are two places on the top of the cap where it looks like
>holes were sealed with solder.

Hmm. I don't think that's anything.

You can test a big cap like that with an analog meter. Unsolder one of
the wires, and put the meter on ohms across the two connectors. If it's
good, the needle will go far to the right and then go gradually to the
left. Then reverse the test leads and it will do the same again.

>Thanks,
>Eric

[et...@whidbey.com]

On Fri, 10 Feb 2017 02:26:07 -0500, micky <NONONO...@bigfoot.com>
wrote:

The grinder is for sharpening tungsten electrodes for TIG welding.
When I got the grinder it needed new bearings so I replaced them.The
wheel guards are cast with the end bells of the motor. The right side
guard had been hacked up by a previous owner which was fine with me
because I wanted the diamond wheel to be completely exposed so the
grinding can be done on the top of the wheel. To that end I machined
the rest of the wheel guard completely away. The electrode is held
basically horizontal when presented to the wheel and then either
tilted slightly and/or moved past the vertical wheel centerline to
get the desired angle on the electrode. The wheel on the left side of
the grinder needs a guard because it is a typical grinding wheel. This
wheel is used to remove any contaminating metal on the electrode prior
to the diamond wheel use. I trued both wheels but there was still
significant vibration so I replaced the large old out of balance wheel
retaining nuts with new small nuts. Now the grinder runs very smooth.
One advantage of this capacitor run motor is the low vibration because
it is always operating as a two phase motor, though imperfectly.
Because the cap is always in circuit and the two windings in the motor
identical the cap is a compromise between a starting cap and a running
cap. The grinder is very well made, but that's no surprise because it
is a Baldor grinder and Baldor made and still makes very good
grinders.
Eric