I have an old Baldor grinder that's at least 80 years old. It is a
cap start and run motor. So the cap is always in the circuit of the
phase shifted winding. There is no centrifugal start switch.
It has always taken a long time to come up to speed but these
types of motors do that. However, it seems to take much longer to spin
up than my newer Baldor grinder motors that are wired the same.
The capacitor is a rectangular shape about 5/16ths of an inch thick
and maybe 1.5 x 2.0 inches. It has a brass case with cloth covered
wires coming out of the tar used to seal the case. I mention the cap
construction just to give an idea of how old it is.
I went through the motor a few years ago because the bearings were
bad. They had rawhide seals that had dried out, shrunk, and let grit
in. I replaced them with modern sealed bearings. The motor runs very
smooth. Just like a grinder motor should.
Today I used the grinder for about 1/2 an hour and it got pretty
hot. I was not loading the grinder very much because I was just
grinding points on tungsten TIG electrodes with a diamond wheel I have
mounted. Still, the motor got pretty damn hot. Could a bad cap do
this?
I suppose there could be some shorted windings. In any case, if a
bad cap could cause hot running I'll take the thing apart again and
replace the cap.
Thanks,
Eric

If the cap is original to the motor, replace it. 80 years is a very long time for such a device, keeping in mind that AC-rated capacitors (80-year-old style) will deteriorate just by sitting on the shelf. A modern polyprop cap of the correct rating and voltage will outlive most of us here.
As to the motor itself, do you have an AC ammeter, with specific reference to the clamp-on type? If so, check each leg of the feed - they should be substantially the same when running, and substantially below nameplate when unloaded. If either of those statements is untrue of your motor, it is on its way out, however slowly.

But, to answer your question directly, yes, hot-running is a symptom of a failed cap - as is slow starting and poor load handling.

Peter Wieck
Melrose Park, PA

On Fri, 14 Aug 2020 05:00:41 -0700 (PDT), "
wrote:

If the cap is original to the motor, replace it. 80 years is a very long time for such a device, keeping in mind that AC-rated capacitors (80-year-old style) will deteriorate just by sitting on the shelf. A modern polyprop cap of the correct rating and voltage will outlive most of us here.
As to the motor itself, do you have an AC ammeter, with specific reference to the clamp-on type? If so, check each leg of the feed - they should be substantially the same when running, and substantially below nameplate when unloaded. If either of those statements is untrue of your motor, it is on its way out, however slowly.

But, to answer your question directly, yes, hot-running is a symptom of a failed cap - as is slow starting and poor load handling.

Peter Wieck
Melrose Park, PA
Yeah, I'm betting that the cap is as old as the motor. I'll take your
advice and replace it. I don't understand how the current can be
different if there are only two wires feeding the motor. What am I
missing?
Thanks,
Eric

On Friday, August 14, 2020 at 6:32:11 PM UTC-4, wrote:

Yeah, I'm betting that the cap is as old as the motor. I'll take your
advice and replace it. I don't understand how the current can be
different if there are only two wires feeding the motor. What am I
missing?

If the capacitor's value is wrong, it doesn't provide the proper phase shift. That causes the motor to require more power to do the same work.

In article ,
says...
Yeah, I'm betting that the cap is as old as the motor. I'll take your
advice and replace it. I don't understand how the current can be
different if there are only two wires feeding the motor. What am I
missing?
Thanks,
Eric



Not likely, but if one of the windings is shorted to ground or the
capacitor is leaking to ground the current could be different.

On 8/13/2020 3:43 PM, wrote:
I have an old Baldor grinder that's at least 80 years old. It is a
cap start and run motor. So the cap is always in the circuit of the
phase shifted winding. There is no centrifugal start switch.
It has always taken a long time to come up to speed but these
types of motors do that. However, it seems to take much longer to spin
up than my newer Baldor grinder motors that are wired the same.
The capacitor is a rectangular shape about 5/16ths of an inch thick
and maybe 1.5 x 2.0 inches. It has a brass case with cloth covered
wires coming out of the tar used to seal the case. I mention the cap
construction just to give an idea of how old it is.
I went through the motor a few years ago because the bearings were
bad. They had rawhide seals that had dried out, shrunk, and let grit
in. I replaced them with modern sealed bearings. The motor runs very
smooth. Just like a grinder motor should.
Today I used the grinder for about 1/2 an hour and it got pretty
hot. I was not loading the grinder very much because I was just
grinding points on tungsten TIG electrodes with a diamond wheel I have
mounted. Still, the motor got pretty damn hot. Could a bad cap do
this?
I suppose there could be some shorted windings. In any case, if a
bad cap could cause hot running I'll take the thing apart again and
replace the cap.
Thanks,
Eric


I suspect your motor has a single capacitor and is a split-phase motor.
Cap start, cap run requires two capacitors and the mechanism to switch
between the two.

A grinder has no reason the have a heavy load when it starts, which is
the reason for motors with two caps.

Paul

On 8/15/20 12:20 PM, Paul Drahn wrote:
A grinder has no reason the have a heavy load when it starts, which is
the reason for motors with two caps.

Well, actually it does.
It has the inertia of both grinding wheel to get up to speed.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

On Saturday, August 15, 2020 at 11:33:43 AM UTC-7, Fox's Mercantile wrote:
On 8/15/20 12:20 PM, Paul Drahn wrote:
A grinder has no reason the have a heavy load when it starts, which is
the reason for motors with two caps.

Well, actually it does.
It has the inertia of both grinding wheel to get up to speed.

The motor could certainly get it up to speed fast with a high torque,
but the user of a grinder can wait a few seconds before applying his
workpiece... so "low startup torque" :== "low startup load" is acceptable in this application.

On Fri, 14 Aug 2020 19:56:04 -0400, Ralph Mowery
wrote:

In article ,
says...
Yeah, I'm betting that the cap is as old as the motor. I'll take your
advice and replace it. I don't understand how the current can be
different if there are only two wires feeding the motor. What am I
missing?
Thanks,
Eric



Not likely, but if one of the windings is shorted to ground or the
capacitor is leaking to ground the current could be different.

I guess the cap could indeed be leaking to ground. It has a brass case
after all and the case may be touching metal. I will be replacing the
cap and that will hopefully solve the heating issue. I'm certain it
will speed up the startup.
Eric

In article ,
says...

Well, actually it does.
It has the inertia of both grinding wheel to get up to speed.

The motor could certainly get it up to speed fast with a high torque,
but the user of a grinder can wait a few seconds before applying his
workpiece... so "low startup torque" :== "low startup load" is acceptable in this application.



I doubt the grinder really has that much inertia to prevent starting.
It may tke some time to get up to full speed. Most hard starting is
like air compompressors and refrigeration compressors unless they have
some sort of unloader.