Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old August 17th 20, 11:14 PM posted to sci.electronics.repair
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Default can bad cap = hot motor?

On Saturday, 15 August 2020 22:19:46 UTC+1, wrote:
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


Maybe you can restuff it rather than dispose.


NT

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Old August 18th 20, 09:34 PM posted to sci.electronics.repair
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Default can bad cap = hot motor?

On Sat, 15 Aug 2020 10:20:08 -0700, Paul Drahn
wrote:

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


Agreed. However, the official name is a "Permanent-Split Capacitor
Motor" which has one permanently connected run capacitor. The "Split
Phase" motor has no capacitors. See:

Two-winding Machines
https://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html

Permanent-Split Capacitor Motor
If run efficiency and vibration are important, but start
torque can be compromised, the capacitor can be left in
the auxiliary circuit at all speeds. Sizing the capacitor
to provide balance at a particular load point, the
backwards field can be eliminated, improving efficiency
and eliminating torque pulsations. Eliminating the
centrifugal switch can reduce the manufacturing cost
significantly. The trade-off is lower starting torque,
since the capacitor is not sized to provide balance at
starting, but for run conditions

The design has the disadvantage of having low starting torque, which
is the cause of the slow start. At 80+ years old, I would guess(tm)
that the capacitor is as dead as the bearings that were replaced. As
it gets old, the ESR starts to climb which I presume was the cause of
the observed heating.

Drivel: My experience with motors was mostly with industrial sewing
machines at my father's lingerie factory. At the time some of the
older motors would exhibit the symptoms of a shorted winding. What
was happening was that some motors were not designed to handle
sustained high temperatures. The copper motor wire would expand
slightly, cracking the old enamel insulation. New enamel was quite
flexible, but old enamel became brittle. My guess(tm) is it took
about 40 years to cause problems. Insulation failures were mostly
around sharp wire bends. I think varnish insulation did the same
thing, but at the time, I couldn't tell the difference between varnish
or enamel insulated motor wire. Therefore, when working with really
old motors, I always look for loose insulation flakes, which might be
an indication of impending shorts.

--
Jeff Liebermann
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http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Old August 19th 20, 05:11 AM posted to sci.electronics.repair
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Default can bad cap = hot motor?

On Tuesday, 18 August 2020 16:54:25 UTC+1, wrote:
On Mon, 17 Aug 2020 14:14:28 -0700 (PDT), tabbypurr wrote:
On Saturday, 15 August 2020 22:19:46 UTC+1, wrote:
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


Maybe you can restuff it rather than dispose.


NT

While I can see the draw of keeping all the old stuff intact or
appearing so this cap is weird. It's a rectangle about 5/16 inch
thick. Modern motor caps are a lot different in shape.
Eric


You gave its dimensions earlier. It is presumably a flat paper cap. I meant restuff with higher v non-motor caps.


NT


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Old August 19th 20, 06:14 PM posted to sci.electronics.repair
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Default can bad cap = hot motor?

On Tue, 18 Aug 2020 20:11:15 -0700 (PDT), wrote:

On Tuesday, 18 August 2020 16:54:25 UTC+1, wrote:
On Mon, 17 Aug 2020 14:14:28 -0700 (PDT), tabbypurr wrote:
On Saturday, 15 August 2020 22:19:46 UTC+1, wrote:
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

Maybe you can restuff it rather than dispose.


NT

While I can see the draw of keeping all the old stuff intact or
appearing so this cap is weird. It's a rectangle about 5/16 inch
thick. Modern motor caps are a lot different in shape.
Eric


You gave its dimensions earlier. It is presumably a flat paper cap. I meant restuff with higher v non-motor caps.


NT

Modern motor run caps are all large. They use a lot of material. It
can't be they do this just because of tradition.
Eric

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Old August 21st 20, 11:26 AM posted to sci.electronics.repair
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Posts: 12,364
Default can bad cap = hot motor?

On Wednesday, 19 August 2020 17:15:06 UTC+1, wrote:
On Tue, 18 Aug 2020 20:11:15 -0700 (PDT), tabbypurr wrote:
On Tuesday, 18 August 2020 16:54:25 UTC+1, wrote:
On Mon, 17 Aug 2020 14:14:28 -0700 (PDT), tabbypurr wrote:
On Saturday, 15 August 2020 22:19:46 UTC+1, wrote:
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

Maybe you can restuff it rather than dispose.


NT
While I can see the draw of keeping all the old stuff intact or
appearing so this cap is weird. It's a rectangle about 5/16 inch
thick. Modern motor caps are a lot different in shape.
Eric


You gave its dimensions earlier. It is presumably a flat paper cap. I meant restuff with higher v non-motor caps.


NT

Modern motor run caps are all large. They use a lot of material. It
can't be they do this just because of tradition.
Eric


You could always look up what properties motor run caps have.
But did you also notice that even a 1930s paper cap lasted 80+ years? Modern film caps are massively better & smaller.


NT
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Old August 21st 20, 06:28 PM posted to sci.electronics.repair
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Posts: 2,161
Default can bad cap = hot motor?

On Fri, 21 Aug 2020 02:26:26 -0700 (PDT), wrote:

On Wednesday, 19 August 2020 17:15:06 UTC+1, wrote:
On Tue, 18 Aug 2020 20:11:15 -0700 (PDT), tabbypurr wrote:
On Tuesday, 18 August 2020 16:54:25 UTC+1, wrote:
On Mon, 17 Aug 2020 14:14:28 -0700 (PDT), tabbypurr wrote:
On Saturday, 15 August 2020 22:19:46 UTC+1, wrote:
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

Maybe you can restuff it rather than dispose.


NT
While I can see the draw of keeping all the old stuff intact or
appearing so this cap is weird. It's a rectangle about 5/16 inch
thick. Modern motor caps are a lot different in shape.
Eric

You gave its dimensions earlier. It is presumably a flat paper cap. I meant restuff with higher v non-motor caps.


NT

Modern motor run caps are all large. They use a lot of material. It
can't be they do this just because of tradition.
Eric


You could always look up what properties motor run caps have.
But did you also notice that even a 1930s paper cap lasted 80+ years? Modern film caps are massively better & smaller.


NT

I know modern caps are much better than old caps. However, why do you
suppose it is that motor run caps are so big for their voltage and
capacitance ratings compared to other non-polarized caps? I'm serious.
Eric

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Old August 21st 20, 07:27 PM posted to sci.electronics.repair
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Posts: 1,625
Default can bad cap = hot motor?

I know modern caps are much better than old caps. However, why do you
suppose it is that motor run caps are so big for their voltage and
capacitance ratings compared to other non-polarized caps? I'm serious.


That is not a bad question at all:
a) Because they are AC-rated caps.
b) Because the nature of a motor starting puts a tremendous load on the cap for a very short time, so they must be robust enough to absorb that shock, repeatedly and reliably, in addition to the actual voltage involved. So, whereas the actual operating voltage may be say.... 240 VAC, the start-cap must be rated (at a minimum) of 150% of the operating voltage.
c) Typical NP caps are used a great deal in audio applications, where the voltages are small, and the frequencies high.
d) Motor Run caps start around 370V, or so, are typically of much lower capacitance than a start-cap (and whereas replacement voltage may be higher, replacement capacitance should remain very close or identical to OEM).

Hope that helps.

Peter Wieck
Melrose Park, PA


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