How to size motor start cap?

[Jeff Liebermann]

On Thu, 19 Jul 2018 16:34:42 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

Maybe because the motor was paired with a compressor with a large
flywheel the pulsing torque from a single phase motor without a run
cap was not considered a problem because the flywheel would smooth
things out.

My guess(tm) is the start winding was intentionally undersized (which
is why the start cap is so large). Exactly why, I don't know.

Ok, I'm wrong. That's not the reason and nothing is wrong.
<https://www.scribd.com/document/6883830/Tutorial-Motor-Basics-Lecture>
See the drawing on Pg 20 of a capacitor start motor (as opposed to a
capacitor start and capacitor run motor). It has only a starting
capacitor and no run capacitor. According to the accompanying text:
- Larger single phase motors with up to about 10 HP.
- A split phase motor with the addition of a capacitor in the starting
winding.
- Capacitor sized for high starting torque.
- Very high starting torque
- Very high starting current
- Common on compressors and other hard starting equipment.

So, adding a run capacitor isn't going to do anything useful with this
type of motor. Sorry for the bad guess(tm) but I'm not familiar with
this type of motor. Everything I've seen has both capacitors.





--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Ralph Mowery]

In article <ak12ldpk619iljm0bbr1lpa3pf6dmqovfu@4ax.com>,
jeffl@cruzio.com says...

Start caps are always non-polarized electrolytics. For the
non-believers see this video where the mad bomber puts 220VAC across a
start capacitor to easily disassemble it.
https://youtu.be/OMd9QkinXz4?t=360
Notice the electrolyte oozing out of the capacitor before it blows
out. Looks like the guy has done this more than a few times in the
past. Run capacitors are also non-polarized, but beside electroltyic,
can also be oil filled. Not the best document on the topic but at
least covers some of the details:
"DIFFERENCE BETWEEN RUN AND START CAPACITORS"
http://www.capacitorindustries.com/wp-content/uploads/Run-and-Start-Capacitors.pdf

I guess it is my time to be educated. Those start capacitors are
common but a special case for electrolytics. Good for a short time, but
will let loose if AC is applied for very long at one time.

 

[Jeff Liebermann]

On Thu, 19 Jul 2018 23:06:54 -0400, Ralph Mowery
<rmowery28146@earthlink.net> wrote:

I guess it is my time to be educated. Those start capacitors are
common but a special case for electrolytics. Good for a short time, but
will let loose if AC is applied for very long at one time.

Maybe. There are different levels of quality in such capacitors.
Since the heat generated is mostly from the ESR (equivalent series
resistance) of the capacitor, a low ESR would heat up less and
theoretically last longer. However, that's not what causes caps to
die:
<https://youtu.be/OMd9QkinXz4?t=345>
He starts out putting a run capacitor across 220VAC and leaves it
connected. It stays cold and nothing happens. Somewhat later in the
video, he does the same to a large value start capacitor. It belches
hot steaming electrolyte and then blows the end off. If the
centrifugal start switch sticks or is delayed in opening, for example
by line undervoltage, the switch stays closed for too long and cap
dies. Part of this is ESR and ripple current heating, but the real
culprit is junk centrifugal start switches with partly welded
contacts. If the switches were good, methinks that even the lowest
cost motor start capacitor would last forever.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Layman]

On 20/07/18 00:34, Jeff Liebermann wrote:

> Impressive but what will they do when the warehouse guy retires?

Some time back there was an old guy whose sole job for 20 years was to
make a product in a 1000+ litre reaction vessel with viewing ports,
using several steps. Just before he retired, he wrote out all the info
needed for the new guy to make the product. But try as he might, with
the new guy the process always failed at a certain stage near the end.
The company called the old guy and asked him to come in and make a batch
with the new guy looking on. He started the process, peering through the
viewing port, and the product came out perfectly at the end. The new guy
asked him how he knew when to add ingredients at the step where it had
always failed before. His answer? "It looked right".

--

Jeff

 

[pfjw@aol.com]

On Thursday, July 19, 2018 at 7:09:27 PM UTC-4, Jeff Liebermann wrote:

Oh, be nice, or would trial by combat be a better way
to settle the matter?

On Trial-by-Combat:

https://www.youtube.com/watch?v=ua_TZ84hmEA

Peter Wieck
Melrose Park, PA

 

[Ralph Mowery]

In article <pis4d6$764$1@dont-email.me>, jmlayman@invalid.invalid
says...

Impressive but what will they do when the warehouse guy retires?

Some time back there was an old guy whose sole job for 20 years was to
make a product in a 1000+ litre reaction vessel with viewing ports,
using several steps. Just before he retired, he wrote out all the info
needed for the new guy to make the product. But try as he might, with
the new guy the process always failed at a certain stage near the end.
The company called the old guy and asked him to come in and make a batch
with the new guy looking on. He started the process, peering through the
viewing port, and the product came out perfectly at the end. The new guy
asked him how he knew when to add ingredients at the step where it had
always failed before. His answer? "It looked right".

--

Sometimes you just have to go by looks or feel than by the book.

Where I worked there was a device to pull a vacuum on the process. It
was very simple. Five seperate stages of a long pipe with what looked
like a trumpet horn in the end that steam was put through. Seemed that
only me and another could trouble shoot them very well. Others just
replaced parts tuil they stumbled on the right one. On more than one
ocassion it went like this. The day shift worked on it most of the day
and would leave about 11 at night. Then me and the other man would get
it going in less than an hour. We had just worked out some tests that
were very simple and easy to make, but were not in the book.

 

[pfjw@aol.com]

On Friday, July 20, 2018 at 3:53:12 AM UTC-4, Jeff Layman wrote:

On 20/07/18 00:34, Jeff Liebermann wrote:

Impressive but what will they do when the warehouse guy retires?

Some time back there was an old guy whose sole job for 20 years was to
make a product in a 1000+ litre reaction vessel with viewing ports,

Sometimes, this is much like making bread. Flour varies from day to day. Yeast will vary, even the dry stuff. Humidity, temperature, salt content. It comes down to the kneading - and getting that right, more importantly, when to STOP kneading.

Peter Wieck
Melrose Park, PA

 

[Jeff Liebermann]

On Fri, 20 Jul 2018 06:49:27 -0700 (PDT), "pfjw@aol.com"
<peterwieck33@gmail.com> wrote:

On Thursday, July 19, 2018 at 7:09:27 PM UTC-4, Jeff Liebermann wrote:
Oh, be nice, or would trial by combat be a better way
to settle the matter?

On Trial-by-Combat:
https://www.youtube.com/watch?v=ua_TZ84hmEA
Peter Wieck
Melrose Park, PA

At a former employer, I would sometimes bring my fencing kit (foils,
mask, gloves, conductive jacket, helmet, buzzer, etc) to work. The
kit would usually live in my truck, and only be dragged inside for a
quick show and tell, or to repair the badly designed contact buzzer.
Someone walked into my office, saw the foil, and decided that dueling
must be my favored method of settling disputes. Within days, everyone
knew that I was armed to the teeth and that I was prepared to do
battle with anyone that criticized my decisions. I had no idea that
the story had become so distorted.

At the next design review meeting, I was asked to leave all weaponry
at the door and that company policy forbids bloodshed during working
hours. I still didn't understand what was happening, until someone
mentioned the fencing foils. When I proclaimed that my kit was in my
truck, everyone seemed to visibly relax.

About a year later, I gave a quick fencing demonstration with a friend
in the parking lot during lunch. It went well and we all went back
inside when lunch was over. However, someone apparently called the
police. At some point, fencing morphed into dueling, which the police
interpreted as using dueling pistols. When the SWAT team eventually
arrived, there was nothing to see, so they surrounded the building and
started to evacuate the neighboring businesses. I'm going to skip
over the next hour and just say the police left looking rather
disappointed. Of course, management was not thrilled, but eventually
concluded that it wasn't really my fault that things escalated out of
control.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

On Thu, 19 Jul 2018 16:48:30 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Thu, 19 Jul 2018 16:34:42 -0700, Jeff Liebermann <jeffl@cruzio.com
wrote:
Maybe because the motor was paired with a compressor with a large
flywheel the pulsing torque from a single phase motor without a run
cap was not considered a problem because the flywheel would smooth
things out.

My guess(tm) is the start winding was intentionally undersized (which
is why the start cap is so large). Exactly why, I don't know.

Ok, I'm wrong. That's not the reason and nothing is wrong.
https://www.scribd.com/document/6883830/Tutorial-Motor-Basics-Lecture
See the drawing on Pg 20 of a capacitor start motor (as opposed to a
capacitor start and capacitor run motor). It has only a starting
capacitor and no run capacitor. According to the accompanying text:
- Larger single phase motors with up to about 10 HP.
- A split phase motor with the addition of a capacitor in the starting
winding.
- Capacitor sized for high starting torque.
- Very high starting torque
- Very high starting current
- Common on compressors and other hard starting equipment.

So, adding a run capacitor isn't going to do anything useful with this
type of motor. Sorry for the bad guess(tm) but I'm not familiar with
this type of motor. Everything I've seen has both capacitors.

I thought that maybe the really large capacitance was for higher
torque, now I know it that it is. Thanks.
Eric

 

So, if you parallel a mess

of capacitors to make a run capacitor with the correct value for PF=1,
then the capacitors will dissipate zero power.

not true.
if there is current flowing through the cap, ESR of the cap will dissipate power.
The purpose of a motor cap is not to correct the PF.

There is no such thing as a single phase motor.
Small motors have shaded poles or other tricks to create a phase shift.
Medium size motors motors use a cap.
Large motors are typically fed with 3 phase.

I think the optimum cap value, is where the current through the aux winding is 90 deg out of phase with the main winding.

m

 

[Jeff Liebermann]

On Wed, 25 Jul 2018 13:07:31 -0700 (PDT), makolber@yahoo.com wrote:

So, if you parallel a mess
of capacitors to make a run capacitor with the correct value for PF=1,
then the capacitors will dissipate zero power.


not true.

I agree. I screwed up and forgot about ESR.

if there is current flowing through the cap, ESR of the cap will dissipate power.
The purpose of a motor cap is not to correct the PF.

There is no such thing as a single phase motor.
Small motors have shaded poles or other tricks to create a phase shift.
Medium size motors motors use a cap.
Large motors are typically fed with 3 phase.

Also true. However, the original problem is with a 3HP motor, with no
run capacitor. I guess for high torque starting, such as with an air
compressor, this would be an exception.

I think the optimum cap value, is where the current through the aux
winding is 90 deg out of phase with the main winding.

Agreed. This explains it better than I could:
<https://www.quora.com/What-is-a-capacitor-start-capacitor-run-in-a-motor>




--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558