How to size motor start cap?

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
  Relative to my previous post today I need new starting caps for a 3
HP single pahse 230 volt motor. Dayton (Grainger) no longer has any
info about this motor.  
   The caps that blew were connected in parallel. One was  560-552 MFD
and the other was 540-648 MFD. The caps were not original and they
were from China. From looking on the web it seems like this much
capacitance is a little high.  
   The motor is an old motor. Maybe 40 years old. Maybe older.
   The cap housing is large enough for two caps and the wires
connecting the caps in parallel looks exactly like the wires going
into the motor from the caps. Same kind and color of insulation and
same look from aging. So the big cap housing appears to have been
meant to hold two round caps and not one big rectangular cap.
    Maybe it was hard to get as much capacitance in the same space
years ago and that's why the two caps.
   If the capacitance is too high can that stress the caps enough to
blow their guts out?
Thanks,
Eric

Re: How to size motor start cap?
On 2018/07/18 1:18 PM, snipped-for-privacy@whidbey.com wrote:
Quoted text here. Click to load it

I would contact a motor rebuild shop and see what they recommend. They  
are the pros in this situation...

John

Re: How to size motor start cap?
On Wed, 18 Jul 2018 13:18:42 -0700, snipped-for-privacy@whidbey.com wrote:

Quoted text here. Click to load it
Well, I tried a smaller value, about 1/5 of what was connected before,
and the motor tries to start but just can't. So I know I need more
than 200 MFD.
Eric

Re: How to size motor start cap?
What's the model#/part# on the nameplate of the motor?  Also, many  
nameplates specify the required capacitors for the motor.  Have you looked  
there yet?

Dave M

Quoted text here. Click to load it



Re: How to size motor start cap?
wrote:

Quoted text here. Click to load it
I looked up the motor number and Grainger, which sells Dayton Motors,
no longer has data on the motor. Neither did an internet search yield
anything. And the motor nameplate doesn't specify the caps. I though
that maybe a newer Daytom motor would have the info but Grainger does
not list the caps required for the motors on their website or on the
motor nameplate. I can't understand why.
Eric

Re: How to size motor start cap?
On 19/07/2018 4:18 AM, snipped-for-privacy@whidbey.com wrote:
Quoted text here. Click to load it
Not sure which model you have but the caps are shown as in series on  
this model ( Cap start - run )
https://www.grainger.com/product/DAYTON-3-HP-General-Purpose-Motor-6K145

Re: How to size motor start cap?
On Wed, 18 Jul 2018 13:18:42 -0700, snipped-for-privacy@whidbey.com wrote:

Any reason for not disclosing the motor model number?  Yes, I know
it's 40 years old and Granger can't find it.

There are two caps in most such motors.  One is the "starting
capacitor".  The other is the "run capacitor".
<
https://www.youtube.com/watch?v=qsuzfz5qcIE

<
https://www.youtube.com/watch?v=OMd9QkinXz4

I'm fairly sure they should NOT be wired in parallel.

For 220VAC, the starting cap should be about 30 to 50uF/kW.
3HP(mechanical) = 2.2kW so try about 90uF.

The run cap is usually about 5 - 20uF.

For the starting capacitor calculations, see:
<https://www.electricneutron.com/electric-motor/single-phase-capacitor-sizing/
<https://www.quora.com/How-do-I-calculate-capacitor-value-for-single-phase-motor
More:
<https://www.google.com/search?q=calculate+motor+starting+capacitor

I couldn't find anything on calculating the run capacitor.


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.


Re: How to size motor start cap?
wrote:

Quoted text here. Click to load it
The model number is 90653-N. The motor has only two wires coming out
to connect to the caps, so there is no starting cap. I tried a known
good cap that is about 200 MFD and it won't start the motor. The motor
tries to turn and I bet if I took the belts off it would start
spinning. The motor does spin easily enough but with the compressor
load it takes so long to spin up with the 200 MFD cap I'm afraid the
motor will overheat or the breaker will pop and so I turned it off
after a few seconds.
   The compressor is a two stage air compressor that came with the
motor as a set. The compressor is unloading properly so the motor is
not trying to spin up a huge load and the motor has been spinning the
compressor just fine for the last 40 or 50 years.
   I find it odd that there is no running cap but there just plain
aren't connections for one.
Eric

Re: How to size motor start cap?
wrote:

Quoted text here. Click to load it
Looking again I see that there are TWO model numbers on the motor
nameplate. The top one is 5K483-D, which is a good number as I found
it on the Grianger website. The other model number is on the bottom of
the ID plate and it says Motor Model Number whereas the top model
number just says Model. I have never seen this before. Anyway, I
called Grainger because the cap(s) weren't listed. The guy at Grainger
had to pull a file but he did find the caps and there are two. Both
caps are 485-582 MFD and are wired in parallel. They are both start
caps and the motor has no run cap. I have two cap coming tomorrow but
they are 540-685 MFD. Will them  maybe be a problem? In the meantime I
will try to get the proper caps.
Thanks,
Eric

Re: How to size motor start cap?
OK - keep in mind that the tolerance in electrolytic caps is typically +100%/-20% unless noted otherwise.  

Your larger caps will be just fine.  


Peter Wieck
Melrose Park, PA  

Re: How to size motor start cap?
snipped-for-privacy@gmail.com says...
Quoted text here. Click to load it

There are two things wrong here.

First this is an AC motor with AC capacitors.  Electrolytics will not  
work on AC as a general rule.

Newer capacitors, especially motor capacitors are much closer now in  
tollorence.

I have one here that is rated +- 5% I keep as a spare for my heat pump.  
Many are rated for a range of capacitance of about +- 10 % now.



Re: How to size motor start cap?
On Thursday, July 19, 2018 at 2:10:21 PM UTC-4, Ralph Mowery wrote:

Quoted text here. Click to load it


From Wiki:


olytic capacitors with non solid electrolyte and therefore they are only ap
plicable for the short motor starting time. ... If a motor does not start,  
the capacitor is far more likely the problem than the switch.

A non-electrolytic cap, AC or DC of that rating would be as big as a footba
ll, or larger.  

The motor is 40+ years old. That is not 'newer'.  

That covers both *wrong* things.  

Peter Wieck
Melrose Park, PA

Re: How to size motor start cap?
snipped-for-privacy@gmail.com says...
Quoted text here. Click to load it

Quoted text here. Click to load it

Winki lets anyone post anything.

My 50 uF capacitor is not electrolytic and rated for AC.  It is no where  
near footbal size.  Not even beer can size.  Even the 500 uF or so  
capacitors for motor starters are no larger than a beer can if that  
large.

People are quoting very old data.  Maybe in 1950 the capacitors were  
larger, but no today.  

Re: How to size motor start cap?
On Thursday, July 19, 2018 at 3:21:30 PM UTC-4, Ralph Mowery wrote:
Quoted text here. Click to load it
  
Quoted text here. Click to load it

ectrolytic capacitors with non solid electrolyte and therefore they are onl
y applicable for the short motor starting time. ... If a motor does not sta
rt, the capacitor is far more likely the problem than the switch.
Quoted text here. Click to load it
otball, or larger.  
Quoted text here. Click to load it
  
Quoted text here. Click to load it

Go to any site, any manufacturer - and you will find motor-start caps to be
 electrolytics. Really. Better yet, just open one up. And, exactly, how do  
you think they are made?  


Peter Wieck
Melrose Park, PA

Re: How to size motor start cap?
snipped-for-privacy@gmail.com says...
Quoted text here. Click to load it

This will be my last post on this thread.  There is no use in me trying  
to educate a person that will not be educated . Unless you can point me  
to a page that specifies an electrolytic capacitor for a run and/or  
start capacitor for an AC motor.  And I do not mean the capacitors used  
in a varitabble speed drive, just an ordinary AC motor.

If you go to the last sentence or two it will tell you that they are not  
suited for use on AC lines.  


Here is a quote from your belovied Wiki

https://en.wikipedia.org/wiki/Electrolytic_capacitor


Reverse voltage

An exploded aluminum electrolytic capacitor on a PCB
Standard electrolytic capacitors, and aluminum as well as tantalum and  
niobium electrolytic capacitors are polarized and generally require the  
anode electrode voltage to be positive relative to the cathode voltage.

Nevertheless, electrolytic capacitors can withstand for short instants a  
reverse voltage for a limited number of cycles. In detail, aluminum  
electrolytic capacitors with non-solid electrolyte can withstand a  
reverse voltage of about 1 V to 1.5 V. This reverse voltage should never  
be used to determine the maximum reverse voltage under which a capacitor  
can be used permanently.[51][52][53]

Solid tantalum capacitors can also withstand reverse voltages for short  
periods. The most common guidelines for tantalum reverse voltage are:




These guidelines apply for short excursion and should never be used to  
determine the maximum reverse voltage under which a capacitor can be  
used permanently.[54][55]

But in no case, for aluminum as well as for tantalum and niobium  
electrolytic capacitors, may a reverse voltage be used for a permanent  
AC application.

To minimize the likelihood of a polarized electrolytic being incorrectly  
inserted into a circuit, polarity has to be very clearly indicated on  
the case, see the section on "Polarity marking" below.

Special bipolar aluminum electrolytic capacitors designed for bipolar  
operation are available, and usually referred to as "non-polarized" or  
"bipolar" types. In these, the capacitors have two anode foils with  
full-thickness oxide layers connected in reverse polarity. On the  
alternate halves of the AC cycles, one of the oxides on the foil acts as  
a blocking dielectric, preventing reverse current from damaging the  
electrolyte of the other one. But these bipolar electrolytic capacitors  
are not adaptable for main AC applications instead of power capacitors  
with metallized polymer film or paper dielectric.

Re: How to size motor start cap?
On Thu, 19 Jul 2018 18:09:05 -0400, Ralph Mowery

Quoted text here. Click to load it

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

Quoted text here. Click to load it

Are these sufficient?

<http://www.keltroncomp.org/index.php/prdcts/ac-motor-start-capacitor
Note the lack of polarity markings.
   Keltron Aluminium Electrolytic Motor Start capacitors  
   are manufactured...

From the Gainger catalog under Dayton:
<https://www.grainger.com/category/capacitors/capacitors-and-accessories/motor-supplies/motors/ecatalog/N-19eb
   These electrolytic, nonplarized capacitors are designed for  
   normal intermittent service on single-phase AC motor starting
   circuits.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: How to size motor start cap?
On Thursday, July 19, 2018 at 7:09:27 PM UTC-4, Jeff Liebermann wrote:

Quoted text here. Click to load it

On Trial-by-Combat:  


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


Peter Wieck
Melrose Park, PA  

Re: How to size motor start cap?
On Fri, 20 Jul 2018 06:49:27 -0700 (PDT), " snipped-for-privacy@aol.com"

Quoted text here. Click to load it


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     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.
Re: How to size motor start cap?
On Thu, 19 Jul 2018 10:29:29 -0700 (PDT), " snipped-for-privacy@aol.com"

Quoted text here. Click to load it
Thanks Peter. Do you know why the MFD value  of motor starting caps is
such a wide range? The specified caps are 485-582 MFD. Is that the min
and max of the cap? Do they spec them that way instead of using a
tolerance?
Eric

Re: How to size motor start cap?
On Thu, 19 Jul 2018 09:14:06 -0700, snipped-for-privacy@whidbey.com wrote:

Quoted text here. Click to load it


Bingo.  
<https://www.grainger.com/product/DAYTON-3-HP-General-Purpose-Motor-5K483
More:
<https://www.google.com/search?q=dayton+5K483+motor
There are TWO wiring diagrms for suffix BB and BA with different
capacitor wiring schemes.  I guess you have the BB suffix since it has
2 caps in parallel.
<https://www.grainger.com/ec/pdf/5K483_4.pdf BB suffix  
<https://www.grainger.com/ec/pdf/5K483_6.pdf BA suffix

I couldn't find a reference with the exact capacitor value(s).  The
parts list should have the Dayton capacitor part numbers on it
somewhere.  Possible source to double check the values:
<http://www.emotorpro.com/capacitors.aspx

Quoted text here. Click to load it

I've never seen anything like that but I guess it's possible,
especially since Grainger was able to find documentation on the motor.
The capacitance range of the new caps overlaps about 1/3 of the range
of the originals.  Not great, but it will probably at least start.
Start caps are only used for increasing starting torque until the
motor is up to speed.  The motor might complain a little starting with
too much capacitance, but once it starts, it should be ok.  I've also
never seen a 3HP motor without a run capacitor.  Weird.

To cover some of the other comments:

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?t36%0
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

According to various web piles, the listed tolerance on most motor
starting caps is usually +/-6%.  When a range of capacitance is
specified, that includes this tolerance.  However that doesn't seem to
be the case here as the 540-685 cap is a much wide tolerance range. If
the capacitor is a nominal 612uF, then that range would be the same as
612uF +/-12%.  


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
We've slightly trimmed the long signature. Click to see the full one.

Site Timeline