How to size capacitor for capacitor run fan motor

[Andy]

I have a small single phase blower motor which requires a capacitor
for running (not starting). The motor nameplate says that I need 7.5
microfarads. I have a pair of 5 microfarad caps in the shop. I
hooked up the motor with just a single 5 uF and it didn't start. Then
I put the pair in parallel and the motor did start. If I use 10 uF I
will draw more current into the directional winding of the motor. So
my question is, how closely are these caps sized? Is the difference
between 7.5 and 10 uF enough to do damage?

thanks

 

[JW]

On Thu, 31 Dec 2009 04:39:47 -0800 (PST) Andy <andrewkgentile@gmail.com>
wrote in Message id:
<a13a138e-e049-43d1-94d5-d8d221c40054@z41g2000yqz.googlegroups.com>:

I have a small single phase blower motor which requires a capacitor
for running (not starting). The motor nameplate says that I need 7.5
microfarads. I have a pair of 5 microfarad caps in the shop. I
hooked up the motor with just a single 5 uF and it didn't start. Then
I put the pair in parallel and the motor did start. If I use 10 uF I
will draw more current into the directional winding of the motor. So
my question is, how closely are these caps sized? Is the difference
between 7.5 and 10 uF enough to do damage?

Can't answer your question, but Digikey carries 7.5uF AC run caps. For $10
or so, better safe than sorry I always say.

 

[William R. Walsh]

Hi!

When I needed a start capacitor for an old furnace fan motor, I had to
go to an electric motor repair shop to get one. (You'd really think
that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store
would have these sorts of things, but...)

I had to get a capacitor slightly larger than the one I was replacing.
However, I was told that the tolerance is 10% either way.

A run capacitor might be a little tigher in tolerance, but I don't
think you've got enough of a difference there to cause a problem.

William

 

[hr(bob) hofmann@att.net]

On Dec 31, 9:01 am, "William R. Walsh" <wm_wa...@hotmail.com> wrote:

Hi!

When I needed a start capacitor for an old furnace fan motor, I had to
go to an electric motor repair shop to get one. (You'd really think
that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store
would have these sorts of things, but...)

I had to get a capacitor slightly larger than the one I was replacing.
However, I was told that the tolerance is 10% either way.

A run capacitor might be a little tigher in tolerance, but I don't
think you've got enough of a difference there to cause a problem.

William

Can you hear a click as the motor gets up to speed? If you do, then
the centrifugal switch is working and disconnecting the starting
capacitor and the extra 2.5 uF isn't going to be a problem. The
manfacturer uses the smallest value they can get away with as it saves
them a few cents. Happy heating.

 

[William R. Walsh]

Hi!

Can you hear a click as the motor gets up to speed?

I can now.

The motor in question was treated badly and left outside in the
weather. Rust soon covered the contacts of the centrifugal switch and
all of the electrical contacts. It couldn't work properly, so I took
it all apart and cleaned it. Even if you helped it, it didn't want to
run properly. More often than not it would just wind down and start
humming again.

Then the switch worked, but the motor almost always needed help to
start. After that it would run fine. So I started to look at the cap,
and $7 later it was replaced. All problems were then solved.

This is a converted fan, set up for standalone use wherever air flow
is needed. I have another that uses a run capacitor. It is much
fancier, with multiple speeds.

William

 

[Andy]

On Dec 31, 11:34 am, "William R. Walsh" <wm_wa...@hotmail.com> wrote:

Hi!

Can you hear a click as the motor gets up to speed?

I can now.

The motor in question was treated badly and left outside in the
weather. Rust soon covered the contacts of the centrifugal switch and
all of the electrical contacts. It couldn't work properly, so I took
it all apart and cleaned it. Even if you helped it, it didn't want to
run properly. More often than not it would just wind down and start
humming again.

Then the switch worked, but the motor almost always needed help to
start. After that it would run fine. So I started to look at the cap,
and $7 later it was replaced. All problems were then solved.

This is a converted fan, set up for standalone use wherever air flow
is needed. I have another that uses a run capacitor. It is much
fancier, with multiple speeds.

William

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required. Where do you
go from there? I'm sure there must be a means of establishing that
you do have the correct cap. I found one document which states that
"the voltage across the cap should be about 5-10% higher than the
voltage across the winding, with the rotor locked." Has anyone any
other rules of thumb?

 

[William R. Walsh]

Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required. Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

 

[Andy]

On Dec 31 2009, 7:50 pm, "William R. Walsh"
<newsgrou...@idontwantjunqueemail.walshcomptech.com> wrote:

Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required.  Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge. I prefer to understand rather than
to ask someone and then believe them. I want to know how to solve
this riddle myself. What is the basis for capacitor sizing? There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding. If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.

 

[Wild_Bill]

There are many small motors in use that utilize a low capacitance value
capacitor. The most common type smaller than 1/8 HP is the PSC permanent
split capacitor type motor.
PSC motors have 2 windings connected in series (3 connections or wires), and
don't have centrifugal switches for starting.
PSC motors are commonly labeled with; thermal and/or impedance protection.

The method used to determine if a motor is a PSC, is to measure the
resistance of the windings. A PSC motor's windings resistances will be
nearly identical for boh windings.

The value of the capacitor affects the torque and speed of the motor. I don'
know of any formula for determining the (best) value of the capacitor.

Years ago, there was a reference to a capacitor value selection chart,
related to PSC motors used in HVAC servicing, IIRC.
The chart suggested capacitor values for different sized motors, for
increasing torque and/or speed.

Many of the small PSC motors that I'm familiar with will operate with
different capacitor values, and a couple of them use different capacitor
values to make the motors operate at different speeds (low-medium-high).

One PSC motor I was using for a machine application actually ran much cooler
when the cap value was changed to a higher value (about 3x the
manufacturer's specified value).

I'm surprised that your motor didn't run with just one 5uF cap connected.
Marked values on caps can vary as much as 20% from the actual measured
value, but 10% is probably a much more likely worst case situation.

The best indicator from a standpoint of harm, is probably going to be the
motor temperature. If the motor doesn't run excessively hot, there wouldn't
be any reason that the cap's value could harm the motor.

--
Cheers,
WB
..............


"Andy" <andrewkgentile@gmail.com> wrote in message
news:a13a138e-e049-43d1-94d5-d8d221c40054@z41g2000yqz.googlegroups.com...

I have a small single phase blower motor which requires a capacitor
for running (not starting). The motor nameplate says that I need 7.5
microfarads. I have a pair of 5 microfarad caps in the shop. I
hooked up the motor with just a single 5 uF and it didn't start. Then
I put the pair in parallel and the motor did start. If I use 10 uF I
will draw more current into the directional winding of the motor. So
my question is, how closely are these caps sized? Is the difference
between 7.5 and 10 uF enough to do damage?

thanks

 

[Michael A. Terrell]

Andy wrote:

On Dec 31 2009, 7:50 pm, "William R. Walsh"
newsgrou...@idontwantjunqueemail.walshcomptech.com> wrote:
Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required. Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge. I prefer to understand rather than
to ask someone and then believe them. I want to know how to solve
this riddle myself. What is the basis for capacitor sizing? There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding. If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.

This isn't a 'site' It is 'Usenet', even if you are using the lame
'Google Groups' interface.

There are formulas to calculate the required capacitor. You need some
basic information to use them, so stick your smug attitude where the sun
doesn't shine.


--
Greed is the root of all eBay.

 

[mm]

On Fri, 01 Jan 2010 17:29:15 -0500, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Andy wrote:

On Dec 31 2009, 7:50 pm, "William R. Walsh"
newsgrou...@idontwantjunqueemail.walshcomptech.com> wrote:
Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required. Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge. I prefer to understand rather than
to ask someone and then believe them. I want to know how to solve
this riddle myself. What is the basis for capacitor sizing? There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding. If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.


This isn't a 'site' It is 'Usenet', even if you are using the lame
'Google Groups' interface.

There are formulas to calculate the required capacitor. You need some
basic information to use them, so stick your smug attitude where the sun
doesn't shine.

I'm just a visitor here, but I agree with Michael's reply.

And Andy, why are you trying to figure out what size the cap should be
when you said that the motor nameplate says 7.5 uF?

I thought your question was, is it safe to use 10 uF?

I think JW and Wild Bill answered those questions before your last
post, and I would add that, though my info is 45 years old and
probably for a different type of cap, I was told the real value for
caps ranged from -10% of the rated value to +50 or +100%.

 

[Andy]

On Jan 2, 11:21 am, mm <NOPSAMmm2...@bigfoot.com> wrote:

On Fri, 01 Jan 2010 17:29:15 -0500, "Michael A. Terrell"



mike.terr...@earthlink.net> wrote:

Andy wrote:

On Dec 31 2009, 7:50 pm, "William R. Walsh"
newsgrou...@idontwantjunqueemail.walshcomptech.com> wrote:
Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required.  Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge.  I prefer to understand rather than
to ask someone and then believe them.  I want to know how to solve
this riddle myself.  What is the basis for capacitor sizing?  There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding.  If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.

  This isn't a 'site'  It is 'Usenet', even if you are using the lame
'Google Groups' interface.

  There are formulas to calculate the required capacitor. You need some
basic information to use them, so stick your smug attitude where the sun
doesn't shine.

I'm just a visitor here, but I agree with Michael's reply.

And Andy, why are you trying to figure out what size the cap should be
when you said that the motor nameplate says 7.5 uF?

I thought your question was, is it safe to use 10 uF?

I think JW and Wild Bill answered those questions before your last
post, and I would add that, though my info is 45 years old and
probably for a different type of cap, I was told the real value for
caps ranged from -10% of the rated value to +50 or +100%.

I'm trying to figure it out because I want to know. I didn't think of
that as smug, but I guess it is.

 

[Andy]

On Jan 2, 11:21 am, mm <NOPSAMmm2...@bigfoot.com> wrote:

On Fri, 01 Jan 2010 17:29:15 -0500, "Michael A. Terrell"



mike.terr...@earthlink.net> wrote:

Andy wrote:

On Dec 31 2009, 7:50 pm, "William R. Walsh"
newsgrou...@idontwantjunqueemail.walshcomptech.com> wrote:
Hi!

So, let's suppose you have a motor, and the nameplate is damaged to
the point you cannot see what size of cap is required.  Where do you
go from there?

I'd start by checking the cap that came with the motor. In the case of the
fan I'm talking about, the capacitor was clearly labeled as to its ratings.

If that wasn't possible, I'd try looking the motor manufacturer up on the
web (either through their web site or from a web site belonging to someone
who sells those motors) for more information. You could also try telephoning
the motor manufacturer.

Still nothing? I'd have to visit an electric motor repair shop at that
point. (That's the only way I know of if everything else is missing or does
not pan out.)

William

This can't be that difficult that we are all dependent on the keepers
of the secret capacitor knowledge.  I prefer to understand rather than
to ask someone and then believe them.  I want to know how to solve
this riddle myself.  What is the basis for capacitor sizing?  There
must be some relationship between the amount of current in the main
winding and the current in the secondary winding.  If someone on this
site doesn't beat me to it, I will figure out how to size a cap and
then post the solution.

  This isn't a 'site'  It is 'Usenet', even if you are using the lame
'Google Groups' interface.

  There are formulas to calculate the required capacitor. You need some
basic information to use them, so stick your smug attitude where the sun
doesn't shine.

I'm just a visitor here, but I agree with Michael's reply.

And Andy, why are you trying to figure out what size the cap should be
when you said that the motor nameplate says 7.5 uF?

I thought your question was, is it safe to use 10 uF?

I think JW and Wild Bill answered those questions before your last
post, and I would add that, though my info is 45 years old and
probably for a different type of cap, I was told the real value for
caps ranged from -10% of the rated value to +50 or +100%.

I am trying to figure it out because I want to understand. I think of
this as curiosity. Others call it smug.