# properly sized capacitor



## AustinDB

what is the consequence of using an improperly sized cap for a fan or condensor motor? for instance, if speced is a 5.0Mfd 370vac, and a 5.0Mfd 460vac is used-how does that effect the longevity or starting of the motor? How about if a 6.0Mfd were speced but a 5.0Mfd were installed? 

I'm trying to run down a problem on a 5 ton ducane unit which shuts off after 10-15 minutes b/c of overheating (top of unit is very hot-radiates heat). Is there typically an overcharge protection-if the charge is too high, would it overheat or shut off from a sensor?


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## Railman

First off, I'm not an hvc guy, but:

If you use a 5mfd cap at a lower voltage, it won't hold the rated mfd. So if your 5mfd @460 is used at 370v, it won't reach full capacitance (mfd storage rating).

As far as overheating while running, I don't think the capaitor has anything to do with it, since caps are only used at start up.

My guess would be that the a-coil is dirty, or some other air flow restriction inside, resulting in excessive head pressure, & heat.

Just my .02
Joe


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## AustinDB

thanks for the reply. The AC tech came back out...twice today. One to swap out the improperly sized 5Mfd cap for the required 7.5Mfd cap, but the unit overheated later. Second time he found a rusted terminal on the solenoid which showed signs of heating up. He cleaned the terminals (spade) and we'll see if the problem is fixed.


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## naptown CR

Is the fan running and the top is still very hot?


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## beenthere

First.
The voltage rating of the capacitor, only tells you how much internal insulation it has. has nothing to do with its capacitance.

Second.
Its a run capacitor, its has a lot to do with how the blower motor runs.

Using a higher or lower valued uF can damage the motor.


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## Leo G

beenthere said:


> First.
> The voltage rating of the capacitor, only tells you how much internal insulation it has. has nothing to do with its capacitance.


Thank you for saying that. I was going to mention it.:thumbup:


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## MechAcc

You can always use a higher rated voltage capacitor in lieu of a lower voltage one, but not a lower voltage for a higher one.


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## Railman

Ok, I was mistaken about the run capacitor vs the start capacitor. Most of my stuff is 3ph which don't use caps. I've only replaced start up capacitors. When they go, you can still start the motor up by helping it spin. I have a brush sander that I've done this on.

As far as capacitor ratings are concerned, volts x mf rating = capacitance. The formula is:
Q= C x V 
where
Q = charge in Coulombs
C = Capacitance in Farads
V = Voltage

That's not to say you can't use a higher voltage cap, but you do need to allow for the change in voltage by also adjusting the mf rating.

If you take a 100v 100mf cap, & charge it with 100v volts, you'll get a 100mf charge.
If you have a 200v x 200mf cap, & charge it to 100v, you'll get a 100mf char

If you you over charge a cap with too much voltage, it can blow up.
If you under charge, it won't reach rated value.

So, yes you can use a higher rated votage cap, but you got to do the math to get the right capacity in use.

Joe


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## beenthere

Thats not the formula to calculate capacitance.

Since "C" is the capacitance, not "Q".

The formula is C=Q/V
Because you want to find "C" Capacitance

Run and start caps, have their voltage insulation value printed on them.
Thats all that voltage rating means. That rating has nothing to do with the capacitors uF rating.

Most manufacuters use 370 volt run capacitors. Simply because they are cheaper then 440 voltones.
And on 1,000,000 units. That saves them a lot of money.


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## Railman

MechAcc said:


> You can always use a higher rated voltage capacitor in lieu of a lower voltage one, but not a lower voltage for a higher one.


This might, or might not be true. For the same capacity, you need to multiply the used voltage times the rated mf to get the effective system capacity. System capacity is directly proportional to the applied voltage. 

Joe


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## beenthere

Railman said:


> This might, or might not be true. For the same capacity, you need to multiply the used voltage times the rated mf to get the effective system capacity. System capacity is directly proportional to the applied voltage.
> 
> Joe


You don't know what the run capacitor does, do you. 

Or where it gets its voltage from.


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## Leo G

The capacitance listed on the capacitor is its rating. If it is 200uF it doesn't matter how much voltage you put on it, it is still and will always be 200uF. The voltage rating is generally the thickness of the dielectric material separating the positive and negative plates in the capacitor. If you exceed this voltage the capacitor can become damaged and cease being a capacitor and turn into a resistor or worse and flame ball. I don't know how you learned this, but you need to unlearn it.


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## Railman

beenthere said:


> Thats not the formula to calculate capacitance.
> 
> Since "C" is the capacitance, not "Q".
> 
> The formula is C=Q/V
> Because you want to find "C" Capacitance
> 
> Run and start caps, have their voltage insulation value printed on them.
> Thats all that voltage rating means. That rating has nothing to do with the capacitors uF rating.
> 
> Most manufacuters use 370 volt run capacitors. Simply because they are cheaper then 440 voltones.
> And on 1,000,000 units. That saves them a lot of money.


beenthere,
You just corrected me with the same formula as I posted. You just solved for a different factor.

I'm not sure, but you may be confusing a caps rated value with it's in place system value. A capacitor will only reach it's rated mf value if you charge it with the rated voltage. You can charge a 460v cap to 230v, but it will result in 1/2 the rated mf value, or capacity.

Manuf. use higher voltage rated caps to make them more durable, not just because they are cheaper. You are right in saying that higher volt caps have a higher insulation value.
A couple of helpfull sites:
http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html
http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/hvac/d/d.htm

Caps are simpley storage devices, that act much like a battery. If you charge a 1.5 v battery with 12 volts, it will overheat & blow up. If you charge a 12v battery with 1.5v, it will only reach 1.5v. Caps are the same in this respect.

Leo,
You got nothin?

Joe


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## beenthere

Railman said:


> beenthere,
> You just corrected me with the same formula as I posted. You just solved for a different factor.
> 
> I'm not sure, but you may be confusing a caps rated value with it's in place system value. A capacitor will only reach it's rated mf value if you charge it with the rated voltage. You can charge a 460v cap to 230v, but it will result in 1/2 the rated mf value, or capacity.
> 
> Manuf. use higher voltage rated caps to make them more durable, not just because they are cheaper. You are right in saying that higher volt caps have a higher insulation value.
> A couple of helpfull sites:
> http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html
> http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/hvac/d/d.htm
> 
> Caps are simpley storage devices, that act much like a battery. If you charge a 1.5 v battery with 12 volts, it will overheat & blow up. If you charge a 12v battery with 1.5v, it will only reach 1.5v. Caps are the same in this respect.
> 
> Leo,
> You got nothin?
> 
> Joe


Mine is for capacitance. Yours wasn't. :thumbup:

And your own link says higher voltage rating is not a concern.



> Voltage rating must always be the same or greater than original capacitor whether it is a start or run capacitor.


Capacitors used with motors, are not used for voltage storage per say.
Rather to alter the sine wave.

Don't confuse capacitors for elecgtronics with ones used for motors. Different purposes. And different charging methods.


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## Leo G

The capacitance rating on the capacitor relates to how many coulombs of energy it can store. A coulomb is a rating of how many electrons are in a given space. A capacitor can hold so many electrons and then it is full. Voltage is a differential charge, a potential. The higher the voltage the more potential it has. It can bust through the dielectric material if the potential between the plates is great enough, hence the voltage rating. If you charge a 200uF capacitor with 10v it will hold so many coulombs of electrons with a potential at 10v. If you take the same capacitor (and its voltage rating is not exceeded) and charge it with 20 volts it will hold the same amount of electrons as its 10v counterpart did. It will however have more power/joules. But it still will have the same amount of electrons in the capacitor which is what the capacitance rating is.

But we are talking about a capacitor on an AC circuit. A starting or running capacitor. This operates differently in the fact that it works based on its impedance in the circuit. AC capacitance can be complex depending on the frequency the system capacitor is reacting with.

I took all electronics courses from the 7th grade to the 1st year of college. I am rusty. But I know that the capacitance rating on the side of the capacitor doesn't change because you are applying a different voltage to it.


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## Railman

Leo G said:


> The capacitance rating on the capacitor relates to how many coulombs of energy it can store.


Exactly, with the addition of the rating being at a certain voltage. A lower applied voltage will not result in a full charge, just like in a battery.



Leo G said:


> A coulomb is a rating of how many electrons are in a given space. A capacitor can hold so many electrons and then it is full. Voltage is a differential charge, a potential. The higher the voltage the more potential it has.


True



Leo G said:


> But I know that the capacitance rating on the side of the capacitor doesn't change because you are applying a different voltage to it.


Not exactly. The rating doesn't change, but the effective capacity does, again, just like a battery. A 12v battery charged with 1.5v will only result in 1.5v charge.

The confusion is between ratings, & effective in system use. They are different animals. The charge is directly proportional to the voltage applied, with the high limit being the rated cap voltage. Again, just like a battery.
This is fun!
Joe


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## beenthere

It does NOT work like a battery.


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## Railman

First off guys, thanks for the discussion.
It's why I go here.:thumbsup:



beenthere said:


> Mine is for capacitance. Yours wasn't. :thumbup:
> 
> And your own link says higher voltage rating is not a concern.
> 
> Capacitors used with motors, are not used for voltage storage per say.
> Rather to alter the sine wave.
> 
> Don't confuse capacitors for elecgtronics with ones used for motors. Different purposes. And different charging methods.


That makes perfect sense, but it doesn't change the fact that using a cap at a lower voltage than designed rating alters the capacity in place. 

If they are used to alter the sine wave, it seems to me that a change in capacity could alter the timing of the wave. 
If so, then using a higher voltage rated cap with the same mf rating, will lower effective capacity. Shouldn't the mf rating also be bumped along with the volt rating to get the same capacity in use? 

On one of the sites I posted, they had a tutorial of how using the wrong value caps results in diferent results. One was a noisy motor, one was overheating, etc.

This is a quote from said site:

"Two basic types are used in electric motor: 1) *Run capacitors* are rated in a range of 3-70 microfarad (mfd). Run capacitors are also rated by voltage classification. The voltage classifications are 370V and 440V. Capacitors with ratings above 70 microfarad (mfd) are starting capacitors. Run capacitors are designed for continuous duty, and are energized the entire time the motor is running. Single phase electric motors need a capacitor to energize a second phase winding. This is why sizing is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat. "

Joe


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## Railman

beenthere said:


> It does NOT work like a battery.


 As far as capacity in use it most certainly does. A cap will only charge to the level of the input voltage, just like a battery.
Joe


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## beenthere

You will never admit your wrong.

So, swhy don't you explain.
Why 1000s and 1000s of A/C compressors are running with 440 volt caps, when the factory spec is 370volt.
And there is no amp draw change.

If I lowered or raised the capacitance, there would have to be a change in amp draw.


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## Leo G

Railman said:


> Exactly, with the addition of the rating being at a certain voltage. A lower applied voltage will not result in a full charge, just like in a battery.
> 
> Joe



It retains a full charge when the two voltages of the source and the capacitor are equal. You stated if you only apply 1/2 the rated voltage to the capacitor it would cut its Farad rating in half. This is totally incorrect. Potential (voltage) has nothing to do with how many electrons a capacitor can store. What you are referring to is joules, the energy equivalent of the electrons stored (Farads) and there potential (voltage), this gives you the total amount of energy that the capacitor has currently stored within itself. 

I don't know what we need to do to convince you that your thinking is a bit wrong. You keep saying capacity and then you mix farads with it.


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## Railman

Leo G said:


> It retains a full charge when the two voltages of the source and the capacitor are equal. You stated if you only apply 1/2 the rated voltage to the capacitor it would cut its Farad rating in half. This is totally incorrect. Potential (voltage) has nothing to do with how many electrons a capacitor can store. What you are referring to is joules, the energy equivalent of the electrons stored (Farads) and there potential (voltage), this gives you the total amount of energy that the capacitor has currently stored within itself.
> 
> I don't know what we need to do to convince you that your thinking is a bit wrong. You keep saying capacity and then you mix farads with it.


Leo,
Are you telling me that if I put 100v to a 460v capacitor, that it will reach the caps rated mf?

Also, isn't 1 farad a measurement of a caps electron capacity?

Joe


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## Railman

Leo,
Maybe this might convince you what I'm talking about:
http://www.faradnet.com/deeley/chapt_01.htm

Drop down to capacitors & read.

A caps mf rating is a capacity of electrons, AT A RATED VOLTAGE.
What don't you understand about this?

Joe


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## beenthere

You don't work on motors, do you?


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## mdshunk

The nameplate voltage on a cap is just the "heavy duty-ness" rating of the cap. It has nothing to do with how it will perform in terms of its micro-farad rating. You need a cap with a voltage rating at least as high as the one it replaces. If you replace a 440V rated cap with a 370V rated cap, the cap's life will be much shorter lived. It's not built as heavy duty inside. The voltage rating is more or less how heavy it's built inside. 5mfd is still 5mfd, no matter how heavy duty the cap is. 

Think of cap ratings like trucks. 

A 350 horsepower 1/2 ton truck is akin to a 5mfd, 370V cap
A 350 horsepower 3/4 ton truck is akin to a 5mfd, 440V cap

Both trucks will get your load there at the same time, but it will be harder on the first truck if you have a 3/4-ton payload in each truck.

Installing a cap with a higher voltage rating can be compared to trucks also by considering that there's nothing really wrong with using a 350 horsepower 3/4-ton truck to carry a sack of groceries. It's just more truck (in terms of payload capacity) than you need for the job.

DO NOT CONFUSE a cap's voltage rating (max rating) with the actual applied voltage when you're doing this fancy math. A 5mfd cap is still 5mfd, no matter its voltage rating, will still be 5mfd when you install it in the same piece of equipment.


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## Leo G

Railman said:


> Leo,
> Maybe this might convince you what I'm talking about:
> http://www.faradnet.com/deeley/chapt_01.htm
> 
> Drop down to capacitors & read.
> 
> A caps mf rating is a capacity of electrons, AT A RATED VOLTAGE.
> What don't you understand about this?
> 
> Joe


An electron is an electron. It takes up a finite space. A capacitor of said size can hold so many electrons. Its voltage has nothing to do with it. The voltage and the capacitance in farads will give you a rating in joules, which is a measure of power. The more voltage you put onto the capacitor (up to its rating), the more joules of power it will be able to provide to to you at discharge. Its capacitance in farads will not change. you are confusing capacity with capacitance.


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## Williamair

I meet a guy today real cocky he seen i was in the hvac buisness and he is as well. So i could tell that he wanted to conversate in the line at mc Donalds and we got to talking and he told me he's an liscense contractor for mechanical and electrical. Which i told him thats awsome im studying for my mechanical as well. He then told me he dont get his hands dirty though he just has his crews to do all the work and he just does the buisness which i understand theres a buisness law part of the test but this guy said he never touched this kind of stuff it's just revenue!!! I MEAN C-MON GUYS THERE GOTTA BE SOMEONE OUT THERE THAT KINDA FEELS LIKE YOU GOTTA GRIND RIGHT?


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## Williamair

*A bigshot!*

I meet a guy today real cocky he seen i was in the hvac buisness and he is as well. So i could tell that he wanted to conversate in the line at mc Donalds and we got to talking and he told me he's an liscense contractor for mechanical and electrical. Which i told him thats awsome im studying for my mechanical as well. He then told me he dont get his hands dirty though he just has his crews to do all the work and he just does the buisness which i understand theres a buisness law part of the test but this guy said he never touched this kind of stuff it's just revenue!!! I MEAN C-MON GUYS THERE GOTTA BE SOMEONE OUT THERE THAT KINDA FEELS LIKE YOU GOTTA GRIND RIGHT? Today 07:01 PMI meet a guy today real cocky he seen i was in the hvac buisness and he is as well. So i could tell that he wanted to conversate in the line at mc Donalds and we got to talking and he told me he's an liscense contractor for mechanical and electrical. Which i told him thats awsome im studying for my mechanical as well. He then told me he dont get his hands dirty though he just has his crews to do all the work and he just does the buisness which i understand theres a buisness law part of the test but this guy said he never touched this kind of stuff it's just revenue!!! I MEAN C-MON GUYS THERE GOTTA BE SOMEONE OUT THERE THAT KINDA FEELS LIKE YOU GOTTA GRIND RIGHT? Today 07:01 PMI meet a guy today real cocky he seen i was in the hvac buisness and he is as well. So i could tell that he wanted to conversate in the line at mc Donalds and we got to talking and he told me he's an liscense contractor for mechanical and electrical. Which i told him thats awsome im studying for my mechanical as well. He then told me he dont get his hands dirty though he just has his crews to do all the work and he just does the buisness which i understand theres a buisness law part of the test but this guy said he never touched this kind of stuff it's just revenue!!! I MEAN C-MON GUYS THERE GOTTA BE SOMEONE OUT THERE THAT KINDA FEELS LIKE YOU GOTTA GRIND RIGHT?


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## Sailordude

Hi guys,
I have been in the AC trade for quite some time and can say that a lot of my suppliers do not even carry a 370 volt cap. As a hobby venture in my spare time I am learning about super caps and they are only 2.5 or 2.7 volts. You can put them in series to bring the voltage up and the mfd down, put them in paralell and the voltage rating increases. It is very important to not use higher voltage to charge them or they can vent ( the term explode may better describe the reaction) The same holds true with these motor capacitors. 
As far as the original question about the motor, after making sure the capacitor was correct and the motor continues to run hot I would suspect the bearings of the motor. I have installed capacitors and got the motor going but the motor heats up again after a few minutes running or after I am gone for an hour or so.


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