Properly Sized Capacitor

72chevy4x4

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?

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

72chevy4x4

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.

naptown CR

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

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.

Leo G

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

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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

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.

Railman

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

beenthere

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

Or where it gets its voltage from.

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,
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...p/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

beenthere

Mine is for capacitance. Yours wasn't.

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.

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

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.

True

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

beenthere

It does NOT work like a battery.

Railman

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

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

Railman

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

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.