Properly Sized Capacitor

Leo G

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,
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

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

beenthere

You don't work on motors, do you?

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.

Leo G

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?

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? 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?

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.