Capacitor run motors

Al reeem,

The answer is that electrolytic capacitors act as capacitors for DC only. They pass AC, and therefore are commonly used as filters for DC power supplies: They smooth the DC and pass any AC components across the + and - sides, thereby eliminating it from the output.

Oil filled capacitors are the most economical type of AC capacitors at those voltages and capacitances to perform phase shifting for motors. Electrolytic capacitors would not work at all.

Regards, Greg

Many years ago an electrolytic capacitor inadvertently connected across a lov-voltage AC supply failed spectacularly by explosion, spreading its former contents over a wide area, including most of the experimenter's clothing. Fortunately, no-one was hurt on that occasion.

I see from the posts here that I should have treated the question in more than an academic fashion because of the danger of hooking up an electrolytic capacitor in an AC circuit of some power: The capacitor is likely to explode violently!

I apologize for any additional misunderstandings that may have occurred due to my omissions, especially regarding bipolar (AC) electrolytics.

In power applications electrolytic capacitors are for DC only!

When properly biased with a DC voltage, they will "pass" (or absorb) a smaller in magnitude AC "ripple", and some are optimized for this function, combining two capacitors in series opposition in a single package, hence the term bipolar or AC electrolytic. These are designed for coupling AC signals only, NOT in power applications.

Under no circumstances are electrolytic capacitors intended for use on AC power alone! They require a DC voltage of the correct polarity in order to function.

For use in an capacitor start/induction run or capacitor run AC motor, the capacitor would likely explode spectacularly, catch fire, or at a minimum internally self destruct with a healthy puff of smoke. In no case would it work.

As it turns out, Wikipedia has a good link on electrolytic capacitors:

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

Regards, Greg

Try to connect an eletrolytic (polarized) capacitor, of appropriate rated voltage (remember the peak-value), to 110 or 230 AC mains: you will see a little explosion (due to the distruction of the thin dielectric and consequent short-circuit) and strewind of strong-smelling material everywhere.

Incidentally: when not used for large amount of time, the electrolytic dielectric reduces his thickness, and hence the applicable voltage.

So, I use to "soft-start" when forcing AC voltage to old equipments like radios or record-players (a Variac is well suitable).

Not only will you trash the cap, with paper and gook everywhere, the smell is horrible.

Been there. Done it. Secondhand T-shirt, covered in humungacite from a wrecked capacitor, now available on eBay...

If it produced an image of someone famous on it, I'm buying.........

The writer is infamous, rather than famous, some might say...

What power rating?What voltage? Low horsepower motors certainly use AC electrolytic caps

Bingo! There are AC electrolytics and DC ones and it is important to select the correct type for the application.

Generally, DC ones store more charge-per-unit-volume than AC ones.

Right!

And if you don't have an AC electrolytic cap you can make one from 2 DC caps connected in series like + - - + or - + + -.

This is a common practice in audio too.

I dont get it not that I really need to but. How can you connect DC caps ( polarity

+-,-+ would this be pointless, maybe?

If in parallel C1+C2+C3= CT

If in series 1/C1+1/c2 +1/c3= CT

I have limited knowledge here but why wouldn't they act like a battery connected in series +- -+ having a canceling effect with alot of heat.

Omberdonk,

"+-,-+ would this be pointless, maybe?"

"I have limited knowledge here but why wouldn't they act like a battery connected in series +- -+ having a canceling effect with a lot of heat."

Capacitors are not like batteries. Batteries produce a voltage by their electrochemical potential, and a current by the actual chemical reaction within the battery. Rather than explain capacitors, I refer you to my post #11 in which I have a link explaining and describing electrolytic capacitors.

Capacitors connected in series opposition: +--+, or -++- are used to couple signals capacitively. This differs from most other applications of capacitors. Depending on the particular circuit, single capacitors are also sometimes used as couplers. When used as couplers, they isolate DC but pass AC (or DC biased AC). This avoids any direct electrical connection yet permits a "signal" to pass through.

Regards, Greg

We should link this to the time travel forum and talk about the "Flux capacitor"

hi there al; there are caps for dc and ac voltages / all built similar / only voltages they handle are MUCH different / they can and do explode when over stressed /
the metal can type are two voltages we use in the usa 370 v and 440 volt / mircofarad [mfd] is low 2 to 90 ~ / they are use in motor curcuits in continous operation / the bakelite case type are starting caps used only to get the motor started then they need to be removed from the curcuit / klystron

"there are caps for dc and ac voltages / all built similar / only voltages they handle are MUCH different "

WRONG!

This will only confuse people who don't understand the different kinds of capacitors.

All caps are NOT built similar at all!

Greg

Just a story, yesterday we found a mistery voltage while pm. we pulled the fuses and disconected the " nutral" in a 1/2 inch grc pipe with two 14 thhn wires suppling 120

after "turning the power off" we found 60 volts on one leg and 20 on the other

not relly a big deal but we are in a gas refinery sparks of any kind have a very bad impact on wellll................ boom big boom........ maybe

end result I discharged the "cap" with my finger clearing the fault

capacitence made with round pipe and two wires caused by inductance. caps are made very differently but application is important.

Not only is voltage rating important, for high power applications, current handling is also important .Whilst metalised film types e.g. PEPT may have sufficient voltage rating and low leakage, the metal film is very thin and hence has a limited curren carrying capacity as I found out the hard way by specifying this (wrong) type as a replacement for an obsolete type in a radar system. High power types are made by coiling up a layer of metal foil (much thicker than metalisation film) with the insulating material. It is best to use a type specifically intended for motor applications, making sure that the type has the correct voltage rating as well as capacitance value.

Regards,

A.L.

Freedom is fragile. Guard it well!

It has to be a non polarised type for ac use and the oil acts as the insulation, and to cool the capacitor as well; they can get rather hot with big motors.