Motor start capacitors?

wrote:


There are all kinds of limitations, explicit, implicit or even unmentioned. For example, how many cheap capacitors are rated for high frequency inductance? Energy storage capacitors such as used for large pulse lasers have all kinds of limitations. RMS current, ringing, etc. Sometimes these have to be checked out by the user. Even then, change in manufacturing can upset results after years of satisfactory performance.
Bill
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That is what I would expect. It is possible,however, that there may be some unwanted resonant effect.
Let me ask the question some other way. Is the inductance of the start winding too small or too great to resonate? A quick calculation indicates to me that it takes about 50mH to resonate with a 140F capacitor at 60Hzm
Bill
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wrote:

Hmmm? I never thought of it as a resonator. A simple calculation shows the impedance to be about 19 Ohms. This means the winding resistance will likely swamp the inductor reactance and soften any resonance (low Q). I don't know what the typical winding inductance is, I suspect it would be higher than 50mH. What we need is actual values from a real motor of L, R and C to answer the question.
Of course if it were truly in resonance there would be "0" deg of phase shift and the field would not rotate. That means it has to be off resonance to get phase shift.
In a so called split phase motor there is no capacitor and all required phase shift is provided by the starter winding's L and R.
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----------------------------
wrote:

--
Actually not. The main winding current at start will be highly reactive- say
70-80 degrees lagging so a resonant circuit in the starting winding would
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Salmon Egg wrote:

I assume that when you say "electrolytic capacitor" you mean a motor start capacitor. These are non-polarized electrolytics specifically designed for starting motors. They should never be across the line (actually in series with the start winding) longer than 1 or 2 seconds. They will burn up if they are connected continuously. These capacitors sometimes have a 50% tolerance to begin with, so it is unlikely that a small difference in the capacitance value would cause any problems. Be sure the voltage rating is at least as high as the original, as there is a higher voltage on the start winding than the operating voltage. The fact that two failed leads me to believe there is a problem with the starting relay or a short in the winding.
Ben Miller
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wrote:

I havenb't seen a motor cap that was anything but oil and paper. They are usually a few microfarads and fairly large silver cans. The 140uF just doesn't sound right to me.
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snipped-for-privacy@aol.com wrote:

Those are run capacitors, and can remain connected continuously. Start capacitors are electrolytic, and can have values up to several hundred uF. They are only intended for momentary connection. Either the centrifugal switch or a starting relay disconnects them once the motor is turning.
The design of the motor dictates which type(s) are used, and their values.
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Benjamin D Miller, PE
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They are most often in a black hard plastic case, but not always. All of them should be under a base or in a metallic cover. They sometimes explode and that hard plastic makes for dangerous shrapnel.
Chuck P.
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Salmon Egg wrote:

Electrolytic? With an AC voltage across it? That's not something I think I've ever seen.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Paul Hovnanian P.E. wrote:

You've never seen the crossover in a stereo speaker? Horizontal deflection circuit in a CRT monitor? Cap start, cap run motor with two humps? Non-polar electrolytics are everywhere, they're vastly outnumbered by the polarized DC type, but they're not uncommon.
My spa pumps have the start capacitor mounted under the plastic cap on the end bell, the run cap is under a metal hump on top, it's only needed for power factor correction.
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Why are some capacitors polorized and some aren't. They both pass AC.
Does it only depend on the value of the capacitors? Small capacitors are non-polorized while larger values are?
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Steve wrote:

They'll pass AC, but put any kind of load on a polarized electrolytic that results in it being charged backwards and it'll blow up.
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| |> start capacitor | | Why are some capacitors polorized and some aren't. They both pass AC. | | Does it only depend on the value of the capacitors? Small capacitors are | non-polorized while larger values are?
The electrolytic can be optimized to work better when one side is going to always be charged positive and the other side charged negative. This is the case for smoothing DC in a PSU. Where the capacitor is going to get AC, then this advantage is not available. I don't know the specifics, so you and your friend Google are on your own to find out. You might want to also get a Digi-Key catalog at http://digikey.com/
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Paul Hovnanian P.E. wrote:

They're electrically equivalent to two capacitors, with reversed polarity, in series. The effective capacitance is, of course, only one half of each unit, and they are not intended to pass AC current for more than a short time.
--
Virg Wall, P.E.

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I believe these capacitors are made from TWO aluminum foils with oxide form forming on both foils.
Bill
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----------------------------
remove the X to answer
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