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| Re: Motor start capacitors? |
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Group: alt.engineering.electrical · Group Profile
Author: phil-news-nospamphil-news-nospam Date: May 13, 2008 19:52
On Tue, 13 May 2008 11:10:14 -0400 John Gilmer crosslink.net> wrote:
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|> None, I don't think Digikey stocks capacitors intended for current
|> handling
|> purposes. That's why you can't find current ratings. There are all kinds
|> of
|> high power, high current caps but you won't find them at Digikey, Mouser
|> or
|> any other electronic supply house. Caps for motor starting, motor running,
|> power factor correction, high current RF work, energy storage, and a whole
|> host of power applications are available from suppliers in those fields.
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| Hey, if you know the capacitance and the voltage (& frequency) then the
| current is also known. If a cap is rated for the frequency and voltage
| then it can handle the current. The reason that the AC electrolytics are
| rated as either "starting" or "running" is that the "starting" caps can only
| handle the voltage (& current) for a short time before they start getting
| warm/hot. "Run" type caps can handle the current indefinitely. IOW: you
| can safely hang them across the AC mains (with some overcurrent protection
| in case the cap fails.)
Capacitors aren't rated as to frequency, either. So for a capacitor, that is
not known. A capacitor used for smoothing DC would have to have a high voltage
for the DC, being able to carry a high charge voltage, but yet not have a lot
of current due to limited ripple at that point. You can build at capacitor for
a given value of microfarads and a given peak voltage, with varying sizes for
heavier or lighter conductors (and conducting foil inside).
There are some very high farad capacitors intended to store power in parallel
with batteries to allow changing the batteries. They work on CMOS or other
circuits that draw very little power. Apparently my universal remote control
for the TV has one, as it says I have 10 minutes to change the batteries, but
if I press any button without batteries present, the stored codes are lost.
The circuits I have seen for these have resistors between the batteries and
capacitor to avoid them charging up too quick if batteries are inserted with
no charge. Sounds like capacitors with extremely thin plates that cannot take
a high current.
--
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