[Boatanchors] Using ac motor start capacitors...

[Carl]

I see several areas of disagreement with your post.

1. Electrolytic motor run capacitors are passe. Modern construction is 
polypropylene film, metallized film or fluid filled (aka oil filled as in 
the old days).

2. Typical temperature ratings are 55 to 70*C  Any quality electrolytic is 
either 85 or 105*C

3. Dissipation factor can reach .70 % at values hams would consider for a 
PS.

4. Maximum capacity ranges from 50 to 70uF. Not really useful in a long 
series string. A 440VAC may be useful in a TX running 807's, 4D32, etc. or a 
tetrode screen supply.

5. Life increases rapidly with reduced voltage voltage. Example: Running at 
only 90% of rated voltage the life is 225% of the 100% rating. Its 375% at 
85%.

6. Life increases rapidly at reduced temperature. Example: At 50% of rated 
temp life goes up to 5000%.

Certainly not my choice for any amp.

Sources: American Radionics, CDE and GE spec sheets

Carl
KM1H



----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <boatanchors at mailman.qth.net>
Sent: Wednesday, September 30, 2009 2:57 PM
Subject: Re: [Boatanchors] Using ac motor start capacitors...


> In answering a question posted by Greg Mijal, Marvin Match wrote:
>
> <snipped>
>
> "BUT, on the next half cycle, the cap will see minus 520 volts in
> addition to the plus 520 volts from the previous half cycle, so the
> DC breakdown voltage has to be at least 1040 volts for a cap rated
> 370 volts AC."
>
> This would be for a cap used in voltage doubler power supply
> service.  Greg's question concerned translating the AC voltage
> withstand rating of a capacitor to a DC voltage withstand
> rating.  The peak voltage that the cap would see in sine wave
> AC service would be 1.414 times the RMS value of the applied
> sinusoidal AC voltage.  Therefore, in 370 VAC sinusoidal
> service, the cap would see 1.414*370 or about 520V. That
> 520 volts would be the withstand rating for the cap expressed
> either as the peak of an AC waveform or as a steady DC voltage.
>
> Marvin Match wrote further:
>
> "Gotchas? The only one I know of is that some AC caps are not just
> caps, but have a series resistor integral to the unit. These will be
> labeled "protected" or something similar, and I'm not sure that a
> series resistor is the only way to "protect" a motor cap."
>
> Some motor run caps do indeed have a resistor, either internal
> or external.  This is a parallel bleeder resistor to protect a
> hapless technician from a real Gotcha!  In DC filter cap service
> you therefore get a bleeder resistor for free, but its power
> rating is based upon the aforementioned RMS case, not upon the
> actual withstand rating. If the resistor were at rated
> dissipation at the cap's rated RMS voltage, then it would be
> underrated by a factor of sqrt(2)^2 or 2 when continuously
> subjected to the cap's withstand voltage.  So, in cases where
> the supplied resistor is underrated (above 370VDC in this case)
> one might desire to yank if possible the supplied resistor and
> then substitute one of adequate power rating.
>
> One responder wrote of avoiding use of electrolytic AC motor
> start capacitors for DC power supply filter service.  A friend
> of mine (note that I evade responsibility here) has successfully
> used scrounged caps of that type in that manner.  In DC supply
> service one sometimes might be concerned with ripple current
> rating; the ripple current might cause excessive heating of
> some electrolytic caps.  In motor start or run service, however,
> "ripple current" is all there is and there is lots of it! So
> in addition to being voltage rated, these caps are also designed
> to handle AC current.  One would not see any problems in normal
> DC power supply service from the use of these caps.  The one
> "problem" might be the increased bulk and cost of these capacitors.
> The former is of less concern to us because we often have room to
> spare in our beloved boatanchors, and the latter is of no concern
> to the scrounger (that's us).
>
> Drew
>
>
>
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