[Boatanchors] Using ac motor start capacitors...

[ronw8]

Thanks John for putting this strange discussion to bed.
---
Ron


J. Forster wrote:
>> On 1 Oct 2009 at 12:00, boatanchors-request at mailman.qth.net wrote:
>>
>>     
>>> 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.
>>>       
>> No It's not. Drew, you missed my meaning completely.
>>
>> Look at it this way: Take a cap and charge it to 100 volts. Now while it's
>> charged, reverse the terminals and connect it again to a 100 volt supply.
>> In
>> this case, what voltage would the dielectric in the capacitor see?
>>     
>
> ========
>
> Essentially infinite current. You HAVE to include a charging resistor in
> the circuit or you see singularities in the equations.
>
> The thing is modeled as a capacitor in series w/ a resistor. A femtosecond
> after you connect the capacitor to the reversed voltage supply, the
> capacitor voltage HAS NOT CHANGED. It is still +V and ALL the doubled
> voltage appears across the resistor. Eventually, after many time
> constants, the voltage across the capacitor will go to -V and the voltage
> across the resistor will drop to zero.
>
> The ONLY case where the capacitor could see more than the peak AC supply
> is if there is an inductor and the thing is resonant.
>
> -John
>
> ==================
>
>
> I contend that it "sees" 200 volts, therefore the cap must be able to
> withstand 200 volts
>   
>> DC. A capacitor rated for 200 volts DC would be safe in an AC application
>> for only 71 volts RMS.
>>
>> Or, in rating a cap for AC service, the dielectric must be able to
>> withstand the
>> peak-to-peak value of the voltage applied, which is 2.8 times the AC RMS
>> voltage, not the peak value.
>>
>> Marvin
>> KA7TPH
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>>     
>
>
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