[Boatanchors] Using ac motor start capacitors...

[J. Forster]

You're very welcome.

-John

=================


> 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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>
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