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RBM
18-09-2012, 09:37 AM
Hello
was wondering if any one can tell me what the voltage rating on capacitors is about ie 240 and 300 volts with the same microfarad rating
Thank you

chemi-cool
18-09-2012, 10:42 AM
Voltage rating must be printed on the capacitor!
In any case, I advise you to use capacitors with the highest voltage rating ,ie, 450V.

Rob White
18-09-2012, 04:23 PM
.

Capacitors arn't realy sized by voltage.

It is the microfarad rateing that is important.

They must be within a voltage range so you would not choose
a capacitor with a lower voltage range than the one you require.

Always go to the higher voltage, never down to the lower one.

The capacitor might only be in a 115 or 230 v circuit but still
may have a large voltage rating like 430 v.

Also capacitors will work in parallel so if you need a 10 microfarad
and only have a few 5 microfarads in your van then they will parallel
up to 10 This means that 2 or more smaller capacitors can be wired
in parallel to replace a single larger capacitor as long as the total MFD
rating is equal.

Regards

Rob

.

chemi-cool
18-09-2012, 04:57 PM
Voltage is important for capacitance!!
http://www.kpsec.freeuk.com/capacit.htm

Rob White
18-09-2012, 05:32 PM
Voltage is important for capacitance!!
http://www.kpsec.freeuk.com/capacit.htm

You are correct, voltage is important I did not mean to indicate
it was not. I was just making the point and agreeing with you
it is important to always go for the higher voltage and not the lower.

Regards

Rob

.

monkey spanners
18-09-2012, 09:18 PM
240V ones are often start caps or for flourescent light ballasts the ones in the 370V sort of range tend to be your run caps, the high voltage rating is to cope with the back emf from the start winding once the motor is up to speed (not an issue with the start cap as its out of circuit by then)

RBM
19-09-2012, 04:48 AM
Thanks for your replys, mostly I use capacitors on single phase (240 Volt) compressors start and run caps, some capacitors are rated same microfarad but different voltage I always use the higher voltage rating but wondered how important it was.

Tesla
19-09-2012, 06:54 AM
The voltage rating is to with stand the actual peak voltage which is 353V peak for 250V RMS. This is why the capacitor rating would have to be larger than 353V. So for Ausie the minimum rating should be 360V, 380V would be better and 460V is probably the best. These ratings would effect the life of capacitor and may provide better protection under hotter environments. As above there is more protection for Back EMF with higher ratings.

Peter_1
26-09-2012, 08:06 PM
The voltage rating is to with stand the actual peak voltage which is 353V peak for 250V RMS. This is why the capacitor rating would have to be larger than 353V. So for Ausie the minimum rating should be 360V, 380V would be better and 460V is probably the best. These ratings would effect the life of capacitor and may provide better protection under hotter environments. As above there is more protection for Back EMF with higher ratings.
Tesla, this is for me the only right explanation, RMS value isn't the right value to choose your cap.
Chemi, if you connect a 50MF/240 cap or a 50MF/400V to a main of 240V, the capacitance will be the same in teh 2 cases.