# Capacitor Value

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#### codan

##### New Member
I have a Plessey AC Motor Start Electrolytic Capacitor USA from a 240v single phase motor that i am trying to get the specifications for, unfortunately most of the writing is worn off so the exact specs are not known:

It is Black in colour, 52mm dia X 112 long, it's hard to make out but it looks as if it might have said 230v?
There is some of the part number remaining but the first & last digits are hard to read.
It looks like 677-9230-1 or 377-9230-1

Can someone help out or is there a site i can go to to look for it?

Do you know the capacitance?

Have you measured it?

Assuming it's rated for AC you could connect it up to the mains with a 3W 10R series resistor. Measure the voltage across the resistor to determine the current; calculate: the capacitor's reactance at the mains frequency and then the capacitance.

The chances are that most non-polarised capacitors that can handle the current and voltage will be fine.

No, the capacitance value is worn off & is the value i need to know, 10R resistor is that 10 Ohms.

What calculation do i need to work out the capacitance value from voltage across the resistor?

Thanks

Assuming that R is small compared to Xc:
$I = \frac{V_R}{R}\\ X_C = \frac{V}{I} \\ C = \frac{1}{2 \pi X_CF}\\ V_R = \text{Voltage measured across the resistor.}\\ R = \text{Resistor value.}\\ X_C =\text{Reactance of the capacitor.}\\ I = \text{Current}\\ F = \text{Mains frequency}$

Check it with an ohmmeter first.
If you put more than a few volts/amps into the cap, wear face protection.

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