[Boatanchors] CAPACITORS

[brianclarke01 at optusnet.com.au]

9 pF for a motor start capacitor? That must be a very tiny motor, like ~ 1 W.

However, if it's really 9 uF, remove the connecting leads and put it in series with a 100 W lamp. Make up a jury-rig circuit - hot lead to the cap, other cap lead to the lamp, other lamp lead to the cold power lead. Connect AC ammeter in series. Connect to the mains. Switch on; the lamp should glow dimly, ie, not as brightly as when directly connected to the mains. 

Measure the Voltages across the lamp and the capacitor, separately, and the circuit current. Put these Voltages on a right-angled triangle to see if the hypotenuse adds up to your applied line Voltage. 

Estimate the reactance of the capacitor from the Voltage and current; compare with the calculated reactance of the capacitance of the capacitor. At 60 Hz, you should get 295 Ohm; at 50 Hz, 354 Ohm.

If you get a very bright lamp or no glow at all, or if any of these calculations does not get the required answer, your capacitor is probably cactus.

73 de Brian, VK2GCE.



> W4AWM at aol.com wrote:
> 
> This may be a bit off the subject, but can anyone tell me how to check a  
> motor starting capacitor without substitution. I don't have a spare. 
> Capacity is  9pf @ 250VAC.