Capacitor Selection

Ok so i know how to read a capacitors mfd but my question is multi fold regarding a formula i came across. In one video the guy said to measure a capacitor under load take the amps of the wire leading to the compressor start multiply by 2650 and divide by the vac reading on the common to start of the cap and it will give you the actual capacitance of the cap. Another website used the exact same formula except for the number 2650, they used 2652 as the number and said that is the formula to check what cap a motor actually needs.

My questions.

1) Is there any truth to the second formula? Bearing in mind a capacitor has to cause a 90 deg phase shift, is a simple formula accurate to determine what size cap is needed by a motor if you dont have a reliable source to work from ie a manufacturers rating etc.

2) If the first formula is accurate then what would happen if the compressor was drawing more amperage than it should ie from an overcharge or similar factors affecting the compressor amp draw. Not that i wouldnt check these things but for the novice out there who would rely on this i could foresee this formula as a problem?

3) From personal experience ive come across techs using the +- 5% rule a bit too lenient. first cap being a 35uf, tech replaces it with a 40 because he doesnt have a 30 or 35 in the van and he is 50km away from a supply shop. Next guy does the same thing, only putting the next size up before you know it the compressor has failed due to this problem. Is there a formula to determine what a motor needs cap wise?

50 viewers dont all jump at once and damage the pace maker to respond :D

I was one of the 50 but didn't know the answer to you question :D

I fit the size capacitor the motor data plate calls for, the +/- 5% is the manufacturing tolerance for the capacitor, not the motor, nor for the use of techs too lazy to get the correct part. :p

Quote:

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Originally Posted by monkey spanners

I was one of the 50 but didn't know the answer to you question :D

I fit the size capacitor the motor data plate calls for, the +/- 5% is the manufacturing tolerance for the capacitor, not the motor, nor for the use of techs too lazy to get the correct part. :p

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Thanks for the reply monkeyspanners, i checked up it seems you can only do this with an oscilloscope.

I am aware of the vector shift for higher starting torque using start capacitors, and the higher voltage requirement for permanent run capacitors . Short ot testing capacitors with Fluke that is extent of it for me. Somewhere I have description for paralleling and series up caps.
Generally have enough problems with the refrigeration side of things, hence this is a refrigeration forum! But can understand if vector shift is too high with wrong capacitor the motor will stall overload and then fail.

Blew one up once!pretty darn scary believe me.

Yes and I have been whacked by the odd one discharging into my pinkies , geez it hurt