Correctly Sizing a Capacitor

  I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.  Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size!
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  1. OK, so what can one do when one wants to install a capacitor to help a compressor starting when it has not previously had one? How can you then go about calculating and or to use a run or start cap? All I know is I have a freezer rated to use 90watts and is single phase 1/6th horsepower compressor motor. It is a common house freezer 240volt in Australia.

  2. This article is misleading in that it will tell the effective capacitance of the capacitor that is installed. If the wrong cap was installed it will be replaced by a wrong capacitor.
    The correct capacitance value to be used is normally stated on the name plate of the compressor. The measurements/ calculations have to match the spec of the motor label.

  3. Using the capacitance on the name plate is a good general starting point. However, if the motor or compressor has been in operation for some time (years); chances are it has changed its coil characteristics due to heat and/or unbalanced loads (especially when one cap was bad or a power surge/brown out occurred). Chances are you don't have a means to measure the coil inductance. If the motor won't turn over and you suspect the cap you should never go for the smaller cap. The formula above will help to reign in on what capacitance is there (actual value - not labelled value). If the motor has seen some use and efficiency is key; a value slightly (emphasis on slightly) higher or lower value may improve performance.