Tag: capacitor

Knowing how to properly combine capacitors in series and parallel is a great, practical field skill to employ when you need to get a customer up and running and you don’t have the exact size.

Increasing in size is easy. Just connect in parallel and add the two sizes together. For example, if you needed a 70MFD capacitor you could easily connect a 50 and 20 in parallel will add up to 70MFD. Connecting in parallel is as easy as making two jumper wires with connectors and jumping one side of each capacitor to the other and then connecting one side like usual.

Series is a little more tricky, it goes like this

Total Capacitance is 1 ÷ (1÷C + 1÷C) = Total MFD When Wired in Series

The result is that the total capacitance will always be less than the smallest capacitor. Let’s imagine a real-world scenario where you need a 3MFD capacitor and all you have is 5 & 7.5 MFD on your van.

The math would be

1 ÷ (1÷5 + 1÷7.5) = Total MFD

_

1 ÷ (0.2 +.13) = Total MFD

_

1 ÷ (0.33) = Total MFD

_

3.03 = Total MFD

Definitely not something you will run into every day but a nice knowledge tool to have in the noggin toolbox

— Bryan

 


You have seen the C terminal on a dual run capacitor before. You have also seen the C terminal on a compressor.

It stands to reason that they would both connect together right?

Wrong, They don’t connect together and they aren’t even related, at least not in the way that you think.

In both cases, the C denotes a “common point” in the dual capacitor it is the common point between the fan capacitor (fan) and the compressor capacitor (herm). In the compressor it is the common point between the run and start windings (this is why R+C + S+C = R+S if you ohm a compressor)

The C terminal of a dual capacitor is actually fed from the OPPOSITE leg of power as the C terminal on the compressor. This is because you must power the start and run windings with the same leg and common with the other leg.

The way I always said it was “The same leg that feeds start feeds run” and the C terminal on a capacitor is actually the common feeds for the start winding of the compressor and fan (OPPOSITE side from the fan and herm plates on the capacitor)

So compressor terminals

C goes to one leg of power

R goes to the other

S goes to the HERM terminal on a capacitor with the other side of that capacitor (C) going to the same leg that feeds R.

C what I’m saying? Confusing

If you are new to the trade and you see the designation C or the word common don’t assume it is the same as other C and common terminals and start connecting stuff together… Unless you like creating smoke.

— Bryan

When testing a run capacitor many techs pull the leads off and use the capacitance setting on their meter to test the capacitor. On a system that is not running there isn’t anything wrong with this test, but when you are CONSTANTLY checking capacitors as a matter of regular testing and maintenance that extra step of pulling the connectors off can be time consuming and in these cases it is also totally unnecessary. Testing the capacitors UNDER LOAD (while running) is a great way to confirm that the capacitor is doing it’s job under real load conditions which is also more accurate than taking the reading with the unit off.

First, if you are used to doing capacitor checks during the “cleaning” stage of a PM you are going to need to change your practices and do your tests during the “testing” phase. These readings will be made at the same time you are taking other amperage and voltage readings during the run test.

This method is a practical method and is a composite of two different test practices combined –

  1. Read your Volt (EMF) and Amp (Current) readings like usual and note your readings.
  2. Measure the amperage of just the start wire (wiring connecting to the start winding), this will be the wire between your capacitor and the compressor. In the case of 4 wire motors it will usually be the brown wire NOT the brown with white stripe. Note your amperage on this wire..
  3. Measure the voltage between the two capacitor terminals, for the compressor that would be between HERM and C, for the cond fan motor that would be between FAN and C. Note the voltage readings
  4. Now take the amp reading you took on the start wire (wire from the capacitor) and multiply by 2,652 (some say 2650 but 2652 is slightly more accurate) then divide that total by the capacitor volts you measured.  the simple formula is Start Winding Amps X 2,652 ÷  Capacitor Voltage = Microfarads
  5. Read the nameplate MFD on the capacitors and compare to your actual readings. Most capacitors allow for a 6%+/- tolerance, if outside of that range then replacement of the capacitor may be recommended. Always double check your math before you quote a customer. We need to make sure we are accurate when advising a repair.
  6.  Repeat this process on all of the run capacitors and you will have assurance whether they are fully functional under load or not.
  7. Keep in mind that the capacitor installed may not be the CORRECT capacitor. The motor or compressor may have been replaced or someone may have put in the wrong size. When in doubt refer to the data plate or specs on the specfic motor or compressor.

If you need a visual, here are some good videos on the topic. Note that some will use 2650, some 2652 and some 2653. It all depends on how many decimals of pi they are using in their calculation but all of them will result in an accurate enough conclusion for our use.

At first doing it this way may take a few minutes longer but in the long run you will go quicker, have fewer mistakes (forgetting to put the terminals back), have a better understanding of how the equipment is operating and get a more accurate reading.

Once you replace a capacitor always recheck your readings to ensure the new capacitor reads correctly under load.

It is also a good practice to check Capacitors you have removed with your capacitance setting on your meter as a reference point.

While this method is good, it is only as good as your tools and your math. When in doubt, double check… and always be in doubt.

— Bryan

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