How to check a capacitor while the system is running.

Yes with a multimeter and using the microfarad function to check a capacitor maybe nice. But this does not test the capacitor under “load” conditions.

Let’s take a look at a typical high voltage circuit of a residential air conditioner or heat pump outdoor unit.

Now to do this we will need to get a few measurements.

Turn the system on and let the system run for 5-10 mins to stabilize operation.

Lets start collecting amp draws and voltages.

1st get the amp draw of the fan motor on the brown wire connected to the fan terminal of the capacitor. Example 0.5 Amp

2nd get the amp draw of the compressor start wire, the blue wire connected to the herm terminal of the capacitor. Example 4.4 Amps

3rd Now lets get the voltage of the condenser fan motor between the brown wire and yellow common wire at the capacitor. Example 271 Volts

4th lets get the voltage of the compressor between the blue start wire and yellow common wire at the capacitor. Example 297 Volts

Now we can do some math. Here is the formula we will use. Use this formula for each component.

Some formulas use 2650, 2652, 2654. We will use 2652.

Lets do the fan first, 2652 X 0.5 amps / 271 volts = 4.9 mfd

Next the compressor, 2652 X 4.4 amps / 297 volts = 39.3 mfd

If your capacitor is rated for 40/5 mfd, the readings are within 10% and the capacitor is good.

Another Example; if the capacitor is rated 45/5 mfd, an allowable tolerance range for our example is 49.5 mfd – 40.5 mfd.

Compressor 2652 X 6.1 amps / 297 volts = 54.5 mfd. This capacitor is performing at 54.5 mfd, which is greater then the 10% tolerance. This capacitor should be replaced.

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Tech tip the combined average amp draw of the R & C leads from the compressor should equal the amperage of the S lead from the compressor.

If not, the capacitor maybe weak and need to be replaced.

A weak capacitor can cause the compressor to overheat, causing high discharge temperature which causes a breakdown of the oil lubricant & higher amp draw. The COST OF OPERATION INCREASES!

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