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Jameco Challenges

No Need for Expensive Equipment

By Posey Bowers

Build your own inexpensive equipment and save money

I wanted to share my thoughts about the Kill-A-Watt energy consumption meter from P3 International. This is a wonderful capacitor checker for run capacitors. A run capacitor uses the charge stored in the dielectric in order to boost the electrical current providing power to an electric motor.

For oil filled or dry caps with working voltages from 125VAC and up, I can get the true power consumption of a capacitor (capacitors should not consume power) as well as the power factor. High wattage indicates a bad internal connection (resistance) or leakage, and high power factor tells me that something is not right with the capacitor under test. A good capacitor should exhibit very low power factor, very little to no power consumption and a current that, when used in the capacitive reactance formula provided, yields a capacitance (in µF) that is reasonably close to the value indicated on the can.

Of course, the first test is a physical examination of the capacitor for damage or bulging and a DC leakage test to check for leakage. No need to test if there is leaking or bulging as even if it tests well, it should not be relied upon. Also, perform a voltage test to be sure the capacitor is NOT charged as well as a resistance test to indicate a lower and climbing (to infinity) resistance as the capacitor charges. Leakage test is at 125V line voltage so be careful. Put a 2A-rated diode in series with a 125V Christmas tree light (to limit current) and these two in series with a milliamp meter (IEC Group 1 or better) and the capacitor under test. Lamp should be bright at first and dim as capacitor charges and leakage current falls off.

Please connect the Kill-A-Watt to a short extension cord that plugs into a grounded 125VAC outlet. Better yet, limit the current through the Kill-A-Watt with a 500W (or equivalent) incandescent light bulb in series with the HOT (black) lead for safety. Connect the run or PSC capacitor under test to the Kill-A-Watt via an ordinary 2-pole line cord (18 AWG or 16 AWG will be fine) that terminates in insulated alligator or crocodile clips.

Starting with the Kill-A-Watt unplugged, make the connection to the capacitor, plug in the Kill-A-Watt and take the current, voltage, power factor and wattage readings. Should the large wattage bulb in series with the Kill-A-Watt light brightly, you have a shorted capacitor. Never short such capacitors to discharge them as many have an internal fuse that will be blown.

With the capacitive reactance formula, you get a value to compare with the printed value on the capacitor. Say we have a 10µF, 330V capacitor connected to 125VAC line. How much current (in amps) should it draw? 1 / (2π x FC). That is 1 / (2 × 3.14 × 60Hz × 10 x 10-6) and I get 265.3Ω of capacitive reactance. The 10µF cap with an XC of 265.3Ω should draw (I=E/Z); 125V/265.3Ω = 0.47A (at 60Hz). The formula tells me 9.964µF.

Good luck Jameco and keep up the good work. Your readers with big money may be tempted to just buy expensive test equipment though I suggest building your own and you will be well rewarded.

Posey Bowers
New Orleans, LA

Warning: This testing procedure involves dealing with high voltages so be sure you know what you are doing, or consult someone who does. Please take all necessary safety precautions.