Motor Run Capacitor Test Chart

Motor run capacitor, Click to see the full size image
Click to see the full size image
    You can test a motor run capacitor using a 100 watt light bulb and a digital ac volt meter.
There are testers on the market but they are not very conclusive or they are expensive. This test cost $5-10 in parts plus a volt meter which most anyone who has an ohm meter also has an ac volts scale and just about anyone can do this test. Believe it or not, this is a fairly accurate test. A more accurate test is below.

    Never use an ohm meter to test a motor run capacitor. It is a very inconclusive test and only shows that there is "something" in the circuit. You are essentially "shooting blind".

    The test consists of connecting the capacitor under test in series with a 100 watt light bulb on a 120 volt ac line. If you have a single pole contactor then you can test your run capacitors in the unit without removing them and use the case of the unit as ground. A two pole contactor system may not work unless one of the line leads is hot to the compressor at all times (in the highly unlikely event that you are running a single phase unit on a 240 (not 208) volt 3 phase system usually found in a commercial setting, this test will not work)! Contact me for more information. Check the circuit by connecting the light bulb from one side of the line to the case to make sure the bulb and circuit is good before proceeding! If the voltage result is for a smaller capacitor than the one under test then it is bad. I used a pigtail socket like the ones used in new construction for temporary lighting. Clip leads are available from Radio Shack. I will add some images when I get a moment.

    I used a 130 volt ac 100 watt rough service bulb because that is what I had on hand and a rough service bulb is less likely to break over an ordinary 100 watt bulb. Results will be slightly different if using and ordinary 100 watt light bulb especially at the higher values.

    To the results: Click here to get the chart by itself.

Micro Farad of run capacitor. Voltage across bulb.
5 mfd. 6-7 volts ac.
7.5 mfd. 20 volts ac.
10 mfd. 27 volts ac.
15 mfd. 70 volts ac.
17.5 mfd 77 volts ac.
20 mfd. 88 volts ac.
25 mfd. 98 volts ac.
30 mfd. 104 volts ac.
35 mfd. 109 volts ac.
40 mfd. 112 volts ac.

    To test motor start capacitors you will need a 200-300 watt bulb depending on the size. I will add a chart on that soon. Caution: capacitors can hold a charge when removed from the circuit unless they have a bleeder resistor. Discharge the capacitor through the light bulb before handling. A capacitor in a working motor circuit cannot hold a charge, it is discharged the instant the power is removed.

Scott Meenen

Test #2 This is the most accurate way to measure a run or start capacitor.

C in MFDs= (2654I)/E

C in MFDs= (2654A)/V

C in MFDs= (2654 * Amps)/voltage.

C in MFDs= 2654 multiplied by Amps through the capacitior then divided by Volts across the capacitor

The 2654 is the number you get when you multiply 60 HZ by 2 PI and divide into 1 Million (6 places).

XC(Cap reactance in ohms)=1/(2PI*F)*(C in Farads) XC=Volts/Amps (C in Farads)=1/(2PI*F*I/E)

The information below is not related to testing capacitors.

It is here for more information.

1819 light bulbs and sockets avaliable from Radio ShackClamp om ac ammeter Sperry Digisnap avaliable at Lowes or The Home Depot

If you need to see the 24 volt control signal this lamp is an easy way do it. This 1819 lamp and sockets are available from Radio Shack. This clamp on ac ammeter is valuable for measuring ac current. The built in "Non Contact Volts" feature is worth the price. About $80 at Lowes or The Home Depot.

Link to White Rodgers that has a good look up table of different manufacturers. You will notice that Lennox's color codes are TOTALLY WEIRD to put it gently.
Link to White Rodgers Clip art that has pictures of their different control products.

I would highly suggest identifying each wire by connecting the hot lead to each function wire with clip leads and a 3 amp fuse and recording the function before installing your new thermostat. While this is more trouble it may save you from ruining your day. 

Related pages:
    Electric furnace trouble shooting and repair.
     Thermostat stuff.
    Heat anticipator setting.
    Oil furnace trouble-shooting and problem solving.
    Blower and fan controls.
    Heat pump defrost cycle
    Heat pump repair stuff
    Defrost control images
    Contactor explanation
    Gas furnace intermittent pilot images
    Outdoor reset added to honeywell 594 thermostat
    Trane Heat pump thermostat wiring question. With more terminal codes.
    Millivolt gas valve electronic thermostat fix.
    Wiring Scheme for a Wood Burning Furnace.

To read about fan motors and furnace blower controls follow these links.

Did you consider Solar Energy?

 If your outdoor unit turns to a block of ice in the heating mode read this.

Thank you for visiting our site Scott Meenen N3SJH

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Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
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Serving MD, DC, and Northern VA

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