testing capacitator with digital meter

[erinj]

I'm not sure if I should have continued in my previous post or started a new thread. I apologize if I am posting incorrectly.

I asked previously about a condenser fan that, about 5 minutes after starting, slows down to almost stopping. Lazypup suggested it might be one of two things...dry bearing or a capacitor. If not, how do we check to see if that's the problem?

Hubby checked the fan..fan spins freely when spun by hand, when unit is on or off, and the shaft is not loose. Does that rule out the problem being a dry bearing?

I've read through other posts and found the procedure for testing the capacitor, but I notice using a needle type meter was recommended. We only have a digital meter. Can you please explain the procedure or difference in procedure for testing the capacitor using a digital meter?

Are there other readings we should take, like the amp going to the motor, etc?

Again I thank you for taking the time to help us. You guys are a godsend! Our very own HVAC gurus!


erinj

[LazyPup]

There are some high end digital meters that have both digital and analog function, but as a rule most digital meters are not suitable for testing capacitors.

Most digital meters use a logic circuit that samples the current and averages it over a modest time. This produces a much more accurate voltage reading, but because it takes time for it all to happen the meter does not function fast enough to see the momentary reaction required to test a capacitor.

[erinj]

thanks for quick response, Lazypup! Neighbor just lent us an analog/needle type meter...and I've downloaded your directions for testing the capacitor from your answer to another member. Thanks again and I'll let you know what we find! erinj

[erinj]

Using analog needle type ohm meter, checked capacitor according to your directions found in this thread:

http://homerepairforum.com/forum/showthread.php?t=1662

(Great directions, BTW, Lazypup, clear and easy to follow)

The needle did rise-- but not all way into mid-range--then dropped down slowly.

Does the fact the needle didn't rise as far as mid-range indicate a problem with the capacitor?

If capacitor is not bad, what would you recommend as the next step in finding where the problem lays?

Again, I want to thank you for taking the time to help me and your willingness to share your expertise.

erinj

[LazyPup]

Technically a capacitor has two internal plates that are separated by an insulator. When a voltage is applied the plates will develope a charge equal to the applied voltage and when the polarity is reversed it will discharge the stored energy.

When testing a capacitor we use an analog OHM meter and it displays the amount of energy required to charge the capacitor plates. When we then reverse the leads the meter is measuring the amount of energy being discharged from the plates. Understading that all OHM meters are different and the condition of the batteries in the ohm meter are different, it then stands that we will not get any exact meter scale reading.

Basically we are looking for one of three possible results.

1. If the meter moves upscale and back, then repeats the same action when the leads are reversed the capacitor under test is Okay.

2. If there is no meter movement the internal wiring of the capacitor is "Open" and the capacitor needs to be replaced.

3. If we get a full scale deflection on the meter indicating infinite resistance the internal wiring of the capacitor is shorted out and again, it needs to be replaced.

When you conducted your tests you got a deflection so we can assume the capactor to be ok.

You stated that when you turned the fan blades they turned fairly easy, but in some cases the shaft may turn freely when cold but as the bearings begin to heat from friction during operation they begin to seize.

Examine the end bells (end caps) of the motor carefully. You may see what looks like a small tube from the bearing to the outer circumference of the motor. If the motor has that molded tube there should be a small plastic cap plugged into the tube on the outer circumference. That is the bearing oil filler tube. Generally the caps on the tube are red plastic but sometimes they are painted the same color as the motor housing or they may be metal pins that look like a rivet head. If you can pry those caps off you can insert the tip of a small oil can and squirt a few drops of light machine oil into the ports. The problem here is that motors that have the oil ports should be periodically oiled, (at least once a year) Generally when the felt oil pads surrounding the bearing get dried out to the point that the bearings are seizing under load it is too late to have much success with oiling and we usually have to change the motor.

in order to confirm bad bearings you could perform an operational AMP test, providing of course that you have access to a "snap on amp meter"

When performing an amp test we must remember that the motor has both a start winding and a run winding, therefore there are two sources of current being fed to the motor.

"Kirchoffs Law" states that the amount of current leaving a circuit is equal to all applied currents, therefore when performing an amp test we must put the amp meter on the common lead to the motor to get a full amp reading.

Begin by checking the motor data plate to find out what the motors rated run amps are. You may find two amp readings on the data plate, LRA & FLA.

LRA is locked rotor amps. When a motor starts it momentarily presents a dead short which results in a very high amp reading until rotation actually begins. That startup current is known as locked rotor current. (locked in the sense that it is static and no rotation is occuring). Typically the LRA about 3 times the FLA but it is only momentary, lasting only a second or two, then as the motor begins rotation the amp draw drops to the FLA- Full load amps, which is the amount of current required to keep this motor in motion when all conditions are normal and the motor is working into its rated load.

If the bearings are overheating and seizing that puts an additional load on the motor which can be seen as an increase in amp draw and the RPM slows down. IF you get the increased amp draw, reduced RPM and oiling does not resolve the problem the only solution is to replace the motor.

[erinj]

You should be a teacher, lazypup! your directions and explanations are great!

I was away last week and will be again shortly. I probably won't be able to do anything with the ac until I return. I didn't want to leave you hanging, after all the help you've been. Or for you to think I didn't appreciate the time you've already taken to help me.

When I get back and have checked the things you listed, I'll be in touch.

Thank you so much, again, erinj

[SSN596]

With an analog VOM, you can get a rough check on the functionality of a capacitor. When the meter ( in the OHM function) is connected across the cap. terminals, the battery in the ohmeter actually causes the capacitor to charge to the battery voltage. You actually see a momentary run up of the needle, then as the capacitor becomes fully charged, the current stops flowing and the meter will read infinity. The time it takes the cap to charge is a function of the RC time constant. The R would be the internal resistance of the ohmeter, and the C of course is the microfarads. The charge is expontential, and the cap is charged to >90% of final voltage in 5 time constants. For example, a 30UF cap with 10K ohms in the meter, the time constant would be 0.3 seconds, 5 TC = 1.5 seconds.

This gives you some idea that the cap is charging, but no indication of the value. The ohmeter will also tell you if there is direct short. So this meter is of some value, but limited.

A DVM has much lower voltages on the ohms scale and much higher internal resistance, so it will not perform this test.

DVM with a capacitor measurement function are not that expensive these days.

[dnpeters]

FWIW - I just had a similar problem with my air conditioning, except my fan shut down after 4 minutes. I removed all the capacitors and had my local dealer check them with a capacitor testor, and they all passed. A service call determined that the fan capacitor (4 MF) was in fact bad, and replacing it solved the problem. When I questioned why the testor didn't catch this, it was explained that sometimes these capacitors only fail when under a load. So, regardless of your tests, you might consider replacing the fan capacitor. The cost of the replacement is a LOT less than a service call, and IMO worth the gamble. Good luck.

[suprheat]

A PSC motor does NOT have a start winding. It's not infinite resistence if you get a full scale reading.

[Canti]

Hi Lazypup,

I just read your thread on " how to test the capacitor" and I understand how to test a 2 terminal capacitor, but how do you test a capacitor with 3 terminals on it? And how do I short the capacitor?

Like this one: