How to check a capacitor?

[Dick Holt]

My Jet air filter takes a LONG time to get up to speed. I am assuming that the capacitor is shot. I have taken the blower assembly out of the unit. There is a small capacitor attached to the motor. I have a meter (digital multimeter) but don't know how to check the capacitor to see if it is in fact bad. What scale do I use and what reading should I get ? If it is the capacitor, do I need to discharge it before changing it and how do I do that. Thanks for your help.

Dick

[Alan Schaffter]

It is not that simple. Google it and you will see you need a special meter, extra components, or a megger. Here is one link to more than you ever wanted to know about testing and other general capacitor info..

If the motor has a centrifugal switch (if it is a small induction motor it won't have one) that is another likely source of your problem. It may have a bad spring or contacts.

[Chris Padilla]

A little bit of theory:

Capacitors are opens to DC (blocks DC) and pass AC (allows AC through them...to a certain extent). However, it takes a certain period of time for the capacitor to charge up once a voltage is applied across it. Whenever you check the resistance between two points, a TINY amount of current is passed through the two points (from lead to lead of your meter).

So when you measure the resistance of your capacitor, it will initially flow current and give you an resistance reading (ohms) and then go back to open or infinite resistance like you would expect.

Set your meter to around 1 kOhm.
Remove the capacitor.
Short the two leads of the cap to discharge it.
Put your meter on the leads and observe the meter giving you a reading and then going to open conditions.
Reverse the leads and observe the same thing.

If the meter doesn't give you a reading and then go to open in both directions, it is likely bad.

[Ken Fitzgerald]

Dick,

Basically you do a resistance check. You should disconnect one lead from the capacitor to get an accurate check. Usually the leads are just slide on leads.

Then set the meter to the highest resistance scale it has. Put one meter lead on each of the leads on the capacitor. The meter should initially show a low value of resistance and charge to infinity over a relatively short period of time.

Usually these capacitors are electrolytic and have polarities.

If you get a constant low resistance reading, swap the meter leads and measure again.

If with the meter leads hooked up one way, the capacitor charges to inifinity or some very large value....several hundred thousands or millions of ohms, the capacitor probably is good.

If in both meter lead hookups ( one direction and then swapped) you get a low constant value, the capacitor is shorted.

If you measure a high value with no charge in either meter lead hook up...the capacitor may be open.

When I use the term "swap" the meter leads....the meter leads are polarized...one is negative and one is postive.....the capacitor is polarized...... when hooked in the wrong configuration..the capacitor may "appear" to be bad and isnt'. So by just swapping the leads and measuring again, you are reversing the polarity of the voltage from the meter and then your measurement should change.

If this isn't clear, I can try to clarify it.

[Chris Padilla]

Excellent, we all pretty much agree! Cool.

[Charles Lent]

You can only do some basic checks with a VOM, but shorted or open conditions can be checked and these will usually show all that you need. This test applies to the larger capacitors that are used in motors. A VOM cannot measure the working capacitance. A special tester is required for this.

Remove the capacitor from the motor. Then set the VOM to the R X 1 or R X 10 scale. Connect the VOM leads to the capacitor while watching the meter. The meter should jump to a low reading and then increase rapidly to the max or OL reading. If it quickly settles out on a low ohm reading then the capacitor is shorted. If it never comes off the max or OL reading when the leads are first connected, then the capacitor is likely open. If your testing makes you believe that the capacitor is bad, take it with you to a motor repair shop so they can test it, an if necessary, sell you a replacement. Motor capacitors for motors of less than 3 hp should cost no more than $20.

The other thing that you should check is the centrifugal start switch. It's located in the non-shaft end of the motor. You will need to dis-assemble the motor to get to it. Remove the 4 long bolts that go through the motor housing from one end to the other. Put a line or scratch across the seam of the motor between the center and the end housing so you can orient the end housing correctly when you replace it. Now using a screwdriver and hammer tap on the edge of the end housing to drive it free. Be careful to note the position of any bearing shim washers so you can replace them in their correct positions. Once the end housing is free you will note a switch contact in the end housing with the motor shaft going through a hole in the center of the arm of the switch. The switch contacts need to be good and they need to be touching. You can use some emery cloth folded to slide between the contacts to clean them up. Next you need to check the centrifugal weight assembly it is on the motor shaft. There is a large washer that slides on the shaft and some
springs and weights behind it. You need to make sure that this washer can slide freely on the shaft. If you push the washer toward the spring assembly you should be able to move it about 1/2" and the springs should force it back when you release your pressure. You can clean the shaft where this washer slides and wipe a very thin layer of oil on the shaft to make the washer slide easier. This washer and weight assembly pushes the switch to open the switch connection when the motor comes up to speed. The switch must be closed when the motor is at rest and must open to remove the capacitor from the circuit when the motor reaches about 1/2 of it's operating speed. It must close when the motor stops so the capacitor is again connected.

Now you can re-assemble the motor, making sure that you put the bearing shims back into the bearing receptacle in the end cover. Line up your mark and install the 4 long bolts to hold the motor together. Turn the motor by hand to make sure that the motor is back together correctly and it turns freely. Make sure that the capacitor has been re-installed, then plug the motor it in and see if it starts and runs correctly. If it does you have successfully repaired your motor. If it doesn't work, re-check everything or take it to the motor repair shop.

Charley

[Alan Schaffter]

The other thing that you should check is the centrifugal start switch. It's located in the non-shaft end of the motor.

Charley

That is what I always thought! A few days ago I was working on a friend's Shop Fox DC and needed to check the centrifugal switch. Guess where it was located? Yup, at the shaft end. We had to remove the blower housing cover plate, use a bearing puller to remove the impeller, then remove the blower housing before we could even remove the end of the motor housing to access the centrifugal switch! Yikes! I should have known since the connection box with capacitor were located near the blower end of the motor.

[paul cottingham]

Might be simpler to just replace it and see if that fixes it.

[Dick Holt]

Thanks for the replies guys. With my meter set on 200 or 2000 ohms (meter does not have a 1K scale) I get no reading with leads in either position. With meter set to 2000K scale the meter charges up to 2000K and then goes blank (it is digital). This happens with the leads in either position. If I understand you correctly, this means the capacitor is bad. Is this correct?

Dick

[Steve Leverich]

Nope, just the opposite - it's probably OK.

Think of a capacitor as a bucket, only instead of water we're filling it with electrons - and instead of overflowing when it's full, it shuts off the "faucet" - in other words, the current flow stops when the "bucket" is full.

So when your ohm meter applies a small voltage to the cap, at first a current flows - when the cap has charged up to the applied voltage, it STOPS the current from flowing - no flow = an "open" circuit, so your meter says it is "open".

If you can see your meter change when you first hook up, this means the cap is charging up. When the meter shows "open", the cap is charged to the value of your meter's power supply (the battery)

Reverse the leads, and you start the process over again, til the cap is charged in the REVERSE direction.

HTH... Steve

[Ken Fitzgerald]

Dick,

Check the manual for your digital meter to see what the blank screen means.

If it means infinity, it means with your meter the capacitor is checking good.

Here's the catch and it's what Charles was referring to. You really need a device that puts out a relatively high voltage....say 300 volts dc or so to get a good check. A device called a "meggar" would work well. However, low voltage Volt-Ohm-meters...multimeters will often find a shorted one. This is the most common type of failure.

I would suggest you check the centrifugal switch to see if it's capable of opening and closing properly. When you first turn the power switch on, power is applied through the centrifugal switch..through capacitor to one motor winding. The capacitor effectively shifts the phase of the voltage to that winding causing the motor to start spinning in a specific direction. Once it gets to about 80% of it normal operating speed, the contacts on the centrifugal switch should open up and remove the capacitor from the circuit. IF sawdust has gotten into the motor preventing the centrifugal switch from closing it's contacts when the motor isn't turning, it will not put the starting capacitor in the circuit. That would be my next recommendation.

Sometimes you can see this switch without disassembling the motor and just use your air compressor to blow it out.

Should that check okay, I'd think about either buying a new capacitor and just wholesale swapping it or take the motor with the capacitor to a motor repair shop and have them check it.

Good luck!

[Van Huskey]

Might be simpler to just replace it and see if that fixes it.

I was scrolling down to say the same thing a little tongue in cheek but the way I often test a cap, since normally when I narrow the causes down there is a 90%+ chance it is the cap.

[Bill Davis]

I think Paul has the best approach for testing this type of capacitor - The Substitution Method. A VOM only applies a low, current limited voltage to the capacitor that is normally being used at much more extreme and harder to test voltages and currents. I vote for substitution.

[Ken Fitzgerald]

Here's the catch for the substitution method.

If you don't have a motor repair shop or an electrical supply house locally, you could wait a week or more to get one only to find out it's not the cause of the problem.

Now with an air filter you can get by.......most people could....

If it was on tablesaw or bandsaw and you had a timeline to meet, it might be wise to continue troubleshooting in case the capacitor is good.

[Bruce Wrenn]

Your air cleaner uses a capacitor RUN (not start) motor. It has oiled sleeve bearings. When was the last time you oiled them. My money goes on a lack of oil in the bearings. Does the owner's manual say anything about oiling the bearings. On our HVAC unit, I oil the bearings twice a year. The blower runs full time. It has run for over THIRTY years now. Only time it''s off is to service unit, and power failure. There are most likely two oil points on your motor, with one being behind the blower wheel. You need a "Zoom Spout" oiler to reach the back on. Many hardware stores along with HVAC supply houses stock "Zoom Spout" oilers.