checking electric motor capactor

Have a 2 hp electric motor that quit. It has two capacitors on the top , one may be the start one . I have a small pencil shaped checker that indicates I have current going to the motor and to and from the capacitor but I need some one to tell me how to check the capacitor with a meter, what readings should I be getting, mechanic I am but nothing about electrical.

First, what is the motor doing?

Not running at all, no hum, can't hand start it?
If so, the caps are not the problem.

If it hums, but doesn't start, could be the cap, the start switch contacts, or the start windings.

Before touching the connections, be sure to short the caps to make sure they are discharged. They can give a nasty shock if charged!

You can visually check the caps for swelling, cracks, loose or arced terminals.

You can test them with an analog ohm meter. Out of circuit, set the meter mid range, touch the leads, the reading should jump up then fall back down. Reverse the leads, it should do the same. If no reading, it is open and bad.

Or you can remove the caps one at a time, connect the wires together, see if it will start. But only run it a second! If it still won't start, the problem is elsewhere, not the caps.

Generally, start caps have a higher rating and are plastic cased. Run caps are lower value and have an aluminum case.

The last comment about the starting and running is spot on and I had been told that already. No hum or run at all so you are saying it is internal. Expected as much , it is really a well built and good looking motor but on a overseas built band saw so probably lasted about as long as it was supposed to. Thanks for the answer and I will put the meter to it but think you are probably right about it being windings or internal. Thanks again

Must remember that a motor that is capacitor start and capacitor run(no relays involved after starting switch) can act as a conductor. So be sure to measure on both sides of the power in to check and see if you have full supply voltage with a meter at the motor. A meter with microfarad setting can usually check a capacitor(may have to hold probes on connections a few seconds), must remove one lead and no resistor across the terminals of the capacitor. + or-10% usually allowed from label ratings. Good luck, and by the way my import has a really small fuse that blew where it was not easy to see on the motor. Also windings of a motor can be checked with a ohmmeter, label and remove wires to the windings. There should be resistance. If some is read it is good. Depending on winding and size 1. to 10 ohms on small motors.

Roger like I said I am a pretty good mechanic but not very knowledgeable on electricity but I first thought a reset or fuse might be the problem because this thing does not work under load and never runs long enough to get warm. Guess I will do a little more looking and then take it to a shop, thanks for the reply .

Another tip. Unhook the belts. Will give you a little more chance to work on it without hurting you. With no load and a bad start cap, sometimes 5he motor will spin up and run. You can try all of the other suggestions too. Nothing like long range diagnosis. Actually have done 5his. 5ake a short piece of string and wrap it around the pulley. Give it a Sharp pull and flip on the power. If nothing happens most likely it is new motor time.

If you have an electronics store near you they will generally test a cap for free, they sell a lot of them that way. But it doesn't sound like your problem. Put your pencil tester away, it's good for finding Romex inside sheetrock walls but not much else. Most of your testing can and should be done with an ohmmeter on this job, with the motor unplugged. Take a screwdriver and short both caps after unplugging and you're sure not to get shocked. Usually a problem bad enough to kill an AC motor is either obvious on close inspection or shows up quickly with an ohmmeter.

jm,
A pencil tester doesn't measure current, it measures voltage.
Use an ammprobe to determine if current is flowing.

The best way to check them is with a meter that can measure micro farads(sp). The cap should have the rating on it. Most Fluke meters have this.

Thanks George that is good to know.

jm.
Many times I find capacitor is good and the end switch that connects cap to start windings is bad. Take motor apart and sand contacts on switch using wet/dry sandpaper.

If there is very little load on the motor just remove the capacitor and hook the 2 wires together, it's not a real necessary component, it just helps them start. If that make it start, then get a new capacitor.

Yepper, one is start the other run. The first one is for starting under a load, unnecessary with a light load split phase installation for example.

The second is the run cap to cancel out the inductive aspect of the motor and return the power factor to something agreeable to the power company, like .85 around here.

Power Factor is the ratio of true power (dissipated) to apparent power (indicated). If V and I are inphase, then you have a resistive load and power dissipated is power measured V x I.

If you have a reactive component like a capacitor or inductor they have reactive impedance, unlike dc resistance. The voltage and current are not in phase and the axiom "ELI the ICE Man" rule applies......E (used to represent voltage in some terminology) leads I in inductive circuits and I leads E in capacitive circuits.

For highly reactive circuits, where current (I) leads or lags voltage (E) significantly, you get the operation of your equipment but the power company doesn't get paid. Purely reactive circuits don't dissipate power and power companies sell power.
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Ok this was unnecessary. Maybe to you but not to me. I am past the ¾ century mark and do not want to loose everything I had nor contact something new like Alzheimer's which, I am told, is assisted by lack of brain exercises. So bear with me while I work my cross word puzzles. Besides, some just might find it interesting. I hope I remembered correctly.

Thanks for giving me the opportunity.
Mark

Thanks Mark form the refresher course in electronics.