?how to check capacitor for 1/2 hp motor ?

Jim In NY · Mar 21, 2011

I have an older air compressor that is operated by a 1/2 hp Packard (GM) electric motor. The compressor brand is "Kellogg", The motor will not turn the compressor fast enough to get it started and up to speed. When I disconnected the belt I could spin the pulley enough to get it going and up to speed okay; and it runs really smooth at that point without any noticable drag. I am assuming that a check of the capacitor is the first order of business, so from that point I need expert advice on the how to's... ? The Motor ID : S7828 And Serial: CW18QK5 . The capacitor markings for ID are: 5526144 and the MFD's are 161-193 any help is as always appreciated. Thanks JIM

Bob · Mar 21, 2011

Try a new cap.

IH2444 · Mar 21, 2011

Yep unless you have a good capicitance tester, try a new cap.

Jim In NY · Mar 21, 2011

Where can I get one of the same that is in there ??

I do have a clamp jaw multi meter... will that do. ?

Ed the wine farmer · Mar 21, 2011

It may be the capacitor or it may be the start winding in the stator. With todays prices for parts, it may be your best bet is to buy a new motor.

Bob · Mar 21, 2011

EXACTLY. It takes a $$$ capacitor tester designed for these heavy-duty caps to ACCURATELY test them.

Bob · Mar 21, 2011

Grainger will have them as well as ANY local electric motor repair shop.

OliLt · Mar 21, 2011

Did you do a visual check? Check the ends, especially where the contacts are located. If they are bulged, the capacitor is probably bad. Any sticky, or oily like material around the seams is another good indication of a bad capacitor. Sometimes bad capacitors can smell. If you detect an acrid, burnt odor, the capacitor is bad.

Capacitors can hold a charge. Always short across the contacts to avoid an unpleasant shock.

gregk · Mar 21, 2011

One way todo it is to take a multimeter (the kind with the needle and dial NOT a digital one) and hook one lead to one post and the other one to the other post, turn the dial to ohms, hold them there for a second and then switch them. If the needle goes down slowly and steadily then it is probably good. If nothing happens try switching them again as this is polarity sensitive. If under no conditions does this work like described above then the capacitor is bad. Hope this helps good luck

Bob · Mar 21, 2011

As you say with the Ohmmeter test "it is probably good".

Trouble is "probably" ain't good enough. Without using a quality specialty 'lytic tester anything else is just a GUESS.

M-MAN · Mar 21, 2011

You need to be very carefull bleeding & unhooking that start cap or you could get lit up like a Christmass tree. If it has a bleeder you're ok but if you have to drain it with a screwdriver watch out.

36 Coupe · Mar 22, 2011

Substituion is the best test.I have a test cap in my tool box.Too much capacity makes a motor start too fast.Under capacity makes the motor slow to start.These starting caps can lose capacity.Take your cap to a motor shop and get a new one.They need the voltage rating and the capacity range to get the right one .Short the terminals before you handle it .

36 Coupe · Mar 22, 2011

If the cap is holding a charge you can kiss your meter good bye.

36 Coupe · Mar 22, 2011

10 bucks will buy a start cap for a 1/2 hp motor in my shop.Dosent hurt to have a spare any way.

Dick2 · Mar 22, 2011

It is more likely to be a faulty starting switch than a capacitor. I'm not familar with that particular motor, but if they used the same switch as the Delco motors, that switch is prone to more problems than other starting switches.

The good news is that sanding the contacts with a finger nail sanding stick will usually get the switch operating again. Also check the springs on the switch, etc.

You will have to remove the end bells to check the starting switch on most motors, although some may have an external electronic starting device. Mark each end bell position with a center punch or scratch mark so you can put the end bells back on in the same position. I would check the starting switch before doing anything with the capacitor.

When you carefully remove the cover on the capacitor, short across the terminals on the capacitor to discharge it. Then examine the end for visible damage. If you get a replacement capacitor, look for one with a microfarid rating close to the one that you remove from the motor. It doesn't have to be exact, just close to the original will do.

Dusty MI · Mar 22, 2011

(quoted from post at 03:22:45 03/22/11) It is more likely to be a faulty starting switch than a capacitor. I'm not familiar with that particular motor, but if they used the same switch as the Delco motors, that switch is prone to more problems than other starting switches.

The good news is that sanding the contacts with a finger nail sanding stick will usually get the switch operating again. Also check the springs on the switch, etc.

You will have to remove the end bells to check the starting switch on most motors, although some may have an external electronic starting device. Mark each end bell position with a center punch or scratch mark so you can put the end bells back on in the same position. I would check the starting switch before doing anything with the capacitor.

When you carefully remove the cover on the capacitor, short across the terminals on the capacitor to discharge it. Then examine the end for visible damage. If you get a replacement capacitor, look for one with a microfarad rating close to the one that you remove from the motor. It doesn't have to be exact, just close to the original will do.

I'm with Dick2. I would just like to add do NOT use emery cloth or board on electrical contacts, emery is a conductor, so do as Dick says and use a sand board.

Also most likely it is the contacts on the starting switch, and Grainger has some of those.

Dusty

greygoat · Mar 22, 2011

I bought a Capaciter Tester from GRAINGERS, not
cheap, but woth it"s weight in gold.
I use it mostly to test Magneto Capaciters, but
it will do the big electric motor ones also

IH2444 · Mar 22, 2011

Nope that won't work.

IH2444 · Mar 22, 2011

Yep I have a high voltage cap tester. Works on motor starter caps. Sucker will bite you though

redtom · Mar 22, 2011

Total novice here, but my old tablesaw wouldn't start and the centrifugal start switch was full of dust. Blew it out and it started. Different story: got a rotary phase converter which uses two starting caps. Wouldn't start, tested caps for continuity with meter. As found on net used meter to test by watching them build then drop. Found they were still bad. Put in two new ones and it worked.

IH2444 · Mar 22, 2011

The reason the caps loose capacity is that the electrolyte paste in the drys out over time.

also if the end on a cap is bulged out get a new one.

dlplost · Mar 23, 2011

To test for a bad start cap, just wire up a 100w light bulb in place of the cap.
If the problem is the cap, the motor will start and run, burning out the bulb in the process.
The bulb will do the function of the cap, but only once per bulb...

Or you could just buy a new cap...

?how to check capacitor for 1/2 hp motor ?

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