Removing Capacitor from table saw motor

[Tom Seaman]

My table saw motor (3 horsepower on Jet JTAS-10 cabinet saw) quit working this week, and after reading several threads here and elsewhere, I believe the problem is a bad start capacitor. After reading those threads, I'm also more nervous about taking it off to get and install a replacement!

I have a couple questions before I take it off. Unlike all the examples I've seen, the motor wires on mine are screwed to the capacitor terminals so I can't just pull them off with pliers w/o touching the metal terminals. Can I just unscrew them one at a time without causing any problems? Do I have to discharge the capacitor first or just take it off and put the plastic cover on for safety (and not discharge it at all)? If needed, I've seen conflicting opinions on whether to discharge with a large screwdriver or with a resistor. This seems like a large capacitor compared to most of the examples I've seen (600 mfd). Thoughts knowing the size of it? Thanks in advance for the help - I like to err on the side of caution (and I couldn't convince my neighbor to take it off for me so i'm going to do it)!

[Cary Falk]

I would discharge it first so there isn't any accidents. Large screw driver should be fine.

[Erik Loza]

I would discharge it first so there isn't any accidents. Large screw driver should be fine.

Yes! ^^^

Please make sure to do this before you start disconnecting it.

Erik Loza
Minimax USA

[Ken Fitzgerald]

Tom,

First, be sure the saw is disconnected from the AC power! If it has a plug, unplug it!

2nd, in theory, it's safer to use a resistor to discharge the capacitor as the resistor limits the instantaneous current when placed across the two leads. As a field engineer and a high voltage specialist prior to my retirement, I have a special discharge tool that consists of a 2' brass rod with a plastic handle and about 10' of #2 cable with a large alligator clip on the end. Inside the handle is a resistor in series with the cable and the brass rod. For high voltage (140,000-160,000 VDC) that was for safety. For the type of voltage involved with this capacitor, a large screw drive is more practical. Be sure the screw driver has an insulating (plastic or rubber) handle and grip it by that handle. Touch both of the capacitors leads with the metal blade of the screw driver simultaneously. Don't be surprised if you see an arc when you short the leads together using the screw driver blade. DO IT MORE THAN ONCE!

3rd, then disconnect the wires from the capacitor.

It wouldn't hurt to discharge the new capacitor before you install it. I have been bitten by new caps which had a residual charge.

[Myk Rian]

How did you determine the capacitor is bad?

[Johnnyy Johnson]

I too have worked 30 yrs in instrument and electrical E&I.. Don't worry about discharge since it has been awhile since it ran. A 1 ho will pull around 28 amps locked rotor or a bad cap. Most likely it a 45 MF cap and is not polarity conceded. Pop it out. .replace and don't sweat the small stuff..Dam I hate the spell check on my cell phone..I'm not in control...

[Ken Fitzgerald]

Myk,

There are some expensive tools used to check capacitors.

In the "old days" of analog multi-meters, often we'd just do a resistance check. A capacitor should never check constantly shorted and typically would show a fairly fast rate of charge until the resistance measure "infinity".

Using digital multi-meters isn't quite as reliable a check but it can be starting point.

The way you do it is place a mulitimeter in the resistance measuring mode of operation and check the resistance between the 2 leads. Then reverse the leads and check it again. A capacitor may show an initial "short" or low resistance but then should charge until it's resistance measures infinity. The leads of the capacitor should be disconnected so it's out of the circuit when you make the check.

Physical evidence of a capacitor being bad or going bad is a swollen, malformed case or leaking material from the capacitor.

[Tom Seaman]

Thanks guys. I got if off without any problems. There was no arc when I shorted the leads.

Myk - I'm not 100% sure it is a bad capacitor, but that seems like a likely culprit and relatively easy/cheap to replace to find out. Here were the symptoms: I hadn't run the saw in a few months, and when I turned it on, it didn't immediately come up to speed. I quickly turned it off, checked to make sure it was clear, the blade spun easily, etc. I turned it on again and it took a little longer to get up to speed (2-3 seconds maybe). It was late so I decided to wait until I had more time to mess with it. A couple days later I took the blade off, looked it over again. When I turned it on, it almost immediately tripped to 20 amp circuit. I checked the outlet, circuit and extension cord by plugging in 2 other 220v machines and they worked fine. That's where I am now.

I saw some youtube videos on how to test the caps with an analog multi-meter. I am going to try that while I wait to get a new capacitor next week. I'll report back anything I learn, and if it isn't the capacitor I'll have a new problem trying to figure out what the problem is!

Thanks again for the help.

Tom

[Tom Seaman]

I got a new capacitor from Jet, installed it last night, and no luck. The motor just buzzed, didn't spin, but did not trip the circuit like it did last time I tried starting it. I think my next step is to remove the motor and take it to a motor repair shop.

BTW, when I tested the old capacitor using an analog multi-meter, it seemed okay.

[Duane Meadows]

Centrifugal switch in the motor bad? Else new motor. Or the motor repair shop may be a good choice also.

[Joe O'Connor]

I had a similar problem with my jet table saw while it was under warranty. The customer service guy thought it was a capacitor and sent one out. When that didn't work they sent a whole new motor. When that didn't work he suggested tying the two wires from the switch together and carefully plugging the saw in to test the switch. The saw fired up with no problems and he sent out a new switch,I'm not sure why that wasn't the first thing he tried but now I've got an extra motor on the shelf.

[Rod Sheridan]

I too have worked 30 yrs in instrument and electrical E&I.. Don't worry about discharge since it has been awhile since it ran. A 1 ho will pull around 28 amps locked rotor or a bad cap. Most likely it a 45 MF cap and is not polarity conceded. Pop it out. .replace and don't sweat the small stuff..Dam I hate the spell check on my cell phone..I'm not in control...

Locked rotor current would be in the range of 3 to 10 times FLA, so it could exceed 100 amperes for a 1 HP motor.

The OP's capacitor is 600 microfarads.......Rod.

[Charles Lent]

More than likely the centrifugal switch is sticking or full of sawdust and can easily be fixed once you get the end cover off of the motor. One of 2 things usually happens

1. the centrifugal flyweights move a plastic collar on the motor shaft and this collar pushes the switch open. The motor shaft gets a little rusty or dirty where this collar needs to slide and it fails to return the switch contact to a closed position when the motor stops. Clean this area of the motor shaft and apply a slight amount of oil.

2 The centrifugal switch contacts get pitted or sawdust builds up on them and prevents the switch contacts from going back together as the motor stops. Cleaning and adjustment is all that is usually necessary. One motor from a Delta Contractor saw that I worked on had a small red rubber bellows around the centrifugal switch contacts that was designed to keep sawdust away from the contacting surfaces. Unfortunately this bellows had managed to get itself between the contacts, preventing them from properly closing. The simple fix was to just remove the rubber bellows since it was significantly deformed and would no longer stay in it's proper place. This motor has been running perfectly for over 3 years now.

Motors that don't fail by letting the smoke out usually only have simple and easy to fix problems. Your's sounds like the centrifugal switch just needs some attention and it will be as good as new again. Be brave and pull the back end of the motor off, with the power disconnected of course.

Charley

[Tom Seaman]

Duane and Charles,
I took the cover off the motor and checked out the centrifugal switch as you suggested. It appeared clean and dust free (it looked to be in great condition to me, but I've never seen one before!). I blew some air on it, and made sure it opened and closed easily. Put it back together and when I tried to start it turned slowly the first 2 times (I only left it ON a second or two). The third time, nothing happened when I pushed the ON button. So, per Joe's experience, I'll check the switch next before taking the motor to a shop. If nothing else, I'm getting a good lesson on motors! :D I appreciate all the help.

[Thomas Hotchkin]

Tom
Was there another capacitor under the motor cover? If so it would be the run capacitor, and it could also prevent motor from running. Sounds like you have gone through just about all there is other then on/off switch.

[Cliff Polubinsky]

Tom,

My SS PCS stopped running last week and Sawstop sent replacements for both the Start and Run capacitors along with the centrifugal switch. As Thomas suggested above, try replacing trhe Run capacitor.

Cliff

[Tom Seaman]

UPDATE:

I swapped out the run capacitor and still tripped the circuit. So I took the motor to a repair shop recommended by my local Woodcraft (where I bought the saw). I learned today the run windings are shorted out. He recommends getting a new motor rather than trying to fix it. So, that is my most likely solution. I'm disappointed of course, as the Jet rep assured me the saw would last longer than me!

Thanks for all the help and advice.