Induction motor failure - trouble shooting

[Michael Kellough]

My table saw just quit right before I planned to install the dado blades. I was using it a few minutes ago and it seemed to be running fine. Needed to do one more rip and the motor wouldn't run, just a meager hum, the blades budges a mm and the shop lights dim a little before I switch it back off.

I need help from you guys in tracking down what could be wrong.

This is an old Walker Turner TS with the original 220v single phase capacitor start motor (at least 45 years old) but it has been performing well and dust is pretty effectively kept away from it.

There is no odor of burned electrical stuff.

I know the power is good as I plugged my 220v band saw into the same outlet and it runs good.

There is no visible sign of a bad electrical connection.

What is the symptom of a bad capacitor?

Any ideas?

 

[Dan Lyke]

I'm not 100% sure, but I believe that you've almost exactly described the symptoms of a bad starting capacitor.

 

[Michael Kellough]

I'm not 100% sure, but I believe that you've almost exactly described the symptoms of a bad starting capacitor.

Thanks for the reply Dan. I'm keeping my fingers crossed but I think you've given me enough encouragement to pursue the capacitor.

I know they can be dangerous (though I guess not if dead but...) so do you have any suggestions on how to handle pulling it out?

 

[Michael Kellough]

Here is something I found on the subject of bad capacitors.

"Capacitors generally fail pretty dramatically, emitting great plumes of acrid smoke when they do. [Didn't happen in my case] A motor with a bad starter capacitor may fail to start (but will hum when power is applied) or will start slowly. If you suspect a bad capacitor, you can easily and cheaply replace it yourself. The capacitor(s) is located outside the motor housing, inside a hump-like metal cover. Make sure the power is turned off. Unscrew the cover and lift it up. If the cap is bad, you can expect the inside to be dripping with the oil that used to be inside the capacitor before it blew. Remove the capacitor (the wires just pull off) and write down the information on the label. You'll need that, as well as the physical dimensions of the capacitor (both length and diameter) to get a replacement. You can order replacements from industrial supply companies such as McMaster Carr. Install the new capacitor and replace the cover. The motor should start up. If not there is some other problem. If the new capacitor blows out in short order then it is likely that the centrifugal starting switch inside the motor is bad."

Came from here.

 

[Dan Lyke]

I once welded a screwdriver to a capacitor when playing with a photographic flash unit, so the things scare the heck out of me...

I'd take a volt meter and measure the potential across the capacitor. If it's 0v, you're home free, and from what little I know about motor starting capacitors, I'd guess it is. If it's something big (ie > 30), I keep a several watt 20 Ohm capacitor around for keeping switching power supplies with minimum load issues running (I'm a computer geek by trade), I'd use something like that to discharge the capacitor. Set up the leads so that you can close the circuit one handed, wear eye protection (for preference, something with UV protection), and use insulated pliers, keeping your other hand behind your back (shocks are bad, but survivable, however it doesn't take much potential at all across your heart to kill you, so it's generally good practice when diddling about with live circuits to do it one handed).

Nothing there that'll kill ya if you treat it with respect, but treat it with respect.

And as I said in my original diagnosis, I could easily be wrong, but a new starting cap costs, what, $5 at your local HVAC supply place? Seems like a cheap easy diagnosis.

 

[Eli]

Juicers. Always at least one hand in a pocket. Grips. Both hands in pockets.  ;D

 

[Michael Kellough]

Juicers. Always at least one hand in a pocket. Grips. Both hands in pockets.  ;D

Hey Eli, at first read I thought you'd gone completely down under jargon wise and I couldn't figure out what you said. Then I remembered that you worked in the film biz and it made perfect sense. 

 

[Michael Kellough]

And as I said in my original diagnosis, I could easily be wrong, but a new starting cap costs, what, $5 at your local HVAC supply place? Seems like a cheap easy diagnosis.

I got a reply from an expert,

"Usually the  motor will buzz/humm loudly when the capacitor has failed.
It could be the problem. The less frequent but more serious failure is an
open winding, there are two "running" windings and one "starting"
winding. Both of these issues will cause an increase in amperage draw,
but not enough to blow a circuit right away. Large amperage draw with a
locked rotor condition is a shorted winding to ground - I believe you
have one of the former conditions. An open field winding is not
repairable, so give the capacitor a shot."

 

[Steve Jones]

WARNING!!!!!

DO NOT TOUCH THE CONTACTS ON THE CAPACITOR

These things hold a charge, if the unit is turned off, unplugged, and the capacitor removed from the circuit (both contacts free) it may STILL hold a sizable charge, treat it with respect.

Steve

 

[Michael Kellough]

WARNING!!!!!

DO NOT TOUCH THE CONTACTS ON THE CAPACITOR

These things hold a charge, if the unit is turned off, unplugged, and the capacitor removed from the circuit (both contacts free) it may STILL hold a sizable charge, treat it with respect.

Steve

I removed the capacitor, nothing shocking to report. Looking for info on testing motor starting capacitors I found this,

"Caution: capacitors can hold a charge when removed from the circuit unless they have a bleeder resistor. Discharge the capacitor through the light bulb before handling. A capacitor in a working motor circuit cannot hold a charge, it is discharged the instant the power is removed."

My capacitor does have a resister across the terminals but I still don't plan to touch them. Besides, I don't know if my motor still qualifies as "working".

 

[Lancashire Fusilier]

If there is a resistor across them you should be fine then now.

 

[Michael Kellough]

Here is something I found on the subject of bad capacitors.

"Capacitors generally fail pretty dramatically, emitting great plumes of acrid smoke when they do. [Didn't happen in my case]

Well, that's is what I read somewhere on-line. The expert I consulted had this to say,

"Capacitor failures are not dramatic, they short out internally and no
longer provide a bias shift of the line phase, so the motor does not
develop a rotating field to start the rotor turn."

I'm off shortly to find a replacement capacitor. I wish I'd given this a more useful title like induction motor failure - trouble shooting.

 

[semenza]

Hi,

      If you click Modify on your thread starting post, you can change the title in the subject box.

Seth

 

[CharlesWilson]

What you are describing is due to a lack of flux in the starter winding which can be due to a failed capacitor. It can also be due to an open starter winding or a bad contact on the centrifugal starting switch.

A 45 year old motor isn't likely to have a burnt out starter winding. They over-designed things back then. It is still a possibility. Since you have the capacitor removed, make sure the starter winding is not open. Depending upon where the centrifugal switch is located, you may be measuring an open switch instead of an open winding.

As mentioned previously, capacitors can fail in a spectacular way. You didn't notice anything in that regard. However, those motors probably used capacitors that were in metal cans and were not electrolytic. Not like that plastic cased one on one of my motors that went postal.

Dirty contacts happen all the time. The centrifugal switch is buried inside the motor. If you have an air hose, you may be able to make some headway without having to disassemble it.

Good luck.

Charles

PS. You can always try using a push stick on the blade to get the motor rotating in the right direction. This, of course, is not for the faint of heart or the lover of life and limb.

 

[Michael Kellough]

Hi,

      If you click Modify on your thread starting post, you can change the title in the subject box.

Seth

Thanks Seth.

 

[Michael Kellough]

Thanks for the interesting info and ideas Charles.

You said, "...make sure the starter winding is not open. Depending upon where the centrifugal switch is located, you may be measuring an open switch instead of an open winding."

Can you explain this, measure where?

"As mentioned previously, capacitors can fail in a spectacular way. You didn't notice anything in that regard. However, those motors probably used capacitors that were in metal cans and were not electrolytic."

My capacitor looks good, no visible signs of failure at all, (no foul aroma either) and is the dry electrolytic form. If it went south should there be some sign?

"Dirty contacts happen all the time. The centrifugal switch is buried inside the motor. If you have an air hose, you may be able to make some headway without having to disassemble it."

I did previously (15 years ago) have a problem with dust in the motor (it smoldered!) which inspired me to  make some changes inside the tub so I could add dust collection. I had to disassemble the motor then to get it really clean. It certainly wouldn't hurt to blow it out and see how much stuff has gotten in there.

"You can always try using a push stick on the blade to get the motor rotating in the right direction."

To try this, would I need to shunt the wires that went to the capacitor, or put the capacitor back in?

Thanks Charles!

 

[CharlesWilson]

Thanks for the interesting info and ideas Charles.

You said, "...make sure the starter winding is not open. Depending upon where the centrifugal switch is located, you may be measuring an open switch instead of an open winding."

Can you explain this, measure where?

If you have an ohmmeter, or continuity tester you can determine if there is low resistance across the starter winding. The starter winding circuit consists of the winding, centrifugal switch and capacitor in series, and is energized when the power is placed across the two motor input terminals.

You know from inspection which two wires are connected to the capacitor, but you don't know how the wires are connected to the input terminals and the winding. If both the switch and the winding are not open, the resistance from each of the wires that were attached to the capacitor to one (and only one) of the input terminals should be relatively low (less than 10 ohms, I would guess). If this is not the case, I would suspect the switch or the winding. A motor connection diagram or physical inspection can help you determine which is the culprit if there is an open circuit. Note, when testing, only use an ohmmeter or something that produces DC (like a battery powered continuity tester). You don't want your motor taking off on you.

"As mentioned previously, capacitors can fail in a spectacular way. You didn't notice anything in that regard. However, those motors probably used capacitors that were in metal cans and were not electrolytic."

My capacitor looks good, no visible signs of failure at all, (no foul aroma either) and is the dry electrolytic form. If it went south should there be some sign?

Also, if you have an ohmmeter, you may actually be able to get some idea of whether the capacitor is working. With a big resistor (eg a megohm) in series with the capacitor, if you measure the resistance of the pair, it will start out at the value of the resistor, and then increase to infinity. For a 20 microfarad capacitor this will occur in 20 seconds or more. For a 2 microfarad cap it will be more like 2 seconds.

"Dirty contacts happen all the time. The centrifugal switch is buried inside the motor. If you have an air hose, you may be able to make some headway without having to disassemble it."

I did previously (15 years ago) have a problem with dust in the motor (it smoldered!) which inspired me to  make some changes inside the tub so I could add dust collection. I had to disassemble the motor then to get it really clean. It certainly wouldn't hurt to blow it out and see how much stuff has gotten in there.

"You can always try using a push stick on the blade to get the motor rotating in the right direction."

To try this, would I need to shunt the wires that went to the capacitor, or put the capacitor back in?

Thanks Charles!

Don't shunt the wires to the capacitor. That will almost surely burn out the starter winding.  Leave them open. The purpose of the capacitor is to phase shift the current going into the starter winding. This produces a relatively small but effective torque to get the motor running in the correct direction. After coming up to speed, the centrifugal switch shuts off the starter winding, which is no longer needed.

When the motor on my old Harbor Freight drill press would no longer start on its own (due to both capacitor and starter winding failure), I would hand start it by spinning the chuck in the correct direction. I don't recommend this 'hands on' method for getting a table saw rotating. Use a push stick.

Charles

 

[Michael Kellough]

Thanks for the detailed information Charles, even though I hope I don't have to do all that I'm glad to know it is here if it is needed (by anyone). I'm up and running now...but it looks like I'll need to keep my fingers crossed.

I put the new ($14) capacitor in and initially got exactly the same behavior as before, a hum, slight budge of the blade, and the shop lights dimmed a little. I wondered if the capacitor needed the motor to run to charge up? I doubt it. But it seemed like a good time to try the push start trick anyway.

I selected a springy 2 foot stick and practiced getting the blade to turn. My saw has three small belts and the shop is cool so the arbor is a bit tough to turn. However, I got the motor to start on the first try. The blade had nearly stopped by the time I could hit the start switch but it cranked and ran fine. I started and stopped a few times though I didn't let it come to a complete stop before restarting in the first couple minutes. Gradually I let the saw come to a full stop for a longer period of time and am now fairly confident that it will start again.

I haven't yet blown out the motor but will do that prior to putting it back to work.

So, Charles, what conclusion would you draw from this? Is the capacitor ruled out? If so, does it sound like clogged starter contacts? Or, it could be a cracked wire in the starter winding? Or, is it still all of the above?

 

[CharlesWilson]

My guess is temporarily gummed up contacts on the centrifugal switch. It is possible that there was enough gunk inside that the switch didn't come back to its original resting place required for starting. Who knows, maybe if you had smacked the motor with a sledgehammer (like they usually do with unresponsive car starter motors before changing them out), it would have reset the contacts.

I'm glad you are up and running.

Charles

 

[Michael Kellough]

My guess is temporarily gummed up contacts on the centrifugal switch. It is possible that there was enough gunk inside that the switch didn't come back to its original resting place required for starting. Who knows, maybe if you had smacked the motor with a sledgehammer (like they usually do with unresponsive car starter motors before changing them out), it would have reset the contacts.

I'm glad you are up and running.

Charles

Still running, thanks again.