Why Is Thickness Sander Motor Stalling

[Gerry Gruber]

I built a thickness sander about 10 years ago, and simply powered it off my table saw for the longest time. Then about two years ago, I bought a standalone "farm duty" electric motor, and now I power the sander with that (1 HP Single Phase electric motor, 1725 RPM, 13.6 Amps on 115 Volt service.). At the start, the motor ran very well. But then after a while it would stall after 15 minutes use or so (i.e., it would trip a 15 Amp breaker switch I reused from my old table saw). If I let it cool down for a few minutes, I could reset the switch, and it would run again. Then, over time, the stalling (tripping) would occur sooner and sooner. Now, I can get maybe 5 minutes of use out of the motor before it "stalls". If I let it cool down for a few minutes, it will run again ... for another 5 minutes.

I tried running the motor without the breaker switch, thinking the switch might be faulty - overly sensitive. As the circuit I was using has its own breaker at the fusebox, I thought I would give it a try. This time, rather than dying, the motor would slow down, with spurts of energy. I turned it off before is shut down completely thinking I might further damage the motor. But clearly it was not running well. But, If it cooled down, it would run OK again for a short while. (I should also mention that I tried a different circuit, with the same results.)

The problem does not appear to be mechanical... the sander drum spins easily and I had used the same sander for years before. When it trips, the motor is warm to the touch, quite warm at the pulley end - but not "hot". The motor is now starting to stall even under no load at all - i.e., running freely, with no wood going through and it trips. I don't think it is undersized either for the sander, as the table saw motor I had previously used was only a 1 HP motor as well.

So, I am wondering what the problem might be? If there is an easy/cheap repair I or a qualified small engine repair guy can make, or should I just replace the motor. (It cost me about $150. So, it wasn't free, but isn't an expensive motor either.)

[Mario Proulx]

Bad bearings? Perhaps they're beginning to seize up as they warm?

You've double-checked all the wiring from the plug itself to inside the motor? If there's a frayed wire/loose connection/whatever, it might flow enough power when cool, but as it warms, you get more and more resistance and eventually she dies.

[Celeste Hall]

If it is not TEFC construction, I would look at dust having jammed the start winding switch closed, but also if it were built with shielded rather then sealed bearings, the bearings would be jammed with dust.

Actually, the more I think about it, it sounds like it is never getting up to speed enough to switch off the start windings.

Is the motor capacitor start and/or run?

[Gerry Gruber]

Thanks for your reply, Mario. The wiring is good up to the motor. Inside... I can check inside the junction box easily enough. Further than that, I'll have to decide whether I'm getting in over my head. I hadn't thought about the bearings, simply because within 5 minutes the motor will run again. I guess if the bearings were bad, I should be able to feel the resistance just after the motor stalls, in comparison to when the motor is cold. Worth a check.

Celeste, thanks for your thoughts too. The motor is TEFC. I don't know enough about motors to answer your question as to whether the capacitor is start or run. I don't have a manual. Additional info on the motor casing: It is continuous duty; Code L; 60 Hz; Fr: 56; Amb: 40 degrees C; Ins Class: B.

When it stalls, it tends to slow down first, and then just when it is about to stall, it does this "spurting", as if there is a short. In other words, the motor does not simply cut off. It loses speed quickly, and then just when it appears that the motor has completely stalled, it will jerk back into action for a split second, and then it trips.

[John Hamlett]

Just guessing here, I don't know "jack" about electricity and motors, but if runs well cold and doesn't run well hot, perhaps it isn't cooling well. Perhaps cleaning the fan and it's surround of dust would help it cool properly.

[David Malicky]

It may help to figure out what type of motor it is. A few questions for that...
- Does it have a centrifugal switch? (Makes a 'click' sound when the motor spins down after turning off. Also visible when taking off the end-cap.)
- Does it have 0, 1, or 2 capacitors? (Usually on the side of the motor under metal cover(s), but sometimes under the end-cap.)

[Clay Schaeffer]

Be careful when examining the capacitors. They hold electricity even when the machine is unplugged.

[Gerry Gruber]

Hi john. My first guess was that too (dust causing heat build-up). But, I've got a pretty good dust collecting system going. The motor is quite clean. Unfotunately, I don't know much about electricity and motors either (otherwise I wouldn't be asking these questions). I am also guessing that a TEFC (Totally Enclosed, Fan Cooled) engine would be protected to some degree from dust.

Hi David. I don't know whether it has a centrifugal switch or not. I haven't noticed a "click' when it spins down. But, then again, with the dust collector humming away, I probably wouldn't have heard it anyways. I'll check into that.

Number of capacitors... ? I googled the model number of the engine and came up with some kind of CSA Certification document, which lists this motor under the following description: "Squirrel cage induction motor, single phase, TEFC, NEMA frame 56, insulation Class B and Class F, 115/230 V, 60 Hz, continuous duty, max ambient 40 deg C, SF 1.15, capacitor start, induction run". So, it appears it does have at least 1 capacitor, right? I did also find a website that talked about how to check whether a capacitor in an electric motor was faulty or not, using a multimeter (which I have - although again - competency may be lacking).

Thanks Clay for the heads up about the capacitors storing electricity.

[Bob Menzel]

Though I doubt it on a farm duty motor, maybe it has thermal protection. You bought it new or used?

[David King]

One HP sounds pretty minimal for a thickness sander. I'd retire this one and look around for something bigger.
It definitely sounds like it has internal thermal protection that's kicking in, it could be that the thermistor is bad now after kicking in so many times. If the bearings are quiet, they are probably fine but Farm Duty suggests to me that this was intend to be a fan motor and not intended to have the side load of a belt and pulleys.

[Gerry Gruber]

OK. This motor definitely has a capacitor... more than 1, I don't know. It also makes a click sound when spinning down. So, to David M's post, I guess this means it also has a centrifugal switch.

A Youtube video I just watched, mentioned that some people will choose to replace capacitors when an electric motor "acts up", simply because it is a relatively cheap and straightforward repair that often will fix the problem. Any sense as to whether this might be worth trying in my case?

To Clay's point about capacitors storing electricity, I am assuming that if I wait a certain time, the electricity in the capacitor will drain, and it would be safe to handle. How long would that be? For example, if I unplug the motor, and wait 5 minutes, would that be safe? 30 minutes? 1 hr?

Hi Bob. I bought the motor new (just over two years ago). About half a dozen guitars through it at the moment. So, not brand new anymore. But, should still have lots of life in it.

Hi David K. I've only got 110 volt service. I think to go beyond 1 HP, I woud probably need to install 220 service - not out of the question, but I would like to explore the less expensive solutions first. As to the bearings, I can spin the pulley easily and it spins smoothly with very little sound, both cold, and after running for 15 minutes (no load). Does that suggest the bearings are OK?

[David Malicky]

Good googling, Gerry. The key info is "capacitor start, induction run" -- that's the type of motor (often just called "capacitor start"), and a good choice for a thickness sander. It has 1 capacitor and a centrifugal switch. Here's how it works...
- Schematic: http://lh4.ggpht.com/_X6JnoL0U4BY/S1cH_ ... humb43.jpg
- At start, the switch is closed and the capacitor is in the circuit, as well as an "aux winding" (usually called "start winding") to help it get up to speed.
- While it is spinning up, at about 2/3 running speed, the switch opens and the capacitor and start winding are taken out of the circuit. The motor just runs on the "main winding".
- When turned off, the switch will close ('click') around 1/4 of normal speed.

So, your capacitor is fine -- if it were bad, it would have trouble starting and not get up to speed. And, the capacitor is supposed to be out of the circuit when running.

I can think of two possible causes for the symptoms:
- Maybe the switch has gone bad: if the contacts aren't disengaging fully or consistently, the motor would be trying to run with both the start and run windings -- that would bog the motor down and increase the amp draw a lot. But as a TEFC motor, the switch is probably sealed inside the case, so harder to service or even inspect. You could try taking off the end-cover and then the fan to see what's accessible. If not much, you'll need to remove the 4 long skinny bolts that hold the motor case together. The switch is usually in the tail end, so try to just remove the tail housing. Then check the springs, mechanism, wiring, and especially the contacts. If this is the cause, I would also replace the capacitor, as it's not intended for steady running and can explode if it sees line voltage for too long.
- Maybe the windings are a little burnt and starting to leak current / short out. A warmer motor would have smaller clearances between wires, and insulation more flexible, increasing the possibility of a short. Generally not worth repairing on small/import motors. Windings can be checked with a good quality multi-meter (ohms and also isolation from ground and to each other).

The 'spurts of energy' symptom is consistent with both of these causes -- the contacts could be making intermittent connection/disconnection, or the windings shorting and then not.

Unfortunately it's probably not the thermal protection switch: the motor is tripping the circuit breaker, implying it's drawing high amps. A bad thermal switch would limit the amps, not increase them. Your bearings also sound fine.

Note: to discharge a capacitor, using the tip of a insulated screwdriver (plastic handle), short the two terminals of the capacitor together.

On replacement motors (the likely 'solution'), many good quality 1.5 HP motors will run on a 15 amp circuit. 1 HP is on the small side, but would be fine if the pulley ratios kept the motor torque on the lower end (slower drum speed). (Farm duty motors are not a problem for belt drives.)

[Gerry Gruber]

Hi David M. Thanks you very much for the detailed post. Much apprecaited. I may have some follow-up questions once I start having a look inside the motor. (At the moment, it is still bolted to my sander.) I'm impressed how you were able to rule out the capacitor as the root cause - even though it may yet need to be replaced - if the switch is the culprit. Thanks also for the info on how to discharge the capacitor. Getting zapped by a hot capacitor doesn't sound like fun.

[Celeste Hall]

"Note: to discharge a capacitor, using the tip of a insulated screwdriver (plastic handle), short the two terminals of the capacitor together."

Expect to lose some metal from the tip of the screwdriver and shaft it the cap is well charged. So don't use your good SnapOn screwdrivers for this

[Clay Schaeffer]

"Farm Duty" motors are usually some of the strongest and toughest for their rating. If you haven't blown the dust out of the motor that is the first thing I would try. Even TEFC motors will accumulate dust around the fan and centrifugal switch (which is usually located behind the fan) and cause problems.
It is also possible that the motor has been over heating and gradually burned the lacquer off the windings. If that is the case then you will want to replace it with a larger motor, or if you replace it with the same size motor, take lighter passes when sanding.

[David Malicky]

Hi Gerry, glad to help. Good tips from Celeste and Clay.

If the switch's contacts are closing normally when the motor is at rest, don't be surprised if you see no spark when shorting the capacitor. With the contacts closed, the capacitor should discharge itself through the windings (see schematic above). Of course, always short the terminals anyway. I also check that it's unplugged at least two times before doing anything.

The centrifugal switch has 2 parts: the mechanism and the electrical contacts. There are many styles--here's one:
http://image.made-in-china.com/2f0j00dv ... -302Y-.jpg
When the weights get thrown out, the black plastic disk is pushed towards the tail of the motor, disconnecting the contacts.
It sounds like the weights and springs are working ok ('click'). Given the amp-tripping:
- The contacts may be ~welded together, or the gap is so small they arc even when the disk is pushed. The contacts are delicate and easily bent during assembly/repair.
- The disk isn't moving far enough or parts are out of sync. In a TEFC, it's hard to know how these 2 parts fit together since they only fit when it's all closed up. Trial and error may help, using a clamp-on ammeter to check current flow.

This page shows the opposite problem (contacts not conducting), but many good pics:
http://www.the-alchemist.com/motor.html

Thinking more about the symptoms, if it's a winding problem it would only show at high temp / voltages. Regular ohmmeter readings may all be normal, despite a problem. Pros use a 'megger' or 'hi-pot' test to check for insulation breakdown. Two good vids:
http://www.youtube.com/watch?v=vJlcKhc8tJo
http://www.youtube.com/watch?v=Fyd3S1EWfYA

[Dick (DT) Trottier]

Hi Gerry,
It sounds like bad bearings. It starts OK so the capacitor is good. That little spurt of energy before the breaker trips? that's probably the centrifugal switch closing and engaging the start windings. To do that the switch had to open at some point so the switch probably good. Bearings are something else. They do things at high RPM's that have little correlation to how easily you can spin the shaft by hand. On the plus side, good quality sealed bearings are are fairly cheap and easy to get. Even without a manufacturer's replacement parts list all you need are inside diameter, outside diameter, length and RPM to pick a suitable replacement.
Good luck...

[Gerry Gruber]

Hi Celeste. Thanks for the heads up about the extent of the spark to expect. I'm not a big fan of surprises.

Hi Clay. Dust is so negligible that I wouldn't have thought it a problem. But a bit of vacuuming is pretty easy to do. If the lacquer is burnt off the windings, I should expect to see black "soot" on the windings, right? As to a bigger motor, I am looking at a website (Princess Auto), which shows a 1.5 HP motor that can be run with 115 service. Problem is, with 115 service the specs say the 1.5 HP motor will draw 17.4 amps at full load. All my existing circuits to my workshop have 15 amp breakers, and I don't think it would be safe to just put in bigger breakers without increasing the guage of the wire (currently 14 guage). So, going with a bigger motor will entail some upgrading to my workshop wiring.

Hi David M. Thanks for the links. Gives me a better idea of what to look for when I open up this motor. I'm not optimistic I'm going to be able to positively identify the problem when I disassemble the motor, let alone fix it, but at this point, there is little downside, as the motor is all but useless in its current state. At least I'll learn a little about electric motor anatomy in the process.

Hi Dick. Thanks for the tip on the bearings, and the cost-effectiveness of replacing them.

[Mario Proulx]

Does your workshop have a breaker panel of its own? If so, you can easily change that same 14 gauge circuit to 220, which draws half the amps for the same HP, and is much safer(wiring runs cooler). At any rate, it's a good idea to run 12 gauge circuits to even your 110v stationary power tools, at which point you can use a 20 amp breaker.

I run a 2hp motor on my thick. sander on 110v; it has tripped the breaker once in a while, but rarely. The peak amp rating is at startup, or at full draw just before it stalls, so your breaker doesn't "see" the max amps but for a short moment. I consider 2hp a absolute minimum; there's a LOT of friction on that drum when it's sanding a full width top or back!

I still think it's a good idea to look at the bearings, too. In your initial post, you say it gets quite warm at the pulley end, and there's no reason for that, other than a bad bearing. I changed the bearings in my jet pump's motor last summer, after the motor literally locked-up(the bearings had been screaming for years, but this is just a sprinkler pump). Got the bearings at a small engine shop, since it turns out they are a standard bearing used on tons of stuff(look for a number like 6105 or 5906 or something like that on the seals); cost well under $10 and the pump has been quiet and smooth since then!

[Mario Proulx]

I just checked the 2hp motor on my sander, and it's rated at 18.4 amps/110v. I've been running it on 110 for 12 years now, no problems whatsoever on a 15 amp breaker, but my outlets are all 12 gauge.

EDIT: why am I running it on 110v? Because it, and the bandsaw, are the two tools that I've never found the ideal location for in my shop, and I have been changing the layout every now and again for almost ten years now...! If I ever find the ideal place for it, I'll run a dedicated 220 circuit for it in a heartbeat, but until then....