Why Is Thickness Sander Motor Stalling

[David Malicky]

On the resistance tests, the motor would need to be hot and showing the problem for a multimeter to have a chance of finding an insulation fault. Even then, as mentioned, only a megger meter (applies 250 volts to simulate in-use insulation breakdown) can definitively test for faults.

Motors running without load consume about 1/4 of their rated HP in electricity. The windings convert that to heat (imagine a ~150W lightbult inside). If the case was hot around the windings, that would be normal. High temp concentrated near the bearing would be a concern. The bearings seem an unlikely cause for reasons in prior posts, but good ones will probably be better than what it came with.

Yes, low line voltage will definitely cause the motor to draw more amps and overheat. I recall you tried the motor on another circuit -- did that one have long runs, too? The voltage measurement would need to be taken with the motor drawing full amps (voltage drop = line resistance * amps). Measuring line voltage has safety risks -- read/study these, and/or find local help before trying it! A normal practice is to put one hand in your pocket, and use the other hand for both probes (or clamp 1 probe), so if there was a shock, it would likely be confined to your hand. Just 30mA though the heart is enough to stop it. Or better, with the motor unplugged, alligator-clip the leads onto the motor inputs; then run the test, then unplug -- that's how I like to do it. Also stand on something insulated, don't stand on any damp spots, and it's best to wear gloves. Some good links:
http://www.allaboutcircuits.com/vol_1/chpt_3/9.html
http://support.fluke.com/find-sales/dow ... ng_h_w.pdf
Measuring motor amps can be difficult since most multimeters can only take 10 amps (fuse).

[Tim Douglass]

That 16 gauge cord will hurt you a lot. The load carrying capacity of a circuit is determined by the smallest wire involved, which is how fuses work - they are smaller than the rest of the wire so they burn out before the wire goes. I bet you will find that your cord gets hot when the sander is working hard, that will mean a significant voltage drop at the end, resulting in a lot more heating of the motor.

[Halgeir Wold]

Hi guys ( and gals ?).......
I'm sorry, but there's some inconcistencies here....
Current drawn is a function of the total resistance in a circuit, supply leads, extension cords etc all included. A 10' 16ga. extension cord will only give an appx 2 volts drop at around 15A, so that is surely not the culprit here. Normal variations in your house supply is probably a lot more, depending on how far from the substation you live. OTOH - 12-14 ga.extension cords for power tools is better - particularly for you guys running on 115V....

A drop in line voltage, extension cord or not, will not increase the current draw, rather the opposite. For a given mechanical load on the tool, however, current will increase as the motor have to work harder to cope with the load.
In this case, this motor still does it's tricks without any work load. This may indicate another mechanical load, - which could be the bearings. There are som soldered points inside, also an overload breaker. If the solder joints were bad, it could possibly make the motor loose power or stall, but that should not trip the circuit breaker, as a bad joint will increase the circuit resistance, and thus reduce the current draw. OK- if the motor jams, that's another story.
I'd still check the solder joints, possibly resolder them, and maybe check and tighten the spade lugs.
The overload breaker is another possible cause, although unlikely.

A thermal driven internal short circuit may indeed be the cause. This may or may not be disclosed with a megger, depending on wether the short is driven by thermal expansion in the windings or something else.
If there's sufficient space around the end caps, so that no points are really close to the casing, that should be OK.
Measuring the resitance between L and N when cold, also between L,N and casing, and then again immediately after stalling when the motor is warm, may reveal something. Start winding and cap are probably not at fault, since the switch works OK, and the problem manifests itself before the speed is reduced enough to close the start switch again.
If everything else is cleared, circuit breakers, supply leads, etc.etc. it's really only the windings and bearings left.

A quick test of the bearings wit the disassembled motoer, could be to heat them with a heat gun, if they haven't already been removed. Sort of 70-80 C ( 160F?) ... and - wear gloves when testing!

[Dick (DT) Trottier]

Hi Gerry,
Is there anything else connected to that 15A circuit?

[Gerry Gruber]

Hi David. Thanks for the tips on measuring voltage and amps. I like the idea of connecting up everything including the meter before applying power. When I ran the resistance checks after the last trial run, I also discharged the capacitor before removing the lugs to check the wiring - not sure if that was necessary or not. At this point, I am going to install the bearings I picked up - as you mentioned, upgraded bearings won't hurt, even if they are not the problem.

Hi Tim. I'm definitely going to pick up some 12ga extension cords to replace the 16ga ones. They did regularly warm up during use - never hot, but definitely warm.

Hi Halgeir. Thanks for the clarification on what happens when voltage drops. I did read elsewhere that amp draw would increase as voltage dropped, which made sense to me, as all dual 110/220 volt motors show about twice the amps at full load when using 110 as opposed to using 220. I'll double check any solder joints I can access. I've already checked the lugs.

Hi Dick. The circuit (normally) does not have anything else on it when running the sander. I use a different circuit for dust collection and lights. But, something also to double check. Mind you if the circuit was overloading, the breaker at the panel would go, which has never happened - it is always the motor that shuts down.

[Halgeir Wold]

Amp draw will increase with lower voltage, but not when idling. Once you present a load to the motor, amp draw will increase as the motor needs more juice to do the job. The 110/220 V double/half amperage, has to do with paralell /series connection of winding resitance.

[Gerry Gruber]

Thanks Halgier for that further clarification. I am finding that the more I read about and learn about electric motors from yourself and others, the less I realize I actually do know. A humbling experience... but also very interesting. And that is one of the things I enjoy so much about guitar-building. It requires that you know a little (and sometimes alot) about so many different subjects, from mathematics (to work out the distances for a particular scale length) to biology (identifying wood species, growth irregularities, etc.) to art (designing a headstock logo). Not many other jobs require expertise in such a broad range of subjects.

[Gerry Gruber]

Just an update for those of you wondering whether I have electrocuted myself yet. No, not yet. : )

I did manage to pull off the old bearings and install the new ones. The biggest challenge in doing that was finding a bearing puller. But, job accomplished. The new bearings feel a bit "stiff" compared with the old ones. But, both new and old are equally solid and smooth when just manually handling and spinning them.

I cleaned the contacts on the centrifugal switch, and then put the motor back together, and reinstalled it in the sander. However, I did not reinstall the old on/off switch with the built-in 15 amp breaker, but rather just used a standard light switch. I also purchased a 14 guage 25 foot extension cord to run the sander. (I found a few 12 guage cords, but all had built-in LED lights... which I do not like, as it means that they constantly draw electricity - yes, I know, very little - but some nonetheless.)

With this settup, I began to work the sander, and after a short while began to feel the sander halting, along with some "clanging" sounds (similar to the sounds before when the motor "spurted"? - not sure). I stopped the sander, and noticed that the pulley on the sanding drum was lose. Wow! It immediately occurred to me that this alone could account for the "spurting" (but not for the engine shutting down). I tightened up the allen nut on the pulley, and sanded for 30 plus minutes without incident. The motor was hot to the touch, as before, but it held up. The 14 guage cord remained cool - unlike the 16 guage cord I had been previously using which was typically warm after use.

Next step: I will reinstall the old on/off switch with the built-in breaker, and see whether the motor shuts down. If it does, then it would appear that this switch/breaker is faulty (has become overly sensitive). If it doesn't, then perhaps the bearing change and/or the extension cord upgrade were enough to "cool down" the motor to the point where it wasn't throwing the breaker.

[Gerry Gruber]

I reinstalled the old built-in breaker (15 amp breaker from my old table saw switch), and as before the motor shut off after 5 to 10 minutes use. Taking the breaker out of the equation, the sander ran continuously without incident. The motor was hot to the touch at the pulley end. But, no other problems.

I thought about measuring the actual current, to confirm whether or not the breaker was faulty. However, I realized that my mulitmeter is not suited to measuring AC current in this range. For that I need to buy a clamp meter. At this point, I am going to hold off on measuring the actual current. The motor has a thermal protection device and the house curcuit I am using also is protected with a 15 amp breaker at the service panel. The service panel breaker has never tripped - I have used two different curcuits, and neither has ever tripped at the service panel.

The more I think about it, the more I am coming to the conclusion, that when I ran the motor without the 15 amp breaker/switch, it did not actually shut down. Instead, what was happening was that the pulley was slipping. Because of the dust collector humming away, and my ear plugs, I thought the motor was spinning down when in fact it wasn't. I felt that the drum had stopped spinning, and concluded that, there it goes again.... the motor has shut down, when in fact, the motor was spinning away, but the pulley was slipping. Then, when I shut the motor off, as it spun down, it grabbed the pulley a bit now and then, producing the "spurting" symptoms I described in previous posts.

So, bottom line is that this 1 hp motor is working hard to run the sander, as evidenced by the heat. But, it does not appear to be drawing an excess amount of current (i.e., more than 15 amps), as evidenced by the service panel breaker not tripping. I will continue to run the sander without the old table saw breaker - just a regular light switch to turn the sander on and off, and protected with the motor's own thermal protection device and the curcuit's 15 amp breaker at the service panel.

I would like to thank everyone for your advice and comments as I went through this process. I learned alot about electricity, electric motors, and mulitimeters in the process. And, hopefully others of you have learned a bit as well through my experience.