Unisaw violent starts and overload

[Kathy Engholm]

I'm looking for troubleshooting advice. My 6-yr-old left-tilt Unisaw has a 240-volt, single-phase motor (2 or 3 hp I think) and a mag switch with overload protection. The last couple of sessions I've used it, it has occassionally failed.

Here's the behavior:
- The first time it happened, I had the blade tilted about 30 degrees and fairly low. When I pressed Start, the whole saw jumped and the motor shut off almost instantly. After unplugging I checked for anything that might have bound up the blade, belts, motor, etc, but everything turned freely. Once the overload switch reset, it ran OK. I suspect that the tilt position is not related to this problem.
- Every few starts, I get a smaller jump or lurch than that first time, but still not the smooth, quiet start I am used to with this saw. Usually, but not all the time, the violent starts are accompanied by the overload protection shutting off the motor.
- Sometimes, the overload cut-off doesn't happen at start-up, but is tripped when I shut OFF the motor. (I only know this if I go to make another cut fairly soon, and nothing happens when I press Start.)
- One time, the overload shut-off was sudden enough that the arbor nut spun loose. A blade spins for a LOOOONNGGGG time after the motor stops if the nut is loose.

I adjusted the belts, cleaned the fan, checked all the electrical connections, including those at the two big capacitors. There doesn't seem to be any rotational play in the shaft. No looseness in the height and angle adjustments. No suspicious smells or heat. I'll pull off the belts this weekend to see if the shaft is loose or the bearings feel rough.

Any ideas why the rough starting and how this would be related to pulling too much current?

[Bruce Wrenn]

My first choice would to replace both start and run capicators. Sounds like one, or both are in the early stages of death. You could also have a bearing that is seizing. When you said that it stopped so fast as to loosen arbor nut, does your saw have an automatic brake? When I taught shop, we had a Delta 14" RAS with auto brake. It would spin the arbor nut off regularly, until the brake was disconnected.

[Kathy Engholm]

I don't believe it has a brake on it. The manual doesn't mention it. The blade spins down over 10 or so seconds.

Thanks for the capacitor advice. I'd definitely prefer to replace them than the motor!

[Mike Heidrick]

I'm looking for troubleshooting advice. My 6-yr-old left-tilt Unisaw has a 240-volt, single-phase motor (2 or 3 hp I think) and a mag switch with overload protection. The last couple of sessions I've used it, it has occassionally failed.

Here's the behavior:
- The first time it happened, I had the blade tilted about 30 degrees and fairly low. When I pressed Start, the whole saw jumped and the motor shut off almost instantly. After unplugging I checked for anything that might have bound up the blade, belts, motor, etc, but everything turned freely. Once the overload switch reset, it ran OK. I suspect that the tilt position is not related to this problem.
- Every few starts, I get a smaller jump or lurch than that first time, but still not the smooth, quiet start I am used to with this saw. Usually, but not all the time, the violent starts are accompanied by the overload protection shutting off the motor.
- Sometimes, the overload cut-off doesn't happen at start-up, but is tripped when I shut OFF the motor. (I only know this if I go to make another cut fairly soon, and nothing happens when I press Start.)
- One time, the overload shut-off was sudden enough that the arbor nut spun loose. A blade spins for a LOOOONNGGGG time after the motor stops if the nut is loose.

I adjusted the belts, cleaned the fan, checked all the electrical connections, including those at the two big capacitors. There doesn't seem to be any rotational play in the shaft. No looseness in the height and angle adjustments. No suspicious smells or heat. I'll pull off the belts this weekend to see if the shaft is loose or the bearings feel rough.

Any ideas why the rough starting and how this would be related to pulling too much current?

This really sounds like belt issue to me. Sure the belt does not slip anywhere?

[Mike Goetzke]

Maybe you should try removing the belts and try several starts. Might tell you if the belts are to blame. Plus you could turn the arbor by hand to see if there is any resistance or play in the bearings.

[Richard Keller]

I would suspect a bearing before anything. A belt won't cause such a violent jump. Something must be binding.

Richard.

[Pete Bradley]

Jumpy starts are often belt-related, but the sudden stop is diagnostic for something binding. Slip the belts off and turn the motor by hand. It would be odd for this to be a bearing, but not out of the question. Slip the belts off both pulleys and spin the arbor and the motor by hand. If one or the other is not smooth as glass, it's time for a pair of bearings. I'm betting on the arbor. You may find the shaft isn't too healthy any more.

Please report what you find. This is an interesting one.

Pete

[Carroll Courtney]

Also check and make sure that the pulley is not wondering on the motor shaft.With the blade tilted,it may be running over to the low side and binding.

[Kathy Engholm]

Just before I logged in here, I took the belts off to see what I might learn.
- Arbor seems smooth, but I don't know if it is spinning as freely as it should. How long should the arbor for this saw freewheel if you give it a smart spin by hand? Mine stops in a couple of seconds.
- I can't find any play or roughness in the motor shaft.
- Pulley is nice and solid. No in-and-out motion on it or the shaft.
- I did several motor starts/stops. After the first set, the motor restarted itself after I released the stop button. I had to hold the stop button in for a minute or two before I could let it go without the motor starting up again.
- With the belts off, the motor is very smooth and quiet. I didn't notice any significant jumps, but then again I only got about 4 or 5 starts before the overload protection tripped.

Maybe 5 starts in less than 5 minutes isn't a fair test. The heaters in the overload circuit don't get a chance to cool down between the high starting surges.

My next step is to put on new belts and see what happens.

[Pete Bradley]

It's not how long it spins, it's how smooth it feels.

After re-reading your post, I'm not sure we've hit on the problem yet. There are a couple of guys on the Old Woodworking Machines forum that might be able to shed some light on this also.

Pete

[Carl Eyman]

This single phase motor undoubtedly has a centrifical start switch internal in the motor. I wonder if the contacts are dirty. If this is true, I'd expect the likely result would be the motor not starting but sitting there humming with the overload relay dropping out in a short time. However, it would be worth a try of blowing out the motor with compressed air. Of couse, if it is a totally enclosed motor this isn't possible. ( It should have something like TEFC on the nameplate, if it is )

[Dave Mcintire]

If the motor won't stop when you press the stop button or restarts itself this has nothing to do with belts, pulleys, bearings or any of that stuff. Its definitely an electrical problem with either the start/stop switch, centrifugal switch in the motor or the magnetic contactor (if it has one). This would also explain the jumping when you try to start. Its pretty hard to guess what it is via Email and with the info you gave but anyone with basic electrical experience should be able to diagnose it pretty easily. Alternately, take the motor and controls to a motor shop.

[Kathy Engholm]

Dave, I wasn't able to think of a reason that a bad start/stop switch or magnetic contactor in the switch box could make the motor jump. That's why I've been trying to diagnose a motor problem. Could you please explain to me how a bad switch/contactor makes a motor jump? I'd much rather troubleshoot and repair a switch than a motor. As a practicing electronics engineer, I probably qualify for "basic electrical experience", but I haven't dealt with motors since Motors&Generators Lab almost 30 years ago.

The motor is totally enclosed (TEFC label and no visible openings). The centrifugal start switch clicks in normally during spin-down after the stop switch. No indication that it is sticky, as the jumping and overload cut-out both occur to fast to ever get into the humming state.

Thanks for the guidance.

[Carl Eyman]

Kathy, your identifying the motor as a TEFC and hearing the sound of the cent. switch clicking in or out pretty much makes my earlier post moot. So here we go on another long shot. Does anyone think that a possible short circuit in the wires that allow you to reverse the direction or rotation could be a factor? The sudden stop you described is so unusual that even a malfunction of the arbor doesn't seem to be a creditable cause. However, a change in the rotation of the magnetic field could explain both the sudden stop and the jumpy starts.

This is one of the most intriguing threads I've seen on the Creek.

As most will know there is a type of switch that allows one to reverse a motor's rotation at will. They are often used on metal working lathes. Those are usually mechanical (non-magnetic) switches. Though I have seen them as magnetic motor starters to reverse dough mixers. By any stretch of the imagination you wouldn't have such an animal on your saw, would you? You'd have to have at least four wires - not counting a ground - between the switch and the motor.

[Richard Keller]

Kathy, your identifying the motor as a TEFC and hearing the sound of the cent. switch clicking in or out pretty much makes my earlier post moot. So here we go on another long shot. Does anyone think that a possible short circuit in the wires that allow you to reverse the direction or rotation could be a factor? The sudden stop you described is so unusual that even a malfunction of the arbor doesn't seem to be a creditable cause. However, a change in the rotation of the magnetic field could explain both the sudden stop and the jumpy starts.

This is one of the most intriguing threads I've seen on the Creek.

As most will know there is a type of switch that allows one to reverse a motor's rotation at will. They are often used on metal working lathes. Those are usually mechanical (non-magnetic) switches. Though I have seen them as magnetic motor starters to reverse dough mixers. By any stretch of the imagination you wouldn't have such an animal on your saw, would you? You'd have to have at least four wires - not counting a ground - between the switch and the motor.

Very doubtful that the saw would have a reverse switch on it, as it would have no use. Kathy also said that she checked the connections in the motor in an earlier post. BUT -- is it possible that one of the leads inside the motor has a crack in the insulation?? This would cause some jumping, the problem would be intermitant, and the overloads would trip...

Richard.

[Kathy Engholm]

I bit the bullet and looked up the schematic for capacitor-start motors. (What would we do without the internet?) My best guess is that the problem is somewhere in the start winding circuit, since that's the only part of the system that is connected during starts and stops but not during run. There are 3 components: centrifugal switch, start capacitor, start winding. I'm pretty sure the switch is OK. The winding would be hard to fix and expensive, but it could easily cause these symptoms. The capacitor is also a reasonable suspect. It will be easy and cheap to replace, once I find a source for it.

[Bill Turpin]

Check the motor and arbor bearings with the longest, heaviest screw driver that you have. Fourteen to eighteen inches is the best. Remove belts, blades etc. that could get you if ScD slips. Put the blade tip on the side of motor or arbor housing as close to bearing as possible. Press the end of the screw driver handle against the flap that closes off your ear canal. You now have a bearing stethoscope. Start the motor, there should be an even whirling sound. Any clicks, ticks, bumps, grinds, taps, knocks, scraps, etc means BAD bearings. Check both ends of motor. Put belts back on and check arbor WITHOUT the blade. This stethoscope works for all things mechanical. Please be careful that moving parts don't get you! :( Every home mechanic should own a screw driver that looks like a crowbar.

Bill Turpin in WNC mountains

[Norman James]

I know I'm really late on this, but hope that my situation may help some one else.

I had the same violent start situation. After some research, I took the motor to the motor repair shop. After doing a full cleaning around the start windings and weights, the problem was solved. I had read a lot about the capacitors, but from what I had read, they are good or bad. This violent motor really does make you wonder about what is going on as the whole saw shakes! I had one start that made it to run speed, it was smooth again. On my next start it was violent, so I stopped before possibly doing a lot of damage. While on the bench, I had them replace the bearings with sealed instead of shielded bearings. I hated to pay all this money for a cleaning and no improvement.