Unisaw motor wind down?

[Scott Sveinson]

Hello,
I have an old unisaw that when I turn it off, the motor sometimes winds down smoothly, and other times it seems as though a brake is applied and the motor stops quicker. It's not a instant stop, but something is going on. Whenever it stops quicker, it also does not start back up on the first push of the Start button. If it winds down smoothly with no braking, it starts back up on the first push of the start button.

220 Single phase, 3HP on 30 Amp breaker.

Anyone seen this before? Thanks for any ideas or thoughts.

[Myk Rian]

Could be either the motor or arbor bearings.

[Gary McKown]

Don't the Unisaws have a magnetic brake (my Grizzly knockoff does)? If I remember correctly, as the motor winds down a circuit (or mechanical switch) applies current to as reverse winding or some such thing, forcing a quicker winddown. Is there a slight *shudder* when it is working properly? If so, then I would suspect an intermittent mag brake circuit or switch problem.

[Scott Sveinson]

I was hoping someone would confirm that the unisaw has a braking mechanism so I could rest a little easier. When the motor windsdown with no break, it doesn't seem to have any *shudder*. I would feel better if the motor started each time I push the Start, but after it seems to *break* to a stop, it doesn't go on the first push. I just hope it's not the motor. I can live with the switch or bearings needing replacement. Thanks for the posts.

[Jeff Duncan]

I have a couple Unisaws and neither has an electric brake, I don't believe they ever did, but I could be wrong. Sounds like an electrical problem but I can't help you with a fix.
good luck,
JeffD

[Carroll Courtney]

Can you take the belts off and try it to see if it stops quickly?If it does w/out the belts then maybe its the bearings.Please let us know what you fine----Carroll

[Scott Sveinson]

I'll be sure and post what I find. thanks for the info. and ideas.

[Alex Shanku]

centrifugal switch sticking?

[Nathan Hawkes]

Well, this probably doesn't help you at all, but I have an early 1970's Rockwell contractor saw, which utilized the same motor as the 1.5HP unisaws, and it DOES have the brake, and stops quite a bit faster than it would without it! I love that feature.

My bandsaw, a Ridgid 14", but with a 2HP baldor motor, will most often stop with its internal brake, but if I don't let it "warm up", or run for at least 30 secs or so, it often will not engage the magnetic brake, and will coast to a stop. Perhaps it has to recharge the capacitor first???

[Tom Veatch]

centrifugal switch sticking?

Wouldn't expect that to be the problem. If it stuck open, the motor would have such a low starting torque that it probably wouldn't start at all. If it stuck closed, after a very short run time, the start capacitor would be toast and the next startup would mimic the "stuck open" case.

I have no real ideas as to what the problem might be, but I'm guessing it's something not quite right in the magnetic starter.

[Greg Crawford]

Could it just be dust in the motor?

[Chip Lindley]

Trying to narrow down your problem, go to the trouble of bypassing the mag starter, and plug the motor directly into 230V. (I have a patch cord with a 230V plug on one end, and insulated alligator clips for just such occasions)

Observe how the motor starts and stops this way. If the erratic behavior continues while bypassing the starter, you know there is a problem inside the motor. (bearings, capacitors or the starter winding contacts) Bearings do not turn silky-smooth, or make a slightly-grinding metallic sound when they are going out. Unless, they are so far gone that they are seizing and do not turn freely.

To my knowledge there is NO internal brake on any Unisaw motors, unless an electronic dynamic brake was installed. But, Rockwell did make an accessory dynamic brake which used the same size metal housing as its Furnas mag starters.

The mag starter should engage each and every time you push the start button, even IF the motor fails to start. IF the mag. starter is actuating (Click/ON...Click/OFF) when you push the start/stop buttons, power contacts OR the pushbutton contacts may be burned or have sawdust in them, and could fail to start the motor every time. A moto-tool with fine brass wire wheel burnishes electrical contacts nicely.

Finally, trying to cover all bases, double check the arbor bearings with the belt off. They should turn smoothly with no noise. These could be seizing part of the time if worn out.

[Scott Sveinson]

Thanks for all the reply. The saw is new to me so I have started with a cleaning. Particularly the switch, which now seems to work on every try. It just had too much saw dust.

The *shuddering* still happens on the stop about 8/10 trys. I'll move on to removing the belts and see how she runs, or I guess stops.

What can I do to check the arbor bearings besides raising the blade and checking it for play? The blade rotates silky smooth with not even a hint of noise or scratch.

Hmm, thinking it's in the motor. We'll see when I remove the belts.

You guy's are a great help.

[Greg Robinson]

This is common with many Unisaws and similar machines with a baldor motor. It's not a motor brake but is a "braking effect" and is causing the shuddering that you describe. I've heard it described as "regeneration" and it has to do with the capacitors discharging I believe. If you look at owners manuals for some Powermatic machines they show you how to rewire their machines to eliminate this, but it's not harmful to the machine or motor. I've grown used to it on the machines I use at work.

[Pete Bradley]

It's interesting that the fast shutdown correlates with problems starting back up. My first bet is bearings. I'd pull the table insert, raise the blade, and slip the belts off. Then turn the arbor by hand feeling for any resistance or roughness. It should be absolutely smooth with no resistance. Remember most bad bearings have no discernable play. Do the same with the motor.

I also wonder if the bearings are heating up excessively when this happens. You may have to test whether the blade spins freely immediately after one of the "bad" stops. Unplug first of course.

Pete