Unisaw motor help

[phillip kerrigan]

Hi all!


Last fall, I acquired a Rockwell Unisaw 34-466 (mid-late 70's). At the time I had no way of testing it out, but took it with the idea that being a project could be fun.


Flash forward to now, and I am so close to having the machine up and running after a _long_ list of replacement parts and repairs. The problem that needs addressing this time is with the motor (Rockwell 83-651: 1ph, 3hp, wired for 230v) / electrical.


The saw has new cords (magnetic switch to electrical box, motor to electrical box, electrical box to outlet). Initially, the motor turned on and off with only a slight lag and hum. I assumed the start capacitor might needed to be replaced and because the motor is still outside the cabinet, now seemed like a good time to do it. Well, the old capacitor specs were printed on a paper label that over time bled and faded and are completely illegible. An internet search revealed someone else with the same motor set up who provided the specs on their capacitor so I purchased a similarly spec'd model. (220-250 VAC, 270-324 mfd)


I installed the new capacitor today and now nothing. Wrong capacitor? If so, what specs should I look for? I'd love to have this saw up and running as soon as possible. Thanks for any advice!

[Dave Cav]

You might try over at OWWM dot org (Old Woodworking Machines) Everything Electrical forum. Lots of experience there.

[Bill Dufour]

Dust inside the starting switch internal to the motor is a common problem.
Bill D.

[phillip kerrigan]

Update: I ended up checking the connection to the new capacitor and it might not have been on so well. Made sure the connection was good and tried again.

I now get the hum again, but nothing moves. I can give the pulley a little push and it does spin, but would take 1½ to 2 seconds to make a revolution. Or it would if the breaker didn't trip before one full turn.

Thoughts, anyone?

[Charles Taylor]

There's some small chance that the new capacitor is faulty as well, but as Bill D. suggested, the centrifugal switch inside the motor may need a good cleaning.

[Mike Henderson]

Try this. Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed.

If so, something is wrong with the starting circuit: Either the start capacitor is bad, the centrifugal switch is not closing, or your starting coil is bad.

If it doesn't spin up to speed, you have some other problem, for which I have no suggestions right now.

Mike

[phillip kerrigan]

"Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed."

Tried this and the motor spun right up! So....

If it's the start capacitor, is the new one I put on even the right one? (See OP for specs)

If the centrifugal switch isn't closing, how do I remedy that? Same for the starting coil?

I'll add that after shutting the motor down, starting it back up again a minute or so later tripped the breaker. Don't know if that helps diagnose anything, but thought I'd add it here just in case. (The wiring in the building should be good. It was redone about 6 months ago and the electrician was able to see what kind of power requirements this motor needed.)

Admittedly, dealing with this kind of stuff is not my forte so I really appreciate any help (particularly when it's descriptive)!

[John TenEyck]

I would take it to a motor shop. They will be able to fix anything short of fried windings, efficiently and correctly. I would have them change the bearings, too, and then you likely won't have to worry about the motor for 25 years or more.

John

[Mike Henderson]

"Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed."

Tried this and the motor spun right up! So....

If it's the start capacitor, is the new one I put on even the right one? (See OP for specs)

If the centrifugal switch isn't closing, how do I remedy that? Same for the starting coil?

I'll add that after shutting the motor down, starting it back up again a minute or so later tripped the breaker. Don't know if that helps diagnose anything, but thought I'd add it here just in case. (The wiring in the building should be good. It was redone about 6 months ago and the electrician was able to see what kind of power requirements this motor needed.)

Admittedly, dealing with this kind of stuff is not my forte so I really appreciate any help (particularly when it's descriptive)!

Let's assume the starting cap you put in is good and you installed it correctly. The next most likely problem is dirt in your centrifugal switch keeping the switch from closing. You have to take the end off the motor (the end opposite the driving shaft) to get to the centrifugal switch. By hand, open the switch and make sure there's no swarf between the contacts. Take some fine sandpaper and lightly burnish the contacts. Put back together and try the motor again.

If that doesn't work, I'd try another start capacitor (make sure it's the right size) and make sure it's connected properly. If that doesn't work, take it to a motor shop.

Mike

[Starting coils don't go bad very often. It's highly likely your problem is either a starting cap (probably 90% of the time) or swarf in the centrifugal switch (more than 9%, but not 10%).]

[phillip kerrigan]

Thanks for the well explained advice!

[Matt Day]

I think you’d spend a few hundred bucks at a motor shop, if they’d even work on a little toy like that (to them). Do some research and fix it yourself. Looks like you going down that road.

[phillip kerrigan]

OP here...

Just wanted to hop back in to share results. It took little time to get around to it, but cleaning up the contacts on the centrifugal switch did the trick. They were not just dirty, but slightly corroded so I sanded just enough to increase the amount of surface area available (using a fairly fine grit sand paper and creeping upon on it slowly). After putting it back together, the motor is now running like a charm! (Still trips the breaker every once in a while, but i'll deal with that later....)

In case someone in the future stumbles across this thread and could also use some help with this problem, I found a video online that does a good job of explaining centrifugal switches as well as showing how to clean them:https://youtu.be/F-jOBxn1w1I. For someone who is not great with motors, I found it very helpful.

Thanks again to all who offered thoughtful advice and responses!

[Eric Arnsdorff]

OP here...

Just wanted to hop back in to share results. It took little time to get around to it, but cleaning up the contacts on the centrifugal switch did the trick. They were not just dirty, but slightly corroded so I sanded just enough to increase the amount of surface area available (using a fairly fine grit sand paper and creeping upon on it slowly). After putting it back together, the motor is now running like a charm! (Still trips the breaker every once in a while, but i'll deal with that later....)

In case someone in the future stumbles across this thread and could also use some help with this problem, I found a video online that does a good job of explaining centrifugal switches as well as showing how to clean them:https://youtu.be/F-jOBxn1w1I. For someone who is not great with motors, I found it very helpful.

Thanks again to all who offered thoughtful advice and responses!

That's a terrific video/tutorial/tip on the centrifugal switch!

I bought a 5 hp unisaw which I originally had a long cord connected to it. It would start and run with no problem. As soon as I shortened my cord I started tripping my breaker. I wanted to keep the shorter cord so I ended up putting some Varistors in the circuit to create a higher starting resistance which quickly becomes low resistance due to the way a Varistor works. This method is ok but determining the right values and such is a bit of a guessing game at first and you have to use the correct components to make this work. There are other devices available to reduce the inrush current at startup and can get rather expensive.

So my easy tip is to use a longer cord. I'm not recommending a smaller gage cord. Do make sure your cord is capable of carrying the sustained current during operation. But a longer cord adds additional impedance to the circuit which can reduce the the inrush some. This method isn't a sure thing but may be enough to keep the inrush current under the trip curve of your breaker at startup.

[Charles Lent]

There is another problem that can occur with the centrifugal start switch that hasn't been covered yet. This is a common problem when motors haven't been used in a long time.

While you have the end cap of the motor off and you have cleaned the centrifugal start switch contacts, you should also service the flyweight mechanism on the motor shaft that operates the centrifugal switch. The weights are spring loaded to hold them against the motor shaft, but swing out as the motor speed increases from stop. When they swing, they move a hard plastic collar along the shaft. It is this collar that operates the centrifugal start switch. Usually what happens is that the shaft gets dirty or slightly rusted in the area where this collar must slide, and the collar can no longer slide easily. A simple cleaning using fine emery cloth of this part of the shaft, plus a very light film of thin oil on this area after cleaning, will fix the problem. Be very careful not to use much oil, since you don't want it flying off when the motor runs.

Now, make certain that the centrifugal switch contacts are clean and going tightly together, then re-assemble the motor and give it a test run.

Charley

[Mike Kees]

This is exactly why I really like three phase motors, so much simpler.

[Charles Lent]

This is exactly why I really like three phase motors, so much simpler.

Yeah, it makes the motors simpler, but increases the cost of the wiring and controls. Problems still happen, but usually in the wiring and control circuits and not in the motors. In factories, 3 phase motors that run nearly constantly are more efficient, so cheaper to run, but in a home shop the cost of a 3 phase installation is not justified by the lower operating costs. Now that phase conversion is more practical in small shops, these tools can be a bargain when bought used, but most of the time, installing 3 phase power from the power company to a home shop is usually not possible and frequently not at all practical.

Charley

[Bill McNiel]

OP here

In case someone in the future stumbles across this thread and could also use some help with this problem, I found a video online that does a good job of explaining centrifugal switches as well as showing how to clean them:https://youtu.be/F-jOBxn1w1I. For someone who is not great with motors, I found it very helpfu

Phillip,
Thank you, thank you, thank you for posting this link. My Laguna dust collector had been down for over a month. Laguna's "assistance" ranged from BS to nothing. The guy your link lead to was intelligent, well informed and explained the centrifugal switch beautifully. Within 20 minutes my dust collector was back up running. You went beyond the norm of saying "thanks, it is fixed" and presented a solid approach to solving a real problem. Again, thank you! SMC rocks!

Regards - Bill

[Rod Sheridan]

This is exactly why I really like three phase motors, so much simpler.

That’s true, however the issue is the use of a centrifugal switch, both my Hammer machines bring the run and start windings to the starter panel.

The start button has 2 contacts, one closes the latching run contactor, the other closes the non latching start contactor, once the machine is up to speed you release the start button.

Everything is in the starter enclosure no need to access the motor, much smarter system....Rod

[Mike Henderson]

That’s true, however the issue is the use of a centrifugal switch, both my Hammer machines bring the run and start windings to the starter panel.

The start button has 2 contacts, one closes the latching run contactor, the other closes the non latching start contactor, once the machine is up to speed you release the start button.

Everything is in the starter enclosure no need to access the motor, much smarter system....Rod

The system you describe works well - the human replaces the centrifugal switch. But centrifugal switches don't give a lot of trouble, especially in TEFC motors. The advantage of using a centrifugal switch is that the motor is brought up to speed "automatically" without any additional action other than applying power.

I'd rather just have to turn power on when I want to use my table saw, without having to hold the button. But I suppose you get used to doing whatever you have to, and it becomes second nature.

Mike

[phillip kerrigan]

OP here again.

Thanks again to all who have offered tips. They have been extremely helpful. It's been a while, but it looks like I'm not out of the woods just yet.

I did take the suggestion of cleaning up the shaft the centrifugal switch moves along. It was mentioned that flyweights swing out as the motor speeds up. The only things that appear to swing out on my motor are two rather light pieces of aluminum. I pulled one off as an example.

460010

Should a weight be attached to this?

Currently, the saw starts up (slightly sluggish) about 50% of the time. When it does, I can make a cut without any bogging down. When it doesn't, it usually trips the breaker (20amp). For every time it does start up, the next time it will trip the breaker. Trying to wait different amount of time between attempted starts yielded the same result. If anyone has had a similar issue, I'd love to hear how you handled it.

Thanks again in advance for the help!

[phillip kerrigan]

OP here.....yet again.

After another hiatus of trying to figure this thing out I decided to start at the beginning. Specifically the start capacitor.

The old one was well past it's prime and needed to be replaced (those who have followed along will remover the paper label containing the capacitor's specs was unreadable). An internet search turned up someone else with the same model motor who shared the specs of the capacitor they were having success with. So I bought one.

Flash forward to about a week ago, I thought "Why didn't I just get in touch with Rockwell in the first place? Surely THEY must know the correct replacement!"

Well, they didn't.

But they did refer me to Baldor, who manufactured this specific motor for Rockwell. (If I looked even the tiniest but closer I would seen 'Made by Baldor' on the motor plate!) So I reached out to them.

Baldor was able to provide me with the actual data sheet for my motor (I now know it was manufactured in Dec. of '77!) including the correct specs for the start capacitor. An online order and a few days shipping later, the correct spec'd capacitor was delivered and installed.

Now this saw starts up quickly (not sluggish like before) and reliably. I just turned the thing on and off for five minutes and it fired up without fail every time! No more walking around the building to flip a tripped breaker six or seven times everytime I needed to cut something. I finally feel like I have a usable machine.

So I guess it's never a bad idea to get in touch with the manufacturer! Despite many helpful tips found here, they have the data to help out for sure.

[Bill Dufour]

so please tell us the specs for future reference.
Bill D

[phillip kerrigan]

so please tell us the specs for future reference.
Bill D

Well, this is sort of the whole point. I got specs from a forum thread like this and it ended up being wrong for me, despite it being even the same model motor. If I list the specs here, someone else might end up getting the wrong part for them.

The point is to check with your motor's manufacturer. They have access to the information about the entire motor to ensure you are using the right part. Hope this make sense.

[Alan Schwabacher]

You have made the valid point that if you can contact the manufacturer, and if that manufacturer is able and willing to provide the information, that's best.

But it would still be helpful if you were to provide the value here, for those who need an idea of the right capacitance. A capacitor is cheap, and knowing something to try today rather than waiting until tomorrow can be valuable.

If you want to provide context, a shot of the motor nameplate would be a good accompaniment.

[Frank Pratt]

Now this saw starts up quickly (not sluggish like before) and reliably. I just turned the thing on and off for five minutes and it fired up without fail every time!

Nice that you found the right part. But I just want to caution that several starts in quick succession could well cause that new capacitor, or worse, the start winding to fail. Both the capacitor & start winding are running a their full load during a start cycle & the number of starts per hour is to be limited to the manufacturer's spec. It's not too bad with a low inertia load like a table saw, but it's generally something to keep in mind.

[Bill Dufour]

Well, this is sort of the whole point. I got specs from a forum thread like this and it ended up being wrong for me, despite it being even the same model motor. If I list the specs here, someone else might end up getting the wrong part for them.

The point is to check with your motor's manufacturer. They have access to the information about the entire motor to ensure you are using the right part. Hope this make sense.


You are correct but what about when the maker goes out of business? I hope this forum, and it's information, will still be available in decades. What about a owner of a Walker Turner tablesaw with a similar size motor. I doubt if Fairbanks Morse has any info about those motors. A real number that is factory approved is a better starting point then just an approximation of what many makers might have used in the past.
Bridgeport invented the vertical metal milling machine and made them for over 90 years. They used a custom long shaft motor. The company is now gone and sold along with factory information.
Bill D