70's Unisaw Start stop issues

[michael roughan]

I acquired a Delta 3hp 240v Unisaw (70's) that cuts great without vibration but has two issues. Every 20th time I start the saw, it instantly trips the breaker. This happens randomly on first use one day and after repeated use the next. Changed the frayed cord, blew out the 30 years of accumulated sawdust from the switch and starter boxes and thought the issue was solved but after 2 weekends of constant use came back the third and tripped on first start. I have a new 20 amp independent circuit, 12 gauge wire (20' from breaker).

I have tracked some of the great (older) threads on this issue and noted the following options:
1. Bad Breaker
2. Bad Starter Capacitor
3. Defective magnetic starter switch
4. Centrifugal switch sticking
5. Motor winding issue

I will go through the diagnostics in the order above unless someone has a better suggestion?

The second issue is more straightforward. While the saw starts up and runs without vibration, on shutdown the arbor shutters. It appears 2 of the 3 belts are matched but one is clearly oversized. Do I remove this belt, try reordering which belt is inside, middle and outside or buy new belts?

First time poster but avid reader.

[mark kosse]

I'm no expert but if I remember right that motor pulls 16a @240V at running speed. Start up would be higher. 20a may not be enough for start up. I believe 30 is more in order.

[Bill Neely]

Curious, did the problem start after you installed the new circuit?

[michael roughan]

No, the circuit was put in specifically for the saw and there are no other devices on the circuit. I do believe I'll try to switch out the breaker next.

[Steve Baumgartner]

You seem to have a reasonable list of things to check for the startup issue. Tripping only occasionally suggests that the startup current is right on the edge of what the breaker tolerates, which could be either a motor issue or a marginal breaker.

The belts have to be matched, hence replaced in sets of three else you are likely to get vibration.

[Mike Hollingsworth]

Installing a larger breaker will certainly solve your issue. Not sure if it's a good Idea though.

[Jim Neeley]

One note.. swapping out the breaker doesn't necessarily require stepping up a size although that's always an option.

There is a level of testing to ensure they trip when they are supposed to but (as far as I know) not precisely tuned to ensure the won't trip prematurely.

At the low pain ($ & time) for breakers, I'd start swapping in the same rated breaker.

[mark kosse]

To expand on my response, a 20a circuit is considered fully loaded @ 80% load, or 16A. Your motor maxes the circuit at running speed. A motor at start up pulls a minimum of 150% of fla and in the case of your motor would be 24A. That would trip your breaker. I have seen motor start up amps estimated as high as 300% of fla.

[Bill Neely]

Code allows for stepping up the size of a circuit breaker to compensate for starting current as long as it's a dedicated circuit. You could certainly go to a 25A breaker.

[michael roughan]

I'll start with swapping out the 20 amp breaker with another 20 amp one. If it still trips I'll up the amperage on the breaker. Stay tuned.

[Tom M King]

I'd run a 30 amp circuit for that saw. The motor will last longer.

[Duane Meadows]

If it only trips that infrequently, I'd blow dust out of, and check the centrifugal switch first. The FLA(the FL is Full Load) rating on a 3HP motor should not be near 16A, let alone idle current... the 20 amp circuit should be fine. 16 amps is 5 HP input. The 3HP should FLA somewhere around 10A, again that's full load, not idle! Check the name plate on the motor if in doubt. Next I would check the actual run current of the motor with an ammeter. 3rd would be the start cap, though I kinda doubt that it is that intermittent, though I suppose stranger things have happened.

[Rod Sheridan]

I'd run a 30 amp circuit for that saw. The motor will last longer.

No it won't.

If the circuit does not have excessive voltage drop it will have no effect on motor life......Rod.

[michael roughan]

Is the centrifugal switch between the motor and the pulleys? Any idea of the best way to clean it out?

296919


If it only trips that infrequently, I'd blow dust out of, and check the centrifugal switch first. The FLA(the FL is Full Load) rating on a 3HP motor should not be near 16A, let alone idle current... the 20 amp circuit should be fine. 16 amps is 5 HP input. The 3HP should FLA somewhere around 10A, again that's full load, not idle! Check the name plate on the motor if in doubt. Next I would check the actual run current of the motor with an ammeter. 3rd would be the start cap, though I kinda doubt that it is that intermittent, though I suppose stranger things have happened.

[Charles Lent]

My 3 hp Unisaw runs on a 30 amp breaker and #10 wire. I think you're trying to use too small of a breaker and wire to handle the start-up current. The breaker is there to protect the wire, not the saw. The thermal heaters in your Unisaw motor starter control box are there to protect your motor. Put in a #10 ga circuit with a 30 amp breaker and connect your saw to it. You will never have any more breaker tripping problems and it will be safe. When sizing the wire and breaker you need at least 150% more capacity than your motor's full load rating. It's allowed by code and it is safe.

Charley

[Duane Meadows]

If the centrifugal switches has issues, it will trip the 30 amp breaker also. It just may do more damage before it does! My 3HP saw has run on a 20 amp circuit for 18 years without ever tripping the breaker. Breakers do not trip instantly and should handle the start current ok...it's only hi for a second or two.

The centrifugal switch is in the motor, under the end cover. Probably 3 screw holding the cover on(maybe only 2).

Again check the nameplate amp rating or check in the saws manual... I really don't think that saw pulls enough amperage to require a 30 amp circuit. As Rod said, it will not extend the life of the motor. Not fixing the problem will shorten it though!

[mark kosse]

Like I said, I'm no expert. I guess rockwell manufacturing wasn't either when they stamped 15.8 amps on the nameplate of my 240v only motor. Your giving bad info out.

[michael roughan]

Thanks all for the responses. I changed out the 20 amp breaker with another circuit and ran the saw from 7:30 am till 6 pm no trips, We'll see how long this lasts?

[Duane Meadows]

Like I said, I'm no expert. I guess rockwell manufacturing wasn't either when they stamped 15.8 amps on the nameplate of my 240v only motor. Your giving bad info out.

Math says otherwise... look it up, don't take my word for it!

Power = volts times Amps
15.8 X 240 = 3782 Watts.
1 HP = 746 watts.
3782/746 = 5.083 HP.

That's raw power input, not taking efficiency and power factor into account. Rockwell in those days was probably a bit more truthful about HP than many manufacturers are today. Older motor with lower effiency, maybe. But that's still likely FULL LOAD amps. How often do you hit full load on a tablesaw?

Still say it will run very well on a 20 circuit. I know mine has.

Michael, was that non stop. or start/stop for 10+ hours? If non stop, only thing that proves is it started that time... once it starts properly the centrifugal switch is out of the equation!

[michael roughan]

Duane,

I am a weekend woodworker so no the saw was not on continuously. I'd say 10-15 starts/stops yesterday and another 1o today. So far so good. If It stays like this I'll focus on the belts issue.


Math says otherwise... look it up, don't take my word for it!

Power = volts times Amps
15.8 X 240 = 3782 Watts.
1 HP = 746 watts.
3782/746 = 5.083 HP.

That's raw power input, not taking efficiency and power factor into account. Rockwell in those days was probably a bit more truthful about HP than many manufacturers are today. Older motor with lower effiency, maybe. But that's still likely FULL LOAD amps. How often do you hit full load on a tablesaw?

Still say it will run very well on a 20 circuit. I know mine has.

Michael, was that non stop. or start/stop for 10+ hours? If non stop, only thing that proves is it started that time... once it starts properly the centrifugal switch is out of the equation!

[Duane Meadows]

Michael, you could have very well had a bad breaker.. it does happen. Hope all stays well with your saw!

[michael roughan]

2 weeks and counting, almost ready to declare the culprit was an overly sensitive breaker. I swapped 2 double breakers and neither have tripped since.


Michael, you could have very well had a bad breaker.. it does happen. Hope all stays well with your saw!

[michael roughan]

Ok, one month since the initial post and the saw is behaving well. I guess changing out the breaker did the trick.

[David Eisenhauer]

I believe I have heard that the shudder is common with Unisaws and mine has done it without any issue (as far as I know) for 30+ years now. Same belts. I use a 10# extension (maybe 8' long?) for my saw that runs to a 20amp circuit without any drop out issues.

[michael roughan]

Dave,

Thanks for the feedback on the shutter. I may defer any attention to the saw until I have some free time (probably months).