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?

Unisaw motor.jpg

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