Unisaw 5 hp Breaker Tripping Issue

[Eric Arnsdorff]

Industrial machines powered by residential electrical isn’t always the desired result.
My 5 hp Unisaw started tripping a 40 amp breaker when I rewired with a shorter larger gage cord to a subpanel next to my main panel. It turns out the extra ability to pull higher current from a less resistive wiring scheme creates a lot of inrush.
Adding a long 10 gage extension cable reduces the inrush enough to stay under the trip curve!
I was concerned that the motor may be going out. I was glad to find that it was working well and no issues. Possibly this will help someone else with a similar issue.

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[Alan Lightstone]

That's really interesting, Eric.

So what is your solution to using the machine going to be? I assume you'd want less restrictive wiring to send the larger appropriate current to your Unisaw.

A bigger breaker with something like 6 gauge wiring to the saw outlet?

[Eric Arnsdorff]

Alan that’s a possibility but there’s some other ways. For this weekend it is simply using the extension cable.
Since this inrush current isn’t needed functionally, I will limit the inrush and go back to my shorter cable. There are several ways to do this.
I’ll take a look this evening at using thermistors - it’s a cheap device and simple to install. It has various uses but in this use it offers a high resistance until it heats up (this happens quickly) and then becomes low resistance. The down side is it does still have some additional resistance.
There are devices to insert in the line which are basically a time delay relay which puts a resistance in line briefly and then switches it out after startup.
There are a few other ways of doing this but I’ll likely utilize one of those in the end.
I’ll likely utilize one of those so the breaker can be reduced.
I thought I’d share this experience since it seemed to be the best case of having minimum voltage drop with short wiring and such. But it backfired on me and caused an unexpected problem. But I’ll take this problem over a motor issue any day.


That's really interesting, Eric.

So what is your solution to using the machine going to be? I assume you'd want less restrictive wiring to send the larger appropriate current to your Unisaw.

A bigger breaker with something like 6 gauge wiring to the saw outlet?

[Ron Selzer]

Thanks for posting this information. I really don't understand the graphs, however I understand your explanation.
Ron

[Jim Becker]

Is there a different type of breaker available that accounts for startup inrush better so you can better take advantage of the newer, heavier, full capacity connection?

[David L Morse]

Nice test equipment!

If I'm reading that right your locked rotor current is 185A at 239V. That seems high. That would make that motor a Code K. Not impossible certainly, just not expected on a table saw.

Sometimes the small increase in insulation life just isn't worth the effort involved in, or the consequences of, squeezing out that last volt or two.:)

[Eric Arnsdorff]

Jim I’m not aware of many options for residential breakers. They may exist. Certainly industrial offerings are available.

But as David said it really doesn’t provide any benefit and a little startup voltage drop in this case is fine. Heck it starts up too fast as is. Adding a bit of startup resistance may provide a bit of a “soft start”. I do want the short cord just to cleanup the wiring so I’ll limit it.

I always tend to over analyze and I have to resist that to be more productive. This is one where professionally I would’ve spent the time looking at all the details but thought it wasn’t needed for my woodworking equipment.

[Eric Arnsdorff]

BTW David - the 185 was with the extra cable. It was over 210 without it!

[David L Morse]

Wow! That would be a Code L. What does the nameplate say?

[Bill Dufour]

Would a line reactor on the input reduce the input rush? I do not think they add much resistance. You could make your own with some waterpipe couplings.
Bil lD

[Eric Arnsdorff]

David the nameplate doesn't say. I wished it did so I could've compared some numbers but no luck. I'm not sure but my suspicion is the older US made Unisaws likely had different motors labeled and listed appropriately but with the overseas construction this may have changed. I'm thinking you would consider a code L motor to be a bit unusual for starting torque. I've seen this on some motors I consider general purpose. But I'm not an expert in motors. I've used a lot and design electrical circuits to support them in many ways. So I was thinking the current was likely normal. Do you think there may be an issue?

Bill I'm sure a line reactor would work. I don't know about the water pipe coupling as a line reactor. Could you explain further?

[Eric Arnsdorff]

Bill would you just wind the wire around the pipe?

[Bill Dufour]

Bill would you just wind the wire around the pipe?


That is my understanding. Simple soft iron is best since it is not that magnetic and will dissipate any spikes. Two or three couplings welded in parallel to a strap. . Wrap them in electric tape then run 2-5 turns of wires through each. Same basic idea as a choke coil. Be careful they do not rub the insulation away from the 60hz hum vibrations.
You are basically making half of a transformer. Make sure the wire gauge is heavy enough to handle the amps.
Bil lD

Quality VFD's will have a load reactor built in so they can handle the return spikes on shutdown.

[Rod Sheridan]

Reduced voltage start on a 5 HP motor, now that’s funny.....Regards, Rod

[Bruce King]

Install an HVACR type 40 amp breaker.
It’s designed to handle high start amps.
You did try a new breaker right ?
On that motor there is likely an internal centrifugal switch that might be stuck or bad but you already know it works on a different type circuit.
The switch disconnects a start capacitor once the motor is spinning enough.
Another thing, it’s ok to put in a larger breaker on a motor circuit without going up in wire size.

[Eric Arnsdorff]

Thanks Bruce! I will look up the HVACR breakers for my panel. That’s good to know. The breaker is “upsized” and yeah the overload protection allows the upsized short circuit protection.
I have a $7 Thermistor on the way and will use it. It’s a 1 ohm which will reduce the startup current to about half and heat up enough quickly to go to a negligible resistance.
It’s relatively small and I have some terminals that will fit in the subpanel for it. So it looks to be the best solution for me.


Install an HVACR type 40 amp breaker.
It’s designed to handle high start amps.
You did try a new breaker right ?
On that motor there is likely an internal centrifugal switch that might be stuck or bad but you already know it works on a different type circuit.
The switch disconnects a start capacitor once the motor is spinning enough.
Another thing, it’s ok to put in a larger breaker on a motor circuit without going up in wire size.

[Eric Arnsdorff]

It doesn’t look like Eaton makes any Definite Purpose breakers for either my main panel or my subpanel.
The NTC thermistor provides a small package low cost solution and the inrush will be reduced.

[Frank Pratt]

It doesn’t look like Eaton makes any Definite Purpose breakers for either my main panel or my subpanel.
The NTC thermistor provides a small package low cost solution and the inrush will be reduced.

I am dubious about your thermistor plan. I've never seen a motor controller that uses that for reduced voltage starting, so that is an indicator. What is the heat dissipation rating of the thermistor? It will need to be pretty high to survive that much current x voltage drop.

[Andy D Jones]

Some thermisters are not designed for repeated activation. Those PTC thermisters designed more for circuit breaker or fuse replacement in small electrics do not return all the way to original on-resistance after activating and cooling.

Check the specs on whatever thermister you choose very carefully.

-- Andy - Arlington TX

[David L Morse]

It doesn’t look like Eaton makes any Definite Purpose breakers for either my main panel or my subpanel.
The NTC thermistor provides a small package low cost solution and the inrush will be reduced.

What Eaton series do you use? BR series does include High Magnetic versions, e.g., BR240H for a dual 40A.

[Frank Pratt]

What Eaton series do you use? BR series does include High Magnetic versions, e.g., BR240H for a dual 40A.

OP will probably have to go to an electrical wholesaler to order one. I doubt the big box stores would have it.

[Bill Dufour]

Or you may have to install the magic breaker onboard the machine or in a subpanel.
Bill D

[Eric Arnsdorff]

I’m not seeing a different trip curve for the BRH’s Maybe I’m looking at a different breaker than you’re referring to.
I pulled up the the trip curves and It shows the same curves. The H’s do have a higher IC - Interrupting Capacity but I couldn’t find anything indicating they had a different trip curve.
I do appreciate the discussion and looking at other options. It’s good to learn what’s available. For commercial/industrial panels there are many offerings of breaker characteristics to coordinate and taylor to a specific purpose. I’m not aware of any being available for these residential grade devices. But maybe I’m not looking at the right items.

The NTC thermistor isn’t the most ideal choice as it does have a couple of things that other more expensive and larger devices avoid.
1) For full resistance recovery it takes 30 secs to a minute to cool down and be fully effective. This is fine for my typical saw operation and that it is usually the cold starts that create the most inrush. I’m ok with this but not ideal for all applications.
2) The device gets hot quickly and this is how it operates. My concern in my shop is the wood dust. I haven’t had an issue inside my panels even though they aren’t dust tight. But it is worth an occasional check inside to make sure. I don’t plan to go to dust proof panels.

Others have commented about board level thermistors or the use of a thermistor at all in this application. They are used in these applications and are made for higher voltage and higher current applications. As a link to the breaker discussion, one of the applications is in hvac equipment. Often VFD manufacturers use them to manage inrush and such. I’ve used them for patient handling equipment in Positron Emission Tomography scanners. They do have drawbacks and are not useful in all applications. There are other means of inrush management. They are not universally applicable.
I’m happy there is this level of technical discussion. I’ve learned a lot from this group in the short time since I’ve joined and look forward to further assistance.
Now if I could just turn this technical knowledge into amazing woodwork I’d be delighted! I’ll keep trying.

[Rollie Meyers]

Install an HVACR type 40 amp breaker.
It’s designed to handle high start amps.
You did try a new breaker right ?
On that motor there is likely an internal centrifugal switch that might be stuck or bad but you already know it works on a different type circuit.
The switch disconnects a start capacitor once the motor is spinning enough.


Another thing, it’s ok to put in a larger breaker on a motor circuit without going up in wire size.

All breakers are HACR now which is why you do not see them marked like they formerly were.

[Eric Arnsdorff]

Thanks Rollie! That explains that!


All breakers are HACR now which is why you do not see them marked like they formerly were.

[Bill Dufour]

There is some interesting code rules about breaker location and also the GFCI breakers. A GFCI outlet ten feet away from the main panel only has to be able to trip and not weld shut under something like 500 amps. A breaker inside the main panel has to be rated not to weld shut under 10,000 amps. This has to do with how much power can go done a wire run before the wire vaporizes. A short fat wire from the unlimited power in the alley can carry a big overload before it frys. A longer smaller wire has enough resistance to slow things down so the breaker can trip open under load. That is why most outside GFCI outlets are at least ten feet from the service panel if they meet code.
My numbers may be off from memory but you get the idea. High current switchgear in a power station will sometimes use air blast to blow out the arc as they are opened.
Bill D.

[Eric Arnsdorff]

As a follow-up, I put the thermistor in and took some measurements.
Inrush without the thermistor: ~210 amps
Inrush with the thermistor: ~140 amps

I’m back to my short cord. One less thing to clutter the shop. But goodness, I have so much more to do!

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[Eric Arnsdorff]

The pics on the last post had the voltage probe before the thermistor. This pic shows the voltage drop from the thermistor and the quick recovery back to full voltage.
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[sean meltvedt]

Eric, I have a heat pump with a pair of 5 ton compressors. LRA of over 170 amps. It was annoying as all get out when they started. So I installed a couple of magic box single phase soft starts that reduced inrush to approx 50 amps. I actually ordered them online, but the manufacturer web site is www.hypereng.com. I’m curious if they have a version that will fit your saw.
Cheers
Sean

[Bill Dufour]

The long cord acts as a line reactor. It takes a few phases for the field to build up to full strength and push the amps through by then then back emf of the motor is resisting the inrush current.
Bill D

[Eric Arnsdorff]

Good point Bill - I keep just calling it resistance, but I should refer to the overall AC effect as impedance.

I'm very happy with the $7 (shipped) NTC Thermistor solution. I did some tests to see the impact of it being heated up and restarting the saw as soon as it spun down. The current was still reduced significantly. I think the combination of the normal running current being much less than the FLA and the time for it to spin down made it very effective for the application. I went with a very low resistance Thermistor (1 ohm) which could likely be increased and lower the inrush more but the 1 ohm provided all I needed.

Thanks to all for the alternate potential solutions. To add to those there are motor soft starters and modules that have SCR and Triacs to do some switching and such to reduce the current. Another potential is reducing the start capacitor. Even using PTC thermistors in the start circuits and such of the motor. Those are all more expensive and more intrusive so I'm happy with the NTC Thermistor solution. I also suspect typically people use longer power wiring and smaller power cords which would keep the inrush down. Therefore, I'm sure this isn't a universal problem. But maybe it could help someone with a similar issue.

Thanks All!!