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Thread: Unisaw 5 hp Breaker Tripping Issue

  1. #1
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    Unisaw 5 hp Breaker Tripping Issue

    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.

    C943113E-DBF3-4D0F-AD13-D11F5E2FF4F7.jpg98C8D8D6-398C-4008-84A2-0CB8F746234B.jpg

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    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?
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

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    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.

    Quote Originally Posted by Alan Lightstone View Post
    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?

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

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    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?
    --

    The most expensive tool is the one you buy "cheaply" and often...

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    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.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

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    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.

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

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    Wow! That would be a Code L. What does the nameplate say?
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

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    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

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    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?

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    Bill would you just wind the wire around the pipe?

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    Quote Originally Posted by Eric Arnsdorff View Post
    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.

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

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    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.
    Last edited by Bruce King; 07-29-2020 at 8:08 PM.

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