3 phase soft start application
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  1. #1
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    Default 3 phase soft start application

    i have a 10hp 3ph platen grinder (used for cyl heads, exh manifolds, etc) bought new 1986. at my present location i am forced to power it from a rotary ph converter (20hp) running current [email protected]
    this machine will not start under load at this point. inrush current [email protected] no load (ie: nothing on the table)
    i have no experience with 3phase soft starts and because this machine is required to start under a load i question if a soft start would cure this. anyone have experience with this?

    fwiw in other shops it would dim the lights for a couple seconds on start up. don't remember it doing that back when i bought it but then the memory is questionable

    i don't believe this is a ph converter issue which is why i didn't post it there

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    You could change the taps on your motor to a "WYE" configuration and see if that helps.

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    Quote Originally Posted by Larry Dickman View Post
    You could change the taps on your motor to a "WYE" configuration and see if that helps.
    Correct, it is a common method to start in WYE and switch to delta after it gets going. Some of the big grinders I work on have load meters and on startup they draw almost exactly the same current as running in delta with a full wheel load. Of course, that requires extra switches, normally two contactors interlocked to avoid having both on at once.

    Bill

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    6 leads must come from motor for yDelta to work. Will still be 60% of Delta starting current. VFD may be best choice.

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    A Wye start won't help you produce any greater torque during start.

    The best question is why are you starting a grinder under load...?

    Anyway, a 'soft starter' modulates only voltage and is a sort of half-ass good device used to limit the impact of a motor start on the system. It does NOT produce any greater torque...it actually limits torque production by limiting voltage.

    What you need is a VFD starter - that will modulate frequency and allow you to produce adequate torque as you come up to speed.

    An unloaded start will still pull the same inrush current, in terms of total amps, but it will pull it for a shorter duration. That often makes a difference on your power supply. An unloaded start also means a motor needs to make less torque during start....which means your power supply can be less stiff.

    But again...I gotta wonder why you are starting loaded.

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    Quote Originally Posted by GregSY View Post
    A Wye start won't help you produce any greater torque during start.

    The best question is why are you starting a grinder under load...?

    Anyway, a 'soft starter' modulates only voltage and is a sort of half-ass good device used to limit the impact of a motor start on the system. It does NOT produce any greater torque...it actually limits torque production by limiting voltage.

    What you need is a VFD starter - that will modulate frequency and allow you to produce adequate torque as you come up to speed.

    An unloaded start will still pull the same inrush current, in terms of total amps, but it will pull it for a shorter duration. That often makes a difference on your power supply. An unloaded start also means a motor needs to make less torque during start....which means your power supply can be less stiff.

    But again...I gotta wonder why you are starting loaded.
    you can't place the cylinder head, block, manifold, etc on the platen with it running. per the manual "NEVER PLACE WORK ON A MOVING BELT"

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    It is a safe bet that his generated phase is somewhere beyond Saturn's orbit when he puts such a heavy load on it. Reducing the starting current may help keep it at a reasonable phase angle. As I have said so many times that folks must be tired of reading it, you need an oscilloscope. People endlessly debate theses issues when a scope picture would settle them. As a guess, he may need a larger RPC and it is probable that he needs a larger starting capacitor, switched out when it gets running.

    Bill

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    Quote Originally Posted by 9100 View Post
    It is a safe bet that his generated phase is somewhere beyond Saturn's orbit when he puts such a heavy load on it. Reducing the starting current may help keep it at a reasonable phase angle. As I have said so many times that folks must be tired of reading it, you need an oscilloscope. People endlessly debate theses issues when a scope picture would settle them. As a guess, he may need a larger RPC and it is probable that he needs a larger starting capacitor, switched out when it gets running.

    Bill
    i didn't think 3 phase used a start capacitor (??)

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    i do have a scope buried in a SUN analyzer. what am i looking for? assuming my scope will tell me what i need to know

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    Quote Originally Posted by 9100 View Post
    It is a safe bet that his generated phase is somewhere beyond Saturn's orbit when he puts such a heavy load on it. Reducing the starting current may help keep it at a reasonable phase angle. As I have said so many times that folks must be tired of reading it, you need an oscilloscope. People endlessly debate theses issues when a scope picture would settle them. As a guess, he may need a larger RPC and it is probable that he needs a larger starting capacitor, switched out when it gets running.

    Bill
    Where can a guy find a reasonably priced scope? Cheapest portable I saw, in a brief sesrch, from Fluke was $3200.

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    still would like to know if a soft start would cure the issue. although this pair looks like the perfect size it's probably not my answer
    YouTube

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    Capacitance is not God's answer to motor starting. It's a crutch at best.

    A loaded start would imply that the grinder is sitting idle with a part in it, and as it accelerates to full speed, it is doing grinding on the fly. I'm not understanding exactly how this grinder works but in any case I doubt that is what is happening. The belt itself imposes very little load on the motor...the load comes from grinding.

    In any case, I sympathize that you don't have what you need - a good supply of 3 phase power. The next best thing is what I suggested - a VFD that takes in single phase and outputs 3 phase. A 10HP output is very do-able....assuming you have enough single phase power. The initial post suggests you have at least 170 amps which is enough. A quality VFD that can do this is around $2K or maybe a tad more...you can probably buy a cheaper Chinese one if you look and need to save money. With a VFD you can run the grinder at the full rated 10HP and it will start as you wish. It will also allow you to start with very little or no inrush current beyond full load amps.

    All the other options are a waste of time, IMO. They're Band-Aids that never get you full performance.

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    And if you notice, Ms. Boobsalot is not starting that chop saw under load. It's unloaded. If she first laid that blade on the metal, it wouldn't start.

    A 'soft start' device has a few legitimate uses but like a lot of products people think it's 'something for nothing'. It ain't.

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    Quote Originally Posted by GregSY View Post
    And if you notice, Ms. Boobsalot is not starting that chop saw under load. It's unloaded. If she first laid that blade on the metal, it wouldn't start.

    A 'soft start' device has a few legitimate uses but like a lot of products people think it's 'something for nothing'. It ain't.
    Not jokin.

    capture.jpg

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    Quote Originally Posted by GregSY View Post
    And if you notice, Ms. Boobsalot is not starting that chop saw under load. It's unloaded. If she first laid that blade on the metal, it wouldn't start.

    A 'soft start' device has a few legitimate uses but like a lot of products people think it's 'something for nothing'. It ain't.
    i realize that 100 percento. but i still watched it twice

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    on another note:
    my platen is 14x40. the company now makes only 12x34 which uses a 3hp motor. not much difference in table size but big difference in motor size. i wonder if i could down size the motor to cure the issue

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    why don't you just retap the motor for Wye and see if it works? It'll take 10 min and won't cost you a nickel. Then you'll know if a Y/Delta soft starter will work.

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    A primary resistance starter may be more applicable than star delta.....there are also auto transformer starts........with marginal power supplies ,you often get the problem of breakers /fuses blowing when the direct line power contacts close,no matter the starting method......Simple answer is the motor is too big for the supply.......Why cant I win Indy with a B&S engine?...I dont want reasons ,I just wanna win.

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    Quote Originally Posted by cg285 View Post
    i didn't think 3 phase used a start capacitor (??)
    Not if you are running on a real three phase line. Except for a few that use only a motor that is rope or pony started, RPCs will have a capacitor from one side of the line to the generated phase. A given capacitor is only the right size for one load and will work to some extent off that load but the quality of the power deteriorates. With that high a starting current, the generated phase will probably be dragged very low. If you start the motor unloaded, even though the generated phase is way off, the motor will start to turn and as it does, the phase will get better, making the motor run faster until it gets up to speed. Remember that a frictional load is greatest before it starts to move and the little torque the motor can generate in that condition will not start it moving. You need a much bigger capacitor in parallel with the normal starting one to get things rolling. If you look at it with an oscilloscope, it is all apparent.

    Bill

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    Quote Originally Posted by cg285 View Post
    i do have a scope buried in a SUN analyzer. what am i looking for? assuming my scope will tell me what i need to know
    Connect the common of the scope input to the line neutral and the input to the generated leg. I don't know what kind of sweep control you have but you want to have it synched to the 60 cycle line. When the motor is running the waveform should be half way between the line. As you load the motor, the generated wave will move toward the right, greater phase shift, and get smaller. I am guessing that in a locked rotor situation the waveform will be far toward the right and very low, in short, not making good three phase voltage.

    The generated leg should be 1.732 times the heights of the readings from the lines and centered horizontally between them. It is possible to adjust the capacitor or load to give that, which will be supplying the motor with voltage the motor cannot tell from regular three phase and producing the same torque. The problem is moving off that load. The main function of the idler is to extend that range.

    I am not familiar with your Sun analyzer, so before you hook up anything, make sure there isn't voltage between the neutrals and that the vertical input can stand the voltage.

    Bill

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