Replacing motor on sliding table saw?
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    Default Replacing motor on sliding table saw?

    Hi all. Iím looking to purchase a used sliding table saw, but most of them are set up for three phase, which I donít have. From what I can tell, VFDís are only good up to around 3HP, and using a rotary converter on a 7HP motor would pull way more amps than what I can provide.

    Has anyone successfully replaced a three phase motor on a sliding saw with a single phase 220 motor? These are mostly European machines and Iím told the bolt patterns are non standard. In addition, Iím not sure if the wiring for the controls would be an issue.

    Any advice or experience greatly appreciated.

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    Generally you will be better off with a vfd. The saw won't be so hard starting that a full amperage vfd is necessary. If you have enough single phase input amps to run a single phase motor, you can put in a vfd with that size input. You may never need a full 7 hp output. A 10 hp vfd down rated to 5-6 hp should work fine. you may be able to find a metric frame motor in single phase to match the existing but most Euro saws use small frame motors packed with a lot of winding to provide high hp. You will need to replace with a lower hp motor anyway so use a slightly underated vfd and avoid swapping the starter, overloads, etc.

    How many input amps are available? Dave

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    Quote Originally Posted by beckerkumm View Post
    How many input amps are available? Dave
    Only 30amps on my 220v circuit, and Iím renting for the next year so Iím not able to run a new one.

    Are you saying that I can run a 7HP 3 phase motor on a 10HP VFD, and somehow dial it down to 4-5HP? Or am I misunderstanding?

    I didnít think you could get much past a 3HP VFD with only single phase input.

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    Full load amps for a three phase 240v motor will be in the 18 amp range. 30 amps single phase is enough to run that motor 95% of the time. Most vfds will allow for single phase imput but you must derate . Most of the time the motor will be pulling less than 15 amps. Dave

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    A vfd will calm the start up "brown out". Get a 20HP high end commercial unit off ebay and never look back.

    I have gravitated to the Toshibas, but have many others with complete satisfaction.

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    I just called a company that sells VFDís, and I think Iím more confused than before.

    He said the largest VFD I can run on 220v 30a in will give me 3 phase 12a out and support a 10.5a 3 phase motor. That was a 3HP VFD.

    But when I look up the amp draw charts, 10.5a 3 phase gets you into 7HP territory.

    What am I missing there?
    Sorry for all the questions, I just donít want to buy an expensive saw I canít use.

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    Quote Originally Posted by johntarr View Post
    Only 30amps on my 220v circuit, and I’m renting for the next year so I’m not able to run a new one.

    Are you saying that I can run a 7HP 3 phase motor on a 10HP VFD, and somehow dial it down to 4-5HP? Or am I misunderstanding?

    I didn’t think you could get much past a 3HP VFD with only single phase input.
    Quote Originally Posted by johntarr View Post
    I just called a company that sells VFD’s, and I think I’m more confused than before.

    He said the largest VFD I can run on 220v 30a in will give me 3 phase 12a out and support a 10.5a 3 phase motor. That was a 3HP VFD.

    But when I look up the amp draw charts, 10.5a 3 phase gets you into 7HP territory.

    What am I missing there?
    Sorry for all the questions, I just don’t want to buy an expensive saw I can’t use.
    You only have 30 amps available, no need to go beyond that.
    You will use it, and if you push it too hard, it will bog, and you'll learn to slow down to keep it running.

    for qty (1) year.

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    Quote Originally Posted by digger doug View Post
    You only have 30 amps available, no need to go beyond that.
    You will use it, and if you push it too hard, it will bog, and you'll learn to slow down to keep it running.

    for qty (1) year.
    Thanks for the reply, but it doesnít really help me understand if a VFD would drive a 7HP saw on 30a or not. I get that if itís close, Iíd have to slow down, but how do I know if Iím in the ballpark?

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    Quote Originally Posted by johntarr View Post
    Thanks for the reply, but it doesn’t really help me understand if a VFD would drive a 7HP saw on 30a or not. I get that if it’s close, I’d have to slow down, but how do I know if I’m in the ballpark?
    What did the drive vendor tell you ?

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    What you quoted from me above, which is that:

    ďThe largest VFD I can run on 220v 30a in will give me 3 phase 12a out and support a 10.5a 3 phase motor.Ē

    Which is confusing because that VFD was only rated 3HP, but 10.5a 3phase is closer to a 7HP motor.

    Iím basically trying to understand the biggest 3phase motor I can reliably run on a single phase 220v 30a circuit.

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    Quote Originally Posted by johntarr View Post
    What you quoted from me above, which is that:

    “The largest VFD I can run on 220v 30a in will give me 3 phase 12a out and support a 10.5a 3 phase motor.”

    Which is confusing because that VFD was only rated 3HP, but 10.5a 3phase is closer to a 7HP motor.

    I’m basically trying to understand the biggest 3phase motor I can reliably run on a single phase 220v 30a circuit.
    and when you were talking to the VFD vendor , why didn't you explain your situation, and ask them ?

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    Because I looked up the charts for amp draw after I got off the phone with them.

    Can always call back, but figured someone on here might know and be able to explain.

    I really wasnít even looking for info on phase conversion, I originally posted here looking for someone with experience swapping out motors. Not to be too cheeky, but if you donít have any advice/info on any of that, I can figure out ďcall the vendorĒ on my own.

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    Look at something like a Hitachi WJ200-110 LF. It will run on single phase and enough amps for 95% of your cuts if not more. Dave

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    Quote Originally Posted by beckerkumm View Post
    Look at something like a Hitachi WJ200-110 LF. It will run on single phase and enough amps for 95% of your cuts if not more. Dave
    Thanks, Dave. How can I determine how many amps that thing would draw with a 7HP motor attached to it?

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    Quote Originally Posted by johntarr View Post
    Thanks, Dave. How can I determine how many amps that thing would draw with a 7HP motor attached to it?
    full load somewhere 34 to 38 amps, real-life probably just under 30

    404: Page Not Found | Valin

    dee
    ;-D

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    Quote Originally Posted by dcsipo View Post
    full load somewhere 34 to 38 amps, real-life probably just under 30

    404: Page Not Found | Valin

    dee
    ;-D
    Thank you. So realistically, 5HP is all I could pull off, given that I need to remain under 80% of the 30a breaker. This is helpful to know.

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    Many people think electric motors somehow "make" power.

    They don't.
    They let enough power in from the grid to do the job at the moment, and convert it to rotary motion for you.

    So if your immediate sawing task in process does not require 7 HP, the motor won't "make" it for you.

    If the immediate in process sawing task takes 3HP, your motor will obligingly convert that much power from the grid to rotary motion and torque, no more. If you push so fast in a 3" thick board that your immediate task takes 5.5 HP and your VFD, or your motor starter is set to trip below whatever amperage that represents, it will trip, and you will learn where to slow down with the feed, or DOC, or both.

    Going the other direction, if somehow the task in process takes 9HP & current is unlimited, even a 5 HP motor will attempt to deliver that until it fries. So best to have a current limit on the input side. Like heaters on the motor controller to trip it, or a VFD. (Breakers are to protect the house wiring & must be sized as such.)

    As Dave notes, you probably will seldom if ever approach that situation with normal blades in normal cuts with typical NA hardwoods and softwoods if there is 30A at 240v on tap.

    smt <----runs 15HP widebelt sander from a 7.5 hp homebrew rotary, along with a 2 HP cyclone at the same time, often with other support equipment including 9HP slider running simultaneously. Not only that, but NYSEG instrumented my set up, logged it for a month, and approved it. Do use correct wire sizes; and breakers, heaters, and other control circuitry appropriate to the loads and equipment.

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    Quote Originally Posted by stephen thomas View Post
    Many people think electric motors somehow "make" power.

    They don't.
    They let enough power in from the grid to do the job at the moment, and convert it to rotary motion for you.

    So if your immediate sawing task in process does not require 7 HP, the motor won't "make" it for you.

    If the immediate in process sawing task takes 3HP, your motor will obligingly convert that much power from the grid to rotary motion and torque, no more. If you push so fast in a 3" thick board that your immediate task takes 5.5 HP and your VFD, or your motor starter is set to trip below whatever amperage that represents, it will trip, and you will learn where to slow down with the feed, or DOC, or both.

    Going the other direction, if somehow the task in process takes 9HP & current is unlimited, even a 5 HP motor will attempt to deliver that until it fries. So best to have a current limit on the input side. Like heaters on the motor controller to trip it, or a VFD. (Breakers are to protect the house wiring & must be sized as such.)

    As Dave notes, you probably will seldom if ever approach that situation with normal blades in normal cuts with typical NA hardwoods and softwoods if there is 30A at 240v on tap.

    smt <----runs 15HP widebelt sander from a 7.5 hp homebrew rotary, along with a 2 HP cyclone at the same time, often with other support equipment including 9HP slider running simultaneously. Not only that, but NYSEG instrumented my set up, logged it for a month, and approved it. Do use correct wire sizes; and breakers, heaters, and other control circuitry appropriate to the loads and equipment.
    Thank you for the detailed reply.

    I think I understood that a motorís power draw varies with load, but I assumed that a 7HP motor draws more than a 3HP motor, even for the same load. Sounds like I may have been wrong about that. Still nervous about dropping $3000+ on a saw and &750 on a VFD and hoping it all works, but this gives me some confidence.

    Out of curiosity, how does your 7.5HP rotary run 17HP of load? Everything Iíve read says a RPC should be twice the HP of the load?

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    Quote Originally Posted by johntarr View Post

    Out of curiosity, how does your 7.5HP rotary run 17HP of load? Everything I’ve read says a RPC should be twice the HP of the load?
    assuming that his motors are connected in star (most common for woodworking stuff) the power drawn by both the sander and the DC is somewhere between 1 ĺ HP and 7.5 HP depending on the loads. (can't go beyond 7.5 HP )
    Motors have no load currents and full load currents, the no-load current of a three-phase motor in star config is about 10% of the full load current. Because at no load motors become very inefficient, the current drawn at some partial load is not going to increase much till the load exceeds the nominal power of the no-load current* voltage - the mechanical and electrical losses of the motor itself. then the current linearly increases until it draws all the current to produce maximum output power. an overloaded motor will attempt to suck more current, and it will either trip a breaker or fry itself. So a 7 HP motor is designed to safely and continually draw about 20 amps at full load, it will only draw 2 amps with no load and something in between depending on how hard it is pushed. You can limit the VFD to only supply 15 amps max and therefore you have a 7 HP motor derated to 5 HP. electricity is a funny thing. a 5 hp motor will take 15 amps to produce 5 HP and that is the max it can produce. a 7 HP motor will produce 5 HP power when you give it 15 amps. But it has 5 amps worth of headroom to give you two more HP when pushed.

    A VFD is also helpful with starting a motor, motors have notoriously large inrush currents the VFD usually has a soft start function to limit the current on start. Not sure how the 7.5 HP motor in the RPC handles the inrush of the 15 HP motor, but Stephen said it works for him, so it is all good

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    VFDs can limit the output current.

    When you have a motor hooked up directly to the power grid, there is nothing to limit the current (apart from the circuit breaker, which is only there to protect the circuit wiring). The motor will draw as much current as it needs to do the work. Too much load = motor overheating -> failure.

    A VFD is in control of it's output. This allows controlled starting and stopping (reducing problems of inrush / starting currents). Also allows limiting output power.

    You can use an VFD rated the same, for more, or less, than the motor rating. It must be programmed appropriately.
    Sized 1:1 - you can get full performance.
    Oversized - you can get full performance. Make sure VFD programmed properly to prevent overheating.
    Undersized - you won't get full performance. VFD will over deliver what it is capable of / programmed to.
    (Too far undersized and you won't be able to do any useful work, or may even fail to get motor going with no load)

    There are multiple previous threads on the topic, e.g.:
    Can I run a 1.5 HP 3 phase motor with a 1 HP VFD?
    1hp VFD for a 1.5hp motor?
    Can a VFD be sized smaller then motor if full power isnt needed?

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