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Thread: VFD for 3 phase table saw

  1. #16
    It will cost more to switch it to single phase.

  2. #17
    Odd that they used a 1-ph motor for the scoring motor, and a 3-phase motor for the main motor...

    The main motor is 16A, so the ebay VFD you linked would work. For $200, give it a shot.

  3. #18
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    Quote Originally Posted by Bill Webster View Post
    I would always go with a rotary phase converter (RPC) over VFD except for 2 cases.

    1. You only have one 3 phase motor to run and it is 3 hp or less.
    2. You want variable speed and can use a motor at least twice the normal size for the machine.

    There are many advantages to a RPC. You can run more than one motor at a time, where VFD's need to be dedicated to a single machine. With a RPC, you can use the machines original switch or motor starter. You can't cut power to a VFD using the original switch or you can damage the VFD. VFD's uses large electrolytic capacitors, which will fail eventually. A VFD for over 3 hp is very expensive too. RPC's are bulletproof, will last a lifetime and only use electrolytic capacitors to start the RPC, not when running. Also, VFD's can be a challenge to program if you have never done it before.

    You should get close to full hp with either a RPC or VFD if properly sized. You will probably never need the full 6.5 hp anyway.
    I have seen VFD's fail for sure, but they are suitable for daily, industrial pounding, for hours on end. I deal mostly with Allen-Bradley, and would bet in a light industrial (10-hr/day) or home shop their VFDs out-live me (maybe 1-2% failure rate in a given year??). Also, you'd typically install the VFD upstream of the existing controls, or better yet re-wire the tool's controls to operate as inputs to the VFD (and use braking), and rarely only cycle VFD power.

    The import VFDs have similar function, and industrial user base would abandon them in a heartbeat if they regularly crashed a multi-million dollar process. I see no such angst among this user group...?

    As for proper size, you are correct, Bill. For full rated power, you'd have to jump the next size up - - and I don't remember ever seeing a 7.5Hp VFD in stock - - so if you need/want 6.5Hp from motor, you gotta look at 10Hp VFDs. And again, set the 'limit' parameters to match the nameplate FLA of the motor.
    Last edited by Malcolm McLeod; 03-13-2018 at 12:58 PM. Reason: 7.5Hp are out there, just no one I know uses them!!
    Molann an obair an saor.

  4. #19
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    With a VFD do not set too fast a deceleration time for a saw or grinder. If you do the nut can spin off and the let the blade or grinding wheel run off. Use at least 3 seconds or more to stop the motor. I use 3 seconds for both accelerate and decelerate on my drillpress.
    Bill D

  5. #20
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    Are you comfortable with wiring, understanding schematics, and programming? Multiple motor machines with multiple stop start and limit switches usually have a bunch of electronics. You will need to bypass the main motor and reroute the switches to the vfd for the main motor. The power will still need to enter the machine to energize the starter for the scoring motor. If the scoring and main motor can be started together, you will need to separate them so the scorer is started on the machine and the main motor from the vfd. If there is a braking module you will need to disconnect it . I'd also want a separate braking resistor added to the vfd so as to not overload the vfd when starting and stopping repeatedly in a given time period. A vfd is great in many applications but the more complicated the machine electronics, the more complicated the conversion to using a vfd. A RPC may be easier to get running if you are new to this stuff. Dave

  6. #21
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    I'll second what Jeff said.

    Jeff, what model AR RFC do you have?

  7. #22
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    David,

    The scoring motor only works when the main motor is energized. Would it not continue to work in the same fashion whether powered by a VFD or RPC? Those are the only two controls on the machine. Start and stop for the main motor and same for the scoring motor.

    Does anyone have a recommendation for a VFD under $500? Most of the 1 to 3phase seem to stop at 3hp.

    James

  8. #23
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    I don't think that running two motors off one VFD at the same time will work. Also, one rule that MUST be followed is NO switching beyond the VFD. The VFD controls starting and stopping. No other switching allowed. As much as I love VFDs (I have 9 installed), this sounds like the case for a high quality RPC. You got the at a good price. Feed it the correct power. Also messing with the controls on this sould like a recipe for disaster.
    CPeter

  9. #24
    James,

    The scoring motor is a single-phase capacitor-start motor. And it looks like it has it's own, independent, on/off button? If so, just plug it (via its switch) into your single-phase circuit.

    Then, buy that eBay VFD (it is correctly-sized for your main motor), and use that to control the main motor. Note that you can't use the on/off buttons on the saw, any longer - you must control the main motor using the on/off buttons on the VFD. But, you could re-wire the saw's on/off buttons to act as remote buttons for the VFD on/off buttons.

    The only caveat is that there may be some goofy electronics to deal with in the saw (for instance, something that only lets you turn the scoring motor on if the main motor is running). You may have to bypass or otherwise deal with those.

    For the cost of that VFD vs a correctly-sized RPC, this is a no-brainer. Get it and try it out.

    As David said, you'll need to be comfortable wiring switches, relays, etc - these are industrial electronics, not consumer products - but to an extent, that's true of the RPC, as well.

  10. #25
    Quote Originally Posted by CPeter James View Post
    I don't think that running two motors off one VFD at the same time will work.
    For some reason, the scoring motor is 1-phase, so I think he can just "ignore" that part of the problem and wire it directly to his supply.

  11. #26
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    My controls are likely more complex but here is an example of what you are taking on when rerouting wires for a vfd. Not saying it isn't possible, but also not nearly as easy as with an old single starter machine.DSCN3577.jpgDSCN3578.jpg Dave

  12. #27
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    Quote Originally Posted by CPeter James View Post
    I don't think that running two motors off one VFD at the same time will work. ...
    Quote Originally Posted by Dan Friedrichs View Post
    For some reason, the scoring motor is 1-phase, so I think he can just "ignore" that part of the problem and wire it directly to his supply.
    I don't own the saw, never worked on one, and don't have the electrical schematic in-hand, so take this with the ignorance with which it's offered - - common sense is that the OEM intends the scoring motor at 240V/1ph, be powered from 2 of the 3-phase legs...?

    Also, Peter with regard to my 'snip' of your statement, a VFD can drive multiple motors (I've seen cases of up to 50-some-odd motors from 1 VFD), but the O/L protection gets increasingly complex to say the least. The total power consumption of the motors just has to stay within the power limits of the drive. Regardless, in the OP's situation, I think Dan's advise is practical and sound. The VFD and re-work of the saw's control wiring seems to be the least expensive option.

    The RPC may be more expensive, but require no modification to the saw's wiring...?? Name your poison.

    *******
    Edit: Reading a couple of the other posts, it occurred to me that the interlock on the scoring motor is fairly easy to duplicate with a VFD: Most, if not all VFDs will have a relay output. You can program the VFD to close this relay when it is 'at speed' (main motor running). Then simply route the scoring motor 'start' signal (wire) thru the 'running' relay. Now you need both the operator start request via pushbutton, as well as the main motor running, in order to start the scoring motor.
    Last edited by Malcolm McLeod; 03-14-2018 at 10:01 AM. Reason: interlock
    Molann an obair an saor.

  13. #28
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    David,
    I usually don't shy away from technical challenges within reason. I bet I could come here for advise if needed.
    I did pass on an Altendorf F45 for $1800 where all of the electronics where "bypassed" because of "probably just needs a switch" so that the main power button controlled the main motor only. It also had .030" of sag in the cast iron table!

    To say I am on a shoe string budget is an understatement. I had to barrow $1500 from my wife to take advantage of the deal on the saw. I have thought of building an RPC. However I don't like the idea of an extra motor idling all day or having a two step starting process for using the saw. Do the off the shelf RPC's start on demand from the saw?


    Quote Originally Posted by CPeter James View Post
    Also, one rule that MUST be followed is NO switching beyond the VFD.
    Thank you for that info! Super important to know going in.

    Dan,

    I may go with the one from ebay. I also like the idea of going thru Amazon for there guaranties. Here is one being sold by the same manufacturer as the ebay one. Many of the seller reviews say great support, supper responsive to technical questions, return shipping reimbursed etc. However they are three phase input or single. How do you derate them? Is the one below big enough?

    https://www.amazon.com/Variable-Freq...requency+drive

    James

  14. #29
    James, the Amazon one doesn't have current specs, so I don't know.

    The formula is that you take the phase current of the 3-phase load (the motor) and multiply by sqrt(3) to get the equivalent single-phase current. If the VFD is rated for at least that much on the input, it would work.

    So in your case, the main motor is 16A, multiply by sqrt(3), and you need 27A from the single-phase supply. The eBay one is rated for 27A input, so it would work. If the Amazon one is rated for >27A input, it would also work.

    FWIW, I think eBay's protections, etc, are as good as Amazon's, these days. But the Amazon one is actually "rated" much higher HP than the eBay one, so it might be a better deal - might be worth asking the seller for a spec sheet or current ratings.

    All that said, the motor will only draw full power if you call for it. Even with a 12" blade, you'd have to be pushing pretty hard to need 6.5HP from that saw. Either of these would work fine, and we're really quibbling over a corner case that you'll probably never see.

  15. #30
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    Dan,

    Thank you for being so helpful. I was concerned that the EBAY offering was $60 in shipping so if there where any issues I lose $60 off the bat. Where as the Amazon offering has free shipping.

    I put in a question to HuanYangthe manufacturer regarding there VFD offerings. Ill see what they say.

    Here is a video where they compare there VFD to similar looking clone.
    https://www.youtube.com/watch?v=tjhP...ature=youtu.be

    James

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