VFD vs. Rotary Phase Converter
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  1. #1
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    Hello to all:
    I just got off the phone with a commercial cnc retrofit company and they said they would not stand behind their product if I use a rotary phase converter on our manual series one Bridgeport. CNC rotary phase converters are not voltage stable enough I was told. I have to use a VFD. Is there a good reason for this? Which is really the better way to go, rotary phase converter or VFD? We have the 2J continously variable speed head on this machine. Thank you for any and all help! (Phase Perfect is to costly at present.)
    Chuck Pool
    Cedar, Michigan
    [email protected]

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    i enjoyed talking with you the other day
    and i will throw my 2 bits in once more

    if you oversize the r.p.c. and have it tied in to only that unit

    i see no problems with your application
    just keep the mfg leg out of the control circut

    Scott

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    Chuck, I have several customers running CNC lathes and mills with RPC's. My recommendation for running CNC equipment is that the converter motor be at least twice the load size. This gives plenty of reserve power and there is less likely to have a sudden voltage changes from a changing loads (which will cause a fault in the CNC controls). However, if the company does not stand behind the RPC powered equipment, you may need to consider that.

    One guy has HAAS mills and lathes. The HAAS technician who set up the machines said as long as the converter meets their power specifications, there is no problem (he would not have set them up if that criteria had not met). Also, by powering the computer and controls from the single phase input (not the generated leg) the controls would not see voltage changes from varying loads.

  4. #4
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    Is this a manual machine converted to CNC or a manual machine? Either way they are nuts. If it is CNC you can not run the electronics on the VFD. If it is manual and you want to use the controls on the mill you will have to rewire the mill. What do they say about a phase perfect converter? They have specs better than the utility company.

    You called it a manual machine but its warranted from the CNC retro fitters so I am not sure what you have. A specific answer will be easier with this clarified.

    In any case I do not believe you will ever have a problem with using a CNC quality RPC. If it is manual, any kind of RPC will work fine.

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    At present, this is a manual Bridgeport Series One Vertical mill.
    Chuck

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    Bill:
    I should of said. This is a manual Bridgeport Series One vertical mill that is going to be converted to 4 axix cnc. Acording to the cnc sales department they will not stand behind their cnc retrofit product if a rotary phase converter is use. Only a VFD may be used.
    Chuck

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    I should also have mentioned that the cnc control and servo motors are not receing any power from the 3 phase spindle motor syetem, i.e. rpc or VFD.
    The cnc control and servo motor would be receiving their power from a separate single phase power source. I don't see why they won't accept a rotary phase converter and only will accept a VFD.
    Chuck

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    Hello to all.
    How does a VFD stack up against a rotary phase converter in the above cnc challenge? Which power source is really the best way to go for cnc?
    Chuck

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    Chuck,
    I and others here think your CNC retrofit vendor is full of soup, especially with regard to using a VFD. Since a VFD outputs variable frequency AC, and only in a form suitable for driving AC induction motors, I can't fathom how the CNC vendor thinks it would be OK to let the VFD power the CNC electronics. The cure I see is to separately power the CNC electronics from a single-phase supply and the spindle motor from a VFD *or* rotary converter. In that case, why should the CNC retrofit guy care how you power the spindle motor? (This assumes the CNC is not going to use a VFD to change spindle speed. You cannot change spindle speed electronically without a VFD or something very much like it.)

    Nevertheless, it's up to the CNC reftrofit outfit to choose what they will or will not warrant. They have just as much right to say that the warranty is void unless you paint the machine pink.

    I power my Deckel FP2NC/Dialog 4 control CNC universal "Euro" mill from a rotary converter without any problems. The control takes the rotary 3-phase and rectifies it to DC, so it doesn't care much about phase balance, which is good nonetheless. A VFD would not work for this application.
    RKlopp

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    RKlopp:
    The cnc electronics (control) and servo motors would receiver their power from a separate single phase power supply and not from the spindle 3 phase power source system.

    This is why I am comfused. Why should a rotary phase converter for the spindle mater. It is not going to be used for the cnc electronics (control) or servo motors.

    Is the VFD realy better than a rotary phase converter?

    Thank you.
    Chuck Pool
    [email protected]

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    Is the VFD really better than a rotary phase converter for the 3 phase spindle power?

    I have a 2J head on this Bridgeport which provides mechanical variable speed control of the spindle.

    Chuck

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    Chuck,
    I think a VFD is better than a rotary for spindle power in a lot of ways. The VFDs offer no moving parts (except a fan, easily replaced if it takes a dump), built-in variable speed capability, built-in braking, built-in motor overload protection, no need for a separate contactor or motor starter, and they often allow speed and/or motor current readout on a display. Try any of that with a rotary!
    RKlopp

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    This answer is based on powering the spindle motor which I assume is a 3 phase fixed speed induction motor.

    The VFD would give some flexability in speed control (small dial vrs big dial)and could gain some higher spindle speeds (probably void warranty on spindle bearings) but can be harder on a motor than an RPC. A VFD puts out fast rise time square wave pulses that can break down the insulation of the windings but this isssue is not significant at 240 Volts (but is an issue at 460). Also if you use a VFD and take advantage of its features and slow the motor down for some operations insuffiecient cooling may occur. This is why they make inverter rated motors.

    Are they changing the motor from what is currently in the mill? Hundreds if not thousands of these things are running on home made RPCs accross the country with out issues. The CNC quality RPCs should be frosting on the cake. I do not understand their reluctance for use of an RPC.

    At worst (verify with them) you would be risking the warranty on the spindle motor and nothing else if you elected to use the RPC and most warranties are not worth the paper they are printed on anyway.

    If you use a VFD, the mill should be rewired so that the VFD operates with input from the standard mill controls, other wise operator confusion will lead to problems.

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    Its more like 10 s of thousands with many being in industrial shops running more than one shift.
    and if the rpc is tied into only the cnc unit and oversized, walking the tight wire power correction/phase balancing act is not a issue.as i stated early on,just keep the mfg leg out of the control circut for many reasons..

    Scott

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    Scott, Howard, Bill and RKlopp thank you for your input regarding our RFC and VFD challenge.
    I appreciate, more thank you realized, all of your help!
    Chuck

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    Scott, Howard, Bill and RKlop thank you for your input regarding our RFC and VFD challenge. I appreciate, more than you realize, all of your help.
    Chuck

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    Being an old post I’m suspecting technology was different 12 years ago. We use an American Rotary AD15 and it powers every machine under 7.5hp just fine and several at once. The one thing we learned is that you need to use a RPC with digital circuitry for CNC machines otherwise you will have voltage fluctuations and potentially bad results. We have better efficiency from our RPC than you get from commercial service. It’s unbelievably smooth and even through all legs. Perhaps the technology wasn’t available then or the company you were dealing with didn’t know. As far as a VFD all of our machines run them with 3ph in/out. Really it depends on your needs but personally I’d never run without a VfD they are priceless in their features. So for anyone presently looking just be sure it’s cnc safe. It did cost $200/$300 more over the standard model.


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