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IndustryArena Forum > CNC Electronics > Phase Converters > Phase convertor needed for 3 HP CNC milling machine (rotary or electronic)?
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  1. #1
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    Question Phase convertor needed for 3 HP CNC milling machine (rotary or electronic)?

    Getting ready to purchase a CNC milling machine that uses a 3 HP 220 V (3 phase) motor. What type of convertor do I need?

    I've done some reading and searching and it seems like a rotary phase convertor with some sort of voltage conditioning is preferred?

    Can an electronic phase convertor work as well?

    The milling machine uses stepper motors and the control cabinet uses 120V. The spindle has a variable dial on the front for speed selection (no changing belt positions). Is this the same as a VFD?

    If a rotary convertor is the way to go, can you go too large? Someone local has a 20 hp rotary convertor he wants to get rid of...

    Thanks for any help...

  2. #2
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    I think as a general rule static phase converters are not recommended for CNC machines. However, it sounds like all you are looking to do is power the spindle motor with the phase converter. I would think a static phase converter would be just fine for that.

    The variable speed spindle is not a VFD. VFD is a variable frequency drive.

    I'm farrrrrrrr from an expert on this stuff so wait for the "smart people" to confirm all of this.

    Allen

  3. #3
    Community Moderator Al_The_Man's Avatar
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    If the mill uses a standard 3 phase motor and is not presently electronically controlled, It sounds like you have a vari-speed pulley, which is mechanical speed adj.
    If so, would go with a 5hp VFD on 240v 1 phase.
    Set up the rest of the machine on 120v 1 phase, most likely there is a 120v control transformer in the enclosure for this already.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  4. #4
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    Quote Originally Posted by Al_The_Man View Post
    If the mill uses a standard 3 phase motor and is not presently electronically controlled, It sounds like you have a vari-speed pulley, which is mechanical speed adj.
    If so, would go with a 5hp VFD on 240v 1 phase.
    Set up the rest of the machine on 120v 1 phase, most likely there is a 120v control transformer in the enclosure for this already.
    Al.
    I believe that is correct, a varispeed pulley (will confirm this weekend).

    I can't seem to find any VFD's rated for 5 hp that can be powered from single phase. Does a VFD need to be overrated as compared to the hp of the motor?

    It seems that most the rotary convertor manufacturers all have a special product for CNC machines. Why does a CNC machine require something different? I'm thinking it is to ensure you have a consistant voltage level for the PC. If this is the case, I can just power the control cabinet from standard 120V and get any 'ol rotary convertor for the motor.

    Can you go too large on a rotary convertor? Any disadvantages?

    Sorry for all the questions... Thanks for the info...

  5. #5
    Community Moderator Al_The_Man's Avatar
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    Usually there is a derating factor when using VFD on 1 phase input, most manuf. top out at 5hp for 1ph and this should be OK for 3hp motor.
    Usually whether using a rotary or static, the 120/240 1phase will come off the main 240v single phase legs, which are passed through the convertor, the manufactured phase is avoided for control puposes, for normal 3ph induction motor, static or rotary should be OK.
    Too large is just overkill, otherwise no ill effects.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
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    Quote Originally Posted by Al_The_Man View Post
    Too large is just overkill, otherwise no ill effects.
    Al.
    I'm thinking if I just get a large one now, it will be able to be used for any other 3 phase large equipment purchased in the future (possibly a welder) and I won't be needing to buy another convertor a year from now. Thanks...

  7. #7
    I'd be a little leery of using it for a welder. Most VFD's tell you to only switch them with the input not the output. With the welder you are abruptly varying the load and could cause problems.
    I used to be appalled, now I'm just amused.

  8. #8
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    Right, I was speaking about using a large rotary for the welder later down the road.

    I thought I read that you don't have to derate a VFD (only static convertors need derated since they essential run the motor on 2 phases instead of 3, and therefore have to be derated 1/3 of the amount). If this is true, then a 3 hp motor would require a 3 hp VFD. Hitachi makes one for about $250. Not too bad.

    The 20 hp rotary I found was $450 and may require too much starting current to get going without a pony motor.

  9. #9
    Community Moderator Al_The_Man's Avatar
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    Quote Originally Posted by dsl-pwr View Post
    I thought I read that you don't have to derate a VFD (only static convertors need derated since they essential run the motor on 2 phases instead of 3, and therefore have to be derated 1/3 of the amount). If this is true, then a 3 hp motor would require a 3 hp VFD. Hitachi makes one for about $250. Not too bad.
    Check the VFD manuf. spec's, they should mention if derating is required, some manuf. also design models that use no derating.
    The reason for derating a VFD is that on 1ph the resulting DC supply is reduced in capacity.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  10. #10
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    Here's the VFD I'm considering:

    http://www.driveswarehouse.com/Drive...4f74f7374e7d7b

    I've looked thru the manuals and there is no mention of derating it. The 3hp model is the largest for a single phase input and is rated at 96% efficient at 100% rated output.

  11. #11
    Community Moderator Al_The_Man's Avatar
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    It appears to be a non-derated model, so you should be OK!.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  12. #12
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    No derating is required on currently available VFD's A 3HP VFD is designed for 3HP regardless , download some manuals and get fimilar with them

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