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Thread: Constant Torque vs. Varaible Torque VFD

  1. #1

    Question Constant Torque vs. Varaible Torque VFD

    I have a vertical knee mill with a 3hp 3 phase motor that I would like to power with a VFD, using mach 3 for speed control. It is a 4 pole, 1725 RPM, 8.9 amp unit.

    Would I need a constant torque VFD, or a variable torque VFD for this application? What is the difference?

    Would the motor lose any power by using a VFD vs. using a rotary phace converter?

    I went to www.driveswarehouse.com and I am a little overwhelmed with the choices. Could someone reccomend a brand / model?

    If I understand correctly, I can use the VFD to increase the maximum RPM of my spindle, by increasing the frequency. How much higher would I be able to increase the frequency without risking damage to the motor, bearings, etc...

    The head is a variable speed model with a low (60-600 RPM) and a hi range (600-4250 RPM). Would I just leave the head in hi range at the maximum RPM setting, and adjust the actual RPM using the software and VFD, or would I still have to shift ranges to meet speed requirements?

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  2. #2
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    robhrzic,

    Constant torque would be the way to go, especially if you want to do tapping. Torque changes with speed/frequency on a variable torque unit.

    As for your motor you should be able to double the speed with out much concern. Normally a motor can run between 20 to 120Hz and still be reliable.

    With so many brands out there I can understand why you would be overwhelmed. Personally I like the AC Tech or Lesson brand VFD's. I have had good results with them in the past.

    Good luck,

    Trevor



  3. #3

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    Quote Originally Posted by robhrzic View Post
    I have a vertical knee mill with a 3hp 3 phase motor that I would like to power with a VFD, using mach 3 for speed control. It is a 4 pole, 1725 RPM, 8.9 amp unit.

    Would I need a constant torque VFD, or a variable torque VFD for this application? What is the difference?

    Would the motor lose any power by using a VFD vs. using a rotary phace converter?

    I went to www.driveswarehouse.com and I am a little overwhelmed with the choices. Could someone reccomend a brand / model?

    If I understand correctly, I can use the VFD to increase the maximum RPM of my spindle, by increasing the frequency. How much higher would I be able to increase the frequency without risking damage to the motor, bearings, etc...

    The head is a variable speed model with a low (60-600 RPM) and a hi range (600-4250 RPM). Would I just leave the head in hi range at the maximum RPM setting, and adjust the actual RPM using the software and VFD, or would I still have to shift ranges to meet speed requirements?

    They used to be called variable Horsepower and constant horsepower, I guess torque works too. You are wanting a constant, variable torque units are for fans and pumps were the faster you spin the more torque they require.
    Please understand that when you decrease below the rating of a motor that you loose torque, and also above the rating for the motor it normally flatlines. To put it simply normal 3 phase motors run best at there name plate rating form there its a give and take. So you will not have a ton of torque at 20 Hz vs 60Hz the drive maybe able to compenstate, depending upon it and the motor, some motors react well to slow speed. There is a disadvantage to running at slow speed, you get alot more heat, and if there is a fan to cool the motor it doesnt work so some secondary cooling might be in order. Your gear box will still be used, I assume that its a variable speed belt since you list a range of speeds if not that then you may consider leaving it in a mid range speed. Needing to shift would fully be depandant upon what you are doing, I can see it not spinning at the 10HZ in high gear and high speed as you tend to need alot of torque to spin at those speeds.
    As for how fast you can get the head to go depends upon how well the motor reacts to being overdriven, I have never had a problem with a motor reaching 80 Hz, after that it can be hit or miss, I have had some go past 120Hz but most don't make it past 90Hz. Since its a 4 pole motor you are normally spinning at 1750 rpm so more then likely you can take more, but you really should check your bearing number and see, sometimes they are listed on the motor nameplate. The spindle on the other hand may not like spinning in excess of 5000 rpm, thats a common rpm rating for bearings, so again you should check your spindle bearings and see if they can handle it. If you are planing on using electronincs in the near this machine you will need a filter/line reactor at that HP, otherwise you will get odd results in the computers.



  4. #4

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    Thanks for the input protman16

    I ordered this VFD today, as I think it will fit the bill.

    http://www.driveswarehouse.com/Drive...V022iC5-1.html

    Quote Originally Posted by in2steam View Post
    Since its a 4 pole motor you are normally spinning at 1750 rpm so more then likely you can take more, but you really should check your bearing number and see, sometimes they are listed on the motor nameplate. The spindle on the other hand may not like spinning in excess of 5000 rpm, thats a common rpm rating for bearings, so again you should check your spindle bearings and see if they can handle it.
    Thanks for the feedback in2steam.

    I really didn't plan on running the spindle faster than about 5000 rpm, I was just wondering what a safe limit would be. I don't see any bearing info on the nameplate, unless I'm not looking in the right place. See attached slighty out of focus pic.

    I was really kind of hoping to find a "sweet spot" for the spindle (for example, 2500 rpm) and then adjust the frequency up or down using Mach 3 and the VFD to attain the necessary speed.

    Is there a way to "feedback" (please excuse the incorrect terminology)the actual spindle speed to Mach 3? I don't see how you can have accurate spindle speed control through the software, if there is no way for it to verify the speed. Especially if there is going to be hi / lo range shifting involved, I would think that this would be difficult to keep track of.

    Quote Originally Posted by in2steam View Post
    If you are planing on using electronincs in the near this machine you will need a filter/line reactor at that HP, otherwise you will get odd results in the computers.
    What is a filter/line reactor? Could you please elaborate, and suggest source/specifications? I am going to have a couple of computers within close proximity of the VFD, due to space constraints.

    Attached Thumbnails Attached Thumbnails -dscn1527-jpg  


  5. #5
    Community Moderator Al_The_Man's Avatar
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    I look at it this way when running a 4 pole motor on 120hz, most manuf. also make a 2 pole motor, It is very likely the motor manufacture uses the same materials (bearings etc) for both style of motor, I can't see them stocking two part numbers to save a couple of ¢.
    One other issue is balance, again, I would think the same techniques apply to both.
    I run my 4 pole motors up to 120hz and so far have not seen any problem.
    If you search for 3 phase choke or line reactor, both motor and VFD re-sellers usually stock them, they are sized according to Max motor/VFD current.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design (Skype Avail).

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


  6. #6
    Community Moderator Al_The_Man's Avatar
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    BTW if you have a 2 speed system (gear/belt) then you need a way of informing the control, usually by a switch if manual change, this should change the scaling of the output to the VFD accordingly.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design (Skype Avail).

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


  7. #7

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    Al_The_Man, what type of switch? Where would I install it? Please elaborate.



  8. #8
    Community Moderator Al_The_Man's Avatar
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    In a system that has auto gear change, there would be a switch back to the control, usually off of a gear shaft to indicate that the gear change had completed, the control would then make the necessary change in the scaling before issueing the S value.
    In the case of a manual gear change, the control should stop the spindle and after the manual change was made a toggle switch or touch screen button etc would be operated manually indicating to the control of the range change and again alter the scaling accordingly.
    The only thing with the manual change, the operator has to make sure it is in the appropriate position when the start button is pressed.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design (Skype Avail).

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


  9. #9

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    I see, thanks again Al_The_Man for your input.



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    Hi, You can control the speed of a VFD with Mach. As most VFD'S have a speed input of 0-10v you will need a converter as Mach uses PWM to control a spindle speed. There is a board called Digispeed (if you do a search on the web) which does this and then you will need a sensor on the spindle to get the rpm fed to Mach.



  11. #11

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    Thanks cjmerlincnc, I have purchased Bob Campbell's Combo Breakout board, which "also includes the spindle speed function that can control a variable frequency drive (VFD) through an optically isolated 0~5 VDC or 0~10 VDC."

    http://www.campbelldesigns.com/Combo-board.php

    In fact, I just received it Thursday. I just hope that it won't be too big of a pain to figure out how to get all of them to communicate. I mean, of course, Mach 3, the Combo Board, the VFD, and the spindle motor.

    Thanks, again.



  12. #12

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    I see your motor is of Chi-com origin, bewarned I have never had alot of luck keeping those motors from going south, esp when in inverter use. Another thing on the several odd off times I have had a failure I took them apart they had sleeve bearings, which don't tolerate higher speeds. Just some words of warning.

    chris



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