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Thread: VFD

  1. #1

    Post VFD

    I recently purchased an older BP mill and am in the process of cleaning, adjusting and getting it set up. I have purchased a VFD to run the 1hp 3 phase 220v motor using single phase 220v and have two questions.

    1. The VFD has three outlet wires - T1, T2, T3 (not counting ground) but the three terminals on the side of the motor/reversing switch do not have the leads identified. Are the leads T1 to T3 left to right? does it matter?

    2. I read in a post on another board that you shouldn't run from a VFD through a switch like a reversing switch, you should use the reverse function on the VFD. I'm not good with electrical stuff but I don't see why this would matter. Is this true? should I just leave the reversing switch in one position?

    Any help for the electrically challenged would be appreciated.


  2. #2
    Join Date
    Apr 2001


    You can hook up the 3 leads in any order. The only thing is, the motor may run backward when it should run forward. If that happens, just reverse any two of them.

    As for #2, yes. Otherwise you can get a spike caused by the motor inductance that can overload the VFD. Switch on/off/reverse ONLY with the VFD. Assuming you got an instruction manual with the VFD, it ought to have a caution about this.
    When I put a VFD on my milling machine, I took the knob off the machine's on/off switch so I wouldn't acidentally use it in a moment of stupidity.

    [This message has been edited by SGW (edited 11-24-2003).]
    Try to make a living, not a killing. -- Utah Phillips
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    Law of Logical Argument - Anything is possible if you don't know what you are talking about.

  3. #3


    Thanks for the reply SGW. I have a book with the VFD and did read through it but missed the part on switches - - guess I was focusing mostly on how to hook it up and basic start/stop type stuff. I'll read through it again.

    Removing the knob sounds like a good idea too!

    [This message has been edited by kruszert (edited 11-24-2003).]

  4. #4


    When I refit my mill with a three phase motor and VFD, I removed the wiring from the existing forward/reverse switch and wired it up to control the VFD. The motor is wired directly to the VFD.

    If you look at the typical drum switch, there are 6 terminals arranged three on one side and three on the other. When the lever is in the OFF position, nothing is connected. When in the FORWARD position, each of the three terminals is connected to the corresponding terminal on the opposite side. When in the Reverse position, the upper two terminals on the left are connected, the upper two terminals on the right are connected, and the bottom terminals on each side are connected.

    I created a diagram that illustrates this. See or

    You can see from the diagram that you can use terminals 1, 2, and A as a single-pole, double-throw switch with center off - exactly what you need command a typical VFD to go forward, reverse or turn off. Wire switch terminal 1 to the common of the VFD, switch terminal 2 to the Reverse terminal of the VFD and switch terminal A to the Forward contact of the VFD and you're set to go.

    This is especially nice if you've been using your mill for a long time and you instinctively reach for the drum switch.

    Don Kinzer
    Portland, OR

    [This message has been edited by dkinzer (edited 11-25-2003).]

    [This message has been edited by dkinzer (edited 11-25-2003).]
    Don Kinzer
    Portland, OR

  5. #5
    Join Date
    Dec 2002


    Make sure your motor is wired to the correct voltage.

    Star-Wye Usually 230-240 volts 4,5,6 together.. Memory fails to the paired off 3 motor connections for T-leads.. (too many knots on my head)

    Delta is usually 460.. 4&7, 5&8, 6&9 T-leads 1,2,3
    I remember that cause I mostly work 460 3 phase jobs..

    Check your motor plate on your motor..

    I know it is a pain, but read your manual for your inverter. Otherwise you will reset faults more then you should. You should have a button to reverse on keypad.. or have connections diagram to remote switch it. NO contactor/drum should be on the exit side of inverter. I did however wire my factory overloads to my Stepper power switch on my bridgeport in the overtravel switch E-stop circuit.


    [This message has been edited by ibewgypsie (edited 11-30-2003).]

  6. #6


    I got the VFD set up and working over Thanksgiving - - between too much turkey. I was getting ready to buy a switch to mount beside the VFD to turn it on/off when I finally got back to the computer and read the additional replies to my queston. Wiring the VFD directly to the motor then using the existing forward/reverse switch sounds like a great idea. Saves me buying and mounting a switch and will look a lot cleaner. I don't have a reverse button on the keypad but the wiring diagram shows how to wire for forward and reverse using a couple of the open terminals.

    I have read that manual three times now looking for something that would tell me not to wire in a switch between the VFD and the motor and I can't find it but I'm not going to risk messing it up!

    Thanks for the replies,


    [This message has been edited by kruszert (edited 12-01-2003).]

  7. #7
    Join Date
    May 2003
    Vancouver's Island


    Regardless of how the motor is powered you should have some sort of emergency stop switch that physically interrupts the flow of electricity to the motor, as in BIG RED BUTTON or similar. Never rely on solid state switching for safety.
    Free software for calculating bolt circles and similar: Click Here

  8. #8


    Hi guys, I just wanted to post the email I sent to Reggie regarding his drive question and the idea of switching.

    >>>>>>>>>>>>&g t;>>>>>>>>>>
    Yes, it is advisable to use the reverse function of the drive for the following reasons.

    1.) It is designed for that purpose with the controlled speed ramp and V/HZ curve.
    2.) If the output is on (I.E. the motor is running) and the output is switched (to reverse or off to on) the drive will see 6 -10 times the FLA of the motor as if it was an across the line starter.
    3.) Out of 3000 calls a year, I get a handful of drives that fail because of this.
    <<<<<<<<<<<<&l t;<<<<<<<<<<<< <<<<<<<<<<<

    You can of course switch the leads when the motor is not outputting, however, it's easier said than done.

    I'm not a salesman...I just do support.

    Any other qustion associated with drives and starters please email me or hit our website at

    Carl Tolbert

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