No announcement yet.

VFD question

  • Filter
  • Time
  • Show
Clear All
new posts

  • VFD question

    Single phase 120/240 power on building. Co-workers want to use a VFD to serve two 3 phase motor loads that are separately controlled. Is that possible with a vfd? We are looking at

    Thank you for any input!

  • #2
    Yes, many drives will accept single phase input power and invert to a three-phase output for the motor. Since you're not using all the rectification components in the input stage, your DC bus ripple will not be as good, plus you'll have to derate the power rating of the drive (can't rectify as much power to feed the bus for inversion). Many manufacturers do this will small drives, up to about 5hp or so. ABB ACS-550 will do it for larger motors that I've seen from other manufacturers. Not sure if you're asking if you can share one VFD between two motors. If so, it is possible to do that, but I'd recommend one VFD per motor.


    • #3
      Thank you for the response.

      What's the risk of running 2 motors with 1 VFD without derating?


      • #4
        You don't run any motors at all without derating. If you reread my post, you'll see that the derating comes from not being able to get enough power to the DC bus, which will still be true no matter how many motors you're feeding. The advice about multiple motors is just instinct about not trying to push the envelope in too many different ways at the same time on the same installation.

        Besides, you said the motors are separately controlled. If they still need to be separately controlled, there's no way I'd EVER do that out of a single VFD, even without the single-phase issue. The case I'm talking about is when you have multiple motors that will all be controlled as a single unit, all wired in parallel (probably with individual overload relays) to the output of the VFD. They all run or they all don't run (and the start and stop is controlled only via the VFD). In that scenario, you don't turn them on and off independently.
        Last edited by ControlsGuy; 02-15-2019, 07:14 PM.


        • #5
          I'm with ControlsGuy, just buy two drives, especially for what they cost. I haven't had to de-rate a drive specifically designed for single phase input though like the Lenze drive mentioned.


          • #6
            Agreed. If a drive is specifically designed for single phase input, as opposed to a nominally three-phase drive that will accept single phase, you shouldn't need to derate. Did not check out the referenced drive.


            • #7
              Two drives: better and smarter than one when running two motors. with that said, if you do choose to run two motors from a single VFD, they will have to run at the same speed and start and stop at the same time. They will also require individual overload relays, and those overload relays must be compatible with the output signals present on VFD motor lines. By the time you include those extra requirements and enclosure to put them in, you have just spent more money than it will cost to buy two drives at that low hp rating.