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Why not a single-phase VFD?

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  • Why not a single-phase VFD?

    Is there a reason why all VFDs seem to have 3-phase output?
    Seems as if with 1/3 of the power electronics you could drive a single phase motor. Is it because the uneven power draw would require bigger power supply capacitors?

  • #2
    Demand maybe low for single phase VFDs. I don't remember seeing many single phase motors in industrial applications. Plus the loss of reverse when using a single phase. Also, it would 2/3 of the power electronics if you have a full sine wave. You have a transistor for + and one - for each of the two legs for a full the single phase motor. So a total of four power transistors vs six for 3 phase.
    Last edited by JPR; 09-14-2006, 04:23 PM.


    • #3

      Two of the main reasons VFD's are mostly three phase, first most are high dollar items aimed at industrial applications. These tend to have 3 phase motors. The second reason is that a single phase VFD does not have the turn down ratio that a 3 phase unit can provide. At low rpm, the single phase voltage cycling becomes very apparent.

      There are single phase VFD's available and the prices are starting to become quite reasonable.


      • #4
        Single phase motors because of their design have limitations as to the frequency that they can operate under.Three phase motors have almost an unlimited range of frequency compatability,this is why they are chosen for applications that require variable speeds.By controlling the frequency output one can control the torque and rpm of the motor,hence the name "variable frequency drive".Also one can control direction and ramp up and ramp down rates as well as many other parameters with a vfd.For lower power applications most vfds will change single phase power to three phase and also have the capabilty to raise voltage levels,such as 115 to 230 volts or 230 to 460 volts 3ph .
        Home, down in the valley behind the Red Angus
        Bad Decisions Make Good Stories


        • #5
          Single Phase VFD

          If you decide to go with a single phase VFD, check to see if your motor is a cap-start / cap-run and if the drive will work with your motor.

          Here is some general information on VFDs.
          Last edited by JPR; 09-15-2006, 12:51 AM.


          • #6
            The main problem would be the disproportionate power loss.

            A single phase motor has ZERO TORQUE. This is inherent in the design.

            The only way you get any torque with a single phase motor is by it "coasting through" part of the cycle on inertia. Or at start, when it starts as a two-phase motor.

            Since the energy stored for "coasting through" is related to velocity (or in this case rotation speed) squared, there is a large loss of energy rate and thus power at lower speeds. You get 1/2 power at 70% speed, 1/4 the power at half speed, etc, etc.

            A 3phase also loses power at lower speeds, but not at a squared rate. Torque is constant, so it isn't MV^2 but rather a force x distance relationship.

            If you re-wired the motor to use the start windiing (at reduced current), with an appropriatly phase-delayed two-phase VFD, it could work possibly a bit better. But you'd be limited by the start winding gauge and allowable current, as well as the "mechanical" phase relation of it vs run winding.

            Keep eye on ball.
            Hashim Khan


            • #7
              I was given to understand that it has more to do with one of the basic characteristics of a single phase induction motor, it has an oscilating (not rotating) field, hence the need for starter windings.
              Your VFD would have to be more complex to deal with starter windings, it would be cheaper to use a standard single to three phase VFD and buy a three phase motor to go with it.
              Not to mention that single phase motor power delivery is lumpy at it's rated speed so slowing it would make it worse still,
              go three phase, you know it makes sense!


              • #8
                Since 1/4-3/4hp three phase motors are cheap and plentiful,and a single phase 115vac input VFD can be had for $120 including shipping there probibly isn't much reason to make a single in single out VFD.
                I just need one more tool,just one!