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VFD yet again! (1 phase question)

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  • VFD yet again! (1 phase question)

    Alright, its time for my monthly stupid question.

    I've been reading alot on the board lately about not being able to drive a single phase motor on a VFD. I realize that VFD's are putting-out three phase power. But why couldn't one just tap two output leads and use those to drive a single phase motor? Is my ignorance showing again?

  • #2
    Most VFD are designed to drive 3phase induction motors, however there are VFD that are specifically designed to drive single phase induction motor. They are however designed only to drive permanent split capacitor (small grinders) or shaded-pole (used in small fans) type of single phase induction motor. Most single phase induction motors used for milling or lathe machines are capacitor start/capacitor run type, which exhibits good starting and run torque. The problem with using VDF with this type of motor is that the centrifugal switch gets in the way. It would keep switch on and off as your rpm increased and dropped.

    In principle, you can use the 2 phases of the 3phase output from the VDF to drive the starter winding and the run winding of a single phase motor (removing capacitor and bypassing the centrifugal switch) but you'd need to limit the current to the starter winding. You'll also need the neutral connection, which on most VFD is not available. In short, lots of work and very little gain.

    I'm beginning to think that for HSM machines, replacing a single phase induction motor with a DC motor is the best solution, both in terms of cost and performance. With DC motor, you'll get smooth and constant torque even at low RPM.



    • #3

      Not wishing to pick holes in an otherwise excellent reply, but did you not mean a 'capacitor start/induction run' single phase is the most common home shop mill/lathe type of motor? These are the motors with the centrifugal switching to drop out the phase 'shift' applied by the capacitor fed start windings. In effect these motors during start up can be considered as a '2 phase' system.

      BTW I once had a motor where the centrifugal switch was damaged and refused to 'drop out' ~ the result being rough running and rapid heating of the motor. I cured it for a short term fix by wiring the start circuit into an external plunger contactor, this was depressed when the motor was started for a second or so to enable the motor to run upto speed, and then released. A similar set-up may be a way around the low speed switching in/out.

      The DC motor option is, in my opinion and endorsing yours, probably the best way to go; closely followed by a VFD/3 phz system.
      The DC motors are powerful, available at reasonable cost (see other recent posting mentioning ~ 2HP for $49.95). My only concern is the cost of a decent controlable power supply? Over in the UK new units are roughly آ£1 per Watt ($1.50/Watt), and second hand units (when available) tend to hold their prices well.

      If they are available at reasonable prices in the US, looks like I will have to pack an extra suitcase on my next trip over!!



      • #4

        There are actually two different types of capacitor start induction motors. In both cases, a starting capacitor is connected in series with the centrifugal switch and the starter winding. What makes "capacitor start/capacitor run" different is the second capacitor which is permanently connected to the starter winding so that it's always energizing it. The run capacitor has a smaller capacitance so the power deliever to the starter winding is relatively small compare to the starter capacitor. Typically, you'll see two lobes on the side of the motor where the capacitors are housed. The reason for the run capacitor is to improve the run torque at the cost of slighly reproduced efficiency.

        I'm not actually sure about the cost of commerical DC motor controllers, but I think one should be able to build one for under $100. You'll need a full bridge rectifier, capacitors, Mosfet, and a PWM driver. I'm sure there's lots of plans for this kind of stuff on the web.


        [This message has been edited by Rotate (edited 09-16-2002).]


        • #5

          See, I told you I qualified for the 1002-1052AD years knowledge!! I stand corrected.

          However, does not the Cap start/Cap run system nowadays tend to do away with the centrifugal switching, replacing it with a piece of solid state gizmo?? I must admit though that AC electrics are not my strong point (some would say neither are my DC electrics).



          • #6
            Contrary to what many people think, mechanical devices are much more reliable in fail safe systems than all solid-state. The last time I was working with power electronics, UL/CSA will not certify all solid-state circuit breakers for safety reasons.

            As for the motor, I think centrifugal switch is very economical and reliable, so going solid-state won't buy you anything.



            • #7
              Most single phase motors I have bought and installed of late have switches. It is much cheaper as Albert says and a better bet over time to work properly in dirty environs then electronics.
              As for using 3 phase drive for single it is not really a good idea. They are ment to output power on all three legs, not just two.
              Baldor makes some single phase ones but they use motor with no starting windings. They also make single to thee phase drives which are more common in home shop use.

              I hope I'm not confusing things, Rick


              • #8
                I don't think this is what Tony was getting at, but Rotate confused me at the start....

                I thought that was the whole POINT of a supply 3 phase to a milling machine or lathe or whatever. The manufacturers I've seen like Superior Phase Converters say their products are designed specifically for this purpose.

                I don't know if I'm heading down the wrong road (I've only run 3 phase with 3 phase) but I imagin if I built a shop I wouldn't pay the $5k to pipe in 3 phase, but wire to a VFD. I thought that was the whole point....

                (I asked this about 18 months ago...about rotary phase converters...and y'all set me onto VFD's)


                • #9
                  I'm afraid to jump into this thread because electronics is not my thing,but, when setting up my toolroom lathe I wanted to be able to control spindle speed quickly and without changing belts gears or gerbils. D.C. seemed like the best way to go but was very expensive, ($800-$1000). Checking on Ebay, I found a brand new MagTech 1 hp motor with vfd. I payed $125 if memory serves. It can be programed to run on 1 phase or 3 phase and controled by a ten-turn pot allows for precise spindle control. It takes a lot of load to cause a drop in rpm btw. It has prover to be a good drive.


                  • #10

                    Perhaps I can clarify few things. Strictly speaking VFD (variable frequency drive) outputs AC with selectable frequency with n phases. You can actually get VFD with 1, 2, 3, 4 or more phase output. For most industrial applications VFDs output 3 phase since it is intended to drive 3 phase induction motors. There are however, as I've noted before, some VFDs that output single phase so as to drive single phase motor.

                    The input of the VFD varies, but internally it needs DC. If all the motors in your shop were connected to VFDs, then the best power to pipe into your shop is pure DC, however since our utility companies can only supply AC, VFDs have rectifiers which convert the AC to DC. Again different VFDs can accept either single phase AC or 3 phase AC as their input. Because single phase AC goes through zero crossing twice a cycle (no power at this point), capacitors are used to smooth out the rectified wave form so that engery can be continuously supply to the inverter (this is the thing that converts DC to AC). With 3 phase AC input, there's is no time when the voltage becomes zero, so when it's rectified the DC that it produces is much smoother and as a result can provide constant power better. Generally for high power VFD, they require 3 phase input.