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  • VFD

    I have been reading about the virtues of a VFD vs. a phase converter. It is my understanding that the VFD does everything that the phase converter will do and does it better.

    A phase converter puts out about 80% of the power sent to it (as I understand it)and therefore requires a unit with about 30% more HP than the motor youi are using to drive the equipment. How does a VFD compare to this?

    I hope I will be needing a phase converter in about 2 weeks as I am supposed to go and look at a South Bend 15x60 in a few days and should it be all that it is supposed to be I shall bring it home. Then I will have to find a way to drive the thing from my single phase current. Should I opt for a VFD or go with a phase converter? How do the 2 compare as far as price is concerned? How would I go about taking advantage of everything a VFD would do?
    Thanks again guys.

  • #2
    I have a VFD on my milling machine and can answer some of your questions. The reason that you get less power thru your static phase converter is because it starts the 3 phase motor with a capacitor on the missing phase and then one phase drops out leaving you to run on two phases or about 66% of your power. My variable freq. drive generates all three phases so you don't lose power. Mine would cost about $350 new and was used at $100. By changing the frequency on the front panel, I can change speeds on my mill w/o changing pulleys. There is a limit to how much change in freq your motor can take w/o overheating--about 25% I think. My drive locks up once in a while if I switch the motor on and off at the mill w/o shutting off the drive first. Maybe that is why I got it so cheap! I think the electric motor type phase converters are bullet proof, altho heavier and not adjustable. Thanks--Mike.


    • #3
      I can't begin to address all your questions about VFD's vs. phase converters, but having made a conversion to VFD's in my home shop I can give you a report of excellent performance and satisfaction.

      Since I lucked into a real bargain, my costs were more favorable than you might be able to get and I bought two VFD's -- a 1 hp unit and a 2 hp unit (based on single-phase power input) -- for my lathe (12 x 24) and my RF-30 mill/drill. Maybe others can help with the price comparisons.

      The first effect I noticed was how much quieter both machines were, with the controlled acceleration of the motors (controlable from the VFD). But the real joy is the ability to control the motor speed and therefore spindle RPMs with quickness and ease. It doesn't completely eliminate changing drive ratios, but makes them necessary much less often.

      There will be some purists that will warn against running a motor at other than the 'designed' frequency of 60 Hz., but I have not noticed any significant loss of power or overheating over a wide range of driving frequencies. Admittedly, I don't have the luxury of operating either machine for 8 straight hours under heavy load, and I doubt that there are many home shops that do. Being able to obtain the proper cutting speed is a far more important consideration.

      The features available will vary from VFD to VFD, but unless you have particular needs, the most important consideration is the ability to power the largest motor size you will be using. I could easily have made do with just the larger of the two I bought, using it alternately for one machine or the other. Bear that in mind if you might one day want to support a larger motor on another machine.

      Good luck!

      Rich Kuzmack

      Pi = 355/113 . . . to
      <85 parts per billion
      Rich Kuzmack

      Pi = 355/113 . . . to less
      than 85 parts per billion!


      • #4
        I am in the process of installing a VFD on my Grizzly 12X36 belt drive lathe. It is an ACTECH unit and to me it was a reasonable price. The motor is a Baldor 1.5hp unit which is rated for VFD use. I will know soon how well everything works and post the news to the board. WARNING, I am old, slow and ugly so don't hold your breath waiting for the information. ACTECH has a web site that will answer your questions. WALT WARREN


        • #5
          I have a Mitsubishi 3HP fully programmable unit. Check your unit because I know mine is derated to 1.5HP (Continuous Duty) when powered from single phase power. To get best performance from the VFD a motor designed for VFD use is the best choice but not an absolute must. My VFD can torque boost, change acceleration, instant brake (bad for a threaded spindle!), and adjust output from 0-400Hz. For continuous duty, 200% motor speed is maximun recommended - you have to be careful doing this as you do not want to exceed safe rpm's for your chucks. It can also be programmed for 3 different configuration profiles from a user interface switch panel. It can accept control spindle speed commands from a CNC program. The display is programmable as well and can display percentage, rpm, or SFPM. It also has to be manually started up after a power failure - a safety feature.


          • #6
            I have a VFD on my lathe, and am very happy with it. I have an older, (mid 60's) 1-1/2 HP motor, and it works well. They still put some iron and copper in them then. Newer motors can be a bit touchier.

            The 25% limit on range is a good suggestion, especially when going to a lower speed. Torque falls off, and overheating may occur.

            One major thing to be aware of when buying used is that most VFD's are for three phase equipment, being used for speed control. If you are getting a surplus one, make sure it is single phase input. If you can inspect it, open, and check the supply and output connections. Also, if you buy used, get the manuals. These things are all different, and change like the wind as technology advances. Set up and operating parameters can vary widely from model to model.
            Jim H.


            • #7
              I've got a TECO SPECON (SPEECON?) VFD on my milling machine. It's great. I expect you can find a suitable surplus VFD for less than the cost of a good phase converter, and it will do more.

              I got mine through
              Friends have used
              (Usual disclaimers.) There are undoubtedly a bunch of other places that sell them.

              The other replies covered pretty much everything, I think.

              Try to make a living, not a killing. -- Utah Phillips
              Don't believe everything you know. -- Bumper sticker
              Everybody is ignorant, only on different subjects. -- Will Rogers
              There are lots of people who mistake their imagination for their memory. - Josh Billings
              Law of Logical Argument - Anything is possible if you don't know what you are talking about.
              Don't own anything you have to feed or paint. - Hood River Blackie


              • #8
                I work at a local Aerospace company.I do electrical/electronic repairs on equipment. All new cnc equipment has vfd's to control axis/spindle motors. The machines are run 24/7 quite a lot of the time. My experience is as folows: they are reliable, they can be a bit difficult to set up because to the number of parameters. The vfd should be of a higher power rating than your largest motor. If you are going to run the motor at a very slow speed add a supplemental fan to cool the motor. Every 10* rise in temperature will cut motor life by 20%. The main parameter to set up is the volts/hertz curve. This is set up by taking the voltage rating of the motor nameplate and rpm and entering them in the appropriate settings. Take your time in setting up the drive and think your way through the instructions.
                Be safe