Variable Transformer VS Rheostat Question

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  • leehljp
    Just me
    • Dec 2002
    • 8429
    • Tunica, MS
    • BT3000/3100

    Variable Transformer VS Rheostat Question

    OK. my ignorance is showing on this question. I have used both in different situations over the years but what is the difference in controlling electric motors?

    On the pen turning web site, (IAP) the question came up about using a rheostat for controlling the speed of non ESV motors. I thought it would be disastrous, but some say it is possible. No one came on and talked the technical terms, but I personally didn't think a potentimeter/rheostat could control a large motor like that.

    So what is the difference in using a 20 amp variable transformer VS a large rheostat to control the speed of a lathe motor. (We are talking about 1 hp or less motors on small lathes.)


    BTW, I tried the 20 amp Variac on my Grizzly lathe. It would not do much of anything until enough voltage came through to make it suddenly start turning at about 450 rpm, where as 600 is the slowest it will go with normal 120V. I tried it once (had to do it) and won't do it again - too much buzzing/humming like it it trying but not doing anything. At 110-120 it works normal.
    Hank Lee

    Experience is what you get when you don't get what you wanted!
  • Slik Geek
    Senior Member
    • Dec 2006
    • 669
    • Lake County, Illinois
    • Ryobi BT-3000

    #2
    A rheostat is a variable resistor. In series with a motor, it will dissipate (as heat) some of the energy going to the motor. The effect will be reduce voltage to the motor and limiting the available current which certainly will affect the motor speed and available torque. On anything but small motors I would expect that a rheostat would glow cherry red and burn up with the serious amount of heat it will dissipate.

    A variable transformer accomplishes the voltage reduction in a very efficient fashion. It actually converts the voltage to a different value instead of just dissipating a portion of the energy as heat. Even so, the motor will have reduced capability in addition to reduced speed. The motor may also have reduced cooling ability so it may not be good for the motor.

    Hence, both techniques are generally not effective for motor speed adjustment. Only certain motors are constructed for variable speed.

    Yes, it is possible to control a motor speed with a rheostat or variable transformer. Recommended? Probably not. Effective? Not really.

    I would get a different motor with a speed controller that is intended for variable speed. For example, DC motor/drive combinations are available. There are other options, but I'm not familiar with those.



    Comment

    • woodturner
      Veteran Member
      • Jun 2008
      • 2047
      • Western Pennsylvania
      • General, Sears 21829, BT3100

      #3
      Originally posted by leehljp

      So what is the difference in using a 20 amp variable transformer VS a large rheostat to control the speed of a lathe motor. (We are talking about 1 hp or less motors on small lathes.
      A pulley analogy - a rheostat is like loosening the belt so it slips on the pulley and sort of slows down a little, a variable transformer is like using a step pulley to generate different speeds.

      A rheostat is a variable resistor, it drops voltage and, to a lesser degree in this application, reduces current. It works OK for a DC motor, not so much for an AC motor.

      A variable transformer changes the amount of voltage change across a transformer by varying the turns ratio of the transformer. It not only is better for AC motor control, it also works only on AC, since a varying voltage is required to operate the transformer.

      An even better choice is an electronic speed control, where the voltage, and current are maintained but either the pulse duration or the frequency is adjusted. VFDs are one example of this. This type of drive can maintain most of the torque and power of the motor while controlling speed.

      --------------------------------------------------
      Electrical Engineer by day, Woodworker by night

      Comment

      • leehljp
        Just me
        • Dec 2002
        • 8429
        • Tunica, MS
        • BT3000/3100

        #4
        My first thought was that a rheostat would probably burn up as mentioned. And with the variac, I have experienced that huge torque drop with voltage drop on AC motors, so I stayed clear of using them except to try them out. I have two 20 amp variacs that I brought back from Japan. I haven't used them like I thought I would. Over there I used them to step up the 100V to 120v to use on my USA tools. That worked well.

        NEXT QUESTION: What is better VFD (frequency) or EVS (pulse)? Which is easier to add onto an existing motor such as on a lathe?
        Hank Lee

        Experience is what you get when you don't get what you wanted!

        Comment

        • Tom Slick
          Veteran Member
          • May 2005
          • 2913
          • Paso Robles, Calif, USA.
          • sears BT3 clone

          #5
          What type of motor is on the lathe?
          Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison

          Comment

          • leehljp
            Just me
            • Dec 2002
            • 8429
            • Tunica, MS
            • BT3000/3100

            #6
            Originally posted by Tom Slick
            What type of motor is on the lathe?
            On the lathe that I will like to add EVS/VFD:

            Type................. TEFC Capacitor-Start Induction
            Horsepower...... 2 HP
            Phase............... Single-Phase
            Amps.................14A
            Speed............... 1725 RPM
            Hank Lee

            Experience is what you get when you don't get what you wanted!

            Comment

            • Slik Geek
              Senior Member
              • Dec 2006
              • 669
              • Lake County, Illinois
              • Ryobi BT-3000

              #7
              With a capacitor-start induction motor, if you lower the voltage (with the variac) you'll reach the point that the starting capacitor remains in the circuit continuously. I believe that the starting capacitors are not designed for continuous use so the capacitor start circuit will likely fail. Then the motor will struggle to get up to speed when started.

              Comment

              • leehljp
                Just me
                • Dec 2002
                • 8429
                • Tunica, MS
                • BT3000/3100

                #8
                Originally posted by Slik Geek
                With a capacitor-start induction motor, if you lower the voltage (with the variac) you'll reach the point that the starting capacitor remains in the circuit continuously. I believe that the starting capacitors are not designed for continuous use so the capacitor start circuit will likely fail. Then the motor will struggle to get up to speed when started.
                So, What is better VFD (frequency) or EVS (pulse)? Which is easier to add onto an existing capacitor motor such as on a lathe?
                Hank Lee

                Experience is what you get when you don't get what you wanted!

                Comment

                • LCHIEN
                  Internet Fact Checker
                  • Dec 2002
                  • 20914
                  • Katy, TX, USA.
                  • BT3000 vintage 1999

                  #9
                  Induction motors can only really be controlled properly by a variable frequency drive.outputting the correct 120VAC. That's if you care about getting maximum torque and all. Or, using various pulley techniques.
                  Induction motors when unloaded turn at the synchronous speed. and slow down by slipping, about 10% as the load goes from 0 to 100%. Past 100% the motor will tend to stall and current go up to locked rotor current.

                  Using a autotransformer is a lossless way of controlling the voltage. But voltage control alone will give extremely poor torque output and erratic speed control
                  Using a rheostat in series with the motor is a another way of controlling voltage by dividing the voltage between the motor and the variable resistance. Aside from doing just a piss poor a job as the autotransformer, it will also dissipate a lot of heat.
                  Using a router speed controller is a phase angle controller. Its just another way of controlling voltage content and won't work well.

                  There's only 2 good ways to control induction motor speed and that VFD and pulleys/gears
                  But quite frankly, VFDs seem to be only availabe configured as 1 ph in 3 phase output at 208 or 230 VAC
                  I dont' see any off the shelf 120V output 1 Ph, although I'm not entirely sure if you can run a 1 ph motor off one of the phases and if it will start right (capacitor start and VFD don't work well together).

                  Good luck.
                  Last edited by LCHIEN; 07-31-2017, 03:07 PM.
                  Loring in Katy, TX USA
                  If your only tool is a hammer, you tend to treat all problems as if they were nails.
                  BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

                  Comment

                  • Tom Slick
                    Veteran Member
                    • May 2005
                    • 2913
                    • Paso Robles, Calif, USA.
                    • sears BT3 clone

                    #10
                    Dart controls makes a 120v speed control but it specially says not for use with start capacitor motors. All of the VFDs I've seen are 3ph output.

                    If you really wanted electronic controled speed I'd look at Automation Direct for a VFD and 3ph motor
                    Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison

                    Comment

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