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What is a variac?

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  • What is a variac?

    I kind of gather they vary voltage, but other than that I am at a lose. The reason I am asking is there is one at a local bid board auction with a low stating price. I seem to remember a post somewhere relating to using one as a variable speed control for a lathe.


  • #2
    A variac is a variable output transformer. Basically, turning the knob varies the ratio of primary to secondary windings. Yup, varies the voltage.

    More tools than sense.


    • #3
      They are an auto transformer where a sliding contact selects the output voltage - think of it as a transformer with a zillion taps. Your voltage varies with dial position. I've used them for very smooth and quiet shaded pole fan control where it was important to not have any "buzz", but they would make a lousy speed controller for a typical lathe motor.

      Yes, they are cheap used... how many do you want ?


      • #4
        You don't want to use a variac in place of a VFD for rotating machinery speed control.

        My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."


        • #5
          A variac is a transformer with a single winding on a round, doughnut shaped core. The primary connection is made between one end of that winding and a "center" tap that is usually around the 75 or 80 percent point.

          The secondary or output connection is made between the same end as used for the primary connection and a wiper that rotates across the windings from one end to the other. This allows, through transformer action, a continuously variable, output Voltage from zero (at the common end) to about 1.5 times the input Voltage (at the far end). Thus, you can select any Voltage output that you want within the range provided. Different designs will have different maximum Voltages available: there is no set number and it could even be many times the input Voltage, although this is rare.

          Unlike a transformer with two coils, there is no isolation between the input and output circuits because they share the same winding.

          I have seen both single phase and three phase units.
          Last edited by Paul Alciatore; 12-22-2014, 08:30 PM.
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.


          • #6
            VARIAble AC. Used to sneak up power on old electronics that have not been turned on for a while. start with 10, 2 hours later run it up to 20, give the dried out capacitors a chance to re-form. used to test electronics to see if they function at 105 VAC and survive at 125 VAC.


            • #7
              Ther are in fact isolated types, but the vast majority are NOT isolated.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.


              • #8
                Variac is a brand name I think originally owned by General Radio. It now seems to be used for the Staco brand. The Powerstat brand is owned by Superior Electric, now Danaher. There are other brands and variations on the design.

                The larger 120V versions usually have taps at 12V and 132V, so you can get 0-132V from 120V, and use them CW or CCW. For 50 Hz you may need to use the full winding without a boost. There are also 240 VAC versions which have a center tap so you can get 0-240 VAC from 120 VAC, but at reduced current. The 240V versions generally have less than half the current capacity as the 120V versions of the same size, so a Superior 116 is 120V at 10A while the 216 is 240V at 3.5A.

                The brushes that ride on the exposed windings of the toroid are made from special grain-oriented carbon, so that when it contacts more than one winding at a time, the current from the two contact points (which have about 1/4 to 1 volt difference) flows preferentially through the brush to the flexible wire and the rotating tap. Otherwise a large circulating current would occur between the windings (like a shorted winding in a transformer). The exposed windings are machined flat, and often coated with gold to minimize oxidation and maximize conduction.

                The output current can be as much as 5 to 10 times the rated current according to a duty cycle curve. At 1.4x this can be as long as 30 minutes (with another 30 minutes for cooling), which is a 50% duty cycle. The duty cycle is proportional to 1/I^2, so at 10x it is 1/100 and typically about 0.6 seconds ON and 60 seconds OFF. We use these devices in our circuit breaker test sets where we need very high current (over 50,000 amps) for only a few cycles to trip a circuit breaker instantaneously.

                It should be understood that the current from the brush (or tap) can be much higher than that of the supply. At 10%, or 12 volts on a 120V supply, 10 amps output corresponds to only 1 amp input. Thus, a fuse or circuit breaker should be placed on the arm for protection.

                Variacs are made from high quality tape wound grain-oriented silicon steel in a toroid, which is very efficient. They can be used at much higher frequencies, at least 400 Hz, and up to 2 kHz or more. At those frequencies, correspondingly higher voltages can be applied, up to perhaps 600V, but beyond that may not be safe because of the insulation of the magnet wire and other components.

                The weak link is usually the brush and the surface where it makes contact. It is possible to replace the brush when it has worn or become damaged, and I have even repaired badly burned windings, although I wouldn't trust it for full power. I have also taken them apart to use only the winding as a primary for a high current transformer. I have a dozen or so damaged units that were rated at 240 VAC and 9.5A, but really could handle only about 6 amps. These cores can handle about 2000 VA, and with 240 VAC applied it will provide 2000 amps continuous at 1 volt through a single turn (bus bar). Multiple turns of welding cable can be wound through the toroid to get higher voltages.

                Here are some in various states of disassembly:

                Here is a high current test set I designed and built with four 1.4 kVA toroidal primaries. It has four turns of 1/4" x 1" bus bar and can provide 5.6 VAC at 1000 amps continuous and short pulses over 10,000 amps:

                Last edited by PStechPaul; 12-22-2014, 09:42 PM.
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030


                • #9
                  Originally posted by Stu View Post
                  . I seem to remember a post somewhere relating to using one as a variable speed control for a lathe.

                  With a bridge on the output you could run a suitably sized DC treadmill motor, this would be quieter at low rpm than the normal SCR bridge types.


                  • #10
                    Thanks for the info guys.



                    • #11
                      I use one for my lathe motor. While you don't get speed regulation, you do get to vary the speed. Without regulation the output rpm will drop with load, but that's not necessarily a bad thing. It can be a good indicator of how much you are loading the lathe, and that can help you find a good operating point. If the rpm drops too far under load, you can just crank up the voltage a bit. So far I haven't found it to be limiting for what I normally do. Bear in mind that you wouldn't run a normal ac motor from a variac, since they are very dependent on having a proper running voltage. Using a vfd is a different thing as you are varying the motor rpm by adjusting frequency, not varying the voltage.

                      I have a few variacs, ranging from a 500 watt unit to a 1500 watt. The 500 watt is the only one which goes from zero to about 135 volts or so- one of the others goes from zero to 285 volts, and the other goes from zero to about 240 or so. The one dedicated to my lathe is seldom run up past about 80 volts. I use a treadmill motor and bridge rectifier, with filter cap, so I'm up around 100 odd volts dc for the most part when I crank it up to the highest rpm I care to use.

                      I was in a second hand store several years ago, where they sold lots of surplus gear, much of it military. I spied the remains of a variac there, just the core with a burnt winding still on it. Figuring I'd wind a custom transformer out of it, I took it to the counter to see how much they wanted. Guy looked it up and asked $200 for it. Told me that was the list price on it and wouldn't budge. A brand new unit of the same size, complete and in total working order at the time was about $180. For some reason they thought that most of their 'stuff' was gold. I got my 1.5kw unit for $15 used but complete and working- I would have given them $20 for the core. Last time I looked in there all the same junk was still there.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                      • #12
                        A vfd varies both the frequency and the voltage.

                        I have a coupe of variacs set up with isolation transformers for testing all sorts of stuff. I also have some tiny (1amp) panel mount variacs. One day I'll find a use for them