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  • #76

    Re: Variable speed

    "Last night turning a 15", 45 pound blank of wettish beech courtesy of hurricane Dorian, I roughed out a blank starting at ~200 rpm in low speed range. Set up worked well. Motor certainly didn't stall, however with a couple catches (one actually bent gouge tang) there was still slipping of 1/2" notched belt on smallest 2" jackshaft pulley. No slippage with 5/8" belt. Guess, one should look at slipping as a safety feature in this situation!"

    The nice thing about your configuration, is that you can go up one step with the pulleys and turn the voltage down to attain the appropriate speed, hopefully providing sufficient grip. If you find that there is not enough torque, you can simply turn the dial to increase the voltage to compensate for the increased load.
    Last edited by tool fan; 02-05-2020, 11:30 AM.

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    • #77

      Re: Variable speed

      Originally posted by guylavoie View Post

      Generally, a DC motor with brushes has a very linear power curve right down to very low RPMs, especially in a series wired configuration. Series wired is rare in a tool application though because they can attain insanely high RPMs if they have no load.
      This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?

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      • #78

        Re: Variable speed

        Originally posted by iamtooler View Post

        This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?
        The apparent "resistance" (ie: power usage, volts x amps) is more complex in a motor. It largely depends on the load. A motor with no load draws relatively little current, while a heavily loaded motor draws a lot of current. Lock the rotor and it draws maximum "locked rotor amps (LRA)". This is true for both AC and DC motors. Think about the following for a moment: The physical winding's resistance and impedance don't change whether the motor is under load or not, yet the current draw changes significantly. So what's happening?

        The answer is that a motor also acts as a generator, and the spinning rotor is generating a current that counteracts the incoming current. This generated current is usually referred to as "counter emf (electro motive force)". At no load, the rotor is spinning at close to the theoretical rpm (eg: 1800 RPM for a 60 Hz AC induction motor) and most of the current is counteracting the incoming current. As load increases, the rotor slows down and the rotating magnetic field starts to "slip" more and more in relation to the incoming current's field. The generated current starts to get out of phase with the supply current and less resulting counter emf is generated...and the resulting current draw increases.

        The current draw of a motor is usually indicated for it operating a full load (eg: requiring an actual 1 HP of work for a 1 HP motor). This also means that a certain amount of magnetic field "slip" is expected to occur. This is why a 60 Hz motor with a rotating magnetic field at 1800 RPM will normally end up rotating at a nominal speed of 1725 RPM.
        Happiness is a new blade!

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        • #79

          Re: Variable speed

          Originally posted by iamtooler View Post

          This means that the amperage draw goes up in line with the voltage (speed ) decrease because volts times amps = watts?
          With the same load I believe that to be true. However when a load is applied, the amperage increases to the limit of the control board circuit, motor or circuit breaker.

          Some of these smaller motors are rated as high as 18 amps and 130 volts or 2340 watts or just over 3 HP. This seems unrealistic. The better ones - the bigger ones - are rated around 20 amps and 90 volts or just under 2.5 HP which seems more realistic. My sense is that the little ones running at top speed under maximum load are on the verge of burning out. Conclusion: use the bigger motors and don't run them for long periods of time at top speed under load.

          The other issue is cooling. When the motor runs faster, more heat is produced. However, the fan(s) turn faster presumably drawing heat away from the motor. Conclusion: either provide external cooling or do not run the motor for long periods of time at low speeds. I have yet to tear apart a treadmill and not find a motor with at least one fan. Someone wiser than me has determined that they are necessary.

          When operating these motors, I think that there is probably a sweet spot somewhere between 70 and 90 volts where the motor is not over heating or over working and fan cooling is adequate.

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          • #80

            Re: Variable speed

            Originally posted by guylavoie View Post


            The current draw of a motor is usually indicated for it operating a full load (eg: requiring an actual 1 HP of work for a 1 HP motor). This also means that a certain amount of magnetic field "slip" is expected to occur. This is why a 60 Hz motor with a rotating magnetic field at 1800 RPM will normally end up rotating at a nominal speed of 1725 RPM.
            I believe that the FLA on the tag is at the listed speed (say 1725) but an idling motor will be closer to it's synchronous speed of 1800 rpm. In this era of digital tachs that would be easy to check. With a universal motor the higher the slip the stronger the magnetic force rotating the armature. With an induction motor torque falls rapidly after 5% slip. Newer cordless tools have brushless motors, I have never had the opportunity to learn what principals they employ. I almost wonder if they are mini vfds!
            Rob

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            • #81

              Re: Variable speed

              They use permanent magnets on the rotor and a number of coils as the stator. A "computer" switches the current in the coils such that the rotor gets attracted to the next coil causing rotation. Since the circuit controls how quickly the different coils get energized you could loosely look at it as a "min vfd" but the motor itself is more complex than a simple 3-phase motor. Brushless DC motors use Hall effect sensors to determine the rotor position so coil switching can maximize torque.

              Some lathe manufacturers offered brushless DC but then went to regular VFDs. Reliability???

              Actually they have been around for a long time in the simple form of stepper motors.

              billh
              Last edited by billh; 02-05-2020, 01:26 PM. Reason: Expanded

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              • #82

                Re: Variable speed

                Well...yes and no. Stepper motors are actually designed to be in a stalled position continuously. For that reason they are almost never used in situations where any kind of speed is required. Think of steppers as rudimentary open loop servo type systems. But I understand the similarity. The variable speed brushless DC motor would be another application of smartly driven coils around a permanent magnet rotor.
                Happiness is a new blade!

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                • #83

                  Re: Variable speed

                  [QUOTE=guylavoie;n1269574The variable speed brushless DC motor would be another application of smartly driven coils around a permanent magnet rotor.[/QUOTE]

                  Does an induction motor have a ''permanent magnet'' rotor? The stator magnets switch between polarities with the +/- of the ac frequency so the rotor magnets must not change polarity?

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                  • #84

                    Re: Variable speed

                    Originally posted by iamtooler View Post

                    Does an induction motor have a ''permanent magnet'' rotor? The stator magnets switch between polarities with the +/- of the ac frequency so the rotor magnets must not change polarity?
                    No. As the name implies, induction motors have their magnetic field induced into the laminated core. Yes the stator magnets switch between the polarities of the AC current...but the rotor spins with the rotating magnetic field, so it "sees" a constant magnetic polarity across the core. The laminated core acts much like the laminated frame of a
                    transformer. An induction motor is actually a simpler, cheaper (and less efficient) way get getting a spinning magnet. But an advantage is the lack of a commutator (brushes), making for a quiet, maintenance free motor.

                    Induction motors depend on the line frequency to induce the magnetic field in the rotor, so they aren't really good for variable speed unless you can keep the voltage constant and vary the frequency itself... which is precisely what VFD drives do. The VFD also allows you to optimally drive a 3 phase motor, which is more efficient than single phase motors because the total power being converted to rotary motion remains constant, while a single phase motor is alternating between powered and free wheeling as the current wave goes up and down sinusoidally.
                    Last edited by guylavoie; 02-05-2020, 09:12 PM.
                    iamtooler likes this.
                    Happiness is a new blade!

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                    • #85

                      Re: Variable speed

                      Jet 1014 Benchtop Midi Lathe

                      Here is another one for those that are interested. This one was a little more difficult simply because I didn't have a cabinet to house the wiring. In the end I struck a balance by mounting the lathe on a 2 x 10 plank and ran the wiring under the plank. I also had a hard time finding a motor with a long enough arbor that was compatible with the step pulley that came with the original 1/2 HP motor. I finally found a serpentine belt drive with the same LH thread pattern as one of my motors. The belt drive is long enough to reach any of the 6 steps in the spindle pulley and the motor easily adjusts up or down to tension the belt. On the second slowest spindle step, functional speeds between 100 and 1900 are possible.

                      Since the belt drive on the motor is threaded on with a LR thread, I decided not to add a forward/reverse switch.
                      Last edited by tool fan; 02-07-2020, 07:13 PM.

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                      • #86

                        Re: Variable speed

                        Originally posted by tool fan View Post
                        Rory,
                        i have been doing a lot of reading today( home sick). One thing I came across was a thread from here a couple years ago where you said you were going to build a controller out of a 400w scr,a bridge rectifier,and a 5ohm potentiometer.
                        How did that work out?
                        I played around with that for a month or so, but could never get something that I was satisfied with. In the end, at that time I concluded that the DC 51controllers were far superior to anything that I could build. Since that time I have discovered that not all DC 51 controllers are created equal. My go to controller is an MC 60 from an old treadmill.[/QUOTE]

                        Rory (or any body else):
                        How about this 10000W SCR controller with a bridge rectifier. modified potentiometer and homemade choke for use with a Treadmill DC motor? You commented in above quote that not much luck with this arrangement but did you try a larger wattage SCR as this? Parts cheap via eBay. People claim to have success using on lathes in Amazon and eBay reviews - for what that 's worth. This is one of several You Tube descriptions (nice mood music):
                        https://www.youtube.com/watch?v=T22pJMIAIRQ

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                        • #87

                          Re: Variable speed

                          Originally posted by DougLA View Post

                          I played around with that for a month or so, but could never get something that I was satisfied with. In the end, at that time I concluded that the DC 51controllers were far superior to anything that I could build. Since that time I have discovered that not all DC 51 controllers are created equal. My go to controller is an MC 60 from an old treadmill.
                          Rory (or any body else):
                          How about this 10000W SCR controller with a bridge rectifier. modified potentiometer and homemade choke for use with a Treadmill DC motor? You commented in above quote that not much luck with this arrangement but did you try a larger wattage SCR as this? Parts cheap via eBay. People claim to have success using on lathes in Amazon and eBay reviews - for what that 's worth. This is one of several You Tube descriptions (nice mood music):
                          https://www.youtube.com/watch?v=T22pJMIAIRQ
                          [/QUOTE]

                          Interesting. Apparently I did not keep the attempt that I made so am not sure of the wattage. What I do remember though was that the DC 51 controller functioned at least as good, if not better and was already contained in a nice useable box. I'll keep looking and see if I can find the one that I made.

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                          • #88

                            Re: Variable speed

                            Thought some might be interested in my recent conversion of Rockwell Beaver 700 drill press to treadmill DC variable speed. Motivation was primarily to slow down for drilling steel. We'll see how it works over long run.

                            There is a couple of videos in this Flickr album:
                            https://www.flickr.com/photos/767624...57713202050737
                            tool fan and iamtooler like this.

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