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Delicacy of VFD's and three phase motors

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  • Delicacy of VFD's and three phase motors

    VFD's and motors have been occasional topics here.

    It was asked elsewhere how sensitive VFD's and induction motors were to frequent starts and reversals. The short answer is not at all. In fact they're practically bullet proof in the absense of deliberate and malicious abuse.

    Once apon a time when motor manufacturers offered real engineering data in their catalogs they included among other things curves representing the number of times their motors could be plug reversed under different conditions of connected inertia. I recall that a 5 HP 1750 RPM three phase induction motor with no inertia but that of its own armature could be plug reversed across the line 30 times a minute and still stay within its thermal ratings.

    "Plug reversing" means applying reverse power to a motor already running at full forward RPM to full RPM in reverse. A "plug stop" means reversing the motor only until it comes to a stop at which time the power is automatically (or manually) shut off.

    A standard item of industrial electrical apparatus is a gadget called a "zero speed switch". EE's in their lab sessions learn to wire reversing starters to zero speed switches as a routine part of their training. The switch has a rotating part that attaches to the back end of the motor shaft. A pair of contacts opens when the motor crosses zero RPM. A zero speed switch wired in an across the line starter circuit will automatically plug the motor to a stop when the stop button is depressed. Great big induction motors are routinely plug stopped and reversed.

    While plugging a motor results in a heat gain to the windings, they are designed for it. There's very little thermal break between a third of the winding's volume and the mass of the iron stator which acts as a big heat sink and radiator. The two thirds that form the exposed portion of the windings is, after all, made of copper, a metal that's a superb conductor of heat as well as electricity. Between the internal fan and the heat sink of the stator, an induction motor is thermally as well as electrically robust. There's not a thing delicate about a three phase motor.

    My discussion is intended to inform and assure people concerned about their VFD/motor spindle drives. So long as the heat input from momentary overloads can escape by convection, radiation, or ventillation the motor will not be harmed provided it doesn't exceed the thermal characteristics of its insulation due to excessive mechanical and, hence, electrical overload - and that is the job of the VFD.

    A VFD is reversed either by the pushbuttons on the digital input keypad on the face of the drive or by wiring it to a start/stop/reverse control station according to the diagram in the manual. No intervening switches or contacts should be present between the VFD and the motor. Starting and stopping and reversing is all the taks of the VFD's control algorithms.

    When you set the accell and decell you're establishing times to achieve rated RPM. You can set the accell to 1 second and the decel to 10 seconds or vice versa or any other figure that works. The time to a given setpoint RPM is proportional to the setpoint divided by the rated RPM. Accel time to 875 RPM is half the time to 1750.

    The VFD dynamic brake extracts energy from the rotating armature by induction and dumps it back into the DC buss via the output transistors. If you set the decel too short, the energy goes into over-charging the capacitor. The dynamic brake circuit is designed to monitor the DC buss voltage and if it exceeds limits dumps excess charge into the dynamic brake resistor. If no resistor is present, the buss overcharges, the safety circuitry trips, and the VFD goes into over-voltage shutdown.

    There's another form of dynamic brake in the VFD called DC injection. This can be used alone or in combination with the electronic braking. DC injection is not as sophisticated or flexible and its possible to fry the armature (not the stator) if very large over-running loads are present.

    VFD's are designed with elaborate thermal protection algorithms and sophisticated current limiting. If the parameters are correctly set to reflect the motor's nameplate ratings it's utterly impossible to fry the connected motor or the VFD by manipulating the control station or physically overloading the motor - ahem, provided the motor is properly ventillated at low RPM.

    If you overload the motor, the VFD automatically reduces the frequency and slows the motor down - to zero if need be. If the overload continues too long the VFD will shut down.

    Furthermore, VFD's do not plug reverse a motor if the accel and decel parameter is set to zero. They accel and decel well within that part of the motor's slip where the frequency leads or lags the armature by an amount that produces the maxiumum torque. A stalled induction motor develops about half the max overload torque.

    Max overload torque occurs at about 150% - 200% of nameplate full load Amps instead of the 7 times inrush Amps you get when starting a motor across the line. A VFD 150% FLA start has more intial torque and draws less start current than an across the line start.

    If you want crisp starts and stops for manual machine tool operation, set the accel parameter for 0.7 seconds and the decel to 3.0. If you don't have a dynamic brake resistor installed you might not be able to stop that quick without going into over-voltage trip. Also remember to consider conected inertia. A big chuck spinning at 1000 RPM stores a lot of energy.

    I've mentioned "dynamic brake resistor" several times. For the 2 to 3 HP machinery most of us work with it's just a 50 Watt resistor. The factory part might cost a $100 - $200. It's a sucker part - like buying ball bearings and standard fasteners under Sears part numbers. A common wire wound resistor of the same resistance and wattage will do exactly the same job for maybe $10 - $15. You choose.

    Most all VFD are made by a single manufacturer, Yaskawa, made to suit the many resellers' concepts of their markets. There's a very good chance that whatever VFD you buy it has close similarities to comparable drives offered by Yaskawa. Your VFD manual is sure to offer a factory part number for the dynamic brake resistor but maybe no resistance or wattage value. I have a book that covers a wide range of VFD accessory data and among the tables are resistance and wattage values for DB resistors.

    Gimme an email with your drive's nameplate electrical specifications and I'll look up a close cousin to the factory dynamic brake resistor; one you can buy at the local major electronics supply house.

    You guys with VFD's with threaded spindle lathes consider the decel issue carefully. VFD's can stop a motor with a helluva lurch. You don't want to spin off the chuck. That makes nasty marks on the ways and gives the operator sore toes.

    My suggestion to you VFD owners is to run the sox of the drive and motor. If you set it up right, you can't possibly hurt it. If you run into problems of nuisance shut downs, tweak the appropiate parameter.

    [This message has been edited by Forrest Addy (edited 02-09-2003).]

  • #2
    Forrest -

    Thank you for this addition to my technical library!



    • #3
      Forrest - I too will save this post for my technical library.

      I have been intrigued by the few posts about VFD's, and have been wondering if the the technology might be useful for a home shop like mine. A question or two or three.

      1. Small lathes, grinders and other small, single phase motors. useful to control speed? Torque OK?

      2. If VFD is useful for a small lathe, would you recommend that the belt be positioned for the lathe's highest speed, then use the VFD to set a lower speed?

      3. My mill is three phase, and I use a static converter to run off single phase. Again, with the VFD, would I set the belts for the highest speed, then use the VFD to set final speed? Could it also be used to expand the table power feed range? Slower not faster.



      • #4
        Fred, VFD's drive three phase motors only. Single phase motors might be made to run in a limited way but it's simply not worth it.

        A VFD/three phase motor will run a drill press and give you variable speeds down to sero.

        That said, remember your high school physics. Induction motors are constant torque machines. Halve the RPM and you halve the HP. You still need the step pulley, gearing what have you for "leverage" so on jobs requireing significant power you can match the motor's power potential to the load.

        I have a 2 HP motor and VFD on my drill press. With it I can tap 1/2-13 holes (using spiral point taps) all day and the VFD's control arramgements make it handy to add a foot switch and reverse. My 1948 Craftsman drill press is now a tapping machine.

        [This message has been edited by Forrest Addy (edited 02-09-2003).]


        • #5
          Thanks Forrest!

          Points taken. Still wish I didn't have fool with belts. Not really lazy, I always seem to pinch my fingers.


          Still might look at VFD for my mill, though.


          • #6
            Yeah.. they are great, but don't plug reverse unless you have to. I used to have to write logic into the conveyor belts programs to stop plugging. The instant reversal with a forward load would explode the gearbox into shrapnel.
            I love my VFD on my bridgeport. Thanks for your post.


            • #7
              Forrest; we were recently cautioned about referring to homemade lashups as real machines. To call a drill press that has been fitted with a variable speed motor and a reversing switch a tapping machine is very misleading.
              A tapping machine is a very rigidly built piece of equipment capable of tapping a hole to a precise depth, and then instantly reversing, usually at twice tapping speed to increase production and eliminate tap breakage. These machines usually incorporate some type of presettable slip clutch to further protect against tap breakage.
              When the term tapping machine is used, this is the type of machine being referred to.
              I don't mean to be a party pooper, but I am sure that you will understand where I am coming from.
              By the way, I do not understand why a person would attempt to make their own tapping machine when Procunier Lead Screw machines are available for under $10,000.
              Jim H.


              • #8
                Touche, point to JC. Now boys let play friendly.
                JHC Dayton, OH


                • #9
                  Arrrgh! I hate it when my own words come back to haunt me.

                  Dammit!! JC, the spurs didn't have to be so sharp and you didn't have to sink 'em so deep didja?

                  Sheesh! :-)


                  Gypsie: did the gearboxes that the conveyors you refer to explode because they happen to be high ratio worm drives? The kind that are irreversible to overhauling loads so that a loaded belt's inertia was sufficient to smash it up? The motor's instant reversal was equivalent to the proverbial monkey wrench.

                  Locally we had a similar problem. In the 60's when the Evergreen Point floating bridge was tested the 10 HP draw span motor opened the 17,000 ton draw section slick as a whistle. When it came time to stop the motor a motor brake provided to decel the span kicked in. 17,000 tons moving at 10 feet a minute Vs a two stage worm reducer rapidly decellerating to zero RPM. No cntest: busted the second reduction to flinders and the drawspan went on to bust up some steel structure.

                  I saw the video and the big beefy cast iron second reduction worm drive case looked like a busted bottle. The installing mechanic they interviewed was full of vindicated outrage: "I told them guys...".

                  In irreversible transmission, high inertia loads have to be driven to decelleration. If the inertia forces over run and the worm drive locks up something has to give. The Evergreen Point lesson of irresistable forces and immovable objects sunk deep into my then young machinist's experience.

                  But there's a story... nah! I babble too much

                  [This message has been edited by Forrest Addy (edited 02-10-2003).]


                  • #10
                    Remind me to never play hardball with JC!
                    Lynn (Huntsville, AL)


                    • #11
                      (dup deleted)

                      [This message has been edited by lynnl (edited 02-10-2003).]
                      Lynn (Huntsville, AL)


                      • #12

                        I just bought a Sheldon 13x36 lathe and it was sold as single phase but turned out to be 3 phase ( would that be a 300% errror factor LOL) so what I was thinking is that I could run the 2 HP 3phase motor on it with one of the VFD instead of a static phase convertor. Do the VFD have a dial that you turn to increase speed or do you have to mess with a lot of buttons? Could you also tell which model or maker would be the best bang for the buck? Also are these units big and where does one mount them? Sorry one more ? do you turn them on with a switch on the unit or do you use the toggle switch on the lathe wired to the VFD?

                        Thanks Dan


                        • #13

                          Wade through this.


                          And look at this


                          Go down the left menu and click on V7 (GPD 315) This is a general purpose drive well suited for home machine tool spindle drives



                          Look at the ads on the home page. If you click on "Inverter drives" in the left menu it will take you to a list of used, clearanced, and discontinured drives. Call them and talk with them.

                          Email me and I'll send you my phone number.