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  • starting and stopping electric motor often?

    Would a 3 phase electric motor be damaged if it is stopped and started repeatedly at 5 second intervals? Or would it be better to use a clutch of some sort and let the motor run?
    Location: The Black Forest in Germany

    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    If the motor is starting from zero ever 5 seconds it will fry form the in rush current.

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    • #3
      Depends on the connected inertia. I recall in the Louis Allis deign guide that an unloaded general purpose three phase motor will take 20 plug reversals per minute. Add a load with inertia and the motor heat goes up increasing the time between allowable reversals.

      Motors are pretty tough. You can't really damage them except by overload or overheating.

      Single phase motors have a weak point wen it come to frequent starts aand that is the startng circuit. 5 seconds starts on an unloaded single phase motor might be asking for it. Betrer run some tests.

      Check with the motor mfgr's field service department. They have engineers for questions like that and their computers can give you charts and tables of motor performance under different operating regimes.
      Last edited by Forrest Addy; 02-16-2010, 01:15 PM.

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      • #4
        Clothes Washers do it.

        Originally posted by Black Forest
        Would a 3 phase electric motor be damaged if it is stopped and started repeatedly at 5 second intervals?
        I understand that is exactly what the "new" clothes washers do, except they reverse direction every 5 seconds also. And no more transmission for the spin cycle, just VFD the Hz up to about 1000 or so.

        You'd be amazed what an expensive enough VFD can do, including current and temperature limiting.

        So there's no inherent technological reason why not, but only the motor manufacturer can tell you what will overheat it, assuming even they know.

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        • #5
          Lightly loaded. No. You'll put significant wear on the switch/relay contacts though unless you're controlling it electronically.
          Southwest Utah

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          • #6
            Newbie VFD mistake #2

            Originally posted by vincemulhollon
            You'd be amazed what an expensive enough VFD can do, including current and temperature limiting.
            ooops I forgot to mention newbie VFD mistake #2. (#1 is obviously wiring it up to rotate backwards, course you don't need a VFD to do that)

            So, you're thinking, I know starting current is 50 amps and that'll smoke it after a minute or so. And I know it'll run "forever" at full 15 amps. Hmm. I know, I'll set my VFD up to current limit at 15 amps peak, and its all good, right?

            Ahh, but the little cooling fan on the shaft only keeps it from frying a 15 amps IF it rotates at full speed.... 15 amps at only 360 RPM will smoke it just as well as 50 amps at 0 RPM.

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            • #7
              Originally posted by jcurrell
              If the motor is starting from zero ever 5 seconds it will fry form the in rush current.
              Nope, Will explain later when i get back from the pub.

              .
              .

              Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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              • #8
                Large 3 phase motors, and probably smaller ones usually have a rated number of starts and a period over which the number of starts can happen.
                Very large motors can be down in the range of a single digit number of starts per hour, or the windings get to hot for the class of insulation and can burn out.

                So it depends on the actual motor and application. A clutch may be better for other reasons than just motor starts.

                Dave
                Just south of Sudspumpwater UK

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                • #9
                  my 2 cents:
                  if the motor has little or no inerta load connected, and is lightly loaded in normal operation, it could be fine..
                  if its a lathe or similar, and you are not using a VFD, you likey have a problem.
                  PS: if you use a clutch, the clutch will have to disipate quite a bit of power too!.

                  Now, if you use a VFD, your braking resistor will have to disipate quite a bit of power! Thankfuly thats as cheap as some heating elements. Also with a VFD the acceleration/deacceleration rate can be precisely controled to prevent overheating the motor or overstressing whats connected to the motor.
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                  • #10
                    Volmer saw sharpening machines, nice machines German but even though they bombed ower chippie I don't hold that against them.

                    They work on a pendulum operation, the pendulum carries the grinding head with small 3~ motor and diamond wheel, it swings by one tooth and grinds it and whilst it's off the tooth the blade indexes one tooth and it swings back. Simple operation provided can cams and ratchets and lasts for years.

                    Because the teeth are offset RH, Lh, RH etc the method is to grind off the tooth, by that I mean the wheel throws the burr off the edge as opposed to onto the edge, because they are staggered the motor has to reverse every swing and this is accomplished by micro switches at the limits of each swing.

                    These saws run day in, day out changing direction every 3 to 5 seconds, we used to have a row of them at the last place I worked and to my knowledge they all still had original motors fitted.

                    .
                    .

                    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                    • #11
                      There is a set up at a local factory were a 220-3phase motor runs a speed reducer that tips a plate about 120' one way stops and then run in the opp direct to 120' stops and so on...runs about 10 hours day.
                      Gearbox gave out, but motor stills runs...
                      please visit my webpage:
                      http://motorworks88.webs.com/

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                      • #12
                        Originally posted by vincemulhollon
                        I understand that is exactly what the "new" clothes washers do, except they reverse direction every 5 seconds also. And no more transmission for the spin cycle, just VFD the Hz up to about 1000 or so.
                        I thought most of the new washers used brushless PMDC motors for efficiency. A good power/control system can recover the inertial energy electrically while slowing the motor down prior to reversal, then use it to accelerate after reversal.

                        Andy

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                        • #13
                          Originally posted by Black_Moons
                          ....

                          Now, if you use a VFD, your braking resistor will have to disipate quite a bit of power! Thankfuly thats as cheap as some heating elements. Also with a VFD the acceleration/deacceleration rate can be precisely controled to prevent overheating the motor or overstressing whats connected to the motor.
                          A VFD has a duty cycle for braking separate from the braking resistor(s). No matter how large you make the resistors(s), the Power FET that drives the braking resistor becomes the limiting factor. The table in the VFD manuals show this, but don't really explain why.

                          For the OP question, a vfd driven motor is preferred, but 5 seconds ramp up and then down continously..? Hmmmm... What power does your motor need to be?
                          Last edited by lakeside53; 02-16-2010, 10:13 PM.

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                          • #14
                            Originally posted by BigJakeJ1s
                            I thought most of the new washers used brushless PMDC motors for efficiency. A good power/control system can recover the inertial energy electrically while slowing the motor down prior to reversal, then use it to accelerate after reversal.

                            Andy

                            My washer has a 3 phase motor and VFD... from 1994. The new model is basically the same.

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                            • #15
                              The answer is, as many have stated, depending on the motor, the load, and how the motor is stopped and started.

                              If plug-reversed every 5 seconds, could be a problem.

                              if it coasts to a stop and is re-started, might easily be fine.

                              if driven by a VFD, likely no problem, since the VFD will ramp it up and avoid starting surges.

                              the question is whether it ever gets much cooling if it runs less than 5 seconds and stops, since it will probably take some time to stop, and that comes out of the 5 seconds.

                              What is the actual "on time" percentage with power applied?

                              Does it start under load? And if so, how MUCH load?

                              Does it get up to full speed? What percentage of the on-time is it at full speed?

                              Originally posted by BigJakeJ1s
                              I thought most of the new washers used brushless PMDC motors for efficiency. A good power/control system can recover the inertial energy electrically while slowing the motor down prior to reversal, then use it to accelerate after reversal.

                              Andy
                              Lots of the brushless PMDC motors ARE 3 phase motors with what amounts to a VFD and hall sensors to time the VFD pulses.
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