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    Motor contactor drop out

    At what voltage and/or cycles does it take for motor starters to drop out? My concern is a fault in the distribution or transmission system.

    I don't want on of those "we had a power surge, it blew a fuse so no power"


    I suspect what happens in a lot of these scenarios is that voltage dips close to zero for a few cycles and things like motors and transformers simultaneously draw inrush tripping OCPDs.

    Is my thinking correct?

    #2
    Why would this situation be more likely to trip the OCPD than any normal power up.
    Bob

    Comment


      #3
      Originally posted by petersonra View Post
      Why would this situation be more likely to trip the OCPD than any normal power up.
      Normal power up does not involve the inrush of every motor in a building.

      Comment


        #4
        191023-1744 EDT

        mbrooke"

        Run an experiment on a sample starter. Use a Triac or SCR solid state switch to control the motor starter coil. The solid-state relay will turn off the excitation to the starter at the first current zero crossing after you remove input to the solid-state switch. Both Triacs and SCRs turn off at current zero crossings. This provides a very controlled turn off with no great effort.

        Run a reasonable amount of DC current thru one of the contactor contacts. 100 mA and a few volts source is probably ok. Silver-cadmium contacts may require more than a few volts to initially conduct. This DC current is too determine when the contacts separate. With a scope monitor the excitation to the solid-state switch, and sync the scope on turn off of this excitation. Monitor excitation voltage to the contactor coil, and the DC switched by the contactor. My guess is that an old AB #2 starter will open in less than 10 milliseconds, and probably more than 1 millisecond after the turn off current zero crossing to the contactor coil.

        .

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          #5
          It sounds like your overloads are doing what they are supposed to, trip at overcurrent of the motor they are protecting.
          Tom
          TBLO

          Comment


            #6
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            Originally posted by gar View Post
            191023-1744 EDT

            mbrooke"

            Run an experiment on a sample starter. Use a Triac or SCR solid state switch to control the motor starter coil. The solid-state relay will turn off the excitation to the starter at the first current zero crossing after you remove input to the solid-state switch. Both Triacs and SCRs turn off at current zero crossings. This provides a very controlled turn off with no great effort.

            Run a reasonable amount of DC current thru one of the contactor contacts. 100 mA and a few volts source is probably ok. Silver-cadmium contacts may require more than a few volts to initially conduct. This DC current is too determine when the contacts separate. With a scope monitor the excitation to the solid-state switch, and sync the scope on turn off of this excitation. Monitor excitation voltage to the contactor coil, and the DC switched by the contactor. My guess is that an old AB #2 starter will open in less than 10 milliseconds, and probably more than 1 millisecond after the turn off current zero crossing to the contactor coil.

            .
            What about AC?

            I guess I should also ask- is this a legitimate concern? Has anyone ever been to a call where OCPDs to other than receptacles have opened without explanation?


            I know it sounds silly, but I want to make sure that absolutely no event internal or external could cause the MCC feeder or main breaker to trip.


            I'm trying to acheive 100% selective coordination in a large building.

            Comment


              #7
              191023-1943 EDT

              mbrooke:

              The reason to use DC on the load contact to run the test is to determine how the contactor mechanically performs. What happens with AC or DC and a large inductive load connected thru the contact is a different issue.

              A small P&B KUP relay probably has a 1 millisecond opening response time.

              Opening time constant is a function of magnetizing force at the start time, mechanical inertia, mechanical force, and how far the armature has to move before the contacts open. All relays have a distance the mechanical components have to move before the contacts open. A spring and/or gravity provide the opening force.

              A dash pot can slowdown opening. With a DC coil and a shunt diode or capacitor you can slowdown opening from the time of removal of excitation voltage. A capacitor can make this very long.

              .

              Comment


                #8
                Originally posted by gar View Post
                191023-1943 EDT

                mbrooke:

                The reason to use DC on the load contact to run the test is to determine how the contactor mechanically performs. What happens with AC or DC and a large inductive load connected thru the contact is a different issue.

                A small P&B KUP relay probably has a 1 millisecond opening response time.

                Opening time constant is a function of magnetizing force at the start time, mechanical inertia, mechanical force, and how far the armature has to move before the contacts open. All relays have a distance the mechanical components have to move before the contacts open. A spring and/or gravity provide the opening force.

                A dash pot can slowdown opening. With a DC coil and a shunt diode or capacitor you can slowdown opening from the time of removal of excitation voltage. A capacitor can make this very long.

                .
                I guess drop out in those few cycles?

                This is what I'm hoping for- it will take someone to hit "start"

                Comment


                  #9
                  Originally posted by mbrooke View Post

                  I guess drop out in those few cycles?

                  This is what I'm hoping for- it will take someone to hit "start"
                  Think beyond that idea!
                  What about: "The equipment ran when the power came back and he was chewed by the equipment into pieces!"

                  Comment


                    #10
                    Complicated to answer but quick and dirty.....50-75% of nominal voltage and 3 milliseconds. Switching to DC coils and adding a cap makes it a little less complicated. But then we have to ask if it is better to not have the load see reduced voltage. Are folks willing to pay for phase loss and under voltage relays?

                    "Has anyone ever been to a call where OCPDs to other than receptacles have opened without explanation?" LOL! At least a few times a week/month, for the last 40+ years. Amount of lightning plays a role. VFD's are the worst offenders.

                    "I know it sounds silly, but I want to make sure that absolutely no event internal or external could cause the MCC feeder or main breaker to trip." Doesn't sound silly to me, just unobtainable (but highly admirable on your part). IMHO biggest cause is requirements on ground fault protection on mains, sometimes dependent on rather or not POCO has such protection, but nobody being willing to pay for same protection on downstream stuff. Electronic breakers with LSIG protection are great right up to the point where they won't reset at all with no prior warning (and they cost so much in the 100+kAIC variety that nobody wants them sitting on the shelf besides me).

                    Once upon a time all my gear (downstream gear too) had main 1A, main 1B and tie breaker 1C plus main 2A, main 2B, tie breaker 2C (draw-out type breakers) with analog panel meters, on both normal and emergency switchboards (in the same room along with multiple ATS's). Every breaker had ground fault modules with visible trip indicators. Made it real easy to see what the problem was and get out of trouble in a hurry. Present management has decided that isn't cost effective to maintain. Heck, some people won't even pay for an extra set of Kirk keys!

                    Best advice I can give is to stop listening to sales people peddling the latest and greatest and go visit the operating engineers and maintenance techs that know what lasts and makes their job easy. Leave the jacket in the car and buy them beers at the local joint where they can feel free to talk.

                    Comment


                      #11
                      Originally posted by topgone View Post

                      Think beyond that idea!
                      What about: "The equipment ran when the power came back and he was chewed by the equipment into pieces!"
                      Conveyors and the like yes- but AHU and pumps I'm feeling confident.

                      Comment


                        #12
                        Originally posted by mbrooke View Post
                        At what voltage and/or cycles does it take for motor starters to drop out? My concern is a fault in the distribution or transmission system.

                        I don't want on of those "we had a power surge, it blew a fuse so no power"

                        I suspect what happens in a lot of these scenarios is that voltage dips close to zero for a few cycles and things like motors and transformers simultaneously draw inrush tripping OCPDs.

                        Is my thinking correct?
                        This report titled "The Effect of Voltage Dips on Induction Motors" discusses some of the things that you mentioned:

                        https://www.google.com/url?sa=t&rct=...EEjVlfaGCVj5EV

                        Comment


                          #13
                          Originally posted by Russs57 View Post

                          "Has anyone ever been to a call where OCPDs to other than receptacles have opened without explanation?" LOL! At least a few times a week/month, for the last 40+ years. Amount of lightning plays a role. VFD's are the worst offenders.
                          Yes! Tell me more about this. Last thing I want is a critical facility tripping OCPDs over a squirrel.

                          Were the lights 120-277 volts? That will do it considering the pull more current.



                          "I know it sounds silly, but I want to make sure that absolutely no event internal or external could cause the MCC feeder or main breaker to trip." Doesn't sound silly to me, just unobtainable (but highly admirable on your part). IMHO biggest cause is requirements on ground fault protection on mains, sometimes dependent on rather or not POCO has such protection, but nobody being willing to pay for same protection on downstream stuff.

                          Unobtainable- well... I'm optimistic. I'm sure there is way. You can't tell me the CIA, Google, Goldman Sachs, Lockheed Martin, Shayne Mountain, Pentagon ect hasn't figured a way to stop everything outside of an actual fault in the zone of protection from clearing just that and nothing more.

                          What has me stuck is how loads behave on open phase(s), grounded phase(s), sags and zero volts for half to 4 cycles.


                          Electronic breakers with LSIG protection are great right up to the point where they won't reset at all with no prior warning (and they cost so much in the 100+kAIC variety that nobody wants them sitting on the shelf besides me).
                          Tell me more about this. I want to avoid breakers and use fuses on everything larger than 225 amps. Some say I'm crazy and maybe I am. But I feel like fuses will beat breakers in many regards. Correct me if I'm wrong.




                          Once upon a time all my gear (downstream gear too) had main 1A, main 1B and tie breaker 1C plus main 2A, main 2B, tie breaker 2C (draw-out type breakers) with analog panel meters, on both normal and emergency switchboards (in the same room along with multiple ATS's). Every breaker had ground fault modules with visible trip indicators. Made it real easy to see what the problem was and get out of trouble in a hurry. Present management has decided that isn't cost effective to maintain. Heck, some people won't even pay for an extra set of Kirk keys!

                          Do you really need ground fault protection?


                          Again, perhaps I am wrong- but in such a case would fused gear not be better? A lot of places have an 'install and forget' policy. Meaning once the gear is installed it does not see a human face for 30 years unless something goes wrong. Or simply only the bare minimum like testing/maintaining the gen and ATS and nothing more.

                          BTW, I don't do kirk keys. Rivet and label is good

                          Best advice I can give is to stop listening to sales people peddling the latest and greatest and go visit the operating engineers and maintenance techs that know what lasts and makes their job easy. Leave the jacket in the car and buy them beers at the local joint where they can feel free to talk.

                          I'll agree here. But I have to admit drinking Kool-aid from Shaw-mut, Eaton Bussman and Little Fuse has been one of my best experience ever.

                          Which leads me to ask- after 30 years of not opening, what have you seen fail more: Fused switches like bolted pressures or molded case circuit breakers? What about power circuit breakers?

                          Comment


                            #14
                            Originally posted by synchro View Post

                            This report titled "The Effect of Voltage Dips on Induction Motors" discusses some of the things that you mentioned:

                            https://www.google.com/url?sa=t&rct=...EEjVlfaGCVj5EV
                            Thats what I was looking for!

                            One thing people do not consider are 345kv transmission line faults which will cause an 85% of more dip in voltage for exactly 4 cycles. Also realistic is fault induced delayed voltage recovery which while a concern on orders of magnitude more for the ultility/ISO, still remains a specter haunting end user equipment and continuity of critical assets.

                            Comment


                              #15
                              I would think that, with a power drop that recovers quickly enough that contactors and controllers remain engaged, the so-controlled motors would still be rotating fast enough that there would be no appreciable inrush current.
                              Master Electrician
                              Electrical Contractor
                              Richmond, VA

                              Comment

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