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  • Wiring a contactor for CNC mill?

    I am using a 24v coil contactor to supply power to my 220v AC drives to my axis motors. I was going to run the neutral also through the contactor along with the 220v L1 hot wire. Is that a good or bad idea?
    Location: The Black Forest in Germany

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

  • #2
    typically, 220 single phase is 2 line leads and a ground, is german single phase different? (your ground should not be switched, neutral can be) Jim

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    • #3
      Originally posted by jmm03 View Post
      typically, 220 single phase is 2 line leads and a ground, is german single phase different? (your ground should not be switched, neutral can be) Jim
      If I understand you yes it is different in Germany. We have three phase power coming into our farm. It consists of three 220v lines and a neutral. If we want just 220v we use just one hot line and the neutral and of course ground.
      Location: The Black Forest in Germany

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

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      • #4
        By NEC in the US the neutral is bonded with ground at the main service panel. At all other subpanels it is not grounded. At equipment I have seen it both ways, switched and not switched. I have noticed in a switched neutral there doesn't seem to be any wear on the contacts. Under some conditions a neutral can become energized and switching it may be for safety. As far as motor operation, I don't think it makes any difference. Consult your local codes for rules in your area. Is there a German blog where you could do a search on the subject? Trouble with a lot of codes they are not meaningful to the average person. Could be they are trying to protect their source of income or keep ignorant laymen from a dangerous situation.
        ​​​​​

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        • #5
          In most jurisdictions the neutral is not switched, particularly if it is a Earth bonded neutral.
          Also generally not necessary, check with the local service company.
          I never have in either UK or in N.A.
          Last edited by MaxHeadRoom; 05-29-2022, 03:23 PM.

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          • #6
            I think that if you have the option, switching both is nicer. The neutral carries some voltage and will often dimly light a power indicating screwdriver, leading to uncertainty of stuff being safe or not.
            I usually switch both.

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            • #7
              In N.A. there is NFPA79 for practices covering Standards for Industrial machinery, and they give examples of Ladder logic control for enclosure wiring which shows the neutral Unbroken throughout.
              Also there used to be a practice of placing the contactor O/L on the neutral side of the coil. This is now frowned on and is considered BAD practice.
              .Likewise, disconnects and fusing are always in the 'Live' conductors only.


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              • #8
                You can switch both, no problem. In Europe, ground and neutral are connected together by the power company. It must be done so that the ground fault circuit breaker can function correctly
                Helder Ferreira
                Setubal, Portugal

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                • #9
                  Why is this a question?

                  Yes, in equipment, the neutral can be opened along with the hot wire. If that is done, the switch will show a different symbol than in cases where it is not done. IIRC, the "O" of the typical "O" and "I" on/off indicator can only be used if the neutral IS opened by the switch. It indicates a complete disconnection.

                  In the US likewise, some does, some does not.

                  In fixed wiring, what the NEC refers to as a branch circuit, the neutral should never be opened, as various sorts of hazard can exist if it is opened. That is, for instance, why a corner-grounded branch circuit must have the grounded conductor (neutral) fuse replaced by a solid jumper.

                  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.

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                  • #10
                    Section 79 of the NFPA79 states that each disconnecting means shall only disconnect all ungrounded conductors of a single supply circuit..

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                    • #11
                      Originally posted by MaxHeadRoom View Post
                      Section 79 of the NFPA79 states that each disconnecting means shall only disconnect all ungrounded conductors of a single supply circuit..
                      it is not at all clear that a legal "disconnecting means" is being discussed. I say it is NOT.

                      A "disconnecting means" is a switch which is normally not on the machine, but may be on it, and which is intended to render the entire electrical system of the machine inoperative and unpowered, i.e. a "safe" condition in which it can be worked on. It is only necessary in the case of a machine which is hard-wired to the branch circuit, not connected by a plug. (A plug is also recognized as a "disconnecting means", and obviously disconnects all wires including ground).

                      A power contactor which simply stops and starts the machine is NOT a "disconnecting means", it has a completely different purpose, and the two are not equivalent. That is what the OP is discussing. A power switch would be similar, and also is not a "disconnecting means".


                      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.

                      Comment


                      • #12
                        In Europe the only time neutral is grounded is at the power company side or at the main switchboard before the GFCB so, it is not part of the ground protection. This is done by the yellow/green wire that's not connected to the contactor. If you connect the neutral to ground at any part of the circuit, the Ground Fault Circuit Breaker will trip.

                        On regular domestic/households we are supplied energy by the power company with either a single phase 230V supply and the power limiting circuit breaker from the company is a 2 pole which limits to a maximum of 45A or we can be supplied 3 phase and neutral with a 4 pole circuit breaker with a maximum of 30A per phase. Small businesses can be supplied with a maximum of 60A per phase with a normal low voltage supply. After that, you need a medium voltage transformer, 3 wire, 3 phase input and 4 wire 3 phase plus neutral output. Around towns the medium voltage is 15Kv and in industrial areas, 30Kv. The very high voltage power lines are for long distance runs.
                        Helder Ferreira
                        Setubal, Portugal

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                        • #13
                          If I get it right it’s just using the contractor terminals as a double pole switch, they are isolated so it wouldn’t matter ( just like the shower pull switch I wired today, and the cooker for that matter, both live L1 and N make or break same time, I read the regs as it’s a requirement in U.K. all your up to is controlling the contacts with the 24v coil.
                          mark

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                          • #14
                            Short answer, it is probably not necessary but it probably will do no harm.

                            Do you know if the neutral is even used? If so, describe how.



                            Originally posted by Black Forest View Post
                            I am using a 24v coil contactor to supply power to my 220v AC drives to my axis motors. I was going to run the neutral also through the contactor along with the 220v L1 hot wire. Is that a good or bad idea?
                            Paul A.
                            SE Texas

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

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                            • #15
                              Originally posted by Paul Alciatore View Post
                              Short answer, it is probably not necessary but it probably will do no harm.

                              Do you know if the neutral is even used? If so, describe how.




                              Unless there is a transformer, to get the 230v, the neutral must be used. Our 3 phase is 400v
                              Helder Ferreira
                              Setubal, Portugal

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