Single or double pole switching for inductive loads

Discussion in 'General Electronics Chat' started by ebeowulf17, Jun 24, 2015.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Here's my situation: we've got a 220VAC centrifugal switch motor (draws about 2 amps, maybe less, when running) which is currently switched with a double pole relay. In the USA, it's split phase power with 110VAC on each leg, and we have to switch both legs so there's no voltage in the motor when it's not running.

    One of our overseas customers wants us to rewire things such that we only switch one leg, because the other wire for them is a neutral and they say that maintaining the neutral connection prevents noise issues from the motor starting and stopping (issues which we think we've recently addressed with a choice of either SSRs or snubbers.)

    I've tried some tests and can't find any improvement in motor related electrical noise by keeping the neutral unswitched, but our customer is quite adamant that it is standard practice and will resolve the noise issues.

    Just to be clear, I'm talking about the neutral, not the ground. In either case we'll have a solid ground connection at all times. Is there any reason to think leaving the neutral connected would improve performance? I just don't see it.

    Even aside from this particular situation, is there a general rule for this? I know that neutrals don't need to be switched, but I never thought any harm could come from switching them.
     
  2. DickCappels

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    Aug 21, 2008
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    I don't know the electrical codes but I would say that their request sounds reasonable. Their AC source is completely different than the two 120 VAC legs you mentioned. Who is responsible for the installation meeting code -the customer or you?
     
  3. ebeowulf17

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    Aug 12, 2014
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    I don't know who bears the actual responsibility for that code compliance, but I know that codes aren't the issue here. We've already done the testing and gotten the extra certifications to sell there many years ago with the double pole switching.

    Really my question is whether, electrically, there's any reason why it's better to leave the neutral unswitched.

    And you're right, it's not really an unreasonable request, but it means more parts, more rewiring, and more customization of the machine on a per-order basis. We're trying to standardize as many components and subsystems as possible and this is a big step in the wrong direction. I know it sounds like a trivial change, but because of the assembly order, etc. this small change would prevent a lot of pre-building sub assemblies and slow us down.

    In the end we may just do what the customer wants even if there's no electrical reason to do so, and in spite of the wasted time and expense. Mostly I just want to know if there's a solid rationale for this request or if they're on a wild goose chase trying to fight electrical noise with the wrong approach.
     
  4. DickCappels

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    Aug 21, 2008
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    Doesn't make sense that leaving neutral connected improves unless in the original design there was a snubber that was wired such that it was disconnected when neutral was disconnected.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    Just wire a short across the neutral contacts for the customer.
    No other parts or changes required.
     
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  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It may even be possible to just double up the wires on one terminal, especially if the wires end with crimp on terminations.

    Just stick the in and the out on the same terminal of the relay.
     
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  7. MaxHeadRoom

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    Jul 18, 2013
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    Never ever heard of that before, and I got all my electrical training in that kind of system.
    The power inputs will have to be labeled as such as presently you may not show any distinction between them, also in N.A. a 240v system/equipment would have dual fusing, if going to a country with 240v L & N, only the live conductor should be fused.
    Max.
     
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  8. ebeowulf17

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    No, no snubbers in the design until pretty recently. In fact, pretty sure there have been no issues with noise on new machines since we started using the snubbers (should've been using them all along, I imagine.)
     
  9. ebeowulf17

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    Aug 12, 2014
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    True, probably simpler than I'm making it out to be. Even then it still means that we have to assemble things to order, or if we're pre-building electronics boxes, we have to maintain stock of two different box assemblies instead of one. We can certainly do it if needed, we'd just rather not if we don't have to.
     
  10. ebeowulf17

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    Aug 12, 2014
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    The labeling is ok - we're already labelled with Line and Neutral. Really our labeling is misleading for N.A. use, but well suited to the rest of the world.

    You bring up an interesting point with the fusing. Right now the motors aren't fused at all. Not sure why it was done that way to begin with and I have no idea how we've gotten away with it for so long (current machine design was done before I started working here.) I take it you don't want the neutral fused because, in the event of some sort of failure, you want the live wire to be shut down instead of breaking the return path and potentially leaving a live wire loose somewhere? Honestly never thought about that issue before, but it makes sense to me.

    With all the discussion of these noise issues and various ways of handling them, we've started brainstorming a major overhaul of the motor control systems. We'll definitely be sure to include fusing, and do the research to make sure we do it right. No doubt I'll be popping in here a lot with questions as our requirements and possible solutions start coming together.
     
  11. MaxHeadRoom

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    You normally only fuse the live conductor when a neutral is used, just as it is in N.A. when 120v L & N is used.
    If both conductors are live WRT to ground or neutral, then both are fused.
    You can however switch both N & L simultaneously if on the same operator, but not necessary.
    Max.
     
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  12. #12

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    Nov 30, 2010
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    "Like" is not enough to convey my concern. On a "one live wire" power configuration, fuse the Line, and don't fuse the Neutral or you will create safety issues, all of your own making. Don't shoot your self in the foot and don't leave defective parts of a machine hot when the repairman arrives. Blowing a fuse should leave the defective parts completely inert.
     
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  13. ebeowulf17

    Thread Starter Active Member

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    Thanks for all the insights! I'm glad that I wasn't crazy for thinking that the switched neutral had nothing to do with the electrical noise issues. The uncertainty around that idea was really bugging me, more than the actual question of whether we should make the change for them.

    We may end up doing what the customer wants anyway since it's harmless, aside from inconveniences for us. In case we do, you all pointed out a few ways to approach it that are better than I had first envisioned.

    Plus we've got a few more things to consider if we do embark upon a real redesign of any of this for the future.

    Thanks everyone!
     
  14. ebeowulf17

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    Thanks for providing emphasis where needed. I promise we'll do our homework and get this right as we make changes. I don't want to contribute to any unsafe situations. There's a lot of room for improvement on our machines in terms of electricity and electronics. I'm learning as fast as I can so that I can help us make the right decisions. I really appreciate all the great help here.
     
  15. crutschow

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    I have a question about fusing a 220 line when both are hot. If one fuse blows then it would seem there is still a shock hazard.
    Wouldn't it be better to have a two-pole breaker that opens both sides of the line?
     
  16. #12

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    1) Yes, that's how amateurs get killed.
    Any repairman that finds one leg of a 2 or 3 wire supply with a blown fuse proceeds with extreme caution (if he deserves to live). He knows something is wrong with a supply that is capable of killing him.
    2) Yes. That's a good reason why 2 pole breakers were invented, and probably why any Two Pole circuit breakers are required by Code to be mechanically latched together.
     
  17. ebeowulf17

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    Aug 12, 2014
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    I had wondered about that too. In fact, we have breakers on our machines for the heating elements, where most of our power goes. As of right now, no protection on our motors, but maybe that would be a better solution.
     
  18. MaxHeadRoom

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    If don't already have a copy, you might want to pick up a copy of NFPA79, Electrical Circuits for Industrial Machinery.
    Although a N.A. publication, it also pertains to other parts of the world to a great extent.
    Another is Grounding and Bonding by Eustace Soares, used by NEC and the Intl. Assoc of Electrical Inspectors.
    Double fusing in both live conductors is allowed for control circuitry under NEC.
    Max.
     
  19. #12

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    @MaxHeadRoom

    Double fusing in both live conductors is allowed for control circuitry under NEC.

    In my world, control circuits float. Either end (or the middle) of the circuit might get connected to real, live, machinery during a fault, like a pinched cable. This would seem to confirm the rule you stated. I only wonder if this was the logic used in making the rule. Not a mind reader? OK, Let's just say my practical experience confirms that this is a good rule. ;)
     
  20. MaxHeadRoom

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    Three phase fuse blocks are common on enclosure disconnects, the alternative is a NFB type (non fused breaker), both are common in industry and allowed under NEC .
    Max.
     
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