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Motor wiring - will phases be unbalanced?

  1. Jul 20, 2011 #1
    Ok an easy post to settle a discussion. I have posted two rough diagrams below. One shows a 3 phase motor connected directly to the supply through a 10Amp MCB for protection and a second single phase load connected via an MCB again.

    Now in theory if I was remove the 2nd MCB and connect the single phase load to one of the motor feed phases after the MCB - what impact would this have?


    One person says that it will cause an imbalance on motor and potentially damage it. It may also damage the motor during a surge as the MCB would not protect it properly.

    Other theory is that the motor will remain in balance due to Kirchhoff law, so the motor will still draw exactly the same current and the remainder of the current will go to the second load if connected. If the current on any phase exceeds 10A the MCB will trip and protect both device.


    Please note this is purely a theory based question, not a regulations one as these may differ from place to place.

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  2. jcsd
  3. Jul 20, 2011 #2
    Electrically, MCBs are nothing more than a short when they are in operation. So, electrically, both diagrams are exactly the same. Only, the protection part is compromised in the second diagram.
    Overloading or shorts in the motor will not only shut down the motor but also the 'Load'. And the reverse is also true. If you can compromise that, nothing is wrong.
     
  4. Nov 2, 2011 #3
    Thanks.

    Would it be correct to say that connecting a load across one phase could cause a volt drop on that phase, which would unbalance the motor, however this would be true in both versions of the circuit shown?
     
  5. Nov 3, 2011 #4
    Assuming that your supply comes from a grid then your load isn't likely to pull down the voltage on one phase. However depending on the current and length of cable from your incoming JB and the point that you take the load off you might get a phase imbalance.
     
  6. Nov 5, 2011 #5
    Except for line resistance issues, the MCB is a short (unless is pops!), so having the extra load on one leg won't bother the motor one bit. The only issue (other than imbalanced line resistance) is that a smart MCB may think something is fishy about the different current on one leg and decide to pop, anyway.

    As always, never alter or access electrical equipment without professional understanding of all safety issues.
     
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