Discuss Neutral mixing on unusual circuit Milling Machine in the Industrial Electrician Talk area at ElectriciansForums.co.uk.

  1. Rupert Young

    Rupert Young New EF Member

    Location:
    Littlehampton
    Hello.

    Can anyone point me to what it might be that requires a neutral generator when using a digital 3-phase converter outputting 400V?

    If a machine has control equipment in its cabinet that wants single line and neutral supply at 230v, how come the supply neutral cannot be commandered for use with one of the synthetic (digitally produced) phases of the converter. I'm guessing it's something to do with unequalizing the 3 phases from the converter maybe causing harmful heating up of the unit. And I'm guessing that a neutral converter somehow bridges between these outputs to keep them somehow equalised.

    Anyone know any theory about these units? I can't phone Drives Direct as I'm not a customer of theirs and it would be unfair to ask them and buy from them.

    The mill is at the back of an A2 office space in a prime retail area, it is not supplied with 3P and so the few machines are due to be powered either 1ph or from VFDs and coverters.

    All the best
     
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  3. Andy-1960

    Andy-1960 Regular EF Member

    Location:
    Dudley
    I assume what you are talking about is a variable speed drive, these units are designed purely to vary the speed of an AC induction motor. This is done by varying the frequency and voltage delivered to the motor which is why you could not tap off it for other purposes.
     
  4. Rupert Young

    Rupert Young New EF Member

    Location:
    Littlehampton
    Thanks Andy 1960 for reply. But negative to your qu., this is not an inverter drive as such, I am aware of the scenario, I suppose I was attempting to frame a question which would elicit an explanation of how a neutral generator taps into the system. But thanks, I'm not really sure of the question I am asking that's been the problem. I am curious, I am not about to do anything not recommended by the manufacturer of the converter
     
  5. Andy-1960

    Andy-1960 Regular EF Member

    Location:
    Dudley
    Do you have more details of the converter? circuit diagram or a pic may help
     
  6. Lucien Nunes

    Lucien Nunes Mercury Arc Rectifier Trusted Advisor

    Location:
    London
    If I understand your question correctly, you are asking why the neutral of the single-phase supply cannot be used to make a 4-wire 3-phase supply from the converter's 3-wire 3-phase output. The neutral of a normal supply is only a neutral by virtue of being connected to the star-point of the substation transformer producing that supply, so that all the lines are at 230V with respect to the neutral. In the inverter, because of the way the circuit works, there is no such star point and therefore nothing that has the required voltage and phase relationship to the three output lines (as with any delta supply), hence it does not automatically provide a neutral on its output side. The single-phase supply neutral is connected to a different point on the input side of the inverter circuit, so it has completely the wrong relationship with the output lines to serve as their neutral. Thus, if the load requires a neutral, the additional 'neutral generator' is needed to derive one from the output lines alone.
     
    drzsta, Des 56, DAvid Prosser and 2 others like this.
  7. Rupert Young

    Rupert Young New EF Member

    Location:
    Littlehampton
    Lucien, thank you very much, that is obvious to me now! Obvious. Thank you I've been occasionally wondering about it for months now, so it's suprising how I overlooked the components of your answer myself. No big mystery, thanks for taking time to answer. R
     
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