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Thread: Utility specs large transformers for motors

  1. #31
    Join Date
    Dec 2007
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    Quote Originally Posted by electrofelon View Post
    Application was put in around December I think. They asked a few questions and told me to go ahead and call them when it was inspected. I thought it was a bit lax for a commercial service, bit figured since it was a small service for two machines maybe it was just easy. Also I hadn't worked with this POCO before. As I stated earlier in the thread, they seemed to size the transformers to the LRA, which I guess kinda makes sense from a "power quality" standpoint, but dosent seem necessary.
    Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

    Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

    If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.
    I live for today, I'm just a day behind.

  2. #32
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    Jun 2004
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    Cherry Valley NY, Seattle, WA
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    Quote Originally Posted by kwired View Post
    Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

    Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

    If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.
    On the utility design plans, there was a field for "flicker" so it seems they take that into account. Just seems a little ridiculous.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #33
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    Quote Originally Posted by electrofelon View Post
    On the utility design plans, there was a field for "flicker" so it seems they take that into account. Just seems a little ridiculous.
    Is that a field for your/owners design decision? If only load is two motors, I wouldn't care about this value.

    If I had large office building and was concerned about flicker when the air conditioning starts up - then it gets more attention.
    I live for today, I'm just a day behind.

  4. #34
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    Jun 2004
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    Cherry Valley NY, Seattle, WA
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    Quote Originally Posted by kwired View Post
    Is that a field for your/owners design decision? If only load is two motors, I wouldn't care about this value.

    If I had large office building and was concerned about flicker when the air conditioning starts up - then it gets more attention.
    No it was something the utility computed and I was never asked for any input.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  5. #35
    Join Date
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    Resting under the Major Oak UK
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    Quote Originally Posted by kwired View Post
    Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

    Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

    If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.
    I’ve had experience of that with upgrading the 550V supply to a plant.

    We changed two 500kVA transformers for two 1000kVA transformers. The 500kVA transformers had a choking effect on the DOL (Direct On Line) high inertia loads. The lights did dim during a plant start up. The larger transformers didn’t give that buffer. It caused problems elsewhere in the works 11kV system due to phase distortion to the point that MBFU lamps were exploding on a remote plant.
    The reports of my death are greatly exaggerated.

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