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One machine supplier shipped us a motor with a power in its plate so different (800 kW) to the one indicated in the electrical diagrams (500 kW). We calculate the wires using the diagrams information of power, but in the motor plate the power printed is higher.

Did this supplier break some regulation?

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  • \$\begingroup\$ I suggest you ask a qualified tradesman who has knowledge of your local electrical code. \$\endgroup\$ – AlmostDone May 17 '18 at 14:36
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Did this supplier break some regulation?

Regulations are not universal worldwide, they vary from one country to another. It makes little sense to ask about regulations without mentioning where in the world you are located. Where I am, in the USA, I know of no regulation that would be broken in this situation, but I think that you would have a good argument that the seller misrepresented the product. If the motor is larger than it needs to be, there may be some advantage as mentioned by Dan Mills, but I believe that the disadvantages far outweigh the advantages. The power factor is likely to be more than slightly lower and the efficiency is likely to be enough lower to make a significant difference in wasted energy and energy cost. Local regulations probably require the wiring to be suitable for the rated power of the motor even if it will not be used at full power. You probably already know that cost. You should certainly have a serious discussion with the supplier of the machine. You should not accept this machine unless the manufacturer can convince you that it is in your best interest to do so.

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No, using an 800 kW motor on a machine that only loads it to 500 kW will make things run very much cooler which is usually a good thing (The motor will (steady state) only draw power equal to the mechanical load plus losses), but the over sized motor will:

  • Run cooler, and therefore last longer all other things being equal.
  • Potentially have higher starting torque (and starting current), you will need to allow for this in your motor starting arrangements, should not be a big deal if running from a VFD, might be if doing something old school.
  • Have a lower winding resistance, hence a potentially higher locked rotor current, again an issue for protection design more then anything else.
  • Possibly have a slightly worse power factor when run at 500 kW, so you might have to deal with a little higher kVA number when sizing wiring and controls.

However, when dealing with the thick end one of a MVA of power, probably at 11 kV or so I would not be listening to some random person on the internet, seek professional advice, this stuff takes no prisoners.

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