First, I'm sure this question is way too easy for this forum but I didn't find a better one. I'm having an argument on the following and we'd like to settle it officially.

On a website about Diesel-Electric cruise ships, we found the following:

The entire ship—not just its propulsion system—can function with only two of the four main generators on line. Each unit generates 11.5 MW at 6.6-kV alternating current. Two electric motors provide the ship's propulsion. The motors are synchronous and run to match the frequency of the supply current. Each motor consumes 14 MW with a voltage of 2.3 kV, so the system requires transformers, as the figure shows.

How is it possible for 2 * 11.5MW (23MW) to generate enough power to feed 2 * 14MW (28MW) motors? As far as my memory of physics goes, the P=UI law, in that case, only allow me to say that

23MW @ 6.6kV ==> 1.74kA and
23MW @ 2.3kV ==> 5kA

Is it possible to say from this that

23MW @ 6.6kV = 66MW @ 2.3kV

and if so, why?

  • $\begingroup$ You are right to question the write up. I would presume that if only 2 generators were on line, each motor would be limited to the available power from one generator. $\endgroup$ – Jon Custer Sep 17 at 14:18
  • $\begingroup$ As noted, the clue is in the power. Transformers do not create any power (in fact they use some in losses); the total power available from the generators cannot be increased without adding more generators. In this case, the motors would not be able to run at full power. $\endgroup$ – Peter Smith Sep 17 at 15:08

because the ship has not two but four generators in it, for a total combined output of 46MW, and the two motors consume 28MW.

Note that the 28MW number pertains to full speed; if the ship has only two of its four generators on line they can still operate the ship but not propel it at full speed.

Note also that the "house load" to run everything else on the ship is big enough that it has to be factored into the generator capacity for the ship. This is why the generators produce 46MW against a propulsion load of 28MW: the "extra" is what powers the rest of the ship's systems, plus a safety factor.

  • $\begingroup$ Thanks but the question was really about the part posted and how 23MW can give 28MW, not really on the ship specs per se. I was trying to know if according to the P=UI law, it is possible for Wattage to increase by playing with Voltage and/or Amperage. But I think that my original question is poorly written... $\endgroup$ – bkqc Sep 17 at 20:27

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