No, not 'what so ever' but continuous rating.
The wording is about not exceeding a continuous power output, not peak/occasional/now and then.
I agree that if you stick the extra (above rated) volts in then the power out is much more than the nominal rating, but would such a motor maintain continuous operation under such conditions? If it would then its continuous rating is more than 250w and thus above the EN limit.
Then of course, what is the definition of continuous as in time duration, ambient temperature etc?
Plus which enforcement authority is going to be checking it anyway!
Just trying to tease out a better understanding here.......
You can't leave out the word "rated". Rated means what the design engineer decides and orders to be put on the label or listing. When the engineer does the rating, they have to take into account the durability of the motor as well as the temperature stability, so any self-respecting engineer would leave a sizeable safety margin. For buildings and structures, it used to be 300%, but that's probably come down now that computers can anayse better. If I were designing a motor, I'd leave at least 100% safety margin.
Incidentally, the standards used to mention maximum continuous output power, but they changed it to add the word "rated" because max continuous power cannot be defined without specifying the conditions. You have to remember that ebikes have to go up hills, sometimes with heavy people on them, and they have to work at different speeds, and efficiency changes with speed, so would you say maximum continuous power when going slowly up a hill, at a steady 12 mph, at 15.5 mph or all of them.
Also, there is no test for "maximum continous rated power". What would be the pass criteria?
All this is academic. Nobody needs to understand it. All you need is a 250w manufacturer's label on the motor.
I was recently involved in a case where the police seized an e-trike from a larg fleet owner after they did some ad hoc power measurement and came to the conclusion that the motor was more than 250w. I wrote a letter for the owner, which was given to his lawyer and then presented to the police. He got his bike back immediately with an apology. The motor was capable of driving a milk float, but it had a manufacturer's label that clearly stated that it was 250w and 25km/h max speed. In this case, the manufacturer considered durability to be more important than temperature when they did the rating, as the trike was designed for fleet owners commercial use rather than occasional/leisure use.