Motor Start

Voltage=400v, motor load=30 kw, when I use a dol start the section of cable=95mm2, add a differential protection the section will be 35mm2, why the section reduced when I add a differential protection?

Because of this 400mtr run of cable using 35mm sq, their must be a high VD caused at motor start up. which will also always be constant on full load...You will need at least copper single cores of 1X95mm or 2x70mm with an earth return of 70mm and 50mm respectively to handle the needful...however if Aluminum core is to be used may be high.

It appears like many contributors treat this application as if it is a distribution board that is to be supplied by a 400m cable connection. That must be the reason why so many calls for a neutral, or earth return core, as well as for the very limited allowed voltage drop during start. In many applications we directly recommend reducing the terminal voltage during start, for a number of reasons, so why should it all of a sudden become detrimental in this situation? The motor has no way of knowing what has caused the reduced starting voltage anyway! As long as there is sufficient voltage to get the drive to start within a reasonable time, the start VD should cause no headache.

One important issue that has only been briefly touched upon by a single contributor is the economy: It should be our professional endeavour to make the total cost of ownership as low as possible, so it would be reasonable to include energy cost into the equation. The heavier cable as proposed by many, will obviously reduce the running losses considerably as compared to the smallest cable that will satisfy the technical requirements.

If this motor is intended to run for only brief periods each day, or maybe for only a few days or months, it will be bad economy to choose the heavy cable just because you can. Likewise, if this installation is part of a quotation, you could very well miss out on a much larger contract, because most clients will only look at the total sum, before addressing any technical issues.

Personally, the Engineer should always choose the technical solution, than if there are more than one, he should do an economical study to choose the shipper solution, but certainly with the same functionalities, that means, we should always seek of the technical solution and not the shipper one.
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