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Maximum size for a UK Direct Online Motor starter

Lee O

Good afternoon,

Is there any practical legislation governing the starting methodology for motor operation in the UK for three phase motors?

I've seen various recommendations or best practice for applying soft starting methodology for motors over certain sizes but I can't find any solid reasoning or evidence for such requirements.

For instance, I've got a 22kW motor and by "best practice" methodology I would generally use a method of soft starting to operate the motor such as Star/Delta or Electronic Soft Start to reduce the inrush where I don't necessarily require speed control.

However a 7.5kW motor I would generally install on DOL unless it requires speed control.

I've come across a customers specification stating that any motor above 5.5kW should use soft starting methodology. I'd like to challenge this as its an expensive solution for a panel full of star/delta starters vs DOL.

The motors are generally operating pumps.

Any guidance would be appreciated.

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    A practical "rule of thumb" is something like the motor inrush should not take the supply over its maximum rating, with any other loads operating. This means that something like 3kW on a 100A sp supply with minimal other loads (domestic), about 90A maximum inrush, or that 22kW motor on a 200A 3ph supply as the only load. A star delta starter approximately halves the inrush, but a VSD is the kindest way to treat the motor and load nicely, as well as limit the supply voltage dip. DNOs would not like heat pumps but that is another story yet to raise its ugly head!

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  • 0

    A practical "rule of thumb" is something like the motor inrush should not take the supply over its maximum rating, with any other loads operating. This means that something like 3kW on a 100A sp supply with minimal other loads (domestic), about 90A maximum inrush, or that 22kW motor on a 200A 3ph supply as the only load. A star delta starter approximately halves the inrush, but a VSD is the kindest way to treat the motor and load nicely, as well as limit the supply voltage dip. DNOs would not like heat pumps but that is another story yet to raise its ugly head!

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