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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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    There is then the further complexity that not all motors of a given wattage have the same inrush, the effect varies with the mechanical load,  not necessarily a much higher  peak inrush current, but certainly the duration of the lights dimming etc so the effect is more problematic.  Some start and stop under full load, but most machines that cut or crush or indeed in a workshop generally , spin up and only then the mechanical load comes on. Other things like compressors and certain designs of pump actually have a rather higher mechanical load at switch on than once spinning, and there the inrush energy is much higher,

    Then there is the no of starts and stops - a lift is more serious nuisance than  something that runs for hours at a time.

    Mike.

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    There is then the further complexity that not all motors of a given wattage have the same inrush, the effect varies with the mechanical load,  not necessarily a much higher  peak inrush current, but certainly the duration of the lights dimming etc so the effect is more problematic.  Some start and stop under full load, but most machines that cut or crush or indeed in a workshop generally , spin up and only then the mechanical load comes on. Other things like compressors and certain designs of pump actually have a rather higher mechanical load at switch on than once spinning, and there the inrush energy is much higher,

    Then there is the no of starts and stops - a lift is more serious nuisance than  something that runs for hours at a time.

    Mike.

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