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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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  • 0

    I am not aware of any limit defined in law or by IET regulations as to the maximum size of motor to be started direct on line. There is a rather vague requirement no to use "equipment that would interfere with the  due supply of electricity to others" that could include voltage drop due to starting currents, but no actual limit is given.

    Large motors should be soft started, but the actual size that constitutes "large" is not stated.

    A lot depends on the size of the supply, the frequency of starting, and the nature of the other loads. Four starts an hour is a lot worse than once a month.

    DOL starting on a 60 amp service will be far more obtrusive than the same motor on a 400 amp supply.

    DOL starting on a service "from passing main" is more likely to be a problem than on a dedicated service to a pumping station.

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  • 0

    I am not aware of any limit defined in law or by IET regulations as to the maximum size of motor to be started direct on line. There is a rather vague requirement no to use "equipment that would interfere with the  due supply of electricity to others" that could include voltage drop due to starting currents, but no actual limit is given.

    Large motors should be soft started, but the actual size that constitutes "large" is not stated.

    A lot depends on the size of the supply, the frequency of starting, and the nature of the other loads. Four starts an hour is a lot worse than once a month.

    DOL starting on a 60 amp service will be far more obtrusive than the same motor on a 400 amp supply.

    DOL starting on a service "from passing main" is more likely to be a problem than on a dedicated service to a pumping station.

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