Maximum size for a UK Direct Online Motor starter

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.

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

    Indeed - I think it's often the DNO that impose specific limits - and those can vary depending on your particular supply situation, The last time I had anything to do with asking about a new (domestic) supply (quite a few years ago now) they were very interested in "disturbing" loads and ended up specifying various maximums for things like heat pumps and other motors.

        - Andy,

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

    Indeed - I think it's often the DNO that impose specific limits - and those can vary depending on your particular supply situation, The last time I had anything to do with asking about a new (domestic) supply (quite a few years ago now) they were very interested in "disturbing" loads and ended up specifying various maximums for things like heat pumps and other motors.

        - Andy,

Children
  • My experience entirely when I had my 3-phase supply put in. The thought of large motors made their man's ears prick up until I pointed out that a 3-phase motor draws 1/3 of the current of a single phase one. In the end, the conclusion seemed to be that on a domestic single phase supply, they don't really want more than 3 kW motors.

    If you scale up to 100 A per phase in a light industrial unit, the OP's 7.5 kW seems perfectly reasonable.