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
  • This is as much to do with the supply capacity as it is to do with the motor.  In principle, the largest motor can be started DOL, but in the extreme case you have to phone up the national grid in advance of doing so.

    (I'm only half joking, some of the test sites for high energy physics have big motor generator flywheels, that they spin up to provide some of the dip when it is time to fire the pulse. and such things are indeed only operated at pre- agreed times such that generation is not going to be over-stressed.)

    I suspect that does not apply in your case. The varying house rules then are set by the appetite to suffer the lights going dim and pulling down the HV feed to an unacceptable degree in cases where the supply to site is at 11 or 33kV. Or the substation transformer regulation /droop.

    If the customer knows that some of their sites have a rather droopy supply, they may require a slow start option. But there is no harm in asking.

    Mike.

Reply
  • This is as much to do with the supply capacity as it is to do with the motor.  In principle, the largest motor can be started DOL, but in the extreme case you have to phone up the national grid in advance of doing so.

    (I'm only half joking, some of the test sites for high energy physics have big motor generator flywheels, that they spin up to provide some of the dip when it is time to fire the pulse. and such things are indeed only operated at pre- agreed times such that generation is not going to be over-stressed.)

    I suspect that does not apply in your case. The varying house rules then are set by the appetite to suffer the lights going dim and pulling down the HV feed to an unacceptable degree in cases where the supply to site is at 11 or 33kV. Or the substation transformer regulation /droop.

    If the customer knows that some of their sites have a rather droopy supply, they may require a slow start option. But there is no harm in asking.

    Mike.

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