1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Motor Overload Setting

Discussion in 'Electrics UK' started by SUNRAY, 8 Jun 2021.

  1. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    Just had a colleague phone from Cambridge, asking advice about troublesome tripping for a vent axia fan. Been like it for a couple of years...

    "Motor plate states 400V 3ph 0.37KW and the overload is..."

    I stopped him before he gave details of overload and did a quick guesstimation in the head, I said the O/L should be set to about 1A.

    "Oh" he says "I calculated 0.54amps and the overload range is 0.4 to 0.63. What does FLC 0.94Amps on the plate mean?"

    Sometimes I really dispair. What is happening with electricians these days?

    Edit: this now shows in 'related threads' below the post:
    https://www.diynot.com/diy/threads/working-out-the-overload-on-this-motor-fan-unit.526239/
    I hope it's not the same one.:LOL:
     
    Last edited: 8 Jun 2021
  2. Sponsored Links
  3. bernardgreen

    bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    25,565
    Thanks Received:
    2,550
    Location:
    Bedfordshire
    Country:
    United Kingdom
    Many seem to have lost the spark.......:mrgreen:
     
    • Like Like x 1
  4. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    Certainly don't seem to be enough live wires these days...;)
     
  5. bernardgreen

    bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    25,565
    Thanks Received:
    2,550
    Location:
    Bedfordshire
    Country:
    United Kingdom
    I am taking up a neutral position. :cool:
     
  6. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    trouble is all the down to earth old timers are gradually retiring.:(
     
  7. fixitflav

    fixitflav

    Joined:
    20 Mar 2018
    Messages:
    1,168
    Thanks Received:
    135
    Location:
    Stoke-on-Trent
    Country:
    United Kingdom
    Maybe in his calculation he divided by 3 instead of sq root 3.
     
  8. Sponsored Links
  9. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    I do it a much easier way for ball park figures.

    Total KW rating divided by 350.
     
  10. fixitflav

    fixitflav

    Joined:
    20 Mar 2018
    Messages:
    1,168
    Thanks Received:
    135
    Location:
    Stoke-on-Trent
    Country:
    United Kingdom
    You mean divide by 0.35 for power in kW.

    That gives 1.06 amp, OK if all you want is a ballpark figure. And for small motors with low PF and efficiency. But it doesn't give a clue about how 0.54 amp was arrived at. I just noticed that 0.94/0.54 = 1.74, quite close to root 3, so thought that could be the explanation.
     
  11. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    power = 0.37KW or 370W

    370/350 = 1.06A as you correctly calculate so my 'ball park figure is about 13% high for 0.94A.

    Now referring to the other thread I linked to there is mention of 'FLC +10%'(1.034A) and 'no more than 115-125% FLC'(1.08-1.175A)

    My own experience is control panels where multiple similar devices are stacked in close proximiity side by side and the heating effect has to be considered and 10% is frequently applied. So in this particular instance my ball park figure is not far out. Certainly when entering a control panel where tripping is occurring the first thing is to look for blatent incorrect settings and my method is a reasonable starting point.

    In the case of my OP I was asked for advice and quickly came up with a figure quite close to motor plate FLC and even quicker was able to advise the overload device was incorrect for the motor.

    I accept my method is not infallible, motors tend to be more efficient as they get bigger but even being 30-40% out is not horrendous, as long as one understands the process.


    When calculating star connected loads, particularly resistive like heaters & lights, I cheat... 370W/230V = 1.61 total/3 = 0.5367A I wonder if...

    EDIT: All of these calculations have ben in head so my apologies if there are any unintended errors.
     
  12. fixitflav

    fixitflav

    Joined:
    20 Mar 2018
    Messages:
    1,168
    Thanks Received:
    135
    Location:
    Stoke-on-Trent
    Country:
    United Kingdom
    OK no problem with that
     
    • Thanks Thanks x 1
  13. SUNRAY

    SUNRAY

    Joined:
    28 Jul 2009
    Messages:
    5,277
    Thanks Received:
    392
    Location:
    Kent
    Country:
    United Kingdom
    Ah, reading again I see your comment.

    Yes divide power in watts by 350 OR divide power in KW by 0.35:notworthy:
     
  14. fixitflav

    fixitflav

    Joined:
    20 Mar 2018
    Messages:
    1,168
    Thanks Received:
    135
    Location:
    Stoke-on-Trent
    Country:
    United Kingdom
    :)
     
Loading...

Share This Page