Motor Overload Setting

[SUNRAY]

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.

 

[bernardgreen]

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

Many seem to have lost the spark.......

 

[SUNRAY]

Many seem to have lost the spark.......

Certainly don't seem to be enough live wires these days...

 

[bernardgreen]

Certainly don't seem to be enough live wires these days..

I am taking up a neutral position.

 

[SUNRAY]

I am taking up a neutral position.

trouble is all the down to earth old timers are gradually retiring.

 

[fixitflav]

"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?"

Maybe in his calculation he divided by 3 instead of sq root 3.

 

[SUNRAY]

Maybe in his calculation he divided by 3 instead of sq root 3.

I do it a much easier way for ball park figures.

Total KW rating divided by 350.

 

[fixitflav]

I do it a much easier way for ball park figures.

Total KW rating divided by 350.

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.

 

[SUNRAY]

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.

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.

 

[fixitflav]

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.

OK no problem with that

 

[SUNRAY]

You mean divide by 0.35 for power in kW.

Ah, reading again I see your comment.

Yes divide power in watts by 350 OR divide power in KW by 0.35

 

[fixitflav]

Ah, reading again I see your comment.

Yes divide power in watts by 350 OR divide power in KW by 0.35