Setting the overload on a direct online starter

[Adam_92]

Hi I’m sitting my AM2 next month and have found out that you need to set an overload on a direct online starter motor circuit.

I have tried asking my supervisor and assessor,tutor at college not one of them have given my a straight answer..

Is this the formula?

Its 3 phase so formula is:
KW/3/(U/√3)/PF

So say a 1kw motor

So 1000/1.73/(132)/0.9= 1.6A per phase?

Thanks in advance for anybody that can shed some light on for for me

 

[Adam_92]

Sorry calculation wrong 1000/3/132/0.9= 1.6A per phase if im right

 

[westward10]

Yes your calculation is correct, did you edit the original?
The relay should never be set above the normal running current of the motor.

 

[Darkwood]

Your overload is usually set to the Full Load Current (FLC) of the motor when talking DOL , motor have many variables that effect there rating, you normally wouldn't calculate it unless you are at a design stage and limited to such info, I guess this is what they are asking you to do, normally you would refer to the motor plate or the manufacturers when setting it up on site.

 

[plugsandsparks]

Motors are rated on their output, so 1KW is the power the motor will provide. So there is also efficiency to consider and a realistic power factor at full load. Hence Darkwoods comments about reading this off the rating plate, its the only method i use to set overloads, then i set the overload as accurately as i can to the FLC stated on the plate.

 

[Darkwood]

I would also add but noting it is beyond what you are been asked to consider in the AM2 is that O/L settings are a complex issue beyond the calculation method, it is often the case I set O/L much lower than FLC, this is because the motor may be running at 75% load under normal conditions, if I set at 85% of FLC then impending issues can be detected long before they become an issue and they effect production, if I get a trip at 85% where it runs at 75% under normal then I program in a maintenance schedule on the possible causes and put the motor O/L to FLC to allow production to continue until an appropriate time it can be addressed and serviced.

 

[Adam_92]

Aparrently you get a chart?
Does anybody know what kind of chart this is so I can have a look into it before the day? Thanks

 

[Darkwood]

Would love to help but I did mine in about 1990, didn't get any chart back then :/.. hopefully another member with more recent experience of the AM2 can help out.

 

[Adam_92]

Okay so apparently you do not get the flc and you have to use a formula to work out the current per phase? Then look it up on a chart?


Would my first formula be correct?


KW/3/(U/√3)/PF


So say a 1kw motor


So 1000/1.73/(132)/0.9= 1.6A per

If we have to take into account power factor that is..

 

[Shane Mullan]

convert motor rating from kW to W (1x1000 = 1000). Route of 3 = 1.73. 1.73x400=692.8. 1000/693=1.44

They may not actually want you to set the overload, but they will at least want to see you write down the calculation