motors-full load current

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buddha

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Hi , another question :)  as a general rule the thermal overload on a starter motor should be set a little higher than the motors full load current right? I'm getting conflicting methods of working out what the thermal overload should be set at,anyone got a simple calculation for this??

 
Set O/L’s to the FLC of the motor. The days of setting at125% are long gone.
thanks Tony, I'm guessing thats a real world answer,I'm doing my AM2 next week, I think I need to show a calculation,my college has given me one example showing the settings to be lower than the flc,and then in another example they show it higher than the flc.

 
I'd say set to FLC too but unfortunately I too am in "The real World"    

I never heard of setting them lower .

You need to know what answer the college wants to see .

The guy to ask is Sidewinder really .   Or go back over the course work.

 
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The only time I would set an O/L unit below the FLC is for a Y∆ starter where they are set to FLC/√3 or 58%.

O/L's for crane duty motors are often set high, but I can’t see them asking that.
Nor me , I used to work on cranes , its a different ball game , BS 7671 doesn,t apply for a start. 

On old cranes with the old tram style speed controllers there would often be , bolted to the floor of the cab , a 2 phase  415V  cab heater ,  no nonsense about switches etc .  If the crane was live , so was the heater.   

A huge Hoist motor fed by what to us , look like cables too small ....I remember thinking blimey shouldn't that be fed with 16mm ....no because the hoist only runs for possibly 15 seconds & stops.  

 
There's a calculation for working out the motors full load current. Or you simply look at the motor name plate. The answer your college may be looking for is either to work it out from a kW rating they give you or a picture of a motor name plate. Either way it's set at the FLC for a standard DOL motor. 

The 110%/125% rule is long gone, and is a hangover from when dash pots were used. I do know a site with some still in action but you'll be hard pressed to find them now. 

 
this is one part of am2 leaflet college gave me,it calculates flc 1.15 A then says set dial to 1.2A so thats 110% I guess.

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this is a question with answer in same leaflet, again it seems to multiply the flc by 1.1

perhaps I should go with this then? Thanks for sharing your knowledge guys,this forum is so helpful.

I noticed there were not many up to date posts on the learning/college section so i posted in this bit hope you dont mind,has the student interest in this site dropped off a bit?

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Ask your lecturer to explain just why you should set the O/L’s at 110%. RoB and I would be most interested in the reasoning.


Not just Rob & yourself Tony, I would like to know, as I suspect would the controls manufacturers I deal with and the motor OEM's, as they all say 100%.

Perhaps the lecturer needs to teach them that they are wrong...

 
You can even point him in the direction of this thread.

It was something that used to be done, but with modern overload protection devices the tolerances are as such that 100% is what you set it at. Even a star delta starter when set at 0.58%FLC is still protecting the motor at it's 100%FLC due to where it is installed in the circuit. 

I know there are some situations where you would set an OL device higher than 100% now, but they're only in specialist situations and wouldn't be an answer in an exam like the one shown, without a hell of a lot more information in the question. 

 
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