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buddha

motors-full load current

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buddha    11
buddha

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??

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Tony S    349
Tony S

Set O/L’s to the FLC of the motor. The days of setting at125% are long gone.

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buddha    11
buddha
10 minutes ago, Tony S said:

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.

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Tony S    349
Tony S

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.

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Evans Electric    2,667
Evans Electric

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.

Edited by Evans Electric

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Evans Electric    2,667
Evans Electric
2 minutes ago, Tony S said:

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.  

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Tony S    349
Tony S

To be honest, I didn’t give BS7671 a 2nd thought.

 

It would help if we knew what the question is.

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Rob.    48
Rob.

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. 

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Evans Electric    2,667
Evans Electric

You mean Allen West  dashpot starters ,  with the oil in them ?    Screw them up & down  to set ?

Edited by Evans Electric

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Rob.    48
Rob.
8 minutes ago, Evans Electric said:

You mean Allen West  dashpot starters ,  with the oil in them ?    Screw them up & down  to set ?

 

The very same

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buddha    11
buddha

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.

400kb.jpg

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buddha    11
buddha

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?

400kb2.jpg

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Tony S    349
Tony S

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.

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Sidewinder    1,942
Sidewinder
5 hours ago, Tony S said:

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...

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kerching    1,873
kerching

Perhaps the 'lecturer' and I use the term in the loosest of a dogs squits way...should learn to spell as well

 

 

.......'from then neutral'..........WTGrape does that mean?

 

pick picky Mc Pick

Edited by kerching

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buddha    11
buddha
13 hours ago, Tony S said:

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.

ok tony,i'll put him on the spot when i see him.

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Tony S    349
Tony S

That would be great. We await his answer with baited breath.

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Rob.    48
Rob.

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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Tony S    349
Tony S
2 hours ago, Rob. said:

 

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

 

 

RoB, that would be the icing on the cake!

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Tony S    349
Tony S

Brain fart time, I’ve written an article on motor O/L’s in KB and forgot all about it.

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Andy™    2,096
Andy™

no wonder new electricians these days dont always know what they are doing when they are being taught the wrong info. at least the OP has enough sense to question what he has been told and look into it further to get the correct answer

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Rob.    48
Rob.

What's worrying is if he answers with the correct answer he may be marked wrong by his tutor!

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steptoe    2,089
steptoe
18 minutes ago, Rob. said:

What's worrying is if he answers with the correct answer he may be marked wrong by his tutor!

we had similar when doing 2391 practical,

iirc, we had to find 12 faults, there were probably about 30 on the board, so the tutor told us to tell him when we had found 12 and he would tell us what 12 to write down,

as if we wrote down the wrong 12 they would be marked incorrect.

 

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Tony S    349
Tony S

Who sets these questions, the C&G or the lecturer?

 

Someone needs their arse kicking.

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Blue Duck    560
Blue Duck

I thought the questions were written in city & guilds English? 

 

Your answers should also be in city  & guilds English.

 

:C

 

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