3 phase motor rating.

[clematis432]

My old engineer used to say..............flc + 10% for o/load setting.
Worked fine until someone put hydraulic oil in the o/load dash pots

Just curious but what do other folk set their motor overloads at. ?
Does anyone actually think about it or just accept what their set at !

I,ve worked in industry quite a lot, and within various factories, and motor overloads have been set from 100% FLC to 130% FLC depending on circumstances but I would say 110% is about the most common I,ve experienced. Trying to get a balance I suppose between protection and nuisance tripping.
Motors are more efficient running around 80% their FLC

 

[Marvo]

If the motor is under constant load we set the overload according to the normal running current and not the FLA marked on the info plate. For example if a motor has an FLA rating of 33Amps but under normal operating conditions it only draws 26.5Amps we'd set the overload at 26.5 + 10%.

 

[Darkwood]

If the motor is under constant load we set the overload according to the normal running current and not the FLA marked on the info plate. For example if a motor has an FLA rating of 33Amps but under normal operating conditions it only draws 26.5Amps we'd set the overload at 26.5 + 10%.

Exactly what I said and do ...this has saved my customers countless down time as I work in the cotton industry a lot and it can wrap around the roll bars and beariings, when it trips I usually have extra available motor power to increase the O/L setting in the short term to overcome the resistance and schedule in a shutdown at convenience.

 

[Knobhead]

DW, Marvo,
Please don’t go in to anything that’s safety critical if that’s you’re method. You wouldn’t survive 5 minutes in the chemical or steel industries.

Oh, look, the plants blown up. The motors OK though so that’s alright, isn’t it?

I’ve shorted O/L’s out, I didn’t give a flying f*** what happened to the motor. It had to keep going to get the process stable to do a controlled shut down. Anything critical had a back up, in some cases two back ups. They had to run no matter what.

O/L’s are set to 100% end of!

Anything critical would be oversized, electrically and mechanically. I had an 800HP blower motor with smoke coming out of it, the little beauty kept slogging on though. By this point all ancillary staff had been told to leg it out of harms way. The plant controller, charge hand fitter and myself as shift engineer had to stay. Common sense would say leg it, try explaining that in an enquiry in to a plant explosion! “You saved yourself, now there’s the body count to tally.”

The motor wasn’t very well afterwards. To rub salt in the wounds, my shift changed it!

 

[Darkwood]

I work in a very different industry which does show from our various sometime conflicting answers but at the end of the day I realise what I do on one machine isn't good on the next its just a judgement call, I also realise your in a very different industry and understand when we have differing views .... i learned all i know from the safety engineer at the Chernobyl nuclear power station ....well on some occasions you could be led to believe that.

 

[Knobhead]

My watch words. "Remember Flixbough."
That was a monumental cock up that circumvented safety to keep production going.

The things I’ve done haven’t been without a lot of soul searching first, but that has time constraints when things are going bang.

 

[Marvo]

Anything critical would be oversized, electrically and mechanically. I had an 800HP blower motor with smoke coming out of it, the little beauty kept slogging on though. By this point all ancillary staff had been told to leg it out of harms way. The plant controller, charge hand fitter and myself as shift engineer had to stay. Common sense would say leg it, try explaining that in an enquiry in to a plant explosion! “You saved yourself, now there’s the body count to tally.”

The motor wasn’t very well afterwards. To rub salt in the wounds, my shift changed it!

The scenarios you're talking about are industrial plant with auto reclosers and first fault override, the OP had a motor that was under a kilowatt in size and smaller than most household appliances, protection considerations will vary drastically between these environments. With very small motors like this there's a valid argument that protection could even outweigh the cost of the motor itself and isn't actually worth installing at all. If he is going to install protection and if the motor is under a predictably constant load range then I would still advise he sets the trip current according to the actual load current rather than at the FLA on the info plate.

 

[GLENNSPARK]

Exactly what I said and do ...this has saved my customers countless down time as I work in the cotton industry a lot and it can wrap around the roll bars and beariings, when it trips I usually have extra available motor power to increase the O/L setting in the short term to overcome the resistance and schedule in a shutdown at convenience.

and if left.....cotton and wool can shear through shafts as well.....

 

[clematis432]

Can,t help thinking this threads getting a little complex. Every individual motor is set to suit it,s OWN use, purpose and conditions. These are many, i.e. plant conditions, flameproof/non flameproof environment,type of motor, type of starting of motor, classification of o/l relay, ambient temperature, etc. Whether the o/l is set by reference to running current or stated FLC is arguable and again individual as to motor duty and purpose. Also the comment made in an earlier thread that "O/l,s are set to 100% end of" is I,m afraid too simplistic and generalized. We could debate all night settings of o/ls for induction motors and nobody would be 100% correct.