Overload device vs. fusing

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Question: Industrial application - is there a way to open a thermal OL device prior to blowing branch circuit fuses during a locked rotor condition or motor stall?

We use 20A "General Use" 480V receptacles and have BUSSMAN LP-CC-20 FUSES. The OL device for this motor is set at 9.6 Amps. According to Table 430.52 fuse size on Class CC can be up to 300% of nameplate FLA which in this case would be 28.8Amps (9.6X300%). Locked Rotor current conversion calculations per Table 430.151 (B) show design code E as 91.5Amps. The only way I can find to detect a locked rotor condition is to upgrade the OL devices in question to a new unit designed to open for such conditions. I assume the amperage increase in time vs. the heat build up in time is the crutial difference as to why this happens.

Anyway, my supervisor (boss) is wondering if there is a way to detect this fault in order to reduce the number of blown fuse replacments. One of our plants switched over from 20A fuses to 20A circuit breakers but I felt that this was not an "answer" to our problem - just a bandaid (per say).

We got in an argument over the theory of how an OL device works opposed to a fuse (I am referencing when a fuse is not intended to be used for overload protection) but he is not willing to accept the fact that OL devices work on heat build up over time and are not intended to be used for short circuit or ground fault protection. He even went as far as calling a local reputable contractor during a plant wide maintenance meeting and in front of everyone asked the contracor if OLs work on voltage spikes caused by stalled motor conditions......... he was trying to embarrase me........ their reply was "an OL device is not intended to be used for voltage spikes and if it is continually opened under such stalled conditions it would eventually fail".

Hmmm, and all this time I was under the impression that heat build up causes thermal OL devices to open and that they are desinged and intended to be resetable for just that - resetting a fault condition during a pre-determined motor overload.......
Voltage spike? pre-mature failures due to overload conditions? where did he come up with that one??? Anyway, my Boss is out to get me and is trying to make me look bad in front of all my fellow maintenance team members. I am one of the only licensed electricians that our company has .......... and my boss is not one of them.

Any feedback or help on where I can find any further information or answers would be greatly appreciated.

INJECTION MOLDING REGRINDER MOTOR - 7.5HP/480V/9.6A 3 phase

CONDITION - Locked Rotor

BRANCH CIRCUIT - 4 General Use 480V 3 phase Receptacles each independantly fused at 20Amps using LP-CC-20 fuses fed from a 60Amp Fused Disconnect. All wiring is THHN and sized accordingly. OL devices are Allen Bradley - properly rated for this motors HP and Current and are set for each motors Name Plate FLA.


Brian G.
Michigan

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Brain,
I looks like you need to pick a fuse with a different time trip curve. The one you have will melt at ~60 amps in 10 seconds. This would be almost the same as a Class 10 over load. Many overloads are Class 20 and if so, the fuse will open before the overload. What are you using for overload devices? Can you change the Class rating? (on some it is user selectable, others it is fixed)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
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retired electrician
Brian,
By the way, for the branch circuit potective device you use the FLA values from Table 430.250 and not the nameplate value. The name plate value is only used for the overload selection. Also if you go to the Bussmann website, they do not recommend that fuse for this application.
Don
 

bdarnell

Senior Member
Location
Indianapolis, IN
Occupation
Retired Engineer
Brian:

Did you read the Exception 2 to 430.52(C)(1) ? That might be applicable.

Also, Table 430.150 lists a 7.5 HP, 460 V motor at 11 amps. IMO, your OCPD could be 40 amps using Class CC fuses.

And, Don is right. LP-CC's are suggested for feeders and branches.
 

tom baker

First Chief Moderator
Staff member
Why do you reference design E?
"Locked Rotor current conversion calculations per Table 430.151 (B) show design code E as 91.5Amps."
There are and were never any design E motors.
 

stew

Senior Member
Tom I am not sure I understand your post. The locked rotor codes are A,B,C,D,E,F,G,H,J,K,L,M,N,P,R,S,T,U,&V. design B motors are normal torue standard type 3 phase if you will. Design C is hi torque,Design D is hi slip and Design E is hi torque Hi slip. At least thats what I recall from my electric motor days. Anyhow you mite want to log on to the EASA home page and download the EASA engineering handbook as It gives tons of usefull info.
 

stew

Senior Member
Tom I am not sure I understand your post. The locked rotor codes are A,B,C,D,E,F,G,H,J,K,L,M,N,P,R,S,T,U,&V. design B motors are normal torue standard type 3 phase if you will. Design C is hi torque,Design D is hi slip and Design E is hi torque Hi slip. At least thats what I recall from my electric motor days. Anyhow you mite want to log on to the EASA home page and download the EASA engineering handbook as It gives tons of usefull info. The locked rotor kva for Design E is 4. 5 to 5 kva per hp or 33.75 to 37.5 kva. My calculations are between 42.4 and 47 amps locked rotor current for thie code letter E motor at 7.5 hp 460 volts. Not sure where the 91.5 amps came from although That would be about right at 230 volts in fact almost exactly correct at 230 volts.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Stew,
It appears that you are confusing the locket rotor code letter with the design code letter. Design E was a planned very high efficiency motor that was never built.
Don
 

dlhoule

Senior Member
Location
Michigan
My question is this: If you are getting stalling or a locked rotor, don't you think there is another problem you should be looking to solve?

Repeated tripping of OLs will sometimes cause failure of OL to be reset. IMHO this is usually from resetting them before they have had sufficient time to cool down.
 
Locked Rotor

Locked Rotor

Yep, sorry, I think I gave you a catalog reference letter of "E"(E-300) - I see the design code letter reference on the nameplate is "H" , CAT E, NEMA B and INS B. So, assuming that 'H" is the design code that would equate to 6.3-7.09 KV-Amperes per HP or 47-53 Amps per table 430.7(B). For some reason I gave you the rating on design code "E" on Table 430.151 (B) which was for disconnecting means and enclosures.

_________________
[DLHOULE/ My question is this: If you are getting stalling or a locked rotor, don't you think there is another problem you should be looking to solve?

Repeated tripping of OLs will sometimes cause failure of OL to be reset. IMHO this is usually from resetting them before they have had sufficient time to cool down.
quote]

As far as the locked rotor condition goes - operators throw anything from bolts to hand tools in regrinders (outside of maintenance control) once in a while and this instantly jams the grinder. This is the reason for the blown fuses in the first place. I guess I am just after a solid reasoning as to why the OL doesn't trip prior to the fuse.............

Now another question, :idea:
If I use the values given in table 430.52 for Class CC (Nontime Delay Fuse Column) for a 7.5HP 480V 3 Phase motor I am allowed up to 300% of 11A (Table 340.150) which equates to 33A. Since 33A is not a standard size fuse from 240.6 I am allowed to size up to the next standard of 35A rather than the suggested 40A that STEW suggested - is this correct? :?: I say this because I do not have a start up trip - but rather a locked rotor trip. Anyway, I think I need to brush up on my code interpretation.

I will try the suggestion of higher rated Fusetrons from STEW and see where we go from there.

Thanks to all of you for the help and suggestions. That is why I love this site so much .............. because of all of you out there contributing your ideas and suggestions. Together, we all learn more.........................

Brian G - Michigan
 
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