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Discuss Mainswitch fuse overheating in the Industrial Electrician Talk area at ElectriciansForums.net

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Hi guys.
I have an ongoing problem with a compressor 166kw variable speed permanent magnet motor. I came across a main Sw fuse upstream from the motor which was extremely how over 100 degrees Celsius and on L 3 which blew fuse. I changed fusecarrier and female recepable whichfuse carrier pins enter to make circuit thinking that possible cause was high resistance at switch . Moving forward a few days same problem has arrised and while I was off site a transformer tripped one day along with the incommer to the main distribution panel feeding the compressor, maybe a coincidence but cause of trip has not been identified either . Also since totally new 400 a main Sw fuse has been installed it still seams to be operating at a high temp and I fear the issue will surface again . Notes to add currents on phases are In the region of 270 to 330 amps per phase I notice a 60 A difference between phases and also the company that supplied the compressor recently changed a stator which has been the only change in layout. I'm going to give the whole system a good going over tomorrow and see can I find any issues . Any advise by members greatly appreciated.
 

James the Spark1976

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Arms Access
Esteemed Member
Check the phase voltages for any differences.
It may highlight why you have a difference in current.
I would normally expect the current readings to be much closer to equal than what you are reporting.

Could there be another load on that circuit, just on 1 phase?

I am thinking
Driers
Fans
Aux heaters etc.
 
  • Thread Starter Thread Starter
  • #3
Iv done some investigation this morning 'll send a picture of the results . The Sw overheating is aprox 7mtrs upstream of the compressor all terminations checked ok with temperatures at terminals as attached. The main distribution panel is another 20mtrs upstream and has temp of 40 to 45 degrees Celsius , so I'm wondering why closest Sw is overheating so much almost 1566295733235807893393.jpgdouble temp .could it be harmonics etc
 

James the Spark1976

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Arms Access
Esteemed Member
Not quite sure about your cable arrangement
Are you running 300A down a 50mm cable?

Also, your supply voltages seem a bit on the low side.
 

Risteard

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TA
Arms Access
Not quite sure about your cable arrangement
Are you running 300A down a 50mm cable?

Also, your supply voltages seem a bit on the low side.
Perfectly normal supply voltage in Ireland. Obviously we've harmonised on 400/230, but the south used to be 380/220.

Also I read it as there are parallel conductors for the supply.
 

James the Spark1976

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Arms Access
Esteemed Member
Perfectly normal supply voltage in Ireland. Obviously we've harmonised on 400/230, but the south used to be 380/220.

Also I read it as there are parallel conductors for the supply.
Agreed, just was not sure exactly what had a parallel supply and what had a single.

Do you normally calculate what the increase in current will be for large motors when running at a reduced voltage?
 
  • Thread Starter Thread Starter
  • #7
It's one area in plant with 380 due to tapping on traffo as rest of site is 230 400. Their is about 10 traffo on site here. The plate on comp is rated 380 400 v either way.
 

darkwood

Moderator
Staff member
TA
Arms Access
Is there any magnetic circulating currents creating excessive heating, have the cables been glanded through metal enclosures correctly?

A pic of the offending switch may be good,
 

Risteard

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TA
Arms Access
I take it some of this installation is quite old given the red/yellow/blue & black cable? I mean it's not even brown/red/yellow & blue.
 
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  • #13
Correct id say 45 to 50 yrs . I personally think it's a harmonics problem as a main incoming mccb tripped other day also at 1600A plus the Rmu feeding 10kv to traffo went also.
 

Strima

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Arms Access
That appears to be hotter where the cable terminates into the ring crimp on L3, have you tried re-terminating?
 

Rob

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Arms Access
Agree with that it looks like a termination issue.

Loading issue would warm up the entire cable length, although I can't tell the differential between the 100deg measurement and the cooler section behind.

Very doubtful it's harmonics causing this issue, not with this amount of loading.

Skin effect isn't that prevalent at low loading, especially with parallel cables. An you have no neutral, this is where harmonics generally become a real problem in these smaller installations.
 

timbobelfast

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Arms Access
Could be possible that the magnets have lost some of their magnetism? Does it run at normal temperature at lower speeds? Has the VFD frequency been changed or adjusted? Does the compressor get regular mechanical servicing?
 
  • Thread Starter Thread Starter
  • #19
That appears to be hotter where the cable terminates into the ring crimp on L3, have you tried re-terminating?
Yes we re terminated and it still existed then moved it to spare 630 a Sw beside and it still followed.
Post automatically merged:

You have not lost a core on one of the parallel supply's have you
No all intact
Post automatically merged:

Could be possible that the magnets have lost some of their magnetism? Does it run at normal temperature at lower speeds? Has the VFD frequency been changed or adjusted? Does the compressor get regular mechanical servicing?
The vsd varies automatically to set point on compressor but we since found a mechanical issue, the on load offload valve was faulty so compressor was flat out trying to reach a set point but dumping half it's air. Since this has been fixed the compressor is now running probably at 75 % capacity and loading on cables has reduced. Also we got a power quality audit done and I'm waiting for the report although it appears there was aprox 18% current harmonics on it . I'll let u know the outcome.
 
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