Wire deterioration

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binney

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I need help settling an argument. We have a motor that is blowing fuses. We've pulled the motor out and it checks out good with the megger. I believe it still may have a probem. Is it possible for the windings to have deteriated. The megger checks for insulation value, but it doesn't check for wire quality. For the record the motor is over 15 years old.
 

kwired

Electron manager
Location
NE Nebraska
More details would be very helpful.

What is HP, voltage, number of phases, type of motor, size and type of fuses being used, type of load being driven.

Do fuses blow during starting, after long run time? Are fuse holders in good condition, or is there heating at the fuses because of poor connection.

Answers to some of these questions can lead to other questions.
 

binney

Inactive, Email Never Verified
The details are kind of irrelevant. The motor is 15 years old and is being replaced. The argument we're having is wether or not the windings can deteriorate over time. More specifically the conductor in the windings not the insulation. I feel the windings can still be going bad although we have a good reading with the megger. The motor was located in an area that is very corrosive and hot.
 

iwire

Moderator
Staff member
Location
Massachusetts
The details are kind of irrelevant. The motor is 15 years old and is being replaced. The argument we're having is wether or not the windings can deteriorate over time. More specifically the conductor in the windings not the insulation. I feel the windings can still be going bad although we have a good reading with the megger. The motor was located in an area that is very corrosive and hot.

If it passes the meg test it is not the windings.

What do you picture is happening to the copper that can cause a blown fuse that the mega can't pick up?
 

big john

Senior Member
Location
Portland, ME
...The argument we're having is wether or not the windings can deteriorate over time. More specifically the conductor in the windings not the insulation. I feel the windings can still be going bad although we have a good reading with the megger....
Yes and no. I absolutely agree windings deteriorate with time, and if you have a current path across poor insulation a megger will find it.

But a megger isn't perfect: If the defect isn't near a conductive path, there will be no reason for current to flow. I've seen a correctly performed insulation-resistance test not find exposed bare copper for just that reason.

When you've tested it are you going between each winding as well as winding-to-ground? For aging or contaminated insulation, a polarization index test will often tell you more than just a spot-check with a megger.

Are you always blowing the same fuses? How many go at a time? What type of overload protection does it have and has it ever tripped?
 

kwired

Electron manager
Location
NE Nebraska
The details are kind of irrelevant. The motor is 15 years old and is being replaced. The argument we're having is wether or not the windings can deteriorate over time. More specifically the conductor in the windings not the insulation. I feel the windings can still be going bad although we have a good reading with the megger. The motor was located in an area that is very corrosive and hot.
One thing a megger can't check is a short from turn to turn in the same winding. But passing the meg test just means there is no short to ground or short winding to winding (depending on what points you test between). A change in load conditions, bad motor bearings, deteriorated switches, controllers, etc. can certainly cause troubles you seem to want to disregard. I have seen many motors much older then 15 years that run just fine so age alone doesn't mean a lot.

If you just replace the motor and still have same problems - then all those details you say are irrelevant suddenly become very relevant.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Yes copper can deteriorate, perhaps a percent or two every thousand years. :roll: But no amount of corrosion or other deterioration of the wire itself is going to cause current through the fuses to increase.
 

electric_cal

Member
Location
California
I need help settling an argument. We have a motor that is blowing fuses. We've pulled the motor out and it checks out good with the megger. I believe it still may have a probem. Is it possible for the windings to have deteriated. The megger checks for insulation value, but it doesn't check for wire quality. For the record the motor is over 15 years old.

A megohmmeter test is only the first step in working to determine the health of a motor system. It is a pass or fail test, and will tell you if the motor winding is grounded, but it will not tell you what the health or condition of the windings are. You would gain more information on the condition of the motor windings by using a volt meter and current probe.

There could be a turn to turn short in 2 of the windings. There may be a coil to coil short. There may be a phase to phase short. There may be a partial ground that only occurs during operation. Etc, etc. A megohmmeter will not find these types of faults.

Surge comparison testing. Overpotential testing. Polirization index testing, will give you greater information on your motor windings overall condition. EASA recommends that a motor be removed for service at least once every 5 years at a minimum. If your motor has been in service for 15 years, I would say that it is long over due for some maintenance.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Nobody thought of measuring starting and running current? Would have been the first thing I did. Phase imbalance is what I'd be looking for.
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
Something I have run into--are you using replaceable link fuses and not torqueing the caps on--Worked at one place that the previous electricians had just grabbed the fuses and "spun them tight" by hand, loose connection, fuses got hot and then blew
 

Smart $

Esteemed Member
Location
Ohio
Yes copper can deteriorate, perhaps a percent or two every thousand years. :roll: But no amount of corrosion or other deterioration of the wire itself is going to cause current through the fuses to increase.
Not absolutely true. First, copper can deteriorate faster than 1-2% per millennium, but the varnish/enamel insulation would have to be compromised first... which may lead to fault current deterioration well before corrosion deterioration... and fault current could be substantially higher than nominal current.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The details are kind of irrelevant. ...
There's the nub of the issue though. The details are ALWAYS important!

But I get it, it was working fine for 15 years and now it isn't so it must not have been the wrong fuse, bad connection, wiring etc. etc. However, I'd be willing to say that almost EVERY time I have been stumped on something, it's because I was ASSuming that some detail was irrelevant...

My money would be on the turn-to-turn short in the windings though. A basic megger test won't see it, as mentioned by the others. Now, if one of the "irrelevant" details is that the motor is being run by a VFD, I'd bet a paycheck that it's a turn-to-turn short!
 
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