Strange Motor Failure

[nightfox1925]

We have a motor with a failed stator winding during the motor load test. There was an open circuit at the stator winding (lower winding cut). The following scenarios took place. There was no overtemperature, no overload..the motor failed during load test.

before testing PI and IR ok

We conducted at no load test (1-hour) the following
1. vibration ok
2. current logged ok
3. Temp ok (body using IR camera) every 15 min
it also passed core test (stator core lamination no hot spot detected)
4. Voltage acceptable (unbalance would just add 1 deg C temp rise as per my computation)

During load test:
1. pulley lazer aligned
2. phase current 90% FLA
3. Voltage acceptable (unbalance would just add 1 deg C temp rise as per my computation), about 6V highest voltage difference (our supply is 480v)
4. Vibration within standard
5. Temp ok
6. Motor failed after running 30 min

Motor failed after 7 months running, the above are conducted after the rewinding
By the way, 3 other identical motor were rewinded with same reading and data of this motor that failed, those other 3 passed.

motor is squirrel cage induction

Motor started by soft starter

What do you think contributed to the failure?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[electricpete]

thread237-203580 ?

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[petronila]

Hello NightFox1925,

Your shop was rewind 4 Exact motors. All start with soft starter, only one failled passed 7 mounths.

The test data don`t show Surge and HIPOT Test.Both tests are important to assure the winding quality works because you can find turn to turn short circuits and phase-phase/Phase-earth insulation failures, stressing the windings.

The least I do if I don`t have a Surge tester is to perform an Impedance Winding reading with a Kelvin Bridge or use a microohmeter to know the winding resistance balance.

In your case your problem could come from a two causes:
1. Some problem in the Costumer`s Soft starter that could inyect some overvoltage or peak to the winding.

2. This specific motor have a problem due to some failure during the winding works at shop(The wire could be hit or the wire have a manufacture defect in some part of the spool.

Regards

Petronila

 

[oftenlost]

I think that a picture of the site of the failure here would be worth all the words.
I gather that you rewound 4 identical motors for some reason, one of which had run for only 7 months before being subjected to your TLC.
Three of the four passed no load and load test, one passed no load and failed under load.
The winding was "cut", the core iron involved, as I read it.
Sounds like it went coil to ground through groundwall insulation.
Possible that you had turn to turn which burned the ground wall. Could be a cracked groundwall sleeve which allowed the core iron to abrade the elastomer wire cover or possibly a bad lead or jumper connection which effectively reduced the cross section of one of your coils causing overheating of one turn after load was applied.
My experience would lead me to suspect groundwall fracture or a bad solder joint. These have been the areas where the influence of the winder is greatest assuming common problems such as a stray wire out of the groundwall cuff have been eliminated.

 

[nightfox1925]

I got some feed backs and the following are the case scenarios:

Soft starter is used during starting only
Soft starter installed 8 years already
Motor drives blower, belt driven, 2plane

Motor rating about 300hp, 480v, 60hz

We have 3 same set up, all three were rewind
They we're rewinded because:
2 motors (6 years running) did not pass surge test but still running (proactive, before motor give up, we rewind)
1 motor failed during running (7 months running) with some weld on the stator stack because of the failure

3 motors under go rewinding
same testing (already mention, same readings, during no load and with load testing the same reading)

all of sudden after 30 min with load, the motor with stack problem failed again, al most the same spot of stator core

By the way the motor undergo stator core re stacking and lamination,
core test to test the stator hot spot, no findings after correction but still failed durring running with load

as per OEM, when stator core is defective and welded, motor should be replaced and not be rewinded.

I'm just wondering, why the motor failed after about only 7 months in service since it is a spare motor only (brand new)
Is it posible that it has manufacturing defects?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[oftenlost]

Way too many variables for me. You wound and restacked failed motor, rewound two good motors based on surge test and the spare installed during this evolution failed earlier than expected and within warranty period. Claim warranty.