Slot failures of motors

[D.D.KORANNE]

In semi-hermetic gas cooled ( winding) , occassionally winding slots are noted to be failing , these slots open up & stator winding fails . What are the probable causes for such a failure of three phase ( 50 hz , 380-420 vac ) part wound motors ?

[GHAZ]

Hello mate i,v seen motor laminations to crack or split on large compressors,60 to 190 hp because they are held in place by a locating bolt ,the bolt wears away the hole after years of service and the stator vibrates side to side has a small split. the slots normally are ok but if the stator is rewound a few times it gets damage during the cleaning process,eg overheating it to knock out the copper wire and insulation paper. the slot burn normally happens with coils vibrating in the slot and wearing away the insulation paper beacause the stator coils hav,nt been hardened enough or they are not tight in the slot

[Magoo]

generally found that slot failures were due to over temp windings. Usually fail close to outer edge of coil laminations. Maybe the suction cooling is insufficient to keep windingsl in temp range.

magoo

[US Iceman]

What are the probable causes for such a failure of three phase ( 50 hz , 380-420 vac ) part wound motors ?


If these are indeed wound for part-winding starting methods the delay timer during transition could be too long causing the motor to experience overheating and additional stresses through the windings.

Repeated starting under this condition might not show up as heat damage, but could show up as accelerated wear.

[Magoo]

Hi US Iceman,
I agree, and add that part wind starts are a con, stalled windings are not good. But I was shot on the water by a Euro from Frascold I think.
Keep the transition timing to max 1 second.

magoo

[D.D.KORANNE]

Thanks ! The motors usually when fail on the slot do not evince any abnormality before dismantling the motor; continuty okay,but put on load and it draws high current . Occassionally it is attributted to a manufacturing defect by motor manufacturers . What could be the manufacturing defect , if any ?

[Yuri B.]

Hello D.D.Koranne.
Some of the stator's laminations may be shifted a bit by the rotor catching them if bearing(s) are worn out.
It, however, remains unclear what the actual defect looks like.

[US Iceman]

...continuty okay,but put on load and it draws high current .


That sounds like an insulation problem. Continuity only shows the winding is not grounded or that the ohm value is within acceptable ranges.

The high operating current though is either a condition of motor stalling (not coming up to speed fast enough) or I think insulation breakdown. The insulation could be tested with a megger, since this is a high potential test (why this is also called a high-pot test).

[D.D.KORANNE]

Us iceman ,
thanks ! Yes at places insulation looks like split open in chutes of the stator winding when seen after the rotor is taken out off the shaft . Since insulation , splits may be ester oil+ moisture could be aggravating problem of its break-down ?

[lowcool]

100% spot on

[US Iceman]

Since insulation , splits may be ester oil+ moisture could be aggravating problem of its break-down ?


That may be. If the failure is related to the winding insulation you have several choices:


Is the insulation suitable for its intended duty (oil or refrigerant types)?
Is something causing the insulation fail? This could cause manufacturing defects in the motor, starting methods, voltage issues, etc.

If a motor fails a hi-pot test then the issue is motor related. Something else could cause it though...