150 HP Trips 400 breaker

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Davebones

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North Of Tampa Fl
Found a 400 amp feed to a controller cabinet tripped last week . Operator's claim the test stand had not been ran in over a week . We megged the feeder to to the controller all ok . We found the feeder to the 150 HP motor megged 0.03 Meg Ohms . Put some lights on the motor over the weekend to see if it was just moisture in the windings . We megged the motor this morning and had 6 Meg Ohms to ground . Phase to phase measured 1.2 ohms each A,B,C . When we tryed to run the motor we immediately tripped the 400 amp feed plus the 1600 amp main for that section . We pulled the motor leads apart and measured the T-leads nothing indicates a short . Phases the same in ohm readings . One winding reads the 6 Meg Ohms to ground , the other two 20+ meg ohms . The motor pump coupling turns freely . Did smell a slight burning smell when pulling the motor terminals apart . 6 lead 480v motor . We are sending the motor out just wondering why we aren't seeing a short ...
 
Davebones said:
Found a 400 amp feed to a controller cabinet tripped last week . Operator's claim the test stand had not been ran in over a week . We megged the feeder to to the controller all ok . We found the feeder to the 150 HP motor megged 0.03 Meg Ohms . Put some lights on the motor over the weekend to see if it was just moisture in the windings . We megged the motor this morning and had 6 Meg Ohms to ground . Phase to phase measured 1.2 ohms each A,B,C . When we tryed to run the motor we immediately tripped the 400 amp feed plus the 1600 amp main for that section . We pulled the motor leads apart and measured the T-leads nothing indicates a short . Phases the same in ohm readings . One winding reads the 6 Meg Ohms to ground , the other two 20+ meg ohms . The motor pump coupling turns freely . Did smell a slight burning smell when pulling the motor terminals apart . 6 lead 480v motor . We are sending the motor out just wondering why we aren't seeing a short ...
Click to expand...
May have short from turn to turn and not to ground.
 
kwired said:
May have short from turn to turn and not to ground.
Click to expand...
Or, if you have split bolt type or other free floating connections in the peckerhead, one of them has been rubbing against the side wall and has worn through the insulation. When you megger it at rest, nothing is touching but when you start it, the motor jerks a little and that connection hits the wall of the peckerhead.
 
We broke the winding's apart . We read roughly 1.8 ohms between 3 & 6 , 1.8 ohms between 1 & 4 , and 1.8 between 2 & 5 . They don't show continuity between them when broke apart . The only thing we saw was winding 2 & 5 showed 5 meg ohm to ground while 1 & 4 , and 3 & 6 showed 30 + meg ohms to ground . I figure winding 2 & 5 was the one that showed the low reading on Friday . ( 0.03 Meg Ohms ) Just never saw a motor where we didn't see the short with the megger ..
 
We broke the motor leads apart no damage at the peckerhead . Just a little bit of burnt electrical smell when we opened it up from inside the motor . Will get a report from the motor shop and post what they find when they open it up for repair ...
 
Davebones...

1) What "trip" flag(s) presented? Ground, Phase


2) Did motor turn freely when uncouplled fro driven machine?

3) What backup trp elements operated?

Regards, Phiol Corso
 
The only flag we saw was the 400 amp feed to the controller cabinet was tripped last Friday . We megged the motor and was reading 0.03 Meg Ohms to ground . Put some lights around it over the weekend and megged this morning . It megged 6 Meg Ohms to ground . We turned the shaft by hand where its coupled up to the pump with no problem. We then started the motor and it imediately tripped the 400 amp feed to the controller and also tripped a 1600 amp feed to that section . Opened the peckerhead to disconnect the motor and could smell a burnt electrical smell .
 
We had checked the main contacts a little burnt from use nothing unusual . Left the motor disconnected and powered evrything up including motor feed and verified voltage everything reads good. Will see what motor shop says after we get it sent out .
 
At 6 megaohms I would have the motor sent out to be rewound. In my experience motors usually test in the several hundred megaohm range or very low, close to zero, making the decision to rewind pretty easy.

I had a 50HP a couple years ago test around 30 megaohms that the customer had laying around and wanted to use(their only spare!). It ran for 2 weeks then blew up.

6 megaohms is way too low for me.
 
6 Meg after how long? What was your DAR/PI? What test voltage?

IR test values corrected to 40 degrees Celsius

Minimum IR readings:
• IR 1 min = 100M> for DC armature and AC winding built after 1970
• IR 1 min = 5M>for most machines and random-wound stator coils and form wound coils rated below 1kV
• IR 1 min = kV+1 for machines made before 1970, all field windings, and others not listed above

Investigate phase to phase stator resistance that vary by more than 10%

Be sure to correct your readings to 40C, that makes a huge difference.
 
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