WE are having GE Frame 9E machines. We recently had a starting motor failure on one of our machines. This is a 11KV, 1MW, 60.7A (nom. current) motor. It tripped on ground fault during start up & later we found one of its phase shorted to ground while other two phases were completely healthy. We are wondering what possibly caused this failure just after 7 years of commissioning as it is a very less running motor.

We observed that during start up, this motor is taking a current up to 6.5 times i.e. around 400A, Is it normal? Motor's bearings were also found OK just after failure. Other than that we are wondering that what it could be e.g torque converter, accessory drive etc?

If any one ever experienced any such failure at their plants, kindly give comments & your opinion regarding this failure.

 

Sunny... to start and presuming protection was of the type that limited ground-current magnitude, then ask your rewind company to determine the cause of the ground fault? Ask if failure occurred:

1) Within stator winding slot, but at slot floor?

2) Within slot, but at slot wall?

3) Where (end-turn) winding emerges from slot.

Regards, Phil Corso

 

This is a perfect example of people operating machines and equipment without a clue as to what normal operating conditions are. Just push the button or click on the target, while sitting down, and sit and wait to push the next button or click on the next target. Oh, and acknowledge those pesky Diagnostic- and Process Alarms; damn those things, anyway! Good thing we don't have to get up out of our chairs to acknowledge those nuisance alarms! And thankfully, someone turned off the Alarm Printer cause that thing just won't stop printing those pesky alarms and it's so damned hard to get up out of the chair and load paper into that printer! (The "operational" emphasis is on sitting, rather than understanding, at most plants.)

It's not uncommon for electric starting motors used for GE-design heavy duty Frame 9E (and Frame 7E/EA) gas turbines to draw as much as 150-160% of rated current during starting and acceleration. And most plants never record a typical start (or shutdown) to have that kind of information available for troubleshooting. I can't begin to count the number of times I've heard an operator or technician or plant supervisor, "It's never done <b>THAT</b> before!" during a re-start after an outage or forced shutdown and be 114% wrong.

These motors are operated above rated current during almost every start and acceleration. If you use the motor for cranking to cool the unit, then every time you do that you are also doing the same thing for a brief period of time (during the purging portion of the crank sequence). If your torque adjuster drive mechanism isn't adjusted correctly it can cause the starting motor to draw even more than "design" current ("design" taking into account the excess current normally drawn during starting and acceleration).

So, some excess current is normal. But, it would seem you have no data from a normal, good start to review and compare current operating data against to determine if there is a problem and what the extent of the problem might be. Most well-operated sites have such information as part of their Standard Operating Procedures so that people using the SOPs can have a reference for a good procedure or sequence.

Motors do fail. These motors, while not continuously operated above rated nameplate conditions are operated during every TURBINE START above rated for some period of time (or periods, as the case may be).

Dirt and humidity can cause problems. And I would suspect, based on this question, that more than once the motor was started more than twice in an hour, which is usually the limit most manufacturers say such a large motor can be started to protect against the damage caused by high inrush currents during starting.

If one or more of the motors is drawing 6.5 times rated current, I would also suspect that the protective relay(s) for the motor(s) (large motors like this usually have one, a digital relay these days) has/(have) been "re-adjusted" to allow such excessive currents and the high stator temps that would likely accompany such high current draws. Because, most motor protective relays, if properly set/adjusted, would likely either annunciate an alarm, or more likely trip the motor's starter to protect against such high currents and temperatures.

 

CSA... of all the failure-modes you described, exceeding the "Number of Starts/Hr" rule is not very likely to cause a ground- fault!

Regards, Phil