Auto transformer starter troubles

[Smoking]

I work on a site that has a 300hp fire pump one day the motor failed short to ground in the windings. I took the motor out had it rewound then installed it and when we tried to test the auto transformer starter failed. This could have been from the motor failure so I replaced that as well, a few week later the new starter was in and ready for testing. When we tested in the auto transformed start to smoke and the contactor welded shut. The company ordered in new parts and I check my feeder and had the motor retested all was good. New parts were installed and test the motor did run but loud noise could be heard from the contactors in each stage now the contacts show sign of pitting for only being use once or twice. I went through the wire from the starter and transform they were all good the only thing I can see that is out of place is the auto transformer tap is on 65% and the print show it should be on 50%. If someone could I would be thankful. :?

 

[electric_cal]

I would disconnect the motor from the autotransformer and go through the start sequence. Make sure your contactors are closing in the right sequence. S1 closes and then pulls in S2. The transition timer will then close the S3, or Run contactor after a set time. You need to note the time from start to transition to run. If it is excessive it means the motor is being held in start too long. This would cause the autotransformer to overheat. A good time from start to run is usually 4 to 5 seconds, depending on the motor size, driven load, or well depth.

 

[kwired]

I would disconnect the motor from the autotransformer and go through the start sequence. Make sure your contactors are closing in the right sequence. S1 closes and then pulls in S2. The transition timer will then close the S3, or Run contactor after a set time. You need to note the time from start to transition to run. If it is excessive it means the motor is being held in start too long. This would cause the autotransformer to overheat. A good time from start to run is usually 4 to 5 seconds, depending on the motor size, driven load, or well depth.

Malfunctioning or improperly set up timer may have been the cause of the initial motor failure and led to everything that happened afterward.

 

[Jraef]

I work on a site that has a 300hp fire pump one day the motor failed short to ground in the windings. I took the motor out had it rewound then installed it and when we tried to test the auto transformer starter failed. This could have been from the motor failure so I replaced that as well, a few week later the new starter was in and ready for testing. When we tested in the auto transformed start to smoke and the contactor welded shut. The company ordered in new parts and I check my feeder and had the motor retested all was good. New parts were installed and test the motor did run but loud noise could be heard from the contactors in each stage now the contacts show sign of pitting for only being use once or twice. I went through the wire from the starter and transform they were all good the only thing I can see that is out of place is the auto transformer tap is on 65% and the print show it should be on 50%. If someone could I would be thankful. :?

From this scenario, I'd be looking for a bad supply circuit, a burned or burning phase connection AHEAD of the Fire Pump Starter somewhere. It sounds like you are single phasing the motor. The problem with is being a Fire Pump circuit is that there would be NO overload protection that would kick in, so a bad connection stays bad until there is a short somewhere. First the short happened in the motor, then when you fixed the motor, the next weakest link was the starter, now that is fixed but the cause is still there, ergo it is AHEAD of that too.