The Motor-Generator (M-G) system is over 50 years old-but so am I. The motors and generators have been rewound several times over their lifetimes. There have been flashes in the past.
50 years ago, equipment was constructed in a much more open fashion than it is today. The dc commutator and brushes were fully exposed. This equipment is located in a locked Motor Control Room, accessible to authorized personnel only. The photo shows the generator just hours before it was destroyed.
Two were in the room - an area electrician and myself. She was about 10m from the generator and I was just behind her when we heard and saw the generator flash. That was a 30cm ring of plasma completely surrounding the 70cm diameter commutator. Sparks flew from the flash, hitting walls on either side of the generator set. Some combustible materials stored on shelving were ignited.
Normally when a motor flashes, the Variable Speed Drive (VSD) detects the fault and trips the motor. On an M-G set, the drive motor on the line tripped due to the fault, but the motor driving the generator saw no fault - only opportunity to keep the generator turning. The flash was fed for several seconds until I was heroically calling for support and the electrician had stopped the M-G set.
A 10m radius from the flash is not too great a distance. The flash made a lasting impression. Electrical flash protection means are uncomfortable, but the energy involved requires a safe boundary distance, safe work procedures and Personal Protective Equipment (PPE).
Tuesday, May 01, 2012 3:28 PM
I recently attended a week long course for the OSHA 511 General Industry. I read (and enjoy) your articles on a regular basis. I wanted to thank you, however, for sharing this particular info about safety. It really is so important to be prepared and know what "could" happen in any situation. I'm glad no one was hurt when YOU experienced the flash!
Tuesday, June 14, 2016 7:57 AM
Sir, I came across your old post. Can you share the root cause for such ring flashing on commutator. We have recently experienced similar problem at our plant's rolling mill. Thanks and regards.
Tuesday, June 14, 2016 6:35 PM
The root cause analysis was inconclusive. The generator set was 60 years old and was replaced with variable speed drives shortly afterwards.
The fault was similar to a flash on any dc motor. However, this was a generator. The flash occurred at the commutator for the generator. The resulting overcurrent tripped the contactor between the generator and motor. Opening the contactor did not eliminate the fault condition.
Further, the overcurrent did not trip the prime mover, so the fault was provided with continuing energy until the generator set was manually stopped and came to a stopped condition (10 seconds, because a qualified electrician was in the room).
Further, the excitation to the faulted dc generator shunt field was not tripped off. This continued to feed full power to the fault.