Discussion on “exciter instability”

W. A. Hillebrand: A rather startling instance of an oscillation such as Mr. Doherty has just described, came within my observation some three years ago. There was a two-pull electro magnet for a Paulson high-frequency arc converter, the magnet weighing approximately 30 tons, and they had only 5-kilowatts of arc converters at the station. Now the magnet itself was perhaps the most powerful that was ever delivered, and it was excited by a 250-kw., 500-volt d-c. generator, direct connected to a 2400-volt induction motor. Now what happened was that the power went off. The energy stored in the magnetic field was sufficient to stop the motor-generator set, reversing it. This reversal of rotation of the motor generator demagnetized the magnet, magnetizing it in the opposite direction, and again brought the set to rest. That is, there was a highly damped oscillation of very low frequency lasting for a cycle and a half. We ran saturation curves under various conditions, and finally after the last run the current was cut off, and I think some three or four minutes after the machine was completely disconnected, while we were stripping it, we still got an arc, showing the current was still flowing. That is, it took several minutes, due to the very slow decay before the current came to zero.