In a couple of earlier posts, I spoke of the effort in upgrading a large dc variable-speed drive with modern control electronics. I will discuss some of the specific commissioning issues.
The first concern was in upgrading the tachometer to be compatible with the new digital drive. The original drive used an analog tachometer with its known issues of zero speed offset and scaling accuracy which changes with temperature. We had the usual problems of getting the new pulse encoder mounted and new tach cables pulled to the drive. Performance improvements are very much noticed with the new tachometers. When the drive is told to stop, it now stops instead of slowly creeping.
The next issue had to do with auto tuning. Auto tuning is much better now than even 10 years ago. However, auto tune did not work with the 40-year-old, 800-HP dc motors. Manual tuning had to be performed. I have seen many cases where the auto tune function cannot cope with an older motor. Years ago, motors had a lot more copper and iron than an equivalent motor today. This affected the armature inductance and time constant.
We also had trouble with simple sounding signal definitions. Consider the signal DRIVE HEALTHY. This signal acted differently between the old drive control and the new digital drive. Drive master programming had to be changed to adapt to the difference. This affected permissives to reset and start the drive.