The purpose of a tachometer attached to the non-drive end of a brush type dc motor is to supply a feedback voltage to the motor`s drive, telling the drive what speed the motor is running. When the drive starts up it supplies a DC voltage to the motor matching the process required set point, thus motor runs at 10% of max speed the tacho will send back to the drive 10% of its maximum voltage (let's say the tacho's nameplate states 90Volts generated max at 1800 RPM and remains at 90V upto 5000RPM) which will be 9 Volt. When the brushes aren`t making good contact or the tacho`s signal path back to the drive gets broken, the drive will keep stepping up the dc output voltage since it would like to get some return voltage from the tacho. The result is a brush type dc motor that will ramp up speed wise too quick at initial start up. We replaced the brushes on the tacho and started the 150HP dc motor up and ramped it up gradually to full speed and after about 3min ramped it gradually down, making sure the drive doesn`t loose control over the motor. The bearings were checked and average wear was noticed. Big question! Will average bearing wear cause the motor to speed off at start up, meaning the tacho attached to the motor failed to generate voltage at start up? NS: the coupling never snapped nor did the bearings seized on the tacho. ANY IDEAS????????