Replacement for DC Tachogenerator in hydroelectric control system
The best speed detector technology uses an inexpensive incremental encoder. There is a wide range of applications for precision frequency and phase control on a rotor position ranging from 1 to thousands of pulses per revolution. Then using a simple pulse voltage averaging method it is converted to DC .  But explicit voltage, current load , tolerance, ripple and accuracy must be defined.

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This method of system power regulation seems to be very poor as described.
If a brush tachogenerator is used to regulate an inductive actuator for power or speed control, then the commutated RPM voltage will certain erode brushes without a suitable RC capactive load to act as a flywheeel to each pulse.  Otherwise arccing will erode the brushes quickly.

Some AC and DC measurements of output would help and a scope photo even better.