I a mechanical engineer working at a mission hospital where we have a relatively new (2013) hydroelectric power plant. The turbine has a speed sensor (DC Tachogenerator) that provides a voltage to a control system to maintain constant RPM.
This Tachogenerator has been going through brushes rather quickly, and even with new brushes there is some fluctuation in the voltage (even though I know from the generator frequency that the system is running close to steady state). The frequency readout that receives input from the Tachogenerator will fluctuate around 3 Hz even though the generator frequency fluctuates under 0.5 Hz. With old brushes the reading is so poor the plant will overcompensate itself outside of the power quality parameters and shut down.
Our maintenance staff will clean the contacts of the rotor whenever they need to change the brushes. There is usually lots of carbon inside the housing.
I would like to know if we can replace this Tachogenerator with a more precise unit, preferably brushless. Long brush life is also important. We can't be shutting down several times per year to change brushes.
This resource is leading me to believe that a DC Tachogenerator is superior for our application. From my understanding an AC Tachogenerator would need a rectifier and capacitor to give a DC voltage with minimal ripple. However, the brushless nature of an alternator seems better to me. http://www.polytechnichub.com/difference-ac-tachogenerator-dc-tachogenerator/
Please see the picture for details on the tachogenerator currently in use.