ProTechTPS Total Protection System
Manual 26710V1
38
Woodward
A passive MPU provides a frequency output signal corresponding to turbine or
equipment speed by sensing the movement of a gear’s teeth past the MPU’s
pole piece. The closer the MPU’s pole piece is to a gear’s teeth and the faster
the gear turns the higher a passive MPU’s output amplitude will be. (Speed
signal amplitude increase with both speed increase and distance decrease.) The
ProTechTPS must sense an MPU voltage of 1 to 35 Vrms for proper operation.
With proper MPU, gear size, and MPU-to-gear clearance, speed measurement
can range from 100 to 32 000 Hz. Standard MPU clearance is recommended to
be 0.25 to 1.02 mm (0.010 to 0.040 inch) from tooth face to pole piece. For
information on selecting the correct MPU or gear size please refer to Woodward
manual 82510. Refer to Figure 2-11a of this manual for wiring information.
Proximity and eddy-current probes may be used to sense very low speeds to
high speeds (0.5 to 25 000 Hz). The speed probe input voltage must be between
16 and 28 Vdc, and the output signal must meet Vlow and Vhigh threshold
values specified in Chapter 3, Inputs and Outputs section. The voltage for the
speed probes must be from the provided voltage port or have its common
referenced (connected) to the provided common pin for proper operation. See
Figures 2-11b thru 2-11c for proximity and eddy-current probe wiring schematics.
An application may use the same or different types of speed probes (MPU,
proximity, eddy-current), between the three different inputs depending on the
specific application’s requirements.
Woodward does NOT recommend that gears mounted on an auxiliary
shaft that is coupled to the turbine rotor be used to sense turbine
speed. Auxiliary shafts tend to turn slower than the turbine rotor
(reducing speed-sensing resolution) and have coupling gear back-
lash, resulting in less than optimal speed sensing. For safety
purposes, Woodward also does NOT recommend that the speed
sensing device sense speed from a gear coupled to a generator or
the mechanical drive side of a system’s rotor coupling.
Figure 2-11a. Example MPU (Passive Magnetic Pickup Unit) Wiring
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Magnetic
Pickup Unit