hydro governor

[shrestha1]

Hello,
Is there anything behind using toothed wheel infront of magnetic pickups for measuring speed in hydro turbine shaft?

 

[davidbeach]

As opposed to doing what?

 

[mgtrp]

The other methods that I have seen are:
- Use of VT signal is the other one that I have seen, as it provides a more stable signal. This doesn't work so well at low speeds prior to application of excitation. It's common to use this in combination with mag pickups (switching over to the VT once a stable signal is achieved).
- A small permanent-magnet generator coupled to the shaft, connected to a small motor in older mechanical governor systems.

 

[davidbeach]

The VT output provides electrical frequency, not mechanical speed. While there's a direct relationship between the two, they're not the same thing, particularly if you're subject to NERC compliance standards. Any over frequency/over speed trip based on voltage is part of a protection system and potentially subject to PRC-005. That mechanical speed indication isn't. Generally there's both.

 

[ScottyUK]

The toothed wheel produces useable output over a wide range of speeds, and it produces a high number of pulses per revolution which allows for finer control of the governors: if you have a single pulse per revolution and you rely on measuring the period between pulses to determine shaft speed, the effective sampling rate of the feedback transducer is likely to be the slowest part of the control loop.