1.4 Electronic governing - part 7
Two types of speed-sensing device are in common use, both employing non-contact sensing of the rotation of a toothed wheel directly keyed to the main turbine shaft. For safety reasons, the toothed wheel is mounted at the HP end of the machine so that in the unlikely event of a shaft or coupling failure, the sudden loss of load torque would be sensed by the overspeed-ing of the HP end of the machine and the governor would act to close all the turbine steam valves.
The first type of speed-sensing device is the magnetic pick-up shown in Fig 2.15. The sensing coil is wound round the pole-piece which directs the magnetic field from the permanent magnet towards the toothed wheel. A voltage is generated whenever the magnetic field is disturbed.
If a tooth on the wheel is opposite the pole piece, the magnetic field is concentrated by the tooth. In the absence of a tooth, the field is less concentrated. The passage of teeth on the wheel past the pole piece causes the flux linking the coil to vary and generate a voltage, which is of near-sinusoidal waveform at normal running speeds. However, at low speeds the waveform will be more distorted and below some well-defined threshold the probe output will be inadequate. This is acceptable for governing purposes since speed integrity checks are built in from an early stage in the run-up. Probe output voltage amplitude depends on the gap setting, which should be checked against the turbine maker's recommendations during overhauls. In general, a triplicated analogue governing system requires three probes; however, a spare probe is often fitted, which can be substituted for a faulty probe by manual connection whilst on-line.
The second type of speed-sensing device is the proximity transducer which consists of a probe connected to an oscillator-demodulator unit by an interconnecting extension cable. The advantage of this system is that it can sense the passage of teeth down to zero speed, which may be important in applications requiring a low barring speed. The equipment is more complex though and relies on the use of specially trimmed interconnecting cables between the two units. The principle of operation is that a radio frequency signal is generated by the oscillator and radiated in the region of the probe tip. As this electromagnetic field is intercepted by the toothed wheel, eddy currents are generated in the material and depending on whether or not a tooth is present, a varying energy loss occurs in the strength of the return signal which is sensed by the demodulator.