1 Hz ie 60 rpm,4 pulses are required and at30 rpm,8 pulses are required.For monitoring of speeds above 240 rpm one pulse/rev is sufficient.The eddy current probe can work using specially fittedspeed wheels or by observing an already fitted gear wheel.Where gear teeth are less than 16mm thick then a lower voltage swing output signal is obtained as shownabove.Most speed monitors have a conventional 4 or 5 digitdisplay and have a number of alarm set points.The update time of the display should be such as to give a steady reading but respond fast enough to speed changes.An update time of 1 second is sufficient for on load andrun-ups but is usually insufficient for overspeed testing.Aspeed monitor that has a faster update time when inoverspeed mode is an advantage:typically 1 00 milli-seconds.When carrying out overspeed testing,to set theemergency stop valves,two speed readings are required:-a) The speed at which the turbine tripsb) The maximum turbine speed reached.

SPEED - OVERSPEED - ZERO SPEED MONITORING

1 second (60 rpm) to 300seconds (0.2 rpm).Alarm voting within speed trip systems is a commontechnique.Typically 2 out of 3 or 2 out of 4 voting methods are used,where multiple measurement heads around a single toothed wheel,generate individual trip alarms,which are further processed to generate a plant trip if any two signals are simultaneously valid.These type of systems also incorporate self test facilities that enable signal injection on individual speed channels for trip verification as well as testing the admissible trip combinations;A+C,A+D,B+C and B+D as an example for a 2 out of 4 system.The test rack provides key switch facilities to enable onload testing.This provides an interlock mechanism,which prevents two channels from being selected at the same time.A typical 3-channel system is illustrated below:- The number of slots used is dependent on the minimumspeed required.Most phase locked loops have a minimum operating frequency.This lower frequency is approximately 4Hz.In order to measure a minimum speed of say The eddy current probe as well as being used for shaftvibration and differential expansion can also be used as a speed monitoring transducer.The eddy current probe gives a large voltage output,which is independent of shaft speed.A pulsed signal is obtained by observing a projection,aslot or series of slots.The size of the slot need only be 1 6mm wide by less than 0.25mm deep.The speed at which the turbine trips is obtained by adisplay freeze on external contact closure.This contact closure is obtained by a micro-switch on the governor valves or some similar method.The maximum turbine speed is obtained by a sample andhold circuit within the speed monitor.It is imperativehowever during the screen freezing or peak holding,that the industrial recorder output follows the speed uninterrupted.A zero speed facility can also be incorporated in a speedmonitoring system or by a separate system entirely.Azero speed facility works by measuring the period between two pulses.When the period exceeds theselected zero speed time limit an alarm is initiated.Zero speed periods range from
Effect of slot width on eddy probe output Turbine speed profile3-channel overspeed trip system

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