Saturday, February 25, 2012

Controls and instrumentation - Engine speed

want to read about :  Controls and instrumentation - Engine torque

Engine speed
21. All engines have their rotational speed (r.p.m.) indicated. On a twin or triple-spool engine, the high pressure assembly speed is always indicated; in most instances, additional indicators show the speed of  the  low  pressure  and  intermediate  pressure assemblies.
22. Engine  speed  indication  is  electrically ransmitted from a small generator, driven by the engine,  to  an  indicator  that  shows  the  actual evolutions per minute (r.p.m.), or a percentage of he maximum engine speed (fig. 12-5).
Fig. 12-5 Engine speed indicators andgenerator.
The engine speed is often used to assess engine thrust, but it does not give an absolute indication of the thrust being  produced  because  inlet  temperature  and  pressure conditions affect the thrust at a given engine speed.
23. The engine speed generator supplies a three- phase alternating current, the frequency of which is dependent upon engine speed. The generator output frequency  controls  the  speed  of  a  synchronous motor in the indicator, and rotation of a magnet assembly housed in a drum or drag cup induces movement of the drum and consequent movement of the indicator pointer,
24. Where  there  is  no  provision  for  driving  a generator, a variable-reluctance speed probe, in conjunction with a phonic wheel, may be used to induce an electric current that is amplified and then transmitted to an indicator (fig. 12-6).  This method can be used to provide an indication of r.p.m. without the need for a separately driven generator, with its associated  drives,  thus  reducing  the  number  of components and moving parts in the engine.

25. The  speed  probe  is  positioned  on  the compressor casing in line with the phonic wheel, which is a machined part of the compressor shaft. The teeth on the periphery of the wheel pass the probe once each revolution and induce an electric current by varying the magnetic flux across a coil in the probe. The magnitude of the current is governed  by the rate of change of the magnetic flux and is thus directly related to engine speed.

Fig. 12-6 Variable-reluctance speed probeand phonic wheel

26. In addition to providing an indication of rotor speed, the current induced at the speed probe can be  used  to  illuminate  a  warning  lamp  on  the instrument panel to indicate to the pilot that a rotor assembly is turning. This is particularly important at engine start, because it informs the pilot when to open the fuel cock to allow fuel to the engine. The lamp is connected into the slatting circuit and is illuminated during the starting cycle.

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