Chapter 5

Driven Equipment

Small gas turbines are employed to drive a number of differing loads and to provide various services which are normally part of an aircraft installation. Small gas turbines may also be used to provide ground based services, these are usually compressed air or electrical power. Mechanical loads take a number of different forms, also in the case of an engine which supplies compressed air directly from its turning compressor, the compressor it self is the load placed upon the engine.

DC Generators
A small gas turbine can be used to drive a DC generator. DC generators of aircraft type are normally quite highly specified producing a relatively high output power for there size and weight. The generator is driven by the engine via a reduction gearbox at a speed usually between 4,000 and 8,000 rpm. Up to 500A at 28V is possible from a DC generator driven by a small gas turbine.

A problem sometimes encountered with DC generators is caused by the brush gear. Due to a high current output, the brush gear contained in the generator is heavily spring loaded producing a machine which is quite stiff to turn. During the starting phase of the gas turbine, the engine may experience difficulty in accelerating under the load placed upon it by the generator. The gas turbine produces virtually no torque until it reaches almost full speed, during this phase it may overheat if loaded by the generator. A centrifugal clutch is sometimes fitted between the engine and the generator, this will allow the engine to start off load and idle before accelerating to running speed and accepting the load of the generator.

AC Generators
One advantage of a gas turbine running at constant speed is that it is suitable for driving an AC generator or alternator, this arrangement will produce an electrical output at a constant frequency. An AC generator is driven by the gas turbine through a reduction gearbox, the speed at which the generator spins and the number of magnetic poles contained in it, will effect the output frequency. Generators originating from aircraft will normally provide a 110/208V output at 400 Hz, for this reason they are required to spin fast at 6,000 rpm, 8,000 or even 12,000 rpm depending on the number of poles. Generators providing a 50 Hz supply for ground based applications normally turn at a slower 3,000 rpm (60 Hz for 3600 rpm).

Generators of up to 60 KW capacity are common, high output types may require an external air supply for cooling. A blower which may be shared with the engine oil cooler can be used to supply air to the generator.

AC generators do not employ heavy brush gear and so are very free to turn. Provided that the machine is electrically isolated during the start up phase of the gas turbine, no load is presented to it.

Many types of AC generators consist of a "brushless" design. Here energy is initially produced by a small permanent magnet generator mounted on the same shaft within the generator housing. This produces electricity to "Excite" a stator field winding which then transfers energy to a rotor field winding which intern couples to an output winding. The energy is transferred magnetically from the stator to the rotor and so brushes are not needed. The rotor also incorporates a rectifier as a DC current is required to excite the rotor.

When operating a gas turbine driven generator, care should always be exercised to ensure that during starting and stopping all electrical load is removed. When starting a gas turbine, the engine should be allowed to stabilise and the exhaust temperature settle before exciting the generator and connecting it to a load. As more electrical load is applied to the generator, the mechanical load on the gas turbine will increase, this will manifest itself as an increase in exhaust temperature. The exhaust temperature should always be watched so that the engine is not overloaded. Except in an emergency, a gas turbine should always be run off load for a few minutes to allow the temperatures to drop before it is shut down.

AC Generators may be used to provide a speed sensing signal for instrumentation. A constant speed engine/generator should provide a 400Hz frequency signal which may be re-calibrated to indicate 100% engine speed. A frequency in excess of 420 Hz indicates the engine is running fast.