Tachometers and Tachometer Generators

The measurement of engine speed is an extremely important parameter, which together with manifold pressure, torque and exhaust gas temperature, allows the performance of an engine to be accurately controlled.

The speed of a reciprocating engine is measured at the crankshaft, whilst with turboprop and turbojet engines the rotational speed of the compressor shaft is measured, which gives a useful indication of the amount of thrust being produced. These instruments are normally referred to as ‘Tachometers', and operate either mechanically or electrically.

In the case of aeroplanes fitted with multi-propeller installations the RPM's are carefully matched to reduce flight crew workload by automatically reducing the noise and vibration during the cruise.

Magnetic Drag Tachometers

This type of tachometer is like a car speedometer and is used on a light aeroplane. It uses a series of small permanent magnets, which are rotated via a flexible shaft at half the engine speed, from a spur gear on the engine.

A highly conductive metal cup or disc (copper or aluminium alloy) is mounted on a shaft, which is free to rotate in very low friction bearings, within the rotating magnetic field. The shaft also carries a pointer, which is positioned by a calibrated hairspring so that it registers zero when the magnets are at rest.

As the magnets rotate the resultant magnetic field will induce eddy currents in the disc, which then interact with the magnetic field, and drag it along with it, hence the name ‘Drag Cup'. The eddy currents are such that the amount of drag increases proportionally with speed, whilst hairsprings apply torque to the system.

The torque produced is proportional to the rotation of the drag cup shaft, and the pointer correspondingly rotates over a linearly spaced dial, as shown on the next page. The cautionary operating range is clearly marked with a yellow arc, whilst a green arc indicates the normal operating range.

Tacho-Generator

This system uses a remotely driven tachometer. The detector (or pick-up) is an AC type generator, which consists of a permanent magnet rotor that rotates within a slotted stator. The AC generator is bolted directly to a mounting pad at the appropriate accessories drive gear outlet from an engine, and the rotor is driven by a splined shaft coupling. In order to limit the mechanical loads on generator, ratio gears are used in the engine drive system to reduce the operating speed of the rotor.

The signal from the detector unit is passed through a synchro system to the indicator unit. A typical indicator consists of two interconnected elements, a ‘driving element' and a ‘speed-indicating element'.

Three wires connect the AC generator to the indicator unit, and as the permanent magnet rotor is rotated within the stator, a three-phase AC supply, whose frequency and voltage is proportional to the engine speed, is generated. The output from the generator is then fed to directly to stator of an ‘AC three-phase Squirrel Cage Induction motor', which in turn drives a cylindrical permanent magnet on a shaft.

As the magnet rotates it induces eddy currents into a drag cup, whose rotation is controlled by a calibrated hairspring, which is attached to one end of the shaft. At the front end of the shaft, a gear train is coupled to two concentrically mounted pointers; a large one indicating hundreds of RPM, and a small one indicating thousands of RPM, as shown below. The indicator may also be designed to read percentage of RPM.