3 Rotational Speed Measurement

3.1 Magnetic Tachometers

3.1.1 Describe the construction and principles of operation of a magnetic tachometer. TQ(2)

The magnetic type of tachometer (also known as the eddy current tachometer) is the most widely used meter on small engines. A relatively small permanent magnet inside the instrument case is driven by a rotating steel shaft coming from the engine at one half the crankshaft speed. Riding on the outside of the magnet, but not touching it, is an aluminium drag cup. A steel shaft is attached to the outside centre of the cup, rides in bearings in the instrument and is free to rotate. An instrument pointer is attached to the end of the shaft and although free to rotate, it is prevented from turning by a calibrated hairspring (not shown).

Figure1: Principle of the magnetic tachometer

When the engine is operating, the magnet is spinning inside the instrument. As it spins, its lines of flux cut across the aluminium drag cup and induce eddy currents in it. The current produces a magnetic field which interacts with the magnetic field of the spinning magnet, and this tries to drive the drag cup. However, the cup is prevented from spinning by a calibrated hairspring, so the needle, which is indirectly attached to the cup, can only turn part of a revolution.

3.2 Electric Tachometers

3.2.1 Explain the function and operation of the following equipment in relation to an electric tachometer:

a. Tacho generator b. Synchronous motor c. Magnets and drag cups d. Indicator unit sensing TQe. Mounting and drive arrangements f. Electrical connections g. Drive gearboxes TQ (2)

a. Tacho generator:

When a flexible drive shaft is used to operate a tachometer, the distance between the engine and the cockpit is a limiting factor. For this reason, electric tachometers were developed so instrument wiring could be used in place of a drive shaft. There are both AC and DC type generators, but the

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DC type is not found on modern aircraft. The DC type had a small DC generator on the engine and a cockpit instrument indicating the voltage output as RPM. A lower than normal voltage would result in a drop in RPM which is a major disadvantage of the DC tacho-generator.

The AC generator eliminates this problem. A typical three phase AC electric tachometer system consists of a 2 or 4 pole (and sometimes 12 pole) generator, sometimes referred to as a tachometer generator or tach generator, mounted on the engine accessory case. An AC synchronous motor is located in the indicator. As the engine runs, the tacho generator produces AC voltage with a frequency that is directly proportional to engine rpm. The method most commonly used for driving a rotor is by means of a splined shaft coupling, the generator as a whole being bolted directly to a mounting pad at the appropriate accessories drive gear outlet of the engine. To limit the mechanical loads the speed of the rotor is reduced by a ratio of 4:1 or 2:1 by ratio gears in the engine drive system.

b. Synchronous motor:

This is part of the electric tachometer and is located inside the instrument. It is the driving element of the pointer in the instrument, receiving signals from the tach generator. The AC voltage produced by the tacho generator drives the synchronous motor in the indicator, causing it to turn at the same rpm as the generator.

c. Magnets and Drag cups:

The synchronous motor in turn spins a magnet and drag cup assembly similar to that of a magnetic tachometer to provide RPM indications.

Figure 2: An electric Tachometer. The 3-phase generator is driven by the engine to produce AC whose frequency relates to engine RPM. The indicator holds a synchronous motor which drives a magnetic drag tachometer magnet.

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d. Indicator unit sensing: TQ

The indicator unit sensing the output from the tacho generator is the synchronous motor which produces the same voltage frequency or speed as the generator which in turn spins the drag cup which is responsible for moving the instrument pointer. The speed indicating element consists of a permanent magnet device which operates on the eddy current drag principle and it utilizes either a drag cup or a drag disc. The primary determining factor in the motor RPM is the frequency of the AC that powers it. A lower voltage caused by loose connections for example would not have much effect on the indicated RPM. Both AC and DC use permanent magnets so that they are totally independent of the electrical systems of the aircraft.

Referring to Fig 2, a permanent magnet is attached to the rotor shaft of the synchronous motor; this is connected to the pointer of the indicator. As the stator field rotates, the permanent magnet keeps itself aligned with the field. A second permanent magnet is attached to the indicator rotor; this is located within a drag cup, see Fig 1. As this second magnet rotates, it induces eddy currents in the drag cup. These currents produce their own magnetic fields in opposition to the rotating magnet. When the rotating magnet increases in speed, the drag (or torque) on the drag cup increases. A hairspring attached to the shaft opposes this torque; the net result is the pointer moves across the scale in proportion to the speed of the engine shaft.

e. Mounting and drive arrangements:

Tachometer generators are small compact units, generally available in 3 types: the pad, the swivel nut, and the screw type. These names are derived from the kind of mounting used in attaching the generator to the engine. The pad-type is constructed with an end shield designed so the generator can be bolted to a

flat plate on the engine frame or accessory reduction gearbox by 4 bolts. (Electric and electronic tachometer installations must be checked periodically to be sure the tach generators and instruments are securely mounted and do not vibrate when the engine is running.)

The swivel-nut type is free to turn in respect to the rest of the instrument. This type of generator can be held stationary while the mounting nut is screwed into place.

The screw-type is constructed with a mounting nut inserted in one of the generator end shields. The mounting nut is a rigid part of the instrument, and the whole generator must be turned to screw the nut onto its mating threads.

Figure 3: Pad-type and screw-type mounting pads for tacho generators

f. Electrical connections:

Electrical wiring should be properly laced and clamped to prevent chafing caused by vibration and looseness. Verify that the wiring bundle is not under tension from being clamped too tightly and is protected from corrosive fluids and heat.

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g. Drive gearboxes: TQ

The tachometer indicating system is a small three phase AC generator connected via a mechanical link to engine accessory gearbox. To limit the mechanical loads the speed of the rotor is reduced by a ratio of 4:1 or 2:1 by ratio gears in the engine drive system.

3.3 Electronic Tachometers

3.3.1 Describe the construction and principles of operation of an electronic tachometer with particular reference to the following:

a. Pulse detection and frequency b. Signal processing (2)

Pulse detection and frequency.

Electronic tachometers produce engine rpm indications by counting electric pulses from the magneto ignition system. A special pair of breaker points in one of the engine magnetos sends pulses to a module which uses digital circuitry to convert the pulses to a voltage for powering the tachometer instrument in the cockpit. The set of points in the magneto open and close like normal points, but they only supply signals to the tacho system. As the engine rpm increases, so too does the pulse frequency and voltage, causing the meter to move and register increased rpm.

A simple electronic tachometer is installed with two electrical connections; one connection to ground and another to a magneto. This system depends on the magneto pulses being strong enough to power the instrument without amplification. A more complex system has an internal solid state amplifier which requires a third connection for 12 volt power from the aircraft electrical system. In this instance an amplifier amplifies a weak magneto pulse and provides a uniform pulse voltage to the digital counter circuitry.

From another source: A special type of point is used in an engine magneto which only supplies signals to the tacho system. The points in the magneto are connected by wiring to the cockpit instrument. Since the frequency of the opening of the points is proportional to the engine rpm, an electronic circuit measures the frequency at which the points open and close and moves the pointer to indicate the proper engine rpm.

Signal processing

Two slightly different types of electronic tachometers are found on turbine engines.

The first type is often used as a fan speed sensor to measure the rpm of the fan section of a turbofan engine. It uses a sensor containing a coil of wire mounted in the shroud around the fan. The coil produces a magnetic field which each fan blade cuts as it passes. As the shaft speed increases the fan tips pass the sensor at an increase rate. The output from the coil is in the form of voltage ‘spikes’. The frequency at which the field is cut is sensed and measured by an electronic circuit. The frequency is proportional to the rpm of the fan (see figure 4).

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Figure 4: Electronic tachometer which measures fan speed for N1 tachometer. The sensors produce an output signal each time a fan blade cuts through the magnetic field. The frequency of this output signal is measured to provide indications of N1 RPM.

Figure 5: A tachometer system for the high pressure compressor of a turbofan engine. The gear-driven rotor from the accessory section has a permanent magnet which induces signals in a pick-up coil.

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The other type has a gear driven shaft on the engine which turns a rotor with a permanent magnet embedded in its rim. A sensor containing a coil is located close the permanent magnet. Each time the field of the permanent magnet cuts across the coil a voltage is induced, the frequency of which is measured by an electronic circuit and used to position a pointer for the correct rpm indication (figure 5).

3.4 Dual Tacho systems

3.4.1 Specify the use of dual tacho systems (e.g. multi engine airplanes and rotorcraft). (1)

The dual tachometer consists of two tachometer indicator units housed in a single case. The indicator pointers show simultaneously on a single dial the RPM of both engines. Some indicators are equipped with a flight-hour meter dial, usually located in the lower centre area. On multi-engine aircraft, each engine may have a separate tachometer or a single tach may be used for both engines. If a single tach is used, the indicating needle for the right engine is identified with an “R”, while the left engine is indicated with an “L”.

Dual tachometers are also placed in the same case with a synchroscope for various purposes. One of these is the helicopter tachometer with synchroscope which shows the simultaneously the speed of rotation (RPM) of the engine crankshaft, the RPM of the rotor shaft and the slippage of the rotor due to malfunctioning of the clutch or excessive speed of the rotor when the clutch is disengaged in flight.

FAR part 27 Airworthiness standards require that all helicopters have at least two tachometer systems. One tachometer indicates engine rpm while the other provides main rotor rpm.

3.5 Maintenance practices

State the following maintenance practices associated with tacho systems: a. Lubrication of drives b. Rectifying erratic indications c. Flexible drive considerations with respect to heat, fluids and bends (1)

a. Lubrication of drives:

Tachometers that are mechanically driven with flexible drive shafts require periodic maintenance to prevent erratic behaviour. The drive shaft must be lubricated with an approved lubricant such as graphite.

b. Rectifying erratic indications:

As an A&P, you are required to check instruments for poor condition. Minor discrepancies such as tightening mount screws or replacing range markings on the glass instrument face are allowed. Discrepancies that require opening of the instrument case, such as cracked or fogged glass, loose pointer or a pointer that will not zero, must be corrected by an approved repair station. As a technician you may replace the instrument.

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The hardware that attaches the flexi-drive shaft to the instrument, airframe and engine should be secure. The difference in readings taken before and after light tapping the instrument should not exceed plus minus 15 RPM. This value may vary by indicator manufacturer. Both generator and indicator should be inspected for tightness of mechanical and electrical connections, security of mounting, and general condition. The manufacturer’s instructions should always be consulted.

An engine equipped with an electric tachometer may fluctuate and read low at idle rpm. This is an indication the synchronous motor is not synchronized with the generator output. As engine speed increases, the motor should synchronize and register the rpm correctly – the rpm at which synchronization occurs will vary with the design of the tachometer system. If the instrument pointers oscillate at speeds above the synchronizing value, determine that the total oscillation does not exceed the allowable tolerances. Check generators and instruments are securely mounted and do not vibrate when the engine is running. Electric and electronic tacho wiring should be checked for:

Properly laced and clamped to prevent chafing caused by looseness and vibration Verify the wiring is not under tension from being clamped too tightly Wires are protected from corrosive fluids and excessive heat.

c. Flexible drive considerations with respect to heat, fluids and bends:

The drive shaft should be installed away from excessive heat or fluids without sharp bends or kinks, and should not impose any strain on the instrument. In addition, the drive shaft should be secured at frequent intervals to prevent whipping, which causes pointer oscillation.

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