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AC Tachogenerators

These tachogenerators all have a wound stator. The voltage and the frequency are both proportional to the speed. These models can also be used as power generators, but their quality intends them for applications of controlling a drive. The absence of contact (rings and brushes) gives these generators a good stability and a good linearity of the output voltage, but the variations of concentricity and symmetry and the irregularities of the magnetic circuit can introduce a certain rate of distortion.

Some alternators can be fitted with a rectifier with which it is possible to provide a DC current proportional to the speed; in this case the polarity is constant and independent of the direction of rotation, so the drive is unable to establish direction of rotation. The linearity is affected by the rectifier and the best results are obtained for a speed higher than a certain limit which depends on the rectifier used, the provided power and the output current.

These types of tachogenerators are either homopolar with stator magnets and rotor with poles of modulation making it possible to obtain very high frequencies for the size of the machines or with rotor magnet and poles of modulation, or heteropolar with rotor magnet.

Some machines can be wound only in single-phase current, others have the possibility of being in three-phase current or even in diphase.

Signal Use

The signal is used either in frequency or in voltage.

The frequency of the pulsations of the voltage is a function of the number of segments of the rotor. It can thus reach high values. Where necessary, the output voltage can be rectified and the use of LC filters smoothes the output signal to have a voltage of good quality.

Output U = f(n) linear and possibility of numerical counting.

Linearity error

It is similar to that of a DC brushed tachogenertor

Distortion rate

It is the level of deformation of the wave of the output signal compared to a perfect wave. It is due to the harmonics of rotation and to the irregularities of the magnetic circuit.

For a good signal it is significant to have a low distortion rate.

Advantages

Use in atmospheres which can deteriorate the copper of the collectors or of the brush holders, supports particularly wet environments and hostile High rotating speed;

No or little maintenance;

Adapted to counting (with low distortion), more robust than an incremental sinus encoder;

Disadvantages

The direction of rotation is only known after the treatment of the signal in polyphase machines.