Tachometer generator
A tachometer generator is a robust electrical generator for recording the speed of electrical and other rotating machines, such as speed-controlled drives or engine test benches . Tacho generators were very common as speed sensors, but are increasingly being replaced by cheaper incremental encoders that can work to a standstill.
The largest German manufacturer of tacho generators is the Hübner company in Berlin.
function
Tachogenerators deliver an electrical voltage proportional to the speed , which however depends on the strength of a permanent magnet , which is temperature-dependent. DC or AC generators are used.
DC voltage generator
In DC tacho generators, an armature rotates at the speed n to be measured in the field of a fixed permanent magnet. The armature is made of soft iron and carries one or more coils , the induced voltage of which is tapped rectified via a commutator . With multiple coils, the signal is smoother. The advantage of the easy-to-use DC voltage signal (the polarity changes when the direction of rotation is reversed) is offset by a measurement error due to superimposed small contact voltages and wear on the mechanical tap.
AC voltage generator
Either the coil of the rotor is contacted via continuous slip rings or the permanent magnet rotates in the coil. The AC voltage measurement is robust against contact voltages. In addition, the frequency of the alternating voltage is proportional to the speed. Their evaluation is even easier with digital technology than measuring the voltage. The direction of rotation can be measured with this method by generating two phase-shifted signals.
Advantages and disadvantages
The main advantage is the robust construction and the ease of use. A check of remotely mounted, stationary encoders for line breaks is easily possible without additional circuits on the encoder (“ringing through”) and a separate power supply is not even required for the direct display of the speed via a moving coil instrument when using a tachometer generator. With lower demands on the dynamics (highest to lowest measurable speed), the signal is robust - tacho generators can usually be loaded with a current of 300 mA. An extended operating temperature range can easily be implemented. Another advantage is that they do not need carbon brushes and are therefore more durable.
The speed range of tacho generators extends up to 12000 min −1 . Very slow rotations cannot be recorded; resolvers that work similarly were used for this . Output voltages of 5 to 200 V / 1000 min −1 are common, depending on the maximum speed . For high dynamics, one must be able to measure small voltages or allow high voltages (observe explosion protection).