Tachometer generators (or tachogenerators) are electromechanical devices which output a voltage proportional to their shaft speed. They are used to power tachometers and to measure the speed of motors, engines, and other rotational devices.
The majority of modern tachogenerators are permanent magnet types. These devices use a rotating armature, one end of which is attached to a machine shaft, to measure rotational speed. The armature rotates within a fixed magnetic field, so that its rotation induces electromotive force (voltage) proportional to the shaft's speed. The armature contacts are connected to a voltmeter circuit, which converts the voltage into a speed value.
Small permanent magnet motors can be used as tachometer generators in some applications.
A direct current (DC) permanent magnet tachogenerator design is shown below. In this particular device, the commutator between the armature and voltmeter circuit serves to convert internally generated AC voltage into DC for compatibility with the voltmeter.
Drag cup tachogenerators are a less common type which use an aluminum cup rotating within a wound electromagnetic stator; the cup is attached to a shaft. Alternating current is supplied to one winding of the stator, generating eddy currents around the cup. The rotation of the cup induces a proportional voltage in the other stator winding.
A drag cup tachogenerator.
Image credit: MachinerySpaces
Use and Applications
Tachogenerator use has numerous advantages. For example, when a corresponding shaft reverses direction, a permanent magnet DC tachogenerator's voltage polarity will also reverse. Tachometer generators are therefore ideal in control or measurement applications which require directional indication.
Tachometer generators are frequently used to measure engine and motor speed, as well as the corresponding speed of powered equipment such as conveyors, mixers, fans, and machine tools. They are therefore employed as feedback components in speed control circuits, as shown in the image below.
A tachogenerator within a closed-loop control system.
Image credit: SPG Motor
Tachogenerators can also be used as feedback sources for motor voltage control. For example, a motor requires substantial voltage to start but can then run on much lower voltage. The use of a tachogenerator allows for a continuous, proportional supply of voltage to the motor, resulting in much higher speed ranges under various loads.
Tachogenerators can be used to power tachometers, which are gauges that display revolutions per minute (RPM). It is important not to confuse the two devices, as they are both related to rotational speed measurement but very different in form and function.
While most tachogenerators share the same form factor (shown in the image at the top of this page), they are typically designed to output either alternating current (AC) or direct current (DC). Both types of current have unique properties.
Direct current flows in one direction; when it reverses direction it is said to reverse polarity. As discussed above, a DC tachogenerator reverses polarity when the shaft rotation direction changes, resulting in an output signal that describes both shaft speed and direction. A DC signal's amplitude determines its output value.
Alternating current changes its polarity twice for every signal cycle, so that it lacks a definite direction. This attribute means that an AC wave's amplitude and frequency determine its output value, and that shaft rotation direction signified by a DC signal is not possible.
Comparison between DC and AC signals.
Image credit: Product Design Funda
Tachometer generators may be governed by standards and specifications like those listed below.
All About Circuits - Tachogenerators