Tachogenerators

Tachogenerators, or tachometer generators, are electromechanical devices that generate an electrical signal proportional to the rotational speed of a shaft to which they are coupled. They are used as speed sensors and as feedback transducers in various control and instrumentation systems.

Most tachogenerators operate on electromagnetic induction. A DC tachogenerator uses a fixed magnetic field and a rotating armature, producing a DC output voltage that is proportional to angular velocity, V = Kω, where ω is the speed and K is a machine constant. An AC tachogenerator, or tacho generator, produces an output whose frequency is proportional to speed, with f = (P/120)N, where P is the number of poles and N is speed in revolutions per minute; the output amplitude can be influenced by the excitation and design.

Construction and outputs vary. DC types may be wound-field or permanent-magnet, brush-commutated devices, while AC types are essentially small alternators. Outputs are typically fed to monitoring instruments, counters, or speed-control loops. They may require field excitation and provide a linear signal over a defined speed range.

Applications include motor drives, servo systems, CNC machines, laboratory test rigs, and automotive tachometers. They offer a direct analog signal suitable for feedback and control, with ruggedness and simple interfacing.

Advantages of tachogenerators include straightforward conversion of mechanical speed to electrical signals and good transient response. Limitations involve mechanical wear from brushes and bearings in brush-type models, temperature and aging effects, and a finite useful speed range. In modern systems, non-contact sensors are sometimes used as alternatives, but tachogenerators remain a classic option for continuous speed feedback.