Tachometers are used to measure the velocity of a rotating object. Tachometers operate on the principle that a driven motor (i.e., a motor operated as a generator) produces a voltage that is proportional to the angular velocity of the motor shaft. The proportionality constant, K that is used to translate mechanical motion into voltage has typical values of 1 to 30 volts per 1000 RPM. Tachometers are typically attached to the output shaft of DC or AC inverter motors requiring close speed regulation. The tachometer feeds its signal to a control which adjusts its output to the DC motor or AC inverter motors accordingly (called "closed loop feedback" control). Tachometers can be of both the contact and noncontact type.
Common configurations for tachometers include handheld, portable, fixed, or modular. Handheld instruments are specifically for using while holding in one hand. Portable instruments have handles/case/wheels etc. to make easy to move, not necessarily held in hand to use. Fixed instruments are fixed or used in one place, for example, benchtop, panel mount etc. Modular instruments have different modules for interfacing to different sensors or input ranges. Tachometers can have batteries for full operation, not just backup.
The two most important parameters to consider when specifying tachometers are operating speed range and accuracy. Operating speed range is the range of rotary speed measurement the tachometer can monitor. The accuracy is typically given in units such as ±RPM, etc. The sensor technology used in tachometers can be contact type, photoelectric, inductive, of Hall effect. In a contact type instrument the tachometer probe or sensor comes in contact with the rotating part. In a photoelectric instrument light beams, visible or infrared, are employed to measure speed. Technology may resemble optical encoders, where beam break frequency is used for speed calculation. Inductive instruments use magnetic elements to induce magnetic fields; frequency of activation is used to measure speed. Hall effect tachometers use Hall effect technology to determine rotational speed.
Features common to tachometers include totalizers or counters, timers, built-in or self-calibration, and self-test diagnostics. Display configurations include analog visual indicators, digital numerical displays or graphic video displays. User interfaces and control types include analog front panels, or digital front panels or computer programmable interfaces. Some tachometers may come equipped with software for running on the hose computer. Many have network or communications interfaces. Available electrical outputs include analog voltage, analog current, analog modulated frequency, serial, parallel, and switch or alarm.