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.