| TachogeneratorsAn electromechanical generator is a device 
                    capable of producing electrical power from mechanical 
                    energy, usually the turning of a shaft. When not connected 
                    to a load resistance, generators will generate voltage 
                    roughly proportional to shaft speed. With precise 
                    construction and design, generators can be built to produce 
                    very precise voltages for certain ranges of shaft speeds, 
                    thus making them well-suited as measurement devices for 
                    shaft speed in mechanical equipment. A generator specially 
                    designed and constructed for this use is called a 
                    tachometer or tachogenerator. Often, the word "tach" 
                    (pronounced "tack") is used rather than the whole word.  
                      By measuring the voltage produced by a 
                    tachogenerator, you can easily determine the rotational 
                    speed of whatever it's mechanically attached to. One of the 
                    more common voltage signal ranges used with tachogenerators 
                    is 0 to 10 volts. Obviously, since a tachogenerator cannot 
                    produce voltage when it's not turning, the zero cannot be 
                    "live" in this signal standard. Tachogenerators can be 
                    purchased with different "full-scale" (10 volt) speeds for 
                    different applications. Although a voltage divider could 
                    theoretically be used with a tachogenerator to extend the 
                    measurable speed range in the 0-10 volt scale, it is not 
                    advisable to significantly overspeed a precision instrument 
                    like this, or its life will be shortened.  Tachogenerators can also indicate the 
                    direction of rotation by the polarity of the output voltage. 
                    When a permanent-magnet style DC generator's rotational 
                    direction is reversed, the polarity of its output voltage 
                    will switch. In measurement and control systems where 
                    directional indication is needed, tachogenerators provide an 
                    easy way to determine that.  Tachogenerators are frequently used to 
                    measure the speeds of electric motors, engines, and the 
                    equipment they power: conveyor belts, machine tools, mixers, 
                    fans, etc.  |