Tachogenerators
An 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. |