Introduction to 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 its 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 its 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

Advantage

Convenient output voltage
The tachogenerator produces a convenient output voltage that can be measured with most conventional DC voltmeters. This means that you do not have to invest in a special voltmeter to measure the speed of the axis of the devices you are testing.

Indicates the direction of the rotation
The output voltage measured with the use of a tachogenerator shows the direction in which the axis is rotating. That said, in addition to information on speed, with these devices you get specific information for the direction of rotation.

Disadvantages

Possible mistakes
Due to variations in contact resistance, you must take into account the probability of errors occurring. That said, the tachogenerators require periodic maintenance and switch brushes.

Non-linearity
It is not uncommon for non-linearity to occur during the measurement of the power speed produced by the tachogenerators. This is the result of the distortions that occur in the permanent magnetic field.

 

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