Tachometer generators and tachogenerators are small AC or DC generators that output a voltage in proportion to the rotational speed of a shaft. They are capable of measuring speed and direction of rotation, but not position. T hey convert a rotational speed into an isolated analog voltage signal that is suitable for remote indication and control applications. Tachometer generators and tachogeneratorsare also used in servo systems to supply velocity or damping signals and may be mounted on or in the same housing as a servo motor. There are two basic types of tachometer generators and tachogenerators: alternating current (AC) generators and direct current (DC) generators. Both types of products develop an output voltage that is proportional to the rotational speed. In the case of DC generators, the phase or polarity depends upon the direction of rotation.
Performance specifications for tachometer generators and tachogenerators include generated voltage, accuracy, maximum speed, ripple, and operating temperature. Generated voltage is expressed as a range and measured in volts (V) per 1000 revolutions per minute (rpm). Accuracy or linearity is the deviation of the voltage output signal expressed as a ± percentage (%). Maximum speed is measured in rpm, revolutions per second (rps), or revolutions per hour (rps). Ripple is a percentage of the output voltage. Operating temperature is expressed as a range and measured in degrees Fahrenheit (°F) or degrees Celsius (°C).
There are several different mounting methods for tachometer generators and tachogenerators. Base mount devices provide attached pads or feet. Generators with machined grooves attach with clamps. Some tachometer generators and tachogenerators have a special flange or plate with holes for mounting. Others have tapped holes in the body of the generator itself. Male and female threaded shafts allow tachometer generators and tachogenerators to be mounted directly to the servo motor. The Society of Automotive Engineers (SAE) take-off method involves the attachment of a threaded, nut-like device on the body close to the shaft that tightens on the motor. Typically, the shaft is keyed for ease of insertion.
Tachometer generators and tachogenerators are available with a variety of features. Explosion proof or intrinsically safe (IS) devices do not produce sparks or thermal effects under conditions that would ignite a specified gas mixture. Weather proof generators are designed to prevent the ingress of moisture or dirt and handle variations in temperature. Some tachometer generators and tachogenerators are built to U.S. military specifications (MIL-SPEC). Others include an auxiliary encoder that is attached to the same axle as the tachometer generator or tachogenerator.