FRS Free-Run Speed Sensor

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Product Description

Magtrol’s FRS Free-Run Speed Sensor is used for accurate measurement of the free-run speed of any motor under test. The FRS Sensor is composed of 3 primary components:


Keyence™ Digital Fiber Sensor

  • LCD digital display: for easy sensor programming and signal monitoring
  • Simple, user friendly functions
  • Select light-operate or dark-operate functions

Fiber Optic Sensor Cable

  • Composed of a single sensing end that emits a sensing beam and receives reflected light
  • Armored ToughFlex™ flexible stainless steel jacket protects fiber from daily wear and resists entanglement or shock
  • More flexible than conventional spiral tubes with a tight bend radius of 2 mm and minimal optical attenuation

Arm and Base



When a motor is coupled to a dynamometer, or any device that has a shaft supported by ball bearings, there will be a small amount of drag load placed on the motor due to friction in the bearings. Ensuring optimal alignment of the motor and dynamometer shafts, as well as using higher-quality bearings and lubricants, can reduce this drag load. However, even when employing these tactics, drag load can never be completely eliminated. All motors will show a slightly greater current draw from the residual drag load, and some motors will show a reduction in free-run speed.


Magtrol’s FRS Free-Run Speed Sensor is designed for applications where it is necessary to acquire speed readings that are unaffected by drag load. Before connecting a motor to the dynamometer, the free-run speed can be obtained from the FRS Sensor. With its reflective sensor, the FRS does not need to be attached to the motor but only placed close to the motor shaft (as shown in the photo to the right). For best contrast, the shaft should be marked with reflective tape.

The sensing end of the fiber optic assembly emits and receives light reflected from the shaft, and sends the speed signal to the fiber optic sensor. The raw speed data is then transmitted to either a Magtrol 3411 Torque Display or DSP7000 Dynamometer Controller where it is converted and displayed in rpm.