Electromagnetic interference: Practical advantages of glass fiber temperature measurement

Electrically demanding environments, such as power converters on electric motors, are a real challenge for precise temperature measurement. imc test engineers have demonstrated this on an imc electric motor test bench.

In this application, the temperature was measured on a supply wire that sends pulsed PWM currents to the electric motor. To do this, sensors were mounted near the motor on a screw terminal of the supply wire. It was protected against contact by means of a transparent shrink sleeve. In the test, the thermocouple was fixed to the shrink tubing from the outside with cable ties. The glass fiber was inserted into the shrink tubing and placed directly at the contact point.

The concrete effects of the electromagnetic interference fields on the thermocouple are visualized in the adjacent diagram (blue curve). It can clearly be seen how the electrical thermocouple is massively disturbed with increasing speed and corresponding motor currents. These kinds of coupling via alternating magnetic fields can be completely avoided with purely optical FBG measurement technology. The technology is completely immune to EMC/ESD influences, as can be seen from the red curve of the recorded temperature signal.

The diagram shows the following measurement data:

  • Temperature via thermocouple (blue)
  • Temperature via glass fiber (red)
  • RPM (orange)
  • Phase current of the motor (green)

Our whitepaper lists further advantages of fiber-optic measurement technology.

Attachment of thermocouple and fiber-optic sensor to the motor supply wire
Comparison of thermocouple and FBG sensor temperature curves.
Report with characteristic curves of the motor
Report with characteristic curves for voltage, rpm, speed and torque