An RTD reading fluctuates only when a nearby motor starts. Likely cause?
A. Calibration error
B. EMI noise
C. Wrong configuration
D. Grounding
View Answer
Answer: B
Explanation:
If an RTD reading fluctuates only when a nearby motor starts, the most likely cause is Electromagnetic Interference (EMI).
Electric motors generate electromagnetic fields during starting, especially large motors, VFD-driven motors, and motor starters. These electromagnetic disturbances can be induced into RTD cables and signal wiring, causing temporary fluctuations in the measured temperature.
Common Sources of EMI
Motor starters
Variable Frequency Drives (VFDs)
High-current power cables
Contactors and relays
Transformers
Typical Symptoms
- Temperature spikes when motors start
- Intermittent fluctuations
- Unstable readings during motor operation
- Readings return to normal when the motor stops
How to Reduce EMI
Use shielded RTD cables
Proper cable grounding and shielding
Separate signal cables from power cables
Use twisted-pair RTD wiring
Ensure proper transmitter grounding
Why the Other Options Are Incorrect
Calibration error
- Causes a constant offset, not fluctuations only during motor startup.
Wrong configuration
- Produces continuous measurement errors, not intermittent disturbances.
Grounding
- Poor grounding can contribute to noise, but the specific symptom of fluctuations when a motor starts is most directly associated with EMI.
Because the temperature reading changes only during motor startup, Option B (EMI noise) is the correct answer.