How to Reduce Electromagnetic Interference (EMI) in Control Loops and Instrumentation Systems. ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Reduce Electromagnetic Interference (EMI) in Control Loops and Instrumentation Systems.

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What is Electromagnetic Interference (EMI)?
Electromagnetic Interference (EMI) means the presence of undesirable interference voltages in a control loop or instrumentation system.  These undesirable voltages are generated by time changing external electric or magnetic fields emanating from such sources as:
  • Electric motors
  • Transformers
  • Power lines
  • Thyristors used in certain electronic devices
  • High frequency radiation
  • Leakage currents due to damaged electrical heaters
Electromagnetic Compatibility (EMC) of Instruments
The ability of an instrument or instrumentation system to withstand or suppress interference is defined as Electromagnetic Compatibility (EMC)

How to Reduce EMI in Measurement and Instrumentation systems
There are a variety of methods that have proven to be successful in combating electromagnetic interference in measurement and instrumentation systems. Some of these methods are briefly discussed below.

EMI Due to Electrical Alternating Current (AC) Fields
The interference due to electrical AC fields can be reduced by adequately shielding the connection wires.
Magnetic Induced EMI
The effect of magnetic induced interference is difficult to reduce using conventional shielding methods. One practical solution that has worked very well is to space the measurement circuit from the EMI sources as far apart as possible. Where interference is still possible after spacing as far as possible, the measurement connection leads should be very close to each other and in parallel if possible.

Twisted pairs or coaxial cables provide good protection against alternating current (AC) magnetic fields interference in instrumentation circuits by shortening the interference sensitive signal path and transmitting the signal over the remaining path by using the milliamps (mA) output signals from a very good transmitter.

EMI Due to Ground Loops
Ground loops are introduced in measurement system due to poor earthing practice. They are caused by currents flowing as a result of the differing ground potentials in a measurement circuit. Ground loops can effectively be suppressed by using grounded metallic sheath for the instrumentation measurement circuit.

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