Don’t be the victim of electrical noise and EMI

Electromagnetic interference (EMI) and radio frequency interference (RFI) in automated machinery can negatively affect operations, and there are options to reduce or remove it. Learn the consequences of electrical noise and benefits of quieting things down.

10/21/2015


Figure 1: A spark is often produced due to high frequency switching and is a very common source of EMI in industrial applications. Image courtesy: AutomationDirectEven with proper precautions, some electromagnetic interference (EMI) and radio frequency interference (RFI) will be present in automated machinery, so components should have appropriate shielding and filtering such that interference will not negatively affect operations. 

Meet electrical noise

EMI and RFI are unwanted electrical noise that can interfere with digital, analog, and communication equipment and processes. The three areas of interest for EMI/RFI are the noise source, the transmission medium, and the noise receiver.

EMI and RFI are often used interchangeably, but there is a difference between EMI and RFI. EMI is the appropriate term for low frequency noise, less than approximately 20 kHz, and RFI is the appropriate term for high frequency electrical noise, greater than 20 kHz.

There are common mode and differential mode components to EMI noise. Common mode EMI noise is transmitted on multiple conductors at the same time and in the same direction on all conductors from source to receiver. Most pulse width modulated (PWM) ac drives produce high frequency common mode noise. Differential mode noise is induced on a conductor and travels in the opposite direction as it does on the grounded conductor. This is similar to a complete circuit with a separate supply and return path for the EMI.

Often depending on the frequency, EMI is emitted as conducted or radiated EMI. Conducted EMI is lower frequency noise that tends to travel in or within close proximity to the conductor path. Radiated electrical noise is higher frequency noise that often uses the component or cable as an antenna, transmitting the noise over a great distance.

There are different ways for EMI to be coupled to a system. Capacitive coupled noise occurs when EMI voltage spikes that are present in the source conductor cause unwanted noise on a parallel conductor. Capacitive coupling is more of a concern at high frequencies. Inductive coupling is typically related to higher currents with magnetic fields that induce current into another conductor.

Inductive coupling is more of a concern at higher currents. Common mode impedance coupling occurs when the current from two or more sources flows through the same conductor. 

Electrical noise sources

There are many sources of EMI in industry, some of which are listed in Table 1. Sources of EMI include the PWM amplifiers often found in many ac motor drives. The faster the rise time on a PWM component, the more noise it creates in the form of harmonics. For example, an ac drive with a 4 kHz switching frequency has many harmonic frequencies which produce problematic emissions. The harmonic frequencies that affect sensitive equipment the most span from 8 kHz to 100 MHz or more.

Table 1: Common industrial sources of EMI

Servo drives can create noise similar to PWM amplifiers due to voltage dips and spikes caused by the electronics switching current flow on and off at a high frequency. Switching dc power supplies also emit EMI and at a much higher level than linear power supplies.

Turning inductive loads on and off rapidly can produce a spark across an electrical contact, which can generate EMI, as can the coil circuit doing the switching (see Figure 1). Opening a current-carrying switch will cause an arc, creating a wide spectrum (or broad bandwidth) of EMI. This arc will have a much greater amplitude when opening the current flow to an inductive component as opposed to a resistive load, resulting in greater generation of unwanted noise.

Even lighting can generate EMI, in this case due to quick changes in voltage or current. Another source of EMI is static electricity and related electrostatic discharge. Nylon or other polymer-based conveyor belts often are used to move material in industrial facilities and can generate high amounts of static electricity. 

Electrical noise victims

There are several types of components often affected by EMI in industrial applications (see Table 2). Encoders rely on low-level signals from rotating equipment and are thus susceptible to EMI. Symptoms include encoder counts changing with no motor rotation and nonrepeatable position moves. Tachometers may show similar symptoms, such as incorrect speed readings and unexpected speed fluctuations.

Table 2: Components affected by EMI

Electrical noise near analog signals and measurement instrumentation often can cause symptoms including unexpected voltage spikes and ripple or jitter causing incorrect or nonrepeatable readings. This occurs more often in voltage-based signals such as 0-10 V dc. The integrity of a current-based 4-20 mA signal is less susceptible to noise.

With communication networks and components, electrical noise symptoms almost always include loss of communication or errors in reading or writing data. And with programmable logic controllers (PLCs) and other microprocessor-based components, symptoms can include loss of communications, faults or failure in the PLC or processor, discrete inputs or outputs triggering unexpectedly, and analog inputs or outputs reporting incorrect values.

Learn how to avoid and mitigate EMI as well as filter and suppression technology.


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