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.
By Jeb Moulton October 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.

With an overview of electrical noise, and with its sources and affected components understood, let’s look at how EMI can be avoided or mitigated. 

Avoid and mitigate EMI

Wiring and cables are a possible source of EMI. Separation of power and signal cables, as well as use of twisted pair cables and reduced cable length, can cut wire and cable contribution to EMI. Many other cable and wire integration and installation techniques can be used to reduce EMI.

A good enclosure design can also reduce EMI. A metal enclosure is a good place to start, along with zinc-plated backplanes for PLCs and industrial PCs. Proper layout is also important by keeping power and signal components and wiring separate. A properly grounded enclosure and a door and back panel will further reduce emitted EMI and susceptibility to electrical noise. 

EMI grounding, shielding

Proper grounding and shielding are the cheapest and one of the most effective methods to reduce EMI in a system. To start, a fully grounded system is one where there is a proper grounding conductor providing a direct low impedance path for common mode noise currents. All grounding points should have large surface areas so as not to impede current flow with higher than necessary resistance.

Braided grounding straps are a good choice when grounding a drive or other noise source to positive earth-grounding studs. Using the shortest braid possible is also good practice. Increasing the surface area of a conductive path is more important than increasing wire diameter. Since high frequency EMI travels on the surface of conductors, more surface area provides a better (lower resistance) path to ground for noise. This means that fine, multi-stranded wire is better than solid wire as a conductor surface area is increased-a braided cable is the best option.

When grounding a cable shield, it should only be connected to ground at the panel or source of the noise. This is also true for analog cables. However, for high frequency sources above 1 MHz, it’s best to connect the shield to earth ground at both ends of the cable.

Central-point grounding reduces the chance for current ground loops, which can occur when two or more grounding points are at slightly different potentials. This can cause high currents throughout the ground network, allowing more noise to couple to the conductors. Sometimes an unshielded cable can be less noisy than one with a shield that is grounded at both ends if there is a significant ground loop present.

Filter and suppression technology

Figure 2: EMI filters can mitigate a wide variety of electrical noise in many types of automated equipment. Image courtesy: AutomationDirectAn ac line filter can remove noise and keep it from getting onto the ac power grid (see Figure 2). An ac line filter should be mounted directly to a grounded frame or backplane. The filter should be mounted as close to the point where the ac power enters the enclosure as possible. Also, to minimize RF coupling, the ac power cable on the line side of the filter should be routed as far away as possible from the load side ac power cable and from all other cable and circuitry. The filter should be properly grounded and placed as close to the noise source as possible, and the ac power cable length should be as short as possible, with the wires twisted together if using individual wires.

Input line reactors can be used to protect ac drives from transient overvoltage conditions. Input line reactors also reduce the harmonics associated with ac drives by increasing line impedance at unwanted frequencies and are one of the most frequently recommended ac drive accessories.

Inductive filters or line reactors added to the output of the drive, in series with the motor, are used to increase the load inductance to meet the minimum load inductance requirement of the amplifier. They also serve to counteract the effects of line capacitance and the reflective wave phenomenon found in long cable runs and in high voltage systems. Line reactors can sometimes be used in place of or in combination with ferrite suppression cores.

It is important to note that motor drives may require their own filter. In some highly sensitive EMI applications, it may be necessary to use an EMI main filter on each drive. Other equipment should not be powered from the load side of these drive filters. It’s also important to keep the current draw balanced between phases. Single-phase equipment connected to a three-phase filter can unbalance the current draw and degrade the ability of the filter to suppress unwanted EMI.

The switching of inductive loads such as contactors, relays, and pneumatic solenoid coils can produce high RFI. The use of suppression diodes across dc component coils and resistor-capacitor (RC) snubbers across ac component coils can greatly reduce voltage and current spikes and related EMI/RFI emissions. Suppression diodes and RC snubbers must be placed directly at the source of the RFI emission or coil.

Regardless of the type of electrical noise and the type of component affected, there are many techniques to avoid and eliminate EMI. Following these recommendations will go a long way to create a system with low levels of emitted electrical noise and high resistance to this noise.

– Jeb Moulton is a product engineer at AutomationDirect. Edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, eeissler@cfemedia.com

Key concepts

  • An ac line filter can remove noise and keep it from getting onto the ac power grid.
  • The switching of inductive loads such as contactors, relays, and pneumatic solenoid coils can produce high RFI.
  • For high frequency sources above 1 MHz, it’s best to connect the shield to earth ground at both ends of the cable.

Consider this

Proper grounding and shielding is the cheapest and one of the most effective methods to reduce EMI in a system. 

ONLINE extra

– See additional articles on electrical noise linked below.