When it comes to the world of AC drives, there are two variations on the topic of noise: audible noise, typically generated by cooling fans, and electromagnetic noise emanating from the AC drives on various frequency bands. In this post we’re focusing on the latter.
Many of us beyond a certain age probably remember seeing random dots and lines (commonly called ‘snow’) on our old-fashioned TV screens when a vacuum cleaner was operating in the same room. Today, although less common, some interference can be heard in speakers when our mobile phone changes its receiving station.
These are minor examples of some symptoms of electromagnetic interference (EMI). But imagine what could happen if EMI were to cause malfunctions, for example, with medical equipment in hospitals: sleep-apnea monitors could fail to sound an alarm when babies stop breathing, anesthetic gas monitors could stop working, ultrasound scans could malfunction resulting in a misdiagnosis… Needless to say, EMI can be a very negative thing.
To mitigate against these negative effects, the aim should be that every piece of equipment is compatible with its electromagnetic environment. This involves operating properly in that environment (being immune) and not introducing intolerable disturbances (fulfilling the emission requirements). The primary challenge here is that, depending on the source, these disturbances take place in a wide range of varying frequency bands.
Standards covering disturbances
Classical rectifiers used in AC drives produce harmonics up to the 50th order (2.5 kHz). Due to the prevalence of AC drives, this frequency band is properly covered by standards such as EN 61000-3-4 and 3-2, because harmonics can produce relevant problems in the grid. We won’t focus heavily on harmonics in this post, but you can find some great background information on harmonics here.
The intermediate range from 2 to 150 kHz (called HF harmonics) is not covered enough by standards. This is because, before the beginning of the last decade, neither significant sources of emission existed nor relevant disturbance problems had, then, been reported. Nevertheless, there are reasons to consider this frequency range when installing AC drives. AFEs, active filters and solar inverters, due to their switching frequency, produce distortion in the 2 to 150 kHz frequency range, which can affect power line communication or lead to resonances. The latter can really be troublesome, because significant heat and interference can be created even at low HF-harmonic magnitudes. As a solution, a modification of the switching frequency could be required.
Disturbances in the range of 150 kHz to 30 MHz (called radio frequency interference, RFI) is properly covered in the IEC 61800-3 standard because these disturbances interfere with public radio broadcasting. IEC 61800-3 differentiates between first (home/office) and second (industrial) environments. And devices are divided into 4 categories (C1 to C4), depending on the environment they are intended for, the EMC know-how of the installer and the voltage/current level of the device. Different emission levels are defined for both conducted and radiated emissions for each category with C1-rated devices having the lowest emission levels. Different immunity levels are defined for first and second environments (lower for first environment) and include immunity against electrostatic discharges, incoming electromagnetic field radiation (radio, mobile communications), burst transients (switching of contactors/relays), surge transients (lightning) and radio frequency common mode.
We’re very aware of the impacts that AC drives have in this frequency range. Steep-edged pulses are generated in the output between the AC drive and the motor due to PWM (Pulse Width Modulation). These pulses contain high-frequency components, which cause undesirable radiation from the motor cable, so it’s necessary to include an appropriate RFI filter in the AC drive to fulfill the emission limits of IEC 61800-3 and also increase the immunity of the drive.
This problem really comes into focus when installing several AC drives in an installation, even in IEC 61800-3-compliant ones. This is because their emissions will add up and could potentially surpass the limits of the generic EMC standards, IEC 61000-6-3 and IEC 61000-6-4, applicable for the installation as a whole. Due to the impact that many sources have on the system, these EMI problems can be, in some cases, quite difficult to detect and even more difficult to solve. Powerful sources may cause EMI over long distances; electromagnetic noise can be transmitted either through conductors or electromagnetic waves, and coupling can be galvanic, capacitive, inductive or electromagnetic. As a result, there are many things to consider when looking to minimize the impacts of EMI on your systems.
Best practices
As a result, the ideal approach is always to follow a code of best EMC practices when installing AC drives, in order to avoid malfunctioning of devices in the installation:
- Use shielded cables for motor and control wiring. The task of the shield is to ‘capture’ the high-frequency components and conduct them back to the interference source, in this case the frequency converter. Among them, much better attenuation is achieved with braided copper shields than with wound copper ones. For control wiring, using shielded cables with twisted conductors improve even more the attenuation.
- Make a correct grounding of the shield in both ends of the cable. Use EMC cable glands for this purpose. They fully surround the shield and connect it to ground over a large area. All other connection methods degrade the effectiveness of the shield. Users often twist the shield braid into a pigtail and use a clamping terminal to connect it to ground. This form of connection creates high transfer impedance for high-frequency signal components. As a result, the shielding effect may be reduced by as much as 90%.
- Interference effects decrease significantly with increasing cabling distance: respect a minimum clearance of 20 cm between control, motor and mains cables. Route them separately as much as possible.
- If cable cross between the power and signal wires cannot be avoided, it should be made with a 90° angle.
- TN-S grid distribution, with separate neutral (N) and protective earth (PE) conductors, is preferred to a TN-C grid in terms of EMC.
- Make a good grounding of the installation. Ensure that metallic surfaces are earthed with low-impedance connections. In terms of EMC, the decisive factor is not the cross-section of the conductor, but instead its surface area, since high-frequency currents flow on the surface due to the skin effect.
- Emissions over 1 MHz can be reduced by installing common mode filters, primarily intended to reduce electrical discharges in the motor bearings.
Selecting the appropriate AC drive and EMC/RFI filter class
When selecting your AC drive, you must consider, in terms of EMC, in which location it is going to be installed and the motor cable length required. The location will carry the compliance of a certain level of emissions, according to a specific IEC 61800-3 category. So, C1-rated AC drives could be requested for EMC-sensitive locations, such as hospitals. C2 would be the standard for all other public grid installations (in residential and commercial areas), and C3 for industrial grid installations, even if C2 could be requested instead in certain industrial areas.
An AC-drive model can be configured with different RFI filter options to match the location and application requirements. For a specific selection of AC-drive model and RFI filter, there is a maximum motor cable length for shielded cables that must be considered in order not to exceed the IEC 61800-3 emission limits for a specific category. For example, an H1 RFI filter in the VLT® AQUA Drive FC 202 allows up to a 150-m cable for C2 conducted-emissions-limit compliance, but a 50-m cable for C1 compliance.
Some AC drives are installed in IT grids, for example in the marine industry. In this case, the drive must be ordered without an RFI filter or it must be disconnected from earth. This is to prevent the intermediate circuit of the drive from being damaged in case of an earth fault on the mains side. This increases the emission level of the drive, and so reduces the maximum motor cable length with EMC compliance.
As you can see, even something as ‘unseen’ as EMI or RFI can cause a large amount of interference issues with the equipment around it if the proper installation steps and filter class selections aren’t made. In most cases, you can remove the internal EMC/RFI filters with a screw or removal of a jumper. But, in many cases, it’s not so simple to add the EMC/RFI protection back in. All in all, considering the installation environment and selecting the proper filter levels from the beginning will always be the safest bet.
Since this is the last post in our “It’s a harsh world…” series, we want to thank you, our readers, for joining us here over the last months. In case you missed any of the posts in our series, you can jump back to Part 1 where we’ve linked to all of the topics in this series.
Authors:
Abraham González Ponce, Application Knowledge Manager, Application & Service Products, Danfoss Drives
Jake Roeder, Global Product Marketing Manager, Danfoss Drives
