VSD noise issues

BY STEPHEN BROAD BENT-REGIONAL MANAGER DAN FOSS NEW ZEALAND

When it comes to the world of Variable Speed AC Drives, there are two variations on the topic of noise: audible noise and electromagnetic noise emanating from the AC drives on various frequency bands. We’ll focus on the latter.

Many of us beyond a certain age probably remember seeing random dots and lines (commonly called ‘snow’) on our CRT TV screens when a vacuum cleaner was operating in the same room or a two-stroke motorbike rides by. Today, although less common, some interference can be heard in speakers when our mobile phone changes its receiving station or a receives a call.

These are minor examples of some symptoms of electromagnetic interference (EMI). But imagine what could happen if EMI were to cause malfunctions i.e. medical equipment in hospitals. Alongside the uptake of communication systems in industry – and particularly the Internet of Things – with devices connected via one common protocol the need for seamless interference-free communication is paramount.

Incorrect data or delays in transmission and control causes productivity loss in industry or serious potential life threatening situations in a hospital. Every piece of equipment should be compatible with the electromagnetic environment it is destined for. (EMC Compliance.) This involves not introducing intolerable disturbances into its environment and also having resistance to incoming disturbances generated from other pieces of equipment. The quality of the installation is vitally important to a device’s EMC compatibility. Close attention should be payed to the manufacturer’s installation guide and the length of motor cable that a VSD can tolerate to confirm to EMC standards.

STANDARDS ARE IN PLACE TO COVER THIS

Disturbances in the range of 150 kHz to 30 MHz (radio frequency interference, RFI) is 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 four categories (C1 to C4), depending on the environment they are intended for, the EMC knowhow 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.

Steep- edged pulses are generated in the output between the AC drive and the motor due to Pulse Width Modulation (PWM) of a drives output transistors. 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 fulfil 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 minimise the impacts of EMI on your systems. It’s not just VSD’s that procedure EMI effects. With the growth in Solar converters, Electrically Commutated motors (EC Motors) and other switch mode power supply devices, our supply grid is ever more stressed with potentially disruptive EMI. This also raises the question of the correct operation of new technologies such as Ethernet transmission over power lines.

BEST PRACTICES INSTALLATIONS

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. As the high frequency noise travels on the outside of a conductor, skin effect, a braid with a high surface area connection is an ideal solution to capture this noise and contain it within the VSD’s filter network. Installers often incorrectly 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 percent.

• Interference effects decrease significantly with increasing cabling distance: respect a minimum clearance of 20cm 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 EMCsensitive 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 Danfoss VLT AQUA FC202 VSD allows up to a 150m cable for C2 conductedemissions-limit compliance, but a 50m 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.