Electromagnetic compatibility (EMC) is a crucial consideration in modern electrical engineering, particularly in the design and implementation of variable speed drive (VSD) systems. When EMC is not properly addressed, systems can suffer from operational disturbances, interference with nearby equipment and non-compliance with legal directives. Here, David Strain, technical director at integrated drive system specialist, Technidrive provides a comprehensive guide to managing EMC challenges in VSD installations.
At its core, EMC refers to a system’s ability to operate effectively in its electromagnetic environment without emitting levels of electromagnetic interference (EMI) that would disrupt other equipment. The term encompasses both emissions from the device and its immunity to external electromagnetic disturbances. EMI, on the other hand, refers specifically to the unwanted disturbances — either conducted through cables or radiated through space — that can cause systems to malfunction.
The distinction between conducted and radiated EMI is especially important in VSD systems, where the high-frequency switching of inverter outputs can generate both forms of interference.
Compliance with the European EMC Directive (2014/30/EU) is not optional. The harmonised EMC product standard for VSDs defines categories from C1 to C4 that set specific emission limits and application conditions, helping ensure products are suitable for their intended environments. Selecting a VSD and associated EMC filters that meet the appropriate category requirements is a key first step towards a compliant and reliable system.
However, ensuring EMC compliance goes beyond product selection — the entire installation must also meet EMC standards. Correct installation practices are essential, including proper grounding, shielded cabling and adherence to manufacturer guidelines. Inadequate installation could degrade EMC performance and result in non-compliance at the system level, regardless of the product’s CE marking. Finally, clear documentation and traceability must be maintained, as failure to demonstrate compliance can expose projects to legal and operational risks.
Installation techniques that ensure EMC compliance
The physical installation of VSD systems plays a pivotal role in controlling EMC performance. Drives should always be mounted inside a well-grounded metal enclosure. The quality of the grounding connection is paramount and the enclosure door must be earth bonded using a short, thick braided cable to maintain shielding integrity.
Additionally, the enclosure should include an EMI conductive seal to prevent radiated emissions from leaking through door seams. If a gland plate is used for cable entry, it should be correctly fitted and either incorporate an EMI conductive seal or ensure bare metal contact at all mating surfaces by removing any paint or coatings that could act as insulators. It’s also essential to note that the grounding system must be tested to ensure low resistance values, as high resistance can degrade the effectiveness of the shielding and compromise EMC compliance.
Another critical area of EMC control lies in the cabling. Both the mains power cables and the motor output cables should be shielded to limit conducted and radiated EMI. In particular, motor cables are notorious for acting as antennas that radiate high-frequency noise. Shield terminations must use 360-degree bonding techniques, such as EMI-compliant glands or properly grounded clips and must be connected to unpainted metal surfaces to maintain continuity.
While pigtail terminations (often used for grounding purposes) are a common installation practice, they can introduce unwanted impedance into the shielding system, potentially reducing the effectiveness of the EMI shielding. This can compromise the overall EMC performance by allowing higher-frequency interference to radiate.
To avoid this issue, it is crucial to ensure that any grounding or shielding terminations maintain a low-impedance connection. Ideally, grounding should be done using a solid, continuous connection to a suitable grounding point, without the need for pigtails. If pigtail connections are necessary, they should be kept as short and direct as possible to minimise impedance and maintain the integrity of the shielded system.
Control cables must also be screened according to the VSD manufacturer’s specifications, with care taken not to connect the screen to control common or 0V lines. While many installers hesitate to ground cable screens at both ends due to concerns over circulating currents, following the equipment manufacturer’s guidance is critical for achieving EMC compliance.
Diagnosing and solving EMC issues on-site
Even when best installation practices are followed, EMC problems can still arise, particularly in complex or high-performance systems. In such cases, using a structured troubleshooting approach is essential. A detailed EMC checklist should be used to verify key aspects of the installation, including the make and model of the VSD, the necessity and category of EMC filters, the length of the motor-side power cables, the grounding and bonding of enclosures, the termination method of cable shields and the carrier frequency setting of the inverter.
Each of these elements can have a significant impact on the EMC profile of the installation. For example, exceeding the cable length limits recommended by the filter manufacturer can lead to diminished filter performance. Likewise, incorrect termination of shields or inadequate grounding can result in elevated emissions or reduced immunity.
Advanced filtering solutions for persistent EMI problems
When standard measures are insufficient, additional EMC mitigation components may be required. One effective solution is the use of line chokes, which are inductive devices installed in series at the input of the drive. These chokes help reduce high-frequency disturbances by increasing the impedance to transients and also assist in reducing harmonic distortion, improving the overall power quality.
Line filters are another vital tool. These are installed on the mains supply side and are designed to suppress conducted EMI generated by the inverter’s switching operation, particularly in the 150 kHz to 30 MHz frequency range. They also offer a degree of protection from disturbances originating from the grid.
On the output side of the inverter, output chokes serve to dampen high-frequency leakage currents that can occur due to the pulsed operation of the VSD. These components are especially useful when the use of motor cable shielding is limited or when long motor leads are necessary.
In more demanding applications, output filters can be employed to convert the square wave output of the inverter into a waveform that closely approximates a sinusoidal signal. These low-pass LC filters allow the fundamental motor frequency to pass while attenuating the high-frequency switching components. Although they are generally larger due to the current requirements of the motor, they are highly effective at minimising EMI and protecting both the drive and motor from voltage spikes.
At Technidrive, we understand the importance of a robust and EMC-compliant VSD installation. Our expertise lies not only in supplying the right components, but also in offering engineering support tailored to each application. We work closely with our customers and with leading VSD manufacturers to ensure that systems are properly designed, installed and compliant with all relevant standards. By assisting with equipment selection, installation best practices and troubleshooting techniques, we help prevent costly EMC-related failures and ensure long-term system reliability.
For assistance with EMC solutions or technical support with your VSD applications, contact Technidrive directly.
