Countering interference

GARETH NUTT, technical manager, Schaffner EMC considers the best ways to minimise EMI

Inverter drive manufacturers are under increasing pressure to develop more compact and efficient drives. But one unfortunate side-effect of this development is the increased levels of electromagnetic interference, the most significant portion of which comes from the output PWM switching circuits which are closely coupled to the AC power input.

While it is usually best practice to reduce EMI at source, in the case of inverter drives attempts to reduce EMI at the switching semiconductor level will inevitably have a negative effect on the performance of the drive.

Motor drives are such significant producers of EMI that the cost of building in suppression is prohibitive. So, generally an inverter drive manufacturer will recommend the use of suppression components as `add-on’ devices. Consequently there is increasing pressure on EMI filter manufacturers to design and produce mains filters for use at the input of inverters. Cost is especially significant at lower power ratings where filters can represent a significant percentage of the cost of the inverter.

Figure 2 shows how interference signals generated within a motor drive are coupled into the earth system within the drive and via the cable from the drive to the motor and via the motor itself. Once in the earth system these interference currents can circulate to other equipment and the higher-frequency components will radiate from the cables into other systems, causing more problems.

The most significant coupling path for interference currents is through stray capacitance in the drive, the motor cable and the motor. The aim of suppression is twofold: to contain the PWM-induced noise frequencies in and around the drive by using suppression components such as filters, and to prevent the higher frequencies from radiating from the motor and connecting cables.

The stray capacitance in the drive and the motor is fixed, but that within the cable is a function of length: the longer the cable the better the coupling path and the higher the EMI levels. Therefore, ensuring that a PDS (power drive system) is EMC compliant is rarely as straightforward as just fitting a filter at the mains input to the inverter: it needs an overall `system’ approach to succeed. A filter that is effective with a short motor cable may be totally ineffective at longer cable lengths and different switching frequencies. Furthermore, inadequate attention to the basic principles of good earthing and screening in any motor drive system will make most `add-on’ suppression components ineffective – no matter how good their performance claims.

The use of filters in suppressing EMI from drives is aimed at preventing the interference from passing down power lines by changing impedance conditions so that EMI in the earth system is redirected back to its source.

Filtering should be applied at the drive input, and sometimes it is also necessary on the output side. At the drive input the use of capacitors and inductor filter networks is usually effective. On the output side, however, the use of capacitors is restricted due to their effect on drive performance, and so the only choice is often a low-value choke.

But an input filter alone may not be the solution to an EMI problem because the EMI may be escaping in other ways. In such cases, it is useless to replace the filter with another higher-attenuation filter unless other measures are taken. Figure 3 shows a typical motor drive system.

The most common source of EMI problems is poor earthing. This may appear illogical as drives are used in Class 1 installations where all accessible conducting parts are connected to an earth point, usually through 1.5mm2 conductors. But such an earth system is designed for safety purposes, and there is a major difference between a safety earth and an EMI earth, especially at the significantly higher frequencies found in interference currents.

Low impedance

What is needed is an earthing system where all ground connection points present a very low impedance to HF. All connections to ground must be as short as possible. `Pigtail’ connections will always cause EMI problems and must be avoided: for example 10cm of 12AWG copper wire has an impedance of approximately 0.9 at 1MHz and 10cm of braid approximately 0.1. Earth bonding areas should be as large as possible: a bond area of 2.5cm by 0.08cm has an impedance of 0.01 at 1MHz. If the inverter is in a cabinet then the cabinet wall is ideal as an earth conductor.

Inadequate screening is another source of EMI problems. When the drive is mounted in a cabinet this can provide the first screen, but it is important that all sectional panels are solidly bonded together. It is often overlooked that paint or corrosion must be thoroughly removed to achieve bonds that present a low impedance at RF frequencies.

The motor housing itself is a solid effective EMI screen. It is almost essential to screen the motor cable, and it is fortunate that in many installations screened or armoured cables are already specified for mechanical protection. A screened motor cable is only effective if it is earthed at both ends, and it is recommended to use 360 clamps on screen braiding as close to the inverter and motor as possible and with no breaks along its length. The aim is to ensure that all screens – cabinet, cable screen, motor housing – are connected together to effectively form one screen.

It is critical to position the input filter as close to the inverter as practical using a short screened connecting cable. To be effective, the filter housing must itself be connected solidly to earth – usually a metal cabinet wall – always ensuring that paint or corrosion is removed first.

A near-perfect option would be to use a so-called `footprint’ input filter. These devices are becoming increasingly popular (but are not available for all drives), and have the advantage of being designed specifically for a particular model of motor drive, matching its EMI characteristics. They offer an easy mechanical fit, directly on to the drive and achieve good EMC performance within limits (usually of motor cable length and switching frequency) specified by the drive manufacturer.

One alternative is to supply input filters in a narrow `bookshelf’ style which are designed to sit next to the inverter drive with a connection system designed to facilitate the shortest possible exposed cable length between the filter and the inverter.

EMC is no longer an option. Power drive systems now have their own `product specific’ EMC standard, BS EN61800-3:1997 which defines minimum EMC requirements, and it is incumbent on both suppliers and installers of such systems to comply. The standard covers systems installed in both industrial and residential environments. It also differentiates between two types (modes) of equipment sale, `restricted’ and `unrestricted’ because some customers may not have EMC expertise.

Schaffner’s 258 series filters use a slimline book-style housing in preference to the usual box shape to facilitate simpler and more compact systems building

Schaffner Tel: 01189 770070