FINDING YOUR WAY IN THE EMC JUNGLE

EMC legislation is just the start to getting the right benefits from drives. Today’s drives are becoming increasingly more versatile and sophisticated. However, as drives switch large currents at high speeds, they are always likely to be regarded as a potential source of electromagnetic disturbance. In addition to becoming more sophisticated, drives are also operating […]

EMC legislation is just the start to getting the right benefits from drives.

Today’s drives are becoming increasingly more versatile and sophisticated. However, as drives switch large currents at high speeds, they are always likely to be regarded as a potential source of electromagnetic disturbance.

In addition to becoming more sophisticated, drives are also operating in an increasingly noisy and crowded electromagnetic environment. This, coupled to the implementation of the EMC legislation and a new EC drives product standard, has made EMC one of the most topical issues at present in drives manufacturing and use.

EMC has been an issue in Europe – largely driven by Germany – since the late 1940s and most manufacturers welcome the benefits that the new standards bring in terms of international trade.

As a result, EMC legislation is also an area where Europe has a significant lead over America, which currently has no specific EMC standards.

EMC rule benefits

Designing to meet today’s EMC needs provides a number of benefits. The first is that the number of internal components – such as microcontrollers and ASICs – which can generate high levels of noise – can be reduced or placed into a smaller space. The number of circuit boards can also be optimised.

Current is diverted away from the boards using high frequency connectors and capacitors from ground to board – ensuring the same HF voltage in both – and from board to board. Usually, the smaller the drive, the easier and cheaper it is to solve the EMC problems.

However, complicating this is the fact that smaller drives are likely to be used closer to residential areas, where even the smallest EMC variation can be critical.

Further, for any drive in any type of application, the further the motor is from the drive, the more problems there are likely to be with the EMC environment.

Poorly installed drives can cause a number of problems. For example, in a manufacturing process, the transducers in a PC or PLC-based system can be disturbed, resulting in incorrect weight or conductivity measurements, which could be critical to quality control in a processes such as food production.

In environmental control applications, a poorly fitted drive can interfere with both the energy management system and computers controlling areas, such as alarms and sprinklers. It can also crash computer systems.

These sorts of problems are almost always caused by poor installations. For example, in the past, suspected EMC problems have been found to be caused by using more than the recommended length of insulated cable between drive and motor, poorly grounding products and, in one notable case, having 750m of cable running to a pump house located in the middle of a sewage works.

It is not enough just to comply with the standards in the EMC Directive. It is best to aim to exceed the EN generic standards, typically by using full metal enclosures and having built-in RFI filters, ensuring a better EMC performance.

To help users ensure that their plant, machine or system lives up to the EMC Directive, it is best to use drives that are not only CE-marked according to the EMC directive, but which also have built-in RFI filters.

RFI filters must be grounded at exactly the same potential as the drive. With an in-built filter, the customer need not worry, but with an external filter, great care must be taken to give correct EMC performance.

To ensure this performance, users should ask the drives manufacturer for a technical construction file, which certifies that a competent body has tested and approved the drive according to the new EMC norms, which are more restrictive than the old EMC standards.

Grounding points

Further, it is vital that all the main component parts of the product have the same galvanic potential. For example, the heat sink on the back of a drive must be attached to the unit using washers with teeth which cut through to the metal.

Large surface area earthing cable should be used to equalise the noise between ports, and cables should not be twisted into a pig’s tail, as this reduces the effect of shielding.

Finally, ensure that the RFI drive combination can still give correct EMC performance with longer motor cable lengths.

* The Author is with Danfoss.