How to make sure your motor power drive system complies with EMC regulations

Electromagnetic compatibility (EMC) is closely regulated as the effects of radio frequency interference (RFI) can be severe and can lead to the malfunction of sensitive electronic equipment. EMC regulations exist to make sure domestic, commercial, medical and industrial electronic devices are protected from RFI and operate as intended.

Pre-Brexit, EMC regulations in the UK were based on various European Union (EU) directives and standards. Many of these standards have been adopted post-Brexit, and EMC regulations still apply to most electronic devices and equipment.

So what does this mean for your motor drive system, and how can you make sure it complies with the latest EMC regulations? We highlight what you need to know in this article.

Will your power drive system be compliant with the latest EMC regulations?

BS EN IEC 61800-3:2018 is the current EMC standard that applies to power drive systems (PDS). Variable frequency drives (VFDs)/variable speed drives (VSDs) or inverters are the basic drive module (BDM) which becomes a power drive system (PDS) when connected to a motor via a power cable. This is a performance standard, and means the devices covered by EN 61800-3 must meet the performance requirements in the regulations to be sold in the UK.

The regulations cover all installations in domestic, commercial, scientific, medical and industrial environments. However, it does not include motor drives in electric vehicles – these have a separate standard.

It has been proven that high frequency conducted emissions from a PDS can cause premature bearing failure in motors. Significantly, it’s also worth remembering that RFI derived from a poorly designed, assembled and installed PDS can lead to catastrophic problems with plant control system performance including digital measurement and display errors, and ultimately machine safety being compromised.

Five ‘Golden Rules’ for controlling RFI from a power drive system

Electronic device and equipment manufacturers, importers and distributors must make sure their products meet the performance standards of EMC regulations when designing and manufacturing their products.

The first step is to specify motor drive system components that meet the performance standards. To do this, check the manufacturer’s data sheets for the relevant standards, such as EN 61800-3, and the levels of RFI filtering included.

You should also do the following when designing and installing your motor drive system:

1. Earth all components to a common point. Earthing should provide a low impedance path for both fault currents and RF currents . Using a common metal earthing block for all the elements in the system helps ensure that there are no potential differences between components therefore reducing the likelihood of stray circulating currents. Be sure never to ‘daisy-chain’ earth conductors between devices, instead taking each earth conductor back to a common earth block or star-point mounted directly onto an unpainted back plate.
2. RFI filters act to reduce the amount of RF currents in the mains power supply and are usually found inside modern VFD’s. For the VFD to synthesize an AC current waveform for the motor, internal power semiconductors (IGBT’s) switch current at several hundred volts DC at high switching frequencies in the order of several kiloHertz (KHz). A by-product of this high frequency switching operation can be the generation of high frequency leakage currents from the motor cable and other parts of the PDS. There can be benefits to installing additional EMI filters such as ferrite cores around the outgoing cables from the VFD to the motor.
3. The use of screened or armoured cables between the VFD output terminals and the motor is very important as is ensuring that the screen is securely connected to ground at both the motor and the VFD. Often the use of this type of cable is overlooked or not used for cost reasons but this can be a false economy if the system builder has to return to the installation site to identify and remedy ensuing EMC problems. Properly shielded and terminated cables will vastly reduce RF conducted emissions from the BDM and PDS.
4. Wherever possible, set the VFD(s) switching frequency the number of operations per second that the IGBT’s operate to form a sine wave) to be as low as practically possible. This practice alone can reduce RFI in many applications but in some the change in audible tone from the motor can not be tolerated so this is a limited option.
5. Physically separate out the components of the system. This may not always be practical in smaller enclosures, but ideally at least 300mm between power inputs including signal cables and the VFD output (motor) cables. Routing VFD output cables inside the same trunking or conduit as power supply cables and signal cables should be avoided at all times.

How to help ensure that your PDS does not interfere with other equipment nearby or that is connected to the same power source.

Conformance to EMC at the PDS design stage then onwards throughout the life of the project through to the final commissioning stage, should be the normal target.
Dalroad can assist you by selecting EMC-compliant VFD’s for your PDS project and then guide you with installation advice and any support with necessary good engineering practice required when you build control panels and when they are being installed on site.

Contact our team for no-obligation advice and recommendations.