Variable Speed Drives control the speed of AC motors by varying the frequency and voltage of the electrical supply to the motor. What a VSD basically does is convert the mains AC input into a DC voltage and then convert that DC voltage back into an AC voltage with an adjustable frequency and amplitude. The vast majority of VSDs today uses Pulse Width Modulation – known as PWM – for the generation of the 3 phase voltage with corresponding frequency. In layman’s terms PWM produces a varying output frequency and voltage by switching transistors on and off at a fast rate (typically at 3,000 times per second and up to 16,000 times per second) and with very short rise times of each pulse (approx 0.2 microsec).
During the early days of digital electronics (around 1970), people used to observe flickering on their television or interference on the radio when switching on a light switch in the house. Just imagine how difficult it would be listening to the radio when a switch (which in the case of a VSD is represented by a transistor) turns on and off at 3,000 times per second or more. Such a phenomenon is a result of radio frequency interference (RFI) or electro-magnetic interference (EMI). The switching of the light switch results in interference on the mains supply which affects the operation of the television or radio.
High levels of RFI
The high frequency switching of relatively high currents by VSD output transistors and the very short rise times of each pulse can result in high levels of RFI noise radiated and/or conducted onto the mains supply or other control cables causing unintentional and unexpectedly interference in other electrical devices, especially when the level RFI noise is too high and more than allowable limits. Hence the control of the Electromagnetic Compatibility (EMC) has been an increasing necessity to ensure reliable operation. It minimises liability risk, reduce project time scales and help to meet with local standards and code of practice.
Remember that ‘prevention is better than cure’. The best time to consider all aspects of EMC is during the upfront design, selection and installation of VSDs to ensure these problems are avoided.
Need to comply to EMC Regulations
The level of allowable conducted RFI noise from the installation of a VSD is defined in local and/or international standards. In some countries the EMC performance of VSDs (and other electrical/electronic equipment) is governed by legislation (i.e. law). For example, in the European Union VSDs and other electrical/electronic equipment must be CE marked to be legally sold. The main purpose of the CE and C-Tick marks is to allow free passage of the equipment withing European Economic and other areas respectively. The presence of CE and C-tick marks provide guarantees that manufacture of the equipment meet the minimum requirements as defined by the standards.
However, it is important to highlight that it is the sole responsibility of the manufacturer to declare conformance with the above marks since typically there is no thrid party verifying compliance with the mark required limits.
Article by Eduardo Gie