August 2021

Drives & Controls & BACK TO BASICS n SPONSORED BY Low-harmonic drives can tackle high levels of harmonic distortion at a relatively low cost. Liam Blackshaw, ABB’s UK product manager for LV drives, argues that concerns over higher-order harmonics and efficiency are usually misplaced. I n musical terms, harmonic distortion is a desirable phenomenon. Think, for example, of a guitar amplifier, with its glowing row of EL84 valves, and the process that adds harmonics to the guitar’s natural sinusoidal signal to introduce warmth and character. In electrical terms, however, excessive total harmonic distortion (THD) is a serious problem, resulting in network disturbances, component failures and higher energy costs. Enter low-harmonic drives, which have an active front-end that uses IGBTs (insulated gate bipolar resistors) to convert the incoming AC power to DC. The active front-end monitors the waveform of the input current, shaping it to be sinusoidal, and uses LCL filters to combat low-frequency (<5kHz) disturbances caused by IGBT switching. Frequencies up to 500kHz are managed by EMC filters, which is comparable standard to six-pulse drives. This reduces THDi to less than 4%. In practice, this allows applications to be specified correctly without needing to over-dimension. Imagine, for example, a plant running a generator to ensure operation during a mains power loss. For plants with soft-starters, the generator will need to be over-dimensioned by 300% to allow for the high initial current demands, but pairing the generator with low-harmonic drives avoids the need for over-dimensioning, thus lowering the upfront cost significantly. There are claims that low-harmonic drives produce significant higher-order harmonics (i.e. above the 50th). However, with the filters integrated into the drive, this issue is mitigated so that levels are comparable with a standard six-pulse VSD. Likewise, any concerns over the efficiency of low-harmonic drives compared to standard diode-rectified drives can be allayed by viewing the bigger picture rather than single components. The technology behind low-harmonic drives helps to cut power losses from the transformer and to the motor, offering potentially higher overall system efficiency and helping to reduce the carbon footprint of an application, while delivering a simpler and easier installation and engineering process. As with any system, it’s vital to understand the bigger picture and not the single pieces alone. Drives are, after all, a single component. Yet for systems where maintenance, running costs and space are an issue, low-harmonic drives offer a practical, reliable alternative to older technologies and mean that harmonics won’t disrupt the harmony of your application. n How harmonics relate to running costs