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EETE MAY 2015

Motor control enable almost anything we do. Your mobile phone charger, your computer’s power supply, your office ventilation system, photocopiers, UPS power, even the 250kW variable speed drive (VSD) controlling the motorised manufacture of your everyday goods, all undergo a process of AC to DC power conversion. The increased use of smaller and more compact power supplies in electronic devices and controlled motors has largely been made possible through the use of switch-mode power supplies (SMPS) in personal usage and VSDs in commercial and industrial usage. This is achieved by manipulating the mains power supply using components in rectifier and chopper circuits in a process of high frequency switching, or pulse width modulation (PWM). Although PWM achieves very low power losses, the process introduces harmonic currents into the supply. Harmonics are essentially multiples of the fundamental 50Hz frequency and are responsible for numerous problems, particularly in industrial environments. Damage The most immediate problem is increased energy consumption. The distortion caused by harmonics results in a non-linear, nonsinusoidal waveform. For example, if the fundamental current drawn is 70A, this capacity may consist of 20A of harmonic frequencies, meaning that the total current the system has to supply is actually 72.8A This increased consumption is often taken for granted and widely goes unchallenged. It’s only when harmonic currents lead to more noticeable component damage that concerns are raised. The most common damage caused by harmonics includes the overheating of transformers and windings. As the problem spreads throughout an interconnected system, bearings can begin to arc at high frequencies, resonance can cause powerfactor correction capacitors to oscillate, resulting in overvoltage spikes. As well as this, hysteresis - the unwanted magnetisation of an inductor core, can lead to energy losses and eddy currents in motors. Collectively, these problems can begin to affect both the local system internally and the wider supply system externally. Internally, electromagnetic interference (EMI) can begin to affect telecommunication equipment and metering apparatus. Significant distortion of the current waveform locally can begin to distort the voltage supply externally, creating symptoms such as flicker on public low voltage networks. And it’s not just at the material level. The prevalence of harmonics can cause operational problems too. System design and equipment purchasing decisions can become more difficult for managers. When the time comes to expand the system it’s quite possible that the power capacity of equipment will have been reached prematurely, meaning that drives, conductors, transformers and fuses may need to be sized larger than necessary to accommodate the harmonics. Directives Even on existing systems, shielded cable becomes mandatory to comply with electromagnetic compatibility (EMC) requirements. Continuous and problematic levels of EMI can constitute EMC issues, rendering some equipment unsafe to use. Standards set at European level, dictate the minimum requirements for safe usage of VSDs in power drive systems (PDS). IEC/EN 61800- 3 directives apply to manufacturers, panel builders and systems integrators. This only serves to add further cost to any given configuration as typical distances between drive and motor can exceed 50m. Here, shielded cable becomes even more necessary as signal attenuation becomes a greater problem over longer distances. The European parliament argues that 70% of all industrial electricity use can be attributed to motor driven systems and that if energy use is not curbed, it could reach 1,252TWh by 2020. The eco design directive covering electric motors, EC 640/2009, goes on to explain that energy use can be reduced by 20-30% and mandates that from January 1, 2015, all new motors not already capable of IE3 efficiency must be equipped with a variable speed drive (VSD). Halting harmonics So what can be done to overcome harmonic currents? Depending on the total harmonic distortion (THD) level of the system and the amount of mitigation required, users can opt for various solutions. For legacy setups that require a high level of redesign, active front end (AFE) or multipulse units can be installed to inject opposing frequencies into the supply, effectively cancelling out the harmonics. The benefit of these systems is that they also provide additional features such as regenerative braking. The downside is that AFE systems may require considerable planning and expense as well as technical expertise and regular maintenance costs. An alternative and much cheaper solution is to use passive filtering. Easily retrofitted into the system, these filters are a quick and effective method of substantially reducing harmonics - to less than 5% THD. Targeting the input and output, various filters offer differing levels of mitigation. From motor chokes and sinus filters to line chokes and dedicated harmonic filters. In recent tests done by REO for a well-known UK manufacturer of drives, the rms current drawn by a 22KW variable speed drive at full load was 42.4A. When a harmonic filter was fitted between the three phase supply and the drive, the same 22KW drive output power was achieved with a 32A supply current; almost a 25% reduction. How was this achieved? By reducing the THD level from 67% down to just 7.6%. Assuming the motor and drive system was running at full load for eight hours a day for 200 days a year and that the price per unit of electricity is 12.7p – the annual saving that could be achieved in the first year, including the cost of the filter, could be £1,099. Even with the labour and downtime costs required to fit the filter, given the increased life of the motor and costs saved by not needing shielded cable, this is an incredible return on investment (ROI). Under the rug For too long the issue of harmonics has been swept under the rug. It seems that the increasingly aggressive EU stance combined with the UK Government’s target to achieve 20% reduction in CO2 emissions by 2020 could be a wake-up call for the industry to take action. 42 Electronic Engineering Times Europe May 2015 www.electronics-eetimes.com


EETE MAY 2015
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