40 n ELECTRICAL November/December 2024 www.drivesncontrols.com How input chokes can help you to tackle VSD issues VSDs (variable-speed drives) control electric motor speeds by varying the frequency and voltage of the power supplied. They consist of a recti er, DC bus and inverter. The drives convert AC power to DC, and back to a variablefrequency AC. This process generates high-frequency RFI (radio frequency interference), which must be ltered to comply with legislation, and harmonic distortion, which compromises power quality (PQ) and system performance. Mains chokes, or line reactors, can help to mitigate these issues. Input chokes are passive inductive components connected between the power supply and the VSD. They provide an impedance to incoming AC power, thus reducing harmonic distortion and limiting inrush current. Their three main roles are: n Mitigating harmonics VSDs produce harmonics due to their non-linear nature. These harmonics can cause overheating and interference with other equipment. Input chokes increase line impedance, reducing harmonic currents and improving power quality. n Reducing inrush currents VSDs can draw large inrush currents at start-up, causing voltage dips and stressing components. Input chokes limit the rate of current change (di/dt), reducing the magnitude of inrush currents and protecting system components. n Protecting against transients Voltage spikes can damage VSDs and motors. Input chokes absorb and dampen these transients, providing additional protection. Benefits of input chokes Using input chokes in VSD systems oers several bene ts. They: n Improve power quality By mitigating harmonics and transients, input chokes help to maintain a cleaner power supply, enhancing system stability and eciency. n Extend equipment lives Reducing thermal and electrical stresses on VSDs and associated equipment prolongs their operating lives. n Enhance motor performance Smoother current and voltage pro les produce more ecient and reliable motor operation with less noise and vibration. n Comply with regulations Input chokes help to ensure compliance with industrial standards such as IEEE 519, avoiding penalties and operational issues. n Reduce RMS current and energy consumption Input chokes are crucial in reducing the RMS (root mean square) current in systems using VSDs. Design considerations Choosing and implementing input chokes requires careful consideration: n Inductance value Choose the inductance based on VSD power rating and desired harmonic mitigation, typically rated to give a line voltage drop of 2-4%. The dierence in these ratings can aect the reduction of harmonics and RMS input current to the drive signi cantly (see Figs. 1-3). n Voltage and current ratings Ensure that the choke can handle the maximum voltage and current. Undersized chokes may overheat, while oversized chokes can be costly. Chokes can be chosen for reduced RMS current, allowing further cost reductions. n Thermal management Provide adequate ventilation or cooling VSDs can introduce harmonics and electrical noise which can a ect power quality and system performance. Mains chokes, or line reactors, can mitigate these issues. Steve Hughes, technical sales manager at REO UK, explores their role in VSD applications, their bene ts, and factors to consider when implementing them. Mains chokes can help to mitigate issues of high-frequency RFI interference and harmonic distortion in VSD installations
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