March 2020

43 www.drivesncontrols.com March 2020 for harmonics content in the network. The answer is to use ultra-low harmonic (ULH) drives designed specifically for HVAC applications because they can maintain low harmonic content, even at partial loads, at the source. With a proper system analysis, adding ULH to chillers, large pumps and fan sets can eliminate the need for external harmonic mitigation for the system as a whole. There is also a need to consider the type of load on the network, because it affects power factor (PF) – a measure of how effectively equipment is using the electricity. Data servers are capacitive loads. Cooling equipment contains large inductive loads. Both are sources of reactive power that impact the power factor and, again, cause energy losses as well as unstable operation for power system equipment, including UPSs and generators. For effective electricity usage and power system reliability, data centre power factors should be close to, or equal to, unity. The final element in power quality is RFI (radio frequency interference). Data centres use large amounts of variable-speed equipment to save energy. All variable-speed equipment generates electromagnetic noise, both radiated to the near field and cable- borne, which is transmitted throughout the data centre and to neighbouring buildings. This radiation might also affect IT equipment and data centre security and performance. Any variable-speed product must comply with EN61800-3, the EMC standard for drive systems. Negligence in working to the correct EMC standards can cause severe issues. Not only will there be a need to install external filters at additional cost later on, but managing high-frequency noise after the installation is completed can be almost impossible. This is because high frequencies within the cables also radiate. If, for example, mains and motor cables are installed in the same cable tray, the RFI filters essentially serve no function, and the only solution is to move one of the cables to a separate tray at substantial cost. There are several approaches to controlling the speed control of data centre fan motors. One approach is to use EC fans – speed controllers packaged with motors and fans. If these are installed in an array, they offer redundancy if one of the EC motors fails. But they are usually controlled via fieldbus, and the drawback is that there is no“hand”mode, so speed control is lost if connection with the control system fails. It is also not possible to use a bypass. An alternative is to use multiple AC motors, each controlled by its own VSD. This offers individual fan control in a fan array application, or control via fieldbus. BACnet can provide an easy-to-use interface.“Hand”and“auto”modes allow fan control even if the control system fails. There is also redundancy so that, if one or several fans fail, the others will continue to work with increased speed. Operability and Serviceability It is also possible to configure fan array controls via a single drive. A redundant drive is kept in standby mode to maintain the required airflow should the main drive fail. Bypass usage to run the fans direct-on-line is also possible if no redundant drive is employed. With fan arrays in particular, it is important to consider what happens should the motor or drive fail. For an EC fan, or a fan with an integrated motor, it is often necessary to replace the complete assembly. This is costly, can involve long lead times, and affects sustainability because it will be scrapped. Should an integrated motor-drive fail, the components can usually be replaced individually. However, the drive and motor spare parts are frequently locked to a package supplier, resulting in long lead times and high replacement costs. The optimum choice for better serviceability is to specify standalone motors, drives and fans. In most cases, each of these components can be replaced on the same day. This also has a positive impact on sustainability. The functions built into VSDs designed for HVAC duties can help ensure availability across the entire cooling and ventilation system. For example, soft starting of motors eliminates the mechanical stress on piping systems as well as on equipment such as pumps, compressors and fans. Additional functions include: indicating incipient bearing failures through torque monitoring; monitoring pipe pressures to raise an alarm if a pipe starts to leak or becomes blocked; and using an override mode to help fire suppression systems to react properly, and to protect server equipment as well as workers in the event of a fire. The server density of data centres will continue to increase. That means that heat loads will also increase. This is where VSDs developed specifically for HVAC applications offer an important advantage, because they are designed with flexibility and scalability built in. n Data centres consume vast amounts of energy, but careful choice of technologies to control a centre’s HVAC systems can reduce the energy consumption and make the systems more reliable. BUILDING SERVICES n

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