October 2019

WATER INDUSTRY n V ariable-speed drives (VSDs) play a key role in many water and wastewater processes, controlling the motors that power a variety of pumps, including intake, transmission, distribution, booster, influent and effluent types. Most of these VSDs are low-voltage (LV) or medium-voltage (MV) systems, but the power range covered by each category can vary from one manufacturer to aniother. For very low or very high power applications, there is often no contest in choosing between LV and MV systems, and the decision is largely determined by the power requirement. For instance, at 250kW or lower, a LV drive will win the day almost every time. However, in the higher power ranges there exists a grey area where the LV and MV technologies overlap, and either type of drive could feasibly be used for the same application. LV drives are far more common, primarily because LV applications are much more abundant in water treatment facilities. At lower power ratings, there is a perception that they are generally smaller, easier to maintain, and carry a lower initial capital cost. Many engineers therefore gravitate towards LV drives because it is the technology with which they are more familiar. There is nothing inherently wrong with this, and indeed it is understandable to favour technologies with which you are familiar. MV drives are often custom-engineered to meet the requirements of an application, and while they need relatively long lead times for delivery – typically from 12 to 20 weeks – this can actually be a blessing. The extra time compared to a typical LV installation means that infrastructure – such as switchboards – can be prepared before the drive goes in, allowing time to ensure that, for instance, the right configuration of supply is available. Cabling and its installation are a significant part of the overall lifecycle costs of a pumping system. Compared to LV drives, MV drives tend to operate at higher supply voltages, with much lower currents for the same output power. Consequently, MV drives require much smaller diameter cables with far less copper content, substantially reducing the cost of the cabling and its installation. It also permits much longer cable runs with lower voltage drops, which multiplies these savings further. LV cabling can be around £150 per metre more expensive than MV cabling, so 250m of three-core, 50mm 2 medium-voltage cable comes out at about half the cost of three-core, 185mm 2 low-voltage cable. Clearly no two applications are the same, but in situations where there is high supply voltage, and long distances between the distribution switchboard, drive and load, an MV drive should be considered. However, for installations over shorter distances, an LV drive’s installation costs will generally be lower. Space requirements LV drives are perceived to have a smaller footprint than MV equivalents, but this is not always the case. In fact, a 1MW MV drive has a smaller footprint than a 1MW LV drive. For instance, an LV system will typically require external components which take up additional space, while MV drives are available with integrated transformer and sine filter options, as well as earthing switches. That said, the MV drive may need extra cooling components and devices. For existing sites, there is often little space available for additional switchgear or large cable trenches and conduits. With greenfield sites, the facility can, to an extent, be designed around the VSD, making it much easier to accommodate larger MV variants. However, in older facilities, the layout of the building, available space and proximity to the distribution cabinet, may well limit the options available for certain power requirements. Difficult to maintain? MV drives are perceived to be more complex and difficult to maintain because they require a “senior authorised person” to ensure that they are properly isolated when carrying out maintenance. However, the actual maintenance requirements, and the time it takes to perform them, are almost the same for LV and MV, and little field servicing is required for either for at least the first five years of their life. In any case, a service contract can ensure that your drive is kept maintained and optimised without the need to invest in upskilling in-house. This can make MV a compelling option, particularly for facilities that already make extensive use of VSD service contracts. LV drives may be more widely used, but that does not mean they are the best option in all circumstances. To compare only the upfront cost is to miss the bigger picture, because the complete lifecycle cost may tell a different story. When choosing a new or replacement drive, it is beneficial to work closely with the VSD manufacturer to gain a better understanding of the application requirements and to help make the best choice. n For many variable-speed drive applications in the water industry, there is often no contest when choosing between an LV system and an MV alternative. However, the choice is not always so clear- cut, as Mark Land, ABB’s solutions specialist for system drives, explains. sales@electric-motors.online 02476 980833 www.electric-motors.online DON’T KEEP ON GOING ROUND AND ROUND Contact EMO for all your Electric Motor and Automation related needs Sales, Service, Maintenance and Mechanical Engineering Electric Motors Online is a family run business located in Nuneaton, Warwickshire. With over 30-years experience in the electric motor and automation industries we are distributors and resellers for a large number of major manufacturers. 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