January 2021

29 www.drivesncontrols.com January 2021 VARIABLE-SPEED DRIVES n PLC. Typical applications might include multi- VSD conveyor systems, or self-contained, duty-assist, standby pumping systems that take advantage of the modern VSD’s pump control capabilities. With the addition of a suitable operator interface, you can have a self-contained automation system. It is also possible to access drives in hard-to-reach locations wirelessly, enabling simple remote monitoring and the ability to fine-tune the VSD without needing to be next to it. Sophisticated diagnostics functions are extending the capabilities of VSDs even further, turning them into predictive maintenance platforms. Some VSDs can monitor their own internal components, including cooling fans, capacitors, power modules and fault contact relays. The VSD tracks operating times and duty cycles and warns when these components are nearing the ends of their lives, giving maintenance engineers time to replace parts during scheduled downtime and eliminating unexpected failures. A diagnostics function unique to Mitsubishi VSDs is the ability to monitor internal components for H 2 S contamination, thus giving warnings of potential drive failures. This is particularly useful in applications where decaying organic matter is present, such as in water treatment or effluent plants. VSDs can also derive information about the health of loads they are driving, comparing the current being drawn with the figures recorded during commissioning – a representation of the relationship between speed and torque under normal operating conditions. In fan applications, for example, an increase in the current drawn might indicate a blocked filter or debris build-up on the fan blades. Conversely, in a conveyor application, a sudden drop in the current drawn could highlight a broken drive belt or trunnion. The VSD can output different warning signals at preset levels of the current being drawn. This can initiate specific remedial functions, such as a cleaning cycle on a pump application, or can stop the system in the event of particular conditions to prevent additional costly damage. Recently, Mitsubishi has extended the diagnostics capabilities of its VSDs by introducing the concept of drive-based condition monitoring. The pre-configured, plug-and-play system includes a Schaeffler SmartCheck vibration sensor and uses the VSD’s built-in PLC to offer drive-based preventative maintenance functions. Operating temperature and vibration feedback from the SmartCheck sensor is combined with other parameters, such as speed, voltage and current. Detailed diagnostics are highlighted on a built-in display. This added diagnostics capability means that it is now possible to monitor not only the health of the VSD, but also that of an attached asset. These extended functions gain additional power when a VSD’s networking capabilities are harnessed. With some drives supporting a host of protocols without needing option cards, and being able to communicate over two different protocols simultaneously, the drive can produce a seamless flow of data. This can encompass plant-level communication via CC-Link, for example, up to controller or edge computing levels, or to the entire enterprise over Ethernet. Special-purpose drives While many VSDs are general-purpose products suitable for a wide range of applications, there is a growing trend to incorporate functions specific to particular tasks – controlling pumps, for example. Even more specialised tasks can be addressed via dedicated parameters or additional hardware within the VSD. For example, sensorless torque control can be used to regulate tension in winding applications such as paper mills, where the VSD adjusts the torque command value depending on the inertia of the paper roll. Such functions can also support taper tension and inertia compensation. Traditionally, this would have been regulated using external tension controllers, with feedback to a PLC and a control signal sent to the VSD, but with sensorless torque control built in, the VSD can handle all of these functions internally by setting a few parameters. For crane and hoist applications where there is a tendency for the load to act like a pendulum, Mitsubishi has developed a set of configurable algorithms that can be used to reduce or even eliminate this effect. Another function can prevent crabbing on travel cranes by synchronising the driven wheels. All of these extended functions represent the here and now for VSDs, delivering enhanced capabilities for some extremely challenging applications. They take drives well beyond just speed and torque control, and highlight how the modern VSD can truly become the basis of highly sophisticated automation platforms. n It is now possible to commission, parameterise, maintain, troubleshoot and monitor drives conveniently from smartphones.