DRIVES, CONTROLS & MOTORS FEATURE
Modern VSDs: meeting automation demands Wayne Turtill, product manager for variable speed
drives, servos and low voltage switchgear at Mitsubishi
Electric UK, highlights how the modern VSD can form the basis of a highly sophisticated automation platform
T
oday, the modern variable speed drive (VSD) is seen as the cornerstone of the automation
environment. Back in the early 1980s it was a revolutionary product, opening up the potential to use low cost AC induction motors in applications where the more expensive DC motor had been the only option. Over the years, its increasing sophistication has seen growth rates little short of phenomenal. This is increasingly a result of the VSDs ability
to address evolving trends or tackle industry issues – from greater energy efficiency to the need for increased safety. Furthermore, the more sophisticated products can operate with an increasingly diverse range of motor types. The latest VSDs from Mitsubishi Electric, for
example, can control not only standard induction and higher efficiency motors, but also multi-pole surface permanent magnet (SPM) and interior permanent magnet (IPM) motors. Later this year there will also be the capability to drive a synchronous reluctance motor, offering ultimate control from a single drive. Furthermore, advanced functionality extends the capability of the modern VSD beyond just speed and torque control. With the addition of an encoder on the motor, it can also offer position control, delivering near servo performance in applications that wouldn’t warrant the cost of the servo motor or where the power requirement is too high – such as for hoists and conveyors. An additional feature is master-slave
synchronisation, where the drive can eliminate the need for mechanical components such as shafts, cams and gears. If two shafts need to be synchronised, the VSD can be electronically locked to an encoder on the first shaft and use this signal to drive the motor on the second. As the master reference changes, the slave has to follow. Used in conveyor applications, this eliminates the need for a separate external controller. There is also the ability to alter the ratio between the master and the slave, providing an electronic gearbox – possible simply with the addition of an encoder option card and the setting of a few parameters.
SMART FUNCTIONS Smart functions, however, are opening up the potential of the VSD as an automation platform. It is, for instance, now common to find a PLC embedded within the VSD, enabling small and even medium sized automation applications to be solved by combining logic control and sequencing with the drive’s ability to control the motor. In multi-drive applications, as long as one of the VSDs has an in-built PLC, it is possible to use
a VSD link function to facilitate communication between multiple drives, allowing a small-scale master-slave control system to be implemented without the need for an additional PLC. Typical applications might include a self-contained, duty assist, standby pumping system (taking advantage of the pump control capabilities of the modern VSD) or a multi-VSD conveyor system. With the addition of an operator interface such as a Mitsubishi Electric GOT HMI, the solution is a completely self-contained automation system. It is also possible to access drives wirelessly, enabling remote monitoring and fine-tuning. Sophisticated diagnostics extend the capability
of VSDs even further. A modern VSD, such as the Mitsubishi Electric A800 series drive, has the ability to monitor some of its own internal components, including the cooling fan, control circuit capacitor, main circuit capacitor, power module and fault contact relay. The VSD monitors operational time and duty cycles and warns when these key components are nearing the end of their expected lifespan. In addition, Mitsubishi Electric VSDs have S contamination monitoring of internal
H2
components, warning of potential drive failure. This is particularly relevant in any application where there is decaying organic matter, such as in water treatment works or effluent plants. The VSD can also derive health information
about the load it is driving, comparing the current drawn with the figures recorded during commissioning – a representation of the relationship between speed and torque under normal operating conditions. In a fan application, for example, an increase in the current drawn might be indicative of 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 current drawn. This can initiate specific remedial functions such as a clean cycle on a pump application or even stop the system in the event of particular conditions to prevent additional costly damage within the application. Most recently, Mitsubishi Electric has extended
the diagnostics capabilities of VSDs by integrating Smart Condition Monitoring (SCM) technology – introducing the concept of drive-based condition monitoring. The pre-configured, plug-and-play solution includes the SmartCheck vibration sensor from the company’s e-F@ctory Alliance partner, Schaeffler, and utilises the VSDs integrated PLC to provide a complete drive-based solution for preventative maintenance.
/ DESIGNSOLUTIONS
Within the VSD, operating temperature and
vibration feedback from the SmartCheck sensor is combined with the monitoring of a full range of other external parameters. These include speed, voltage and current information, with detailed diagnostics highlighted on an integrated GOT display. With this added diagnostics capability, it is now possible to not only monitor the health of the VSD but also the health of the asset. This extended functionality is given additional
power when network capability within the VSD is considered. With a host of protocols supported without the need for option cards and the ability to communicate over two different protocols simultaneously, there is a seamless flow of data from the drive. This encompasses plant level communication, via CC-Link for example, up to controller level, edge computing level, or the higher level enterprise over Ethernet.
SPECIAL PURPOSE DRIVES While often a general purpose product suited to a range of applications, there is a growing trend to incorporate features into a VSD that are specific to given tasks. Specialised tasks can be addressed through dedicated parameters or the addition of specific hardware within the VSD. Examples include sensorless torque control to
regulate tension in a winding application, such as in a paper mill, where the VSD adjusts the torque command value according to the inertia of the paper roll. This function also supports taper tension and inertia compensation. Traditionally, this would be regulated by external tension controllers, with feedback to the PLC and a control signal sent to the VSD. But with sensorless torque control built in, the VSD can handle it by setting a few internal parameters. For crane and hoist applications, where there
is a tendency for the load to act like a pendulum, Mitsubishi Electric has developed a set of configurable algorithms that can be used to lessen or even eliminate this effect. A further function can prevent crabbing on travel cranes by providing synchronisation of the driven wheels. So, taking them beyond just speed and torque
control, today’s VSDs can form the basis of a sophisticated automation platform by delivering enhanced capability to meet both challenging and sophisticated applications.
Mitsubishi Electric Europe
gb3a.mitsubishielectric.com
DESIGN SOLUTIONS | OCTOBER 2020 15
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