January 2021

28 n VARIABLE-SPEED DRIVES January 2021 www.drivesncontrols.com How VSDs have evolved into automation platforms I n the early 1980s, the variable-speed drive was a revolutionary product that opened up the potential to use low-cost AC induction motors in applications where previously more expensive DC motors were the only option. Indeed, there was much talk during that decade and beyond of how the VSD had sounded the death knell of the DC motor. This debate proved something of a red herring, but certainly the benefits of VSDs and their steadily increasing sophistication have seen growth rates little short of phenomenal. This success comes from far more than their ability to turn a low-cost electric motor at a specified speed or torque. Increasingly, it is coming from the VSD’s ability to address evolving trends and to tackle industry issues – from the requirement for greater energy efficiency, to the need for increased safety. Today, all of this and more is taken for granted on all but the most basic of VSDs, while more sophisticated products extend the flexibility of VSDs by being able to handle an increasingly diverse range of motor types. In recent years, VSD manufacturers have added safety functions, sophisticated communications, powerful auto-tune capabilities and intuitive set-up applications, but these capabilities barely scratch the surface of the extended functions that today’s VSDs can deliver. For example, some VSDs can now control not only standard induction and higher efficiency motors, but also multi-pole surface permanent magnet (SPM), interior permanent magnet (IPM) and synchronous reluctance motors. However there is more, with advanced functions extending the capabilities of modern VSDs beyond just speed and torque control. By adding an encoder to a motor, VSDs can also offer position control, delivering near-servo performance in applications that wouldn’t warrant the cost of a servomotor or where the power requirement is too high. Another interesting capability is master- slave synchronisation, where a 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 locked electronically to an encoder on the first shaft and the signal used to drive the motor on the second. As the master reference changes, the slave follows. This flexible control philosophy has been used in countless conveyor applications, eliminating the need for external controllers. There is also the ability to alter the ratio between the master and the slave, providing an electronic gearbox simply by adding an encoder option card and setting a few parameters. Smart functions The latest smart functions are opening up the potential of the VSD as an automation platform. For example, it is now common to find a PLC embedded within a VSD, enabling many small-, or even medium-sized, automation applications to be solved by combining logic control and sequencing with the drive’s ability to control the motor. As long as one of the VSDs in a multi-drive application has an built-in PLC, it is possible to use a VSD link function to facilitate communication between the drives, allowing small-scale master-slave control systems to be implemented without needing an additional Today’s variable-speed drives offer far more than simple speed and torque control functions. Wayne Turtill, Mitsubishi Electric’s product manager for VSDs, servos and low-voltage switchgear, explores some of the extended roles of the modern VSD and looks at its potential to act as a complete automation platform. The latest smart functions open up the potential for variable-speed drives to act as automation platforms for sophisticated applications.