Wayne Turtill of Mitsubishi Electric UK explains how the versatility of a problem solving VSD extends far beyond controlling the speed and torque of a motor. Many problems solving VSDs like the FRA-800 offer PLC level controls and even servo drive like performance.
Modern VSDs are the cornerstone of the automation environment, providing effective and efficient motor speed and torque control. Besides standard induction motors, a single drive can control SPM, IPM and synchronous reluctance motors.
Moreover, in recent years, manufacturers have included comprehensive safety functions, integrated communications, powerful auto-tune capabilities and intuitive software for application set-up.
Problem solving VSDs
With the addition of an encoder on the motor, the VSD can also offer position control, delivering near servo performance. Hoists and conveyors are typical applications that benefit from lower costs. Its master-follower synchronisation does away with mechanical components like shafts, cams and gears. Furthermore, to synchronise two shafts, the VSD is electronically locked to an encoder on the first shaft and uses this signal to drive the motor on the second. As the master reference changes, the follower follows suit, hence simplifying conveyor applications. Changing the ratio also provides a simple electronic gearbox.
A built-in PLC supports medium-sized automation to control both machines and motors as well as external equipment. In multi-drive applications, it is possible to use the VSD link function to communicate with multiple drives. Application examples include a self-contained, duty assist, standby pumping system or a multi-VSD conveyor system.
Smart diagnostics provide a platform for predictive maintenance to track operating time and duty cycles of key components. This helps maintenance engineers replace parts during scheduled downtime, thus eliminating unexpected stoppages.
Problem solving VSDs can also derive health information about the driven load by comparing current data with that from commissioning. For example, in a fan application, 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 also output different warnings at pre-set levels to start specific remedial actions to avoid downtime.
Networking the drives is straightforward as they support most major communication protocols without the need for option cards. The drives also allow two networks to operate at the same time. This allows plant level communication via CC-Link for example and to the higher-level enterprise over Ethernet.
Special purpose drives
Many VSDs are general-purpose products covering a range of applications like fan and pump control. Yet there are benefits and savings from using application-specific drives for specialised tasks? Addressing these handled using dedicated parameters or the addition of specific hardware within problem solving VSDs.
Examples include sensorless torque control to regulate tension in a winding application, such as in a paper winding. Here, 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, regulation uses external tension controllers with feedback to the PLC and a control signal sent to the VSD. Yet with sensorless torque control built-in, the VSD can handle it within its internal parameter settings.
For crane and hoist applications, where there is a tendency for the load to act like a pendulum. Mitsubishi Electric has developed configurable algorithms for lessening or even stopping this effect. A further function can prevent crabbing on travel cranes by providing synchronisation of the driven wheels.
Extended functionality problem solving VSDs support many challenging and sophisticated applications. Having surpassed speed and torque control they form the basis of today’s sophisticated automation platform.