Not just energy savers – Using variable speed drives for better productivity

2013-01-18 - As well as their well-known uses in saving energy, variable-speed drives are also sophisticated control elements in their own right that provide indispensable functions to plant managers. Neil Ritchie looks at these functions and how users have come to expect a lot more from their drives.
For a long time, variable-speed drives (VSDs) have been promoted for their contributions to energy efficiency and rightly so, as they are among the most effective ways to cut the costs of running electric motors, the largest users of energy in industrial applications.

Yet initially, VSDs were mainly valued for their ability to improve process performance and for many users this remains a primary reason for using them on their applications. Many industrial users have come to see VSDs as a major pillar of their quality and safety policies, while also expecting more support from their VSD vendors.

An example is a VSD application for tomato grower John Baarda, which has doubled the packing rate of a tomato handling machine.

Using ABB machinery drives, the speed of the tomato handling conveyors are controlled to match the speed of the packing machine and ensuring that the tomatoes are packed quickly and accurately. Another ABB machinery drive runs rollers on the conveyors, allowing the tomatoes to be turned automatically and inspected for quality.

The two feeder conveyors are each driven by an ABB machinery drive in master-slave configuration, with the master receiving an encoder signal from the wrapper. This ensures that the drive knows where the wrapper is in its cycle and can control the speed of the conveyor precisely to ensure the tomatoes arrive at the wrapper at the correct time. If the speed of the wrapper changes, the drive can alter the speed of the conveyor accordingly to maintain the correct timing.

The new system has allowed far higher production rates than with previous mechanical systems, giving an average of 70 to 80 packs per minute compared to around 60 packs per minute.

Powerful new functionality
From being rather anonymous boxes, VSDs have become much more user friendly, giving more detailed and understandable information on their condition and operating parameters. VSD displays can now present an array of information, for example, showing exactly how much energy they are using on the drive’s screen. These measurements can help ensure that a plant or process is working to its optimum performance, while avoiding the extra cost of external energy measuring equipment.

Within the drive itself, increasing use of software is leading to more intelligent VSDs that can replace many of the functions of traditional PLCs, timers and counters, reducing the complexity of control systems and cutting costs. Some drives include a PID controller, eliminating the need for an external PID controller for flow or pressure control.

Precise control clears jams
Another example of the process improvement capabilities of VSDs is a project for vehicle component manufacturer, GKN, whose plant at Telford produces a wide range of components for major vehicle producers around the world. One of the production lines makes wheels for many of the leading brands of agricultural and construction vehicles, including Caterpillar and JCB.

One part of the line involves a walking beam, which uses a system of reciprocating parallel bars to pass rolled and welded wheel rims from an oil dip to a flare press. As the beam reaches top dead centre, the larger wheel rims can cause the beam to over speed and the momentum carries them too quickly towards the press. This can result in the system jamming, which can take around two hours to clear. GKN was spending around £25,000 a year clearing these jams.

Estil, a systems integration company, and ABB developed a system that employed a 1.5 kW ABB standard drive. At the beginning of the walking beam’s cycle, the drive is started and the internal timer runs the drive for one second at full speed. After this time, the drive is switched to two thirds speed and is then gradually decelerated. The beam, carrying the wheel rim, is slowed gradually as it approaches the press. As the wheel rim reaches the press, the beam triggers a limit switch that stops the drive to complete the cycle.

An integral part of control and safety systems
Specialised software can now be added to make a VSD particularly suited to pump and fan applications, including such functions as pump cleaning that keep pumps clear of debris. Diagnostic software can be used to alert operators if the VSD detects set points or other parameters going out of limits. Process information can be checked on line or the drive can even send an SMS to a mobile phone to alert an engineer to a problem.

More VSDs are capable of communicating over standard communications protocols like Ethernet, allowing easy connection to automation systems through multiple fieldbus adapters and giving reduced installation and programming costs. This can allow plant managers to combine process automation and power automation in the same plant control system, a concept known as electrical integration.

Safety of course is an overriding concern and manufacturers are increasingly turning to drives to form a major part of their safety management philosophy. Because their essential function is to control a process, VSDs can readily contribute to safe working. In the food and beverage industries, for example, VSDs can ensure that an oven is at the right temperature or that a conveyor is moving at the right speed, preventing objects falling from it and causing a hazard.

Built in reliability
Today’s VSDs are also inherently very reliable. They use fewer components than earlier drives and those they do use are based on solid state electronics rather than electromechanical techniques. As they have evolved, VSDs have come to generate significantly less heat than their predecessors, helping to cut a major source of failure.

Yet even the most reliable of VSDs needs regular maintenance to keep it in full working order and to ensure that no surprise failures interrupt production. Plant managers have come to expect that, along with excellent products, drives vendors will supply excellent support. Maintenance activities have also evolved to match this improved reliability with built-in maintenance and predictive diagnostics, backed up by a defined lifecycle management programs to avoid over maintenance and to target maintenance needs precisely. This support has come to extend from before purchase, with help deciding on which drive is right for a particular process, through installation and commissioning, preventive maintenance programs and to the end of the drive’s life, when it is disposed of carefully and recycled according to international environmental standards.

With their sophisticated features, today’s VSDs offer a wealth of functionality, giving process engineers the ability to keep their energy costs in check while also providing the quality, production capacity and safety they need.

John Baarda has doubled the packing rate of a tomato handling machine using ABB machinery drives.