The drive for productivity – variable-speed drives in process improvement


Variable-speed drives have long been known for their abilities to improve the control of industrial processes. Today’s drives offer sophisticated functions that make them an ever more vital element in the drive for efficient processes. John Guthrie looks at these functions and how they are helping companies transform their production


When we think of variable-speed drives (VSDs), probably the first thing that springs to mind is their ability to cut energy use by driving motors at just the right speed to match the process.

The most modern VSDs can give more detailed and understandable information, for example, showing on the drive’s screen exactly how much energy they are using. These measurements can help ensure a plant or process is working to its optimum performance, while avoiding the extra cost of external energy measuring equipment.

Yet when first developed, VSDs were mainly valued for their ability to improve process performance and this remains a large part of their appeal today.

Their capabilities have continued to improve, with new functionality that allows them to replace mechanical based control systems, cutting many of the costs associated with maintenance of gears and other components. Increasing use of software is also 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.

With easily upgradeable software, VSDs can also keep pace with the evolution of the process, adapting to meet new needs quickly and inexpensively.

The major elements of process efficiency can be characterised as productivity, quality and the maintenance effort needed to support the process and VSDs can certainly make major contributions to improving all these factors.

Packing in more product
More productivity was a major benefit of using VSDs on its packing process for tomato grower John Baarda. Two VSDs control the speed of the tomato handling conveyors, matching the speed of the packing machine and ensuring that the tomatoes are packed quickly and accurately. Run in master-slave configuration, the master drive receives an encoder signal from the wrapper. This ensures 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.

With mechanical systems, the company can achieve a maximum rate of around 60 packs per minute, while the VSD system gives an average of 70 to 80 packs per minute.
Product changes are also far easier with the VSD method. The drive is programmed with product menus, allowing different products to be run on the conveyor and with quick changeovers. The previous mechanical system required co-ordination to be changed by adjusting a chain drive, cutting into production time.

Another VSD runs rollers on the conveyors, allowing the tomatoes to be turned automatically and inspected for quality.

Baking a better biscuit
Improved quality was also on the menu for one of the UK’s leading biscuit manufacturers when it investigated the potential of using VSDs in its processes. Fox’s Biscuits of Batley, West Yorkshire, initially looked at VSDs for its energy saving qualities but quickly saw how they could improve the quality of their products and potentially reduce the amount of wastage.

One of the major benefits for Fox’s is the flexibility to run at a wider range of speeds than were previously available. The company uses a lot of other ingredients in its biscuits, such as fruit. Previously, the mixers might overmix, leading to staining where the fruit becomes a mush rather than remaining whole. Good speed control helps maintain a high quality product and avoids a situation where a whole 400 kg batch of dough could be wasted.

The ability of the drive to change the speed of the mixer also allows the company to be more innovative. It can experiment by using different speeds throughout the mix, from as low as two rpm to 50 or 60 rpm, depending on the recipe.

The drive also offers a lot of other facilities, such as the load analyser, which saves process data such as current and torque values that can be used to assess production issues.

Stopping the stoppages
VSDs also have a role to play in reducing maintenance effort, as vehicle component manufacturer GKN found. The company is saving around £25,000 in costs following the installation of a VSD on one of its production lines.

GKN’s Wheels 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.

This type of stoppage can take an average of two hours to remedy, resulting in lost production time and associated costs. To maintain schedules and keep its customers supplied, the plant needed to run overtime following such incidents. With maintenance, materials, and lost production time, costs due to these stoppages were totalling around £25,000 a year.

A system was developed that employs a 1.5 kW VSD. 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. Stoppages have been completely eliminated, cutting the extra maintenance costs.

New functions make drives ever more capable
Controlling the speed of pumps and fans has always been a particular strength of VSDs and specialised software can now be added to make a VSD even more capable in this role, 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.

Increasingly, VSDs are capable of communicating over standard communications protocols like Ethernet, allowing easy connection to automation systems. This can allow plant managers to combine process automation and power automation in the same plant control system, a concept known as electrical integration. Because their essential function is to control a process, VSDs can readily contribute to safe working. For example, in food and drink manufacture, 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.

With their sophisticated features, today’s VSDs offer a wealth of functionality, not only keeping energy costs in check but also providing the quality, production capacity and low maintenance that production engineers need.