Two years ago, a compressor manufacturer took a stride forward in the development of its products by adopting variable speed drive technology. It succeeded in reducing the energy consumption of the new compressors by up to 30% compared with fixed speed types. But the electronics added complexity, pushing up costs, so it switched VSD platform.....
Avelair Compressors of Bury-St-Edmunds, is a fast-growing manufacturer of rotary-screw compressors, offering both standard products and the capability to design and build systems to order. The VSD variable speed range has been offered for two years, using a market-leading inverter drive coupled to a PLC. But, as technical director, Tim Shepherd explains, the need for a separate drive and PLC, plus the associated costs of the complex wiring were making the build costs expensive.
"We had all of the drives companies calling on us and were able to analyse what each could offer to meet our needs. Control Techniques (CT) was the only company that came back with a solution that offered us real cost-savings whilst meeting our specification. Now, with the Unidrive SP, we have been able to standardise on layout to a large extent and, by using the plug-in applications/PLC module, we've been able to eliminate the need for a separate PLC completely, whilst retaining all of the flexibility and functionality we had before."
Unidrive SP models from 11 to 35kW span the standard Avelair VSD range and each is supplied complete with an SM applications module that includes a fully programmable PLC. It was Mark Dunn of local company, Trax Circuits who undertook the programming. "Although this was a first for me, I found the process very straightforward and logical using CT's software tools: CT-Soft for programming the drive and SYPT Lite for the PLC." The PLC is responsible for the complex procedures involved in starting up and closing down the VSD compressor to meet safety requirements, as well as monitoring of line pressure, control faults, cooling fan operation, and the integration of fault conditions from the drive itself.
When starting, the compressor motor is accelerated to minimum speed, the cooling fan is started, the inlet solenoid valve remains closed whilst the control system checks for fault conditions. After 30 seconds, if the compressor internal pressure is above 1bar, the system goes into running mode. In running mode, the compressor motor speed is controlled by the Unidrive SP in closed-loop speed control using its internal PID loop to maintain the set pressure. If the compressor runs at minimum speed (no air required) for more than five minutes, the system automatically goes into idle mode to save energy. The drive also includes a 'secure disable' mode for safe machine maintenance.
The Avelair range of VSD compressors can operate as stand-alone units or work in conjunction with fixed-speed units that provide the base-load required, with the variable speed unit providing additional air, according to demand. With fixed speed systems, the air pressure is usually set just above the required pressure - for example if 8bar is needed, the compressors are set to provide 9bar - to give a margin. With a system that includes a VSD compressor, the pressures would be set at 8bar, resulting in direct energy savings across the system.
"The choice of Unidrive SP, with its integral PLC, has enabled us to offer a more compact, more efficient and more price-competitive product," says Avelair managing director, Brian Wood. It has saved space, eliminated considerable wiring, removed a potential source of failure and cut costs all in one go! We like other features too, such as the through-hole mounting so that heat is dissipated from the cooling fins at the back of the panel. This means that the panel can be sealed against dirty environments and there's no problem of over-heating through blocked filters. The small size of the drive has allowed us to reduce the size of the control cubicles on most of the units too and control panel layouts have been standardised. It's all so much simpler - a very elegant solution all round."