The use of Invertek variable-frequency drives (VFDs) in supermarkets to control compressor and fan motors has become a key factor in reducing energy costs, enhancing product life, and improving equipment service life. For outdoor condensers, drives utilizing variable-speed operation have made important contributions to noise reduction.
Refrigeration systems are designed for full-load conditions. Most of the time, however, their loads are average, not peak, and full motor capacity is not required. During average conditions, motors in traditionally designed systems (without VFDs) either constantly are running at a higher speed than necessary or frequently cycling on and off.
Producing more capacity than needed wastes considerable energy, and frequent on/off cycling accelerates wear and shortens the useful life of motors, contactors, and other components. Frequently starting and stopping motors and continually accelerating them to full speed eliminates opportunities for reducing energy costs.
Although maximum-load conditions — high ambient temperature, high humidity, and fully loaded store fixtures and storage boxes — exist as little as 4 percent of running time, refrigeration systems must be designed to cope with them. However, it is just as desirable for systems to provide as much of the required capacity for expected part-load conditions as possible. Unfortunately, providing capacity to meet peak demands wastes considerable energy when part-load conditions exist if machines simply are switched on and off.
By installing a VFD on a condenser, fans now can run at a lower speed, reducing start-up noise. Also, the whine that is produced as fans start and climb rapidly to full speed also is reduced because the fans are ramped up to the required speed gradually.
The initial cost of VFDs in supermarket installations has decreased markedly. In the earliest days of drive applications, there were control difficulties, but with Invertek Drives, these problems have been solved. Current drives are smaller and easier to program. New drives are easier to configure than earlier drives and can adapt automatically to the motors with which they are working. The number of configuration settings has been reduced by more than 50 percent.
Store owners can expect payback from energy savings in less than 24 months when installing VFDs on condensers and HVAC fans. There are measurable energy savings with refrigeration-compressor installations, but the greatest effects are added machinery life, reduced maintenance costs, and a better storage environment for refrigerated products. Because the refrigeration capacity supplied is always matched to the load, variations in temperature are reduced, lengthening product life. This, coupled with programming simplicity, make modern VFDs attractive.