Technical Bulletin : Variable Speed Drives In Refrigeration

Introduction: As the refrigeration industry seeks to maximize energy efficiency of their systems, while simultaneously reducing operating costs, investment is being focussed on more technologically advanced refrigeration systems to meet this aim. One main component which assists in achieving this is a variable speed drive on a refrigeration compressor. Due to a compressor being a high energy consumer in a refrigeration system, by using a variable speed drive, energy consumption can be decreased while enhancing system performance simultaneously.


Variable speed drives (VSDs) also known as variable frequency drives work by converting the mains input alternating current (AC) to direct current (DC) and from this generating a simulated alternating current (AC) signal at various frequencies. The image below is a simple illustration of this principle. 

Variable Speed Drives are used to control refrigeration capacity by changing the speed of a motor, and thereby accurately matching the compressor capacity to the required load. VSD’s can be used in various applications in the heating, ventilation, air-conditioning and refrigeration industries as they come in many different sizes. 

The difference between standard refrigeration systems and systems with a variable speed drive is in the control of the system capacity at part-load conditions. Refrigeration systems are designed at a “full-load” running condition. However, these systems mostly run at a lower load during their normal operations. The system capacity cannot be reduced though, due to the fact that maximum load conditions, high ambient temperatures as well as high humidity still occur during the operation of the system. This means the additional load would still be required. A normal fixed speed compressor will then either constantly run at full speed or frequently cycle on and off. If the compressor is fitted with a variable speed drive however, the compressor can lower its speed to match the load requirements and avoid wasting energy while simultaneously reducing the frequent on/off cycling.