**1. Introduction**

Electric motor-driven systems (EMDS) are the largest electrical end users and account for 43– 46% of all global electricity consumption [1]. In the United States, HVAC and refrigeration applications consume 91% of motor-driven energy use in the residential sector and 93% in the commercial sector [2]. Reducing unnecessary energy consumption is the most direct and

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effective way to improve building energy efficiency. The increasing advancement of electronic and control technology greatly improves the performance of variable speed drives. Variable frequency drives (VFDs) are being used in the HVAC industry more frequently and in more applications. They can modulate the motor speed very smoothly within a wide range. Reduced motor speed provides a significant reduction in motor power.

In general, a VFD can be used in a variety of applications where the load is variable, such as fans, pumps, and compressors. The energy savings for fans and pumps are about 30–50% over conventional speed applications, and up to 35% for compressors. In the following sections, the working principle of the VFD is introduced and its applications on HVAC systems are presented. Lastly, the current issues and concerns about VFDs are discussed with potential solutions provided.
