**2.1. VFD**

A VFD is an electrical device used to control the rotation speed of an alternating current (AC) electric motor by adjusting the frequency of the electrical power supplied to the motor. It is also called pulse-width-modulated drive (PWM drive), adjustable-speed drive (ASD), adjustable-frequency drive (AFD), AC drive, inverter drive [3], or variable voltage variable frequency (VVVF) drive.

Most VFDs used in HVAC applications are inverters using sine-coded PWM technology. As shown in Figure 1, a VFD works by converting the incoming AC power to DC power using a diode bridge rectifier, then passing the filtered, smoothed the voltage onto the inverting section, and finally controlling voltage and frequency sent to the motor by high-speed bipolar transistors [4].

**Figure 1.** Schematic diagram of a variable frequency drive [4]

The output voltage is adjusted by changing the width and number of the voltage pulse as shown in Figure 2, whereas the output frequency is varied by changing the length of the cycle.

**Figure 2.** Sine-coded PWM waveform [4]

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

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

A VFD is an electrical device used to control the rotation speed of an alternating current (AC) electric motor by adjusting the frequency of the electrical power supplied to the motor. It is also called pulse-width-modulated drive (PWM drive), adjustable-speed drive (ASD), adjustable-frequency drive (AFD), AC drive, inverter drive [3], or variable voltage variable

Most VFDs used in HVAC applications are inverters using sine-coded PWM technology. As shown in Figure 1, a VFD works by converting the incoming AC power to DC power using a diode bridge rectifier, then passing the filtered, smoothed the voltage onto the inverting section, and finally controlling voltage and frequency sent to the motor by high-speed bipolar

The output voltage is adjusted by changing the width and number of the voltage pulse as shown in Figure 2, whereas the output frequency is varied by changing the length of the cycle.

motor speed provides a significant reduction in motor power.

solutions provided.

168 New Applications of Electric Drives

**2. Principles**

frequency (VVVF) drive.

**Figure 1.** Schematic diagram of a variable frequency drive [4]

transistors [4].

**2.1. VFD**

A VFD can provide the follow benefits:


These advantages boost the widespread utilization of VFDs in a variety of industrial processes including the HVAC field. Although equipping a VFD in a new system or existing system increases the initial investment, the reduced VFD cost combined with the increased energy savings derived from a VFD result in short payback period, which is usually less than three years.
