**6. Summary**

First, the motor itself has some limitations. VFD manufacturers often recommend a minimum speed of 30% of their rated speed (18 Hz) to prevent motor overheating due to inadequate airflow [23]. An inverter duty motor can have lower minimum setting as 20% (12 Hz).

For fans and pumps, the minimum speed can be as low as 6 Hz without creating motor overheat issue and other mechanical drawbacks [18]. Meanwhile, the operation factors should be considered as well, such as the indoor air quality (IAQ) requirements and air distribution requirements. If the fan speed is too low, with the same outdoor air damper position, less fresh air is delivered to the space. Therefore, a proper engineering calculation is needed. In addition, the operating mode places limitations on the minimum speed. For a single-zone unit running in cooling mode, a low speed could cause very low velocity at the outlet of ductwork, which may result in the cold air being dumped directly into space without a good mixture. In the heating mode, a speed that is too low may cause the hot air to stagnate on the upper level of space due to the buoyancy effect. Therefore, the actual minimum fan speed may be 20 Hz or so. In chilled water pump applications, the primary pump speed should be high enough to provide sufficient chilled water through chillers. Otherwise, the low-water-flow alarm could

For compressors, their minimum speeds should be determined based on the oil return, as well as structural and safety requirements. For example, the manufacturer recommended a minimum VFD speed of 25 Hz for Discus compressors and 45 Hz for scroll compressors [24]. Most compressors have a vibration resonance issue at certain speeds. This can be solved by programming the VFD to skip this range, or by simply setting up a higher minimum speed to

Most VFDs use pulse-width modulation to control the motor speed. PWM can create a large and rapid voltage swing, or an electromagnetic interference (EMI) because of the fast rise and fall times of the signals used by the PWM control circuits. The interference has adverse effects

**•** Minimize the cable lengths between the VFD and motor. The longer the cable, the greater the potential for reflected voltage. The users shall follow the manufacturer's requirements for power cable installation. Generally, the cable length should be no more than 200 ft.

**•** Use the lowest VFD carrier frequency as it affects the maximum permitted cable length. The lower the frequency, the greater the maximum possible length of cable between the VFD

**•** Use an armored power cable. Metallic outer armor is recommended for the power cable to shield the system components from the high-frequency electric fields. Copper or aluminum should be used because steel does not provide effective shielding at high frequencies.

on the operation of the control system and motor components.

There are several recommendations to minimize interference from VFDs [25].

However, more considerations are needed to ensure effective operations.

trip the operation of the chillers.

182 New Applications of Electric Drives

bypass this range.

**5.2. Interferences**

and motors.

The VFD is an excellent electric device to control motor speed within the allowable operating range. The VFD applications on HVAC systems are presented in detail from the control perspective. Then, the application on FDD is introduced from the metering point of view. Lastly, the existing issues are summarized and recommendations are provided. Overall, VFDs play a great role in the optimal operation of building energy systems. The increased function‐ ality and reliability along with the reduced cost make them more and more widely accepted and used in the HVAC industry. These applications will achieve a tremendous energy savings from motors.
