**4. VFD application on fault detection and diagnosis**

Many researchers have studied fault detection and diagnosis (FDD) on HVAC systems. FDD technique is an effective way to improve the reliability of HVAC systems, and reduce the maintenance costs. There are a variety of methods and strategies on the equipment-level and system-level FDD, including AHUs, RTUs, etc. [19–21]. Almost all of the methods rely on the system operations measurements, such as temperature, humidity, pressure, airflow, and water flow.

Although VFDs are widely used in fans, pumps, and compressors in HVAC systems, most of these applications are focused on how to use the VFD to control motor speed. However, the VFD can measure several useful electrical-related parameters, which could be used for system monitoring and FDD purposes.

A typical VFD can measure and provide the output of speed/frequency, current, power, torque, and many other parameters. These electrical signals have inherent relationships with the system operating performances. For example, Li et al. [10] developed several fault signatures for a single-stage DX rooftop unit using fan power, compressor power, and supply air temperature measurements through an experimental study. With these known parameters, the components and system faults can be identified in advance. These signals can be sent out to an external controller or a BAS system through analog output signals or digital communi‐ cation signals (Modbus, N2, FLN, BACNet, etc.).

Figures 13 and 14 present two configurations of connection between VFDs and the unit controller/BAS. In Figure 13, the VFD controls the speeds of multiple motors, such as fan motor, pump motors, or compressors. The controller monitors the operation of motors and receives motor operating information (such as speed, current, power, and torque) through digital communication. The controllers utilize this information and other system measurement readings (such as temperature) to perform FDD analysis.

It is also very common that each VFD controls only one motor, as shown in Figure 14. The controller communicates with each VFD and performs FDD analysis based on the operations of all motors.

One example is the application of the VFD on FDD in packaged RTUs. The common faults of an RTU include fouling evaporator coil, filter blockage, fouling condenser coil, refrigerant leakage, and improper charge. The common methods to detect these faults are using the measurement of multiple temperature and pressure points and comparing the actual readings to normal state readings. As a matter of fact, electrical signals can reflect the change of system performances. Recent research shows that the electrical signals, such as a VFD speed (fre‐ quency) and power, combined with other temperature parameters, can be used to detect these common faults based on experimental studies [10].

A typical VFD can measure and provide the output of speed/frequency, current, power, torque, and many other parameters. These electrical signals have inherent relationships with the system operating performances. For example, Li et al. [10] developed several fault signatures for a single‐stage DX rooftop unit using fan power, compressor power, and supply air temperature measurements through an experimental study. With these known parameters, the components and system faults can be identified in advance. These signals can be sent out to an external controller or a BAS system through analog output signals or

Figures 13 and 14 present two configurations of connection between VFDs and the unit controller/BAS. In Figure 13, the VFD controls the speeds of multiple motors, such as fan motor, pump motors, or compressors. The controller monitors the operation of motors and receives motor operating information (such as speed, current, power, and torque) through digital communication. The controllers utilize this information and other system

digital communication signals (Modbus, N2, FLN, BACNet, etc.).

measurement readings (such as temperature) to perform FDD analysis.

Figure <sup>13</sup> Configuration <sup>A</sup> showing the connection between the VFD and controller **Figure 13.** Configuration A showing the connection between the VFD and controller

operations of all motors.

Figure 14 Configuration B showing the connection between the VFD and controller **Figure 14.** Configuration B showing the connection between the VFD and controller

To obtain the frequency (speed) and kilowatt for both the fan and compressor, both of them should be equipped with VFD, using a VFD to control both speeds or using dedicated VFD for fan and compressor. To monitor the performance of RTU, an outdoor air temperature (OAT) sensor and a supply air temperature (SAT) sensor are installed in the unit in addition to the VFD. The VFD speed and power are provided by the VFD itself and sent to an external controller or BAS through Modbus communication. The measured system parameters, such as VFD speed, VFD power, OAT, and SAT, are used to perform FDD on the existing RTU. of an RTU include fouling evaporator coil, filter blockage, fouling condenser coil, refrigerant leakage, and improper charge. The common methods to detect these faults are using the measurement of multiple temperature and pressure points and comparing the actual readings to normal state readings. As a matter of fact, electrical signals can reflect the change of system performances. Recent research shows that the electrical signals, such as a VFD speed (frequency) and power, combined with other temperature parameters, can be used to detect these common faults based on experimental studies [10]. To obtain the frequency (speed) and kilowatt for both the fan and compressor, both of them should be equipped with VFD, using a VFD to control both speeds or using dedicated VFD

> for fan and compressors. To monitor the performance of RTU, an outdoor air temperature (OAT) sensor and a supply air temperature (SAT) sensor are installed in the unit in addition to the VFD. The VFD speed and power are provided by the VFD itself and sent to an

> One example is the application of the VFD on FDD in packaged RTUs. The common faults

#### **5. Application considerations** external controller or BAS through Modbus communication. The measured system parameters, VFD speed, VFD power, OAT, and SAT, are used to perform FDD on the

existing RTU.

#### **5.1. Minimum VFD speed 5. Application Considerations**

For a VFD-equipped compressor, the compressor speed is often modulated to maintain the

Many researchers have studied fault detection and diagnosis (FDD) on HVAC systems. FDD technique is an effective way to improve the reliability of HVAC systems, and reduce the maintenance costs. There are a variety of methods and strategies on the equipment-level and system-level FDD, including AHUs, RTUs, etc. [19–21]. Almost all of the methods rely on the system operations measurements, such as temperature, humidity, pressure, airflow, and water

Although VFDs are widely used in fans, pumps, and compressors in HVAC systems, most of these applications are focused on how to use the VFD to control motor speed. However, the VFD can measure several useful electrical-related parameters, which could be used for system

A typical VFD can measure and provide the output of speed/frequency, current, power, torque, and many other parameters. These electrical signals have inherent relationships with the system operating performances. For example, Li et al. [10] developed several fault signatures for a single-stage DX rooftop unit using fan power, compressor power, and supply air temperature measurements through an experimental study. With these known parameters, the components and system faults can be identified in advance. These signals can be sent out to an external controller or a BAS system through analog output signals or digital communi‐

Figures 13 and 14 present two configurations of connection between VFDs and the unit controller/BAS. In Figure 13, the VFD controls the speeds of multiple motors, such as fan motor, pump motors, or compressors. The controller monitors the operation of motors and receives motor operating information (such as speed, current, power, and torque) through digital communication. The controllers utilize this information and other system measurement

It is also very common that each VFD controls only one motor, as shown in Figure 14. The controller communicates with each VFD and performs FDD analysis based on the operations

One example is the application of the VFD on FDD in packaged RTUs. The common faults of an RTU include fouling evaporator coil, filter blockage, fouling condenser coil, refrigerant leakage, and improper charge. The common methods to detect these faults are using the measurement of multiple temperature and pressure points and comparing the actual readings to normal state readings. As a matter of fact, electrical signals can reflect the change of system performances. Recent research shows that the electrical signals, such as a VFD speed (fre‐ quency) and power, combined with other temperature parameters, can be used to detect these

supply water or supply air temperature set point.

180 New Applications of Electric Drives

flow.

of all motors.

monitoring and FDD purposes.

cation signals (Modbus, N2, FLN, BACNet, etc.).

readings (such as temperature) to perform FDD analysis.

common faults based on experimental studies [10].

**4. VFD application on fault detection and diagnosis**

For all VFD applications, the maximum speed or frequency is relatively easy to set. In the United States, the maximum speed is usually 60 Hz. In some cases, a higher speed is used, which is not typical and recommended [22]. In contrast, the minimum speed setup needs more considerations because it has a potential impact on the building energy use and motor performance.

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). However, more considerations are needed to ensure effective operations.

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 trip the operation of the chillers.

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 bypass this range.
