**1 Introduction** On the surface, variable frequency drive (VFD) control is often considered a separate component, a pure capital expenditure. In the design of many new projects, VFDs are often among the first components considered for cost reduction. However, by truly integrating VFDs into the control system, optimal cost-effectiveness and efficiency can be achieved. **2 Cost-Saving Features of VFDs for HVAC Systems** VFDs are widely used in large central air conditioning systems, particularly in standard hotels. Optimal performance of a VFD typically depends on its built-in functions. Numerous manufacturers produce VFDs specifically designed for HVAC systems (e.g., the Danfoss VLT6000 HVAC system VFD). These VFDs possess all the specific functions required for standard HVAC systems, thus often allowing for the reduction of other components in the system. This reduces the initial total cost of the system. This article also details some representative typical applications of frequency converters, how to achieve the best performance of frequency converters, and how to use frequency converters to achieve the best results and lowest cost. However, before describing the specific applications, we will first explain several characteristics of frequency converters specifically designed for HVAC systems and their application in hotel HVAC systems. 2.1 Installation Location of Frequency Converters Nowadays, frequency converters should generally be installed near the operating pumps, air handling units, or fans, and should not be fixed in the central distribution panel or control panel. The main advantage of this installation method is that it can reduce the size of the main distribution panel, simplify installation, and facilitate commissioning. At the same time, this installation method can also ensure the long-term reliability of the frequency converter and the protection of personnel in the equipment room. Therefore, it is extremely important to ensure the "enclosure protection rating" of the frequency converter. Therefore, if the frequency converter is not installed in the distribution panel, but is instead installed directly on the wall of the equipment room or on the side of the air handling unit, it is recommended to place the frequency converter in a sealed enclosure with an IP54 rating, which will help ensure the long-term reliability of the equipment. Many frequency converters specifically designed for HVAC systems are installed in IP54 configuration for this very reason. The smaller switchboard size not only saves costs but also eliminates the need for additional motor starters when using a frequency converter. If flow control is achieved using valves, baffles, or inlet guide vanes, the pump or fan must use a fixed-speed motor and requires a direct starter or star/delta starter, thermal overload protection, autotransformer, etc. If a star/delta starter is used, six cables are required. With a frequency converter, only three cables with three-phase fuses are needed. Frequency converters have built-in motor overload protection. If a fixed-speed motor is used, a power factor correction capacitor may be required depending on the motor's power factor. Most frequency converters have power factor correction, resulting in a power factor of approximately 1 (or at least > 0.95), eliminating the need for a power factor correction capacitor in the system. Comparatively, frequency converter installation is significantly simpler and more cost-effective. 2.2 Electromagnetic Compatibility Inverters control the speed of AC motors by changing the frequency (and voltage) of the motor power supply. Essentially, this is achieved through the rapid switching of transistors (standard value >4500 times/s). Most people experience flickering in their televisions when turning on lights or starting dishwashers, washing machines, or vacuum cleaners at home; this is caused by radio frequency interference (RFI) or electromagnetic incompatibility. To ensure the reliability of a hotel's HVAC system and other electrical/electronic components (e.g., televisions in guest rooms, elevators), it is essential that the switching operation of the inverter's transistors does not interfere with these components. The simplest way to reduce the risk of interference is to install an inverter equipped with an integrated radio frequency interference filter that complies with either "EN61800-3 Class I Unrestricted Distribution" or "EN61800-3 Class I Restricted Distribution" and adheres to the manufacturer's installation guidelines. The performance of frequency converters equipped with integrated radio frequency interference filters, designed and installed by the manufacturer, is now widely recognized, replacing the previous complex practice that required specialized technicians to properly install a separate external radio frequency interference filter. This EMC standard is relatively new, replacing the previously used EN55011 standard. The "Class I environment" definition includes civil buildings and is recommended for hotel installations to ensure the operational reliability of equipment in guest rooms. The "unrestricted distribution" definition is recommended because it requires the frequency converter itself to have better EMC performance, rather than relying on the installer's EMC installation skills. If the installer does possess qualified EMC installation skills, then a "restricted distribution" specification is permissible. EN55011 previously often specified Class 1A performance; the corresponding performance standard in the new standard is "EN61800-3 Class I environment unrestricted distribution requirements." If specifying the EMC performance of the frequency converter, another extremely important factor should be defined: motor cable length. The frequency converter should comply with this standard. For any frequency converter, the longer the motor cable, the worse the radio frequency interference performance. If the frequency converter complies with the Class I environmental requirements of EN61800-3, a 30m long motor cable should be provided; if the cable is 40m, it does not comply with this requirement. If the motor cable length is known during the design phase, it can be specified; if unknown, a general statement should be issued requiring all hotel standard equipment to comply with the unrestricted distribution requirements of EN61800-3 for Class I environments and be equipped with a 50m motor cable, or the restricted distribution requirements of EN61800-3 for Class I environments and be equipped with a 50m motor cable. (Remember: the definition of "unrestricted" or "restricted" depends on the installer's electromagnetic compatibility capabilities.) 2.3 Harmonic Frequency Converters are non-linear electrical devices, similar to most other modern electronic devices. That is, these devices act as sources of harmonic current in the power supply system. Harmonic currents can increase the RMS current value, thereby increasing current losses in cables, power transformers, and other distribution equipment. This can cause another form of electrical interference, known as voltage harmonic distortion. The simplest way to avoid this is to ensure that the inverter is equipped with a harmonic filter (e.g., a DC reactor). Most inverters for HVAC systems are equipped with this filter or are offered as an option. Without this filter, the inverter may cause excessive voltage harmonic distortion, which may affect other power-consuming equipment connected to the same transformer, which is a particular concern in multi-family commercial buildings. Obviously, just like radio frequency interference filters, if the reactor is built into the inverter, it can save time and cost during installation. In addition, if the inverter is equipped with such a harmonic filter, its RMS input current will be lower than that of the inverter without such a harmonic filter. Therefore, smaller fuses and cables can also be selected, which can further save costs. [b]3 HVAC System Functions[/b] The application of inverters in HVAC systems also includes some or all of the following features: (1) Low-noise motor operation This is especially important when the motor drives the AHU fan, because noise can affect the operation of the piping system and cause annoyance to hotel guests. The noise from the cooling tower fan may also cause annoyance to hotel guests, especially at night. (2) Fast start-up reliability Although the fan (e.g., cooling tower fan) may be shut down, it is still very likely to rotate under the action of natural air convection. If the inverter is not equipped with the "fly start" function, it is very likely to cause mechanical impact on the fan and its mechanical connection parts when starting the fan, which will accelerate wear or even cause breakage, and will also increase the motor current, thereby causing the inverter to trip or be damaged. If the inverter for HVAC system is equipped with the fast start function, it can "stop" the rotating fan blades under any speed and direction conditions, and then control it to start smoothly. (3) Automatic energy optimization The inverter for HVAC system has this function. This function can be used to automatically optimize the excitation effect of the motor when it is running under light load. If the inverter has the automatic energy optimization function, it can save 5 to 10% more energy than the inverter without this function, and can automatically compensate for the insufficient power factor of the motor under light load conditions. (4) Built-in [Manual]-[Stop]-[Automatic] buttons: If these buttons are provided, there is no need to install them on the distribution panel/control cabinet, which simplifies on-site operation and commissioning. (5) Ability to display motor current, kWh, pressure and/or related temperature values ​​in engineering units. (6) Fan belt breakage detection: This function is built into the inverter, so there is no need to install a differential pressure switch at the fan to detect the airflow in the AHU. [b]4 Intelligent inverters for HVAC systems in distributed building management systems[/b] 4.1 Key advantages: As mentioned above, dedicated inverters are equipped with all the necessary related functions, eliminating the need to install other components in the system, thus saving costs. Further integration of the inverter with an advanced interface (serial communication) and integration of the inverter with the building management system (BMS) can further reduce the initial cost of the system. In addition, the reduction in operating and maintenance costs can result in a return of at least 4 times the initial cost within 5 years. A single serial communication cable can be used to connect a dedicated HVAC system inverter to a building management system. When using HVAC system protocols such as OnWorks, BACnet, and Metasys N2, multiple points can be monitored via this single cable, minimizing the number of hardware I/O interfaces and associated construction and commissioning costs. Installation and commissioning costs are thus reduced, while the amount of information processed by the building management system can be increased. Typically, if there is no longer a need to control and monitor the inverter using analog and digital I/O interfaces in a hard-wired system, the inverter's I/O interface can be used as a dedicated I/O interface for the building management system. Even if not used in the initial design, this I/O interface can provide a free backup for future expansion. The inverter can receive start/stop, reset, speed setpoint, and setpoint commands via this communication cable, and can also provide the building control system with indications of its operating and fault/tripping status. When a frequency converter is used to maintain a set pressure (e.g., static duct pressure in a variable air volume system), it can perform all necessary closed-loop control operations, allowing the fan to operate at any speed that can maintain that pressure. The building management system can monitor the pressure and adjust the setpoint as needed via the communication cable. The frequency converter can provide relevant diagnostic information, such as motor current; without a frequency converter, a separate shunt and ammeter would also be required. It can also provide information such as power consumption (kWh), operating time, and many other operating variables. 4.2 Examples The following examples, based on actual application costs and load/operation conditions, illustrate how to optimally utilize a single frequency converter for multiple applications, emphasizing the percentage cost savings that can be achieved at various points in the system/installation equipment after installing a single frequency converter. The cost savings are compared based on two factors: the system cost of using a complete IP54 HVAC system-specific inverter, equipped with radio frequency interference and harmonic filters (Danfoss VLT6000), and integrating the inverter with the building management system using three-phase fuse power and serial communication (inverter cost = 100%); and the system cost of using a hard-wired constant-speed pump/fan with other flow control methods, including the control cabinet with three-phase fuse power, and the motor starter with thermal overload, voltmeter, ammeter, and run and trip indicators. These examples emphasize approximate relative costs, expressed as a percentage of the IP54 inverter cost. The actual cost will vary depending on the project, but the following demonstrates that integrated solutions can save on actual system costs. The cost considered also includes all I/O interfaces requiring wiring and commissioning. Furthermore, the speed at which the inverter's "actual cost" is recovered is highlighted when operating costs are lower (compared to not configuring a flow controller or using other flow control methods). (1) Variable Air Volume (VAV) systems are the most efficient way to maintain specific environmental conditions within a building. In a VAV system, the supply air temperature can be kept constant by adjusting the chilled water valves on the cooling coils. The VAV air handling unit can be adjusted according to the zone temperature to change the airflow to each zone. The intake fan flow is controlled in some form (e.g., inlet guide vanes or frequency converters) to maintain the required static pressure in the intake duct when the VAV air handling unit regulator is opened and closed. Depending on the complexity of the design, temperature sensors may be installed in the intake duct, mixing duct, and rooms to control or monitor relevant temperatures. A digital controller can maintain a constant supply air temperature by sequentially controlling ventilation, cooling, and heating (if relevant). Relative humidity can also be controlled by duct humidifiers. If a frequency converter is used in a VAV system, a pressure sensor should typically be installed at 2/3 of the duct's length to measure the static supply air pressure within the duct. This sensor should be directly connected to the frequency converter. If a PID controller is used with the frequency converter, the converter can operate in a closed loop to maintain the static pressure at the desired setpoint. When the VAV unit is shut down, the sensor detects a rise in static pressure, and the frequency converter reacts by reducing the speed/flow rate of the intake fan to maintain the pressure at the setpoint. A particularly useful feature of frequency converters used in VAV systems is the dual-zone PID controller. If the intake duct is divided into two paths, it is possible to install static pressure sensors in both paths. The frequency converter can ensure that the static pressure remains constant at either point. Alternatively, if there is only one intake duct, a static pressure sensor can be installed near the fan to act as a static high-pressure sensor, thus preventing damage to the duct system. In this case, we compare the cost of using an imported guide vane or hardwired general-purpose frequency converter with the cost of integrating the frequency converter with the building management system (BMS) using a serial communication cable. According to the system design concept, the hardwired variable air volume (VAV) system shown above requires approximately 16 input/output points within the building management system, including: 3 analog output points for controlling fresh air, return air, and mixing air dampers; 1 analog output point for controlling the chilled water valve for cooling coil flow; 3 analog input points for monitoring the temperature of the intake duct, mixing duct, and indoor air (and other parameters if there are more than one zone); 1 digital output point for starting/stopping the fan; 1 analog output point for controlling airflow (controlling the position of the inlet guide vanes or controlling the speed of the frequency converter); 1 digital input point (and a differential pressure switch connected to both ends of the fan) for detecting airflow/fan belt breakage; 1 digital input point for switching between [manual]-[stop]-[automatic] states; 1 analog input point for displaying the static pressure of the intake duct; 1 analog input point for displaying the static high pressure of the intake duct; and 2 digital input points for indicating the running/tripping status. One analog input point is used to monitor motor current; the building management system also uses this data to estimate power consumption (kWh). If a dedicated HVAC system inverter is used, and the static pressure sensors for the intake duct and high-pressure intake duct are directly connected to the inverter, serial communication will be used to integrate the inverter with the building management system. In this case, only one cable (one point) is needed to connect all the following points: fan start/stop duct static pressure setpoint (including standard and upper limit values); actual duct static pressure (including standard and upper limit values); fan belt breakage detection; operation/trip status indication for [manual]-[stop]-[automatic] switching; and motor current and power consumption (kWh). This means that if a dedicated HVAC system inverter is used, only 7 I/O points are needed, thus reducing the number of I/O points to 9. The cost calculation of the building management system should obviously include the wiring and commissioning costs of each control point; therefore, reducing the number of required I/O points can significantly save costs. In addition, for example, a system using a dedicated HVAC inverter only requires a three-phase fuse power supply, while older systems require star/delta starters with thermal overload protection, current transformers, and ammeters. This further reduces costs. The above example specifically refers to a location where an AHU (Automatic Heatsink) has been selected for the VAV (Variable Valve Air) system. The system's pressure design value is 1916 Pa, and the flow design value is 17.85 m³/s, maintaining the static pressure in the pipes at the sensor location within the VAV box at 374 Pa. This example uses a 55 kW motor, and the AHU's flow control option is either an imported guide vane or a dedicated HVAC inverter integrated with the BMS (Body Management System) via serial communication. If an integrated HVAC inverter is used, the installation cost savings are equivalent to approximately 52% of the total cost of purchasing and installing the inverter (i.e., the cost of using the inverter accounts for only 48% of the actual total cost). This doesn't even include the savings from not installing imported guide vanes, which typically account for 35-60% of the inverter's cost. Therefore, once this is included, the inverter becomes the lowest-cost option. If we only consider the basic installation cost savings (ignoring the imported guide vanes) and the load under approximately 18 hours of operation per day and 340 days per year, the actual cost of installing the inverter can be recovered within six months of operation due to energy savings. Furthermore, what hasn't been mentioned above is that inverters do not require power factor correction capacitors and only need three cables, while star/delta starters require six. Inverters not only display motor current but also other information (voltage, kWh, running time), facilitating understanding of system operation. Smooth fan acceleration and deceleration minimize fan belt wear, and low-speed fan operation also reduces mechanical wear, thus lowering operating costs—all of which should be considered. In addition, the BMS system can receive and process more system information through a serial communication network. (2) Constant Air Volume to Single-Zone Variable Air Volume (VAV) Traditional constant air volume (CAV) systems do not have any flow control equipment (as the name suggests). The specific space requiring adjustment receives a pre-designed fixed airflow at all times, and the chilled water valves are adjusted according to the indoor air temperature or return air temperature to change the supply air temperature. This approach makes energy saving almost impossible. However, if the CAV air handling unit can only serve a large single zone (e.g., hospitals, airports, hotels, theaters/cinemas, and large shopping malls), a frequency converter can be installed to simulate a VAV system, thereby saving energy. This type of building typically has a large variation, depending on the area occupied. Therefore, the supply air volume can be adjusted according to its area. Only one frequency converter needs to be installed to control the fan speed according to the indoor or return air temperature to adjust the air volume supplied to the single zone (its function is equivalent to the operation of a variable air volume box in a VAV system). In addition, a temperature sensor should be installed in the air inlet duct to regulate the chilled water valve, thereby maintaining a constant supply air temperature, which is equivalent to the function of a VAV system. The supply air temperature can thus be kept constant, while the controlled area needs to be regulated by adjusting the airflow. The minimum speed of the inverter is usually set to about 70% of the rated speed to maintain air quality. Its speed range extends from this speed/flow rate to the maximum value, depending on the temperature (as above) in the controlled area or the air quality sensor installed in the controlled area (e.g., carbon dioxide), and the specific value will vary depending on the area occupied by the controlled area. In this case, if one is new to using an inverter in a traditional application without a flow controller, the drive may seem too expensive. However, it is important to remember that if HVAC-specific inverters are integrated with a building management system (BMS) using serial communication, cost savings can be achieved throughout the system. According to the system design concept, the traditional constant air volume system shown above would require at least approximately 11 hardwired input/output points if used in a building management system, including: 3 analog output points for controlling fresh air, return air, and mixing dampers; 1 analog output point for controlling the chilled water valve of the cooling coil; 1 analog input point for monitoring indoor air temperature/return air temperature; 1 digital output point for starting/stopping the fan; 1 digital input point (and a differential pressure switch connected to the fan) for detecting airflow/fan belt damage; 1 digital input point for switching between manual, stop, and automatic modes; 2 digital input points for indicating running/tripped status; and 1 analog input point for monitoring motor current, which the building management system also uses to estimate power consumption (kWh). If a dedicated frequency converter for HVAC systems is used, and the intake duct and its static high-pressure sensor are directly connected to the converter, and the converter is integrated with the building management system via serial communication, then only one cable (one point) is needed to connect all the following points: l Fan start/stop; l Indoor (or zone) air/return air temperature; l Fan belt breakage detection; l For [manual]-[stop]-[automatic] state switching; l Running/tripping status indication; l Motor current and power consumption (kWh). If a frequency converter is used, only the remaining four analog points, plus one temperature sensor and one analog input point, are needed. This reduces the number of I/O points by six. The cost calculation of the building management system should obviously include the wiring and commissioning costs of each control point; therefore, reducing the number of required I/O points can significantly save costs. As in variable air volume (VAV) applications, in addition to the cost savings mentioned above, systems using dedicated HVAC inverters only require a single three-phase fuse power supply, while older systems require star/delta starters with thermal overload protection, current transformers, and ammeters. This further reduces costs. Consider a hotel conference room equipped with a single-zone constant air volume (AHU) system. The system's design pressure is 1200 Pa, its design flow rate is 15.5 m³/s, and it uses a 30 kW motor. This is a large single-zone application. We evaluated the cost of integrating the inverter with the BMS system via serial communication, including temperature sensors, and the single-zone AHU system. Using an integrated inverter, the purchase and installation costs can be reduced by approximately 60% compared to using imported guide vanes (i.e., the actual cost of the inverter accounts for only 40% of the total actual cost). The inverter operates for approximately 17 hours per day, or 365 days per year in this application scenario. If the minimum airflow velocity/flow rate is limited to 70% to maintain air supply quality, the actual cost of installing the frequency converter can be recovered within 4 months of operation due to energy cost savings. Other cost-saving examples related to variable air volume applications, such as further reducing the amount of motor cables used and simplifying operation, also apply here. (3) Other applications can be analyzed similarly for other applications of the frequency converter, including: l Condensate pump: In the traditional way, the flow rate of the condensate pump is usually regulated by a two-way valve, and the flow rate is set to the design flow rate value of the chiller. If the valve is opened, the flow rate can be set by using a frequency converter to reduce the pump speed. The system can operate more efficiently. l Cooling tower fan: In the traditional way, the cooling tower fan is usually not controlled and is only equipped with a simple on/off controller or a dual-speed motor. If a frequency converter is used, it can be directly connected to the temperature sensor installed on the cooling tower collector or condensate circuit. The frequency converter can regulate the cooling tower fan speed at any required speed/flow rate, ensuring that the return water temperature to the chiller remains at the design point, thus significantly saving energy, reducing mechanical wear, and lowering noise. Primary pump: The primary pump is almost identical to the condensate pump; therefore, the frequency converter can also be used as an "electronic" valve. Secondary pump: By controlling the secondary pump, the differential pressure at the furthest load end of the system can be maintained at a fixed value, thus saving energy even when the cooling coil valves are at any opening (except fully open). 5. Conclusion The above analysis results show that in all these application scenarios, if a dedicated frequency converter for HVAC systems is integrated into the building management system (BMS) using serial communication, the actual cost of installing the frequency converter is only a fraction of the surface cost. Furthermore, replacing other flow regulation or control devices with frequency converters is the most effective energy-saving method. Therefore, this solution provides the lowest initial cost and the highest operating energy efficiency for the system. [b]References[/b] [1] VLT6000 Design Manual [Z]. Danfoss Ltd. About the Author: Andrew Cooper, Business Manager for HVAC Systems, Asia Pacific, Danfoss Drives & Controls Division, HVAC Systems Control Expert, responsible for promoting Danfoss Drives products in the HVAC field in the Asia Pacific region. He has published numerous papers on energy saving and operation optimization of HVAC systems in several domestic and international HVAC magazines.