Advantages Of VFD In Heating And Air Conditioning Systems
One of the most successful energy management tools used in building UVAC systems is Variable Frequency Drive(VFD).
Over the past 20 years, VFD has been successfully installed on fan and pump motors for a variety of variable load applications. Reduce energy consumption by up to 35 to 50 percent compared to traditional constant speed applications. The investment return period is 6 months to 2 years.
The power limitation of the motor originally limited the equipment using VFD, but now VFD can be installed on all RVAC equipment in commercial and residential buildings. The new VFD can work at a higher voltage than the previous generation, providing a readily available, stable and variable power supply for motors with a maximum of 500 horsepower.
Previously, VFD was affected by low power, which may damage distribution capacity facilities, resulting in cost penalties for power companies. VFD can now operate at almost constant power over the entire speed range of the motor.
In addition, the problem of operating noise was corrected. As the drive output frequency responds to the load and decreases, the vibration noise generated by motor engagement can be easily transmitted to the interior space of the building through the motor. But now the operating frequency of the driver is higher, and the operating noise is beyond the range of human hearing. This is just an example. VFD is constantly developing.
From various advantages of VFD to more and more available devices, VFD has proved to be more useful and powerful.
- Main characteristics of VFD
Most traditional buildings use HVAC systems to run fans and pumps at a certain speed. But the load of the building is constant. In existing systems, some type of mechanical throttling system can be used to reduce the quantity or airflow. Whether the system is heating or cooling load, the drive motor runs at full speed, consuming almost the same energy.
Mechanical valve system can provide good control, but the efficiency is not high. VFD has become an effective alternative.
The operational efficiency of VFD is improved through the coordination and cooperation of the three elements.
1.1 On demand operation
The HVAC system of the building is designed for peak load conditions. For typical electrical products, the maximum load condition is only 1% to 5% of the operating time per year. This means that the pump and fan motor consume more energy between 95% and 99% of the operating time.
1.2 Overload system design
The peak load design will cause the RVAC system to consume too much energy in most working hours. The overload design mode further aggravates this situation. Within the normal operation range of HVAC equipment, its power system also allows unknown loads that may be generated by underrated, unexpected road conditions and other uses of the building space.
1.3 Motor energy consumption is a function of speed
Induction motor is the most commonly used motor in building HVAC system. For induction motors, the power consumption varies with the cubic number of motor speed. This means that if the motor can reduce the normal operating speed by 25%, the energy consumption will be reduced by nearly 60%. If the motor speed is reduced by 50%, the energy consumption will be reduced by nearly 90%.
In HPAC applications, the installation of VFD will decrease with the decrease of system load. The VFD controller will reduce the running speed of the motor to meet the flow through the system, but not exceed the load requirements.
Installing VFD on the whole air conditioning equipment can solve the problem of low efficiency caused by the first two factors, and save energy due to the third factor. VFD converts 60 loop currents into D, and then achieves this goal according to the change of load output voltage and frequency of the system. As the system load decreases, the controller in the VFD will meet the load requirements through the system process, but will reduce the motor speed to avoid exceeding.
- Advantages of VFD
The most important advantage of using VFD is to save energy. The system capacity matches the annual actual load, and the goal of saving the energy consumption of the motor in the HVAC system is achieved.
Another advantage is to reduce motor wear. When the induction motor works, its current consumption is higher than that of normal operation. These ammeters can be 3 to 10 times of the total working current of the motor and generate heat and stress on the motor coil and other parts. Frequent starting and stopping will lead to early motor failure.
Conversely, if the motor is connected to a VFD, very low frequencies and voltages are applied. These two devices are gradually restored to normal at a controllable speed, extending the service life of the motor.
VFD also provides more precise control of the equipment. For example, a booster pump system is used in high-rise buildings to supply domestic water to maintain adequate water pressure at each floor of the building. This allows the traditional pump control to maintain the pressure within a range, but VFD can maintain more accurate control within a larger flow range, while reducing energy requirements and pump wear.
Three phase induction motors are most commonly used in building UVAC applications, but single-phase induction motors can be used for smaller applications. Both can install VFDs.
VFD controller can be used in many fields, but its application requires variable speed, so it gains the greatest benefits. For example, use VFD to change the flow rate. The pumps used to build the HPAC system can generate flow rates that match the building load. Similarly, in domestic hot water and cold water systems that do not consider the flow rate and require a certain pressure, VFD controlled by the pressure setting value can maintain the pressure at the most required level.
Most commercial and public UVAC systems use variable intake fan systems controlled by variable intake vanes and variable intake slots to distribute air conditioning. As the load decreases, the variable bellows closes and the positive pressure of the system increases. When the controller on the fan detects this increase, it closes the inlet blades. Using this type of control system can reduce the energy demand of the system fan, but it is not as efficient or accurate as the system using VFD.
VFD can also be used in variable refrigerant flow system. The system connects one or more compressors to a common refrigerant supply system that provides multiple steam. The demand for energy distribution is greatly reduced by replacing air pipes with refrigerants.
The load of the compressor varies with the demand of the compressor. Therefore, VFD matches the load by controlling the speed of the compressor to reduce the energy demand under certain load conditions.
- Other electrical
The advantages of VFD above are only energy saving, but it is very suitable for other electrical appliances, and energy saving is secondary. For example, VFD can control precise speed or torque in some commercial electrical products. Some are dedicated to dual fans or pumps. Due to the precise speed control of VFD, they can run at the required speed.
Technological advances have increased the amount of load that can be driven by one unit. At present, the voltage and current ratings may match three-phase induction motors in most buildings. The equipment with more than 500 horsepower can be installed on the large capacity centrifugal cooling water equipment, which can significantly save energy.
One of the most important changes that has occurred recently is that VFD control is necessary for systems from different manufacturers and is widely recognized. For example, centrifugal chiller manufacturers provide VFD control for multiple devices.
Similarly, manufacturers of household booster pump systems also take control as a part of the system, providing users with better control strategies and reducing energy and maintenance costs.
- Precautions
Facility managers should consider various factors related to application details when evaluating VFD installations. For example, if the VFD sends out a series of fast switching pulses, these pulses can be reflected from the motor end back to the cable connecting the VFD to the motor.
For devices that work for a long distance between the motor and VFD, these reflected pulses will generate a voltage that exceeds the line voltage, causing cable and motor winding stress, thus causing insulation failure. Motors operating below 230 volts are almost unaffected, but motors operating above 480 volts have problems. Use cables specially designed for VFD to minimize the distance between VFD and motor.
In addition, VFD may affect motor bearings. Pulse generated by VFD may cause voltage difference between motor shaft and panel cabinet. If the voltage is high enough, sparks can corrode bearings and surfaces. This can be avoided by using cables.