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Variable Frequency Drives VFDs | Energy Efficiency - In HVAC variable frequency drives add energy efficiency and precise control for fans and pumps. The VFDs offer demand control for airflow and water in chilled water systems, hot water systems, and air handling systems. Additionally, they are used to control airflow in a variable air volume system. More on how VAV boxes work.
VFDs require inverter duty motors and variable frequency drives can be purchased for three-phase and single-phase applications although the vast majority of variable frequency drives are used in commercial three-phase applications.
Variable Frequency Drives VFDs | Energy Efficiency
Variable frequency drives are also used for conveyor systems and machine tooling equipment like CNC machining. The drives or VFDs work by changing the frequency or hertz of the electricity serving the electric motor. VFD’s typically have a local interface for programming parameters for specific applications and features the specific frequency drive has to offer. VFD’s are also controlled remotely by DDC or SCADA systems where the DDC system or the SCADA system receives control input from a remote device and responds by controlling the drive to a certain speed based on the control input from the remote device.
An example of that would be a pressure sensor located in a chilled water pipe where a specific pressure needs to be maintained. If the pressure falls the variable frequency drive responds by increasing the speed of the pump motor and if the pressure rises above the set point the variable frequency drive responds by decreasing the speed of the pump motor.
Furthermore, if the piping has 10 air handlers with chilled water coils inside them and the actuators open the chilled water valves (based on demand) the pressure in the loop will drop. When the pressure drops the control program will send a signal to the VFD to speed up to maintain the setpoint pressure. If demand drops and the valves close or close off since many are modulating then the control program will slow the pump down using the VFD.
Pressure Control
The pressure in the water loop will rise and fall when the valves on the loads open and close. The valves on a set of CRAC units will open when the CRAC units determine that the temperature needs to be reduced in the space so the valves will open up to provide more chilled water to the coils. When this happens the pressure transmitter senses a drop in pressure. This pressure drop is fed back to the DDC program and it sends an output signal to the VFD to speed up the pump to pump more chilled water to handle the demand.
For those who don’t know - a CRAC Unit is a Computer Room Air Conditioner usually found in telecom buildings and data centers.
When the CRAC units cool the temperature down on the data center floor the chilled water valves begin to close. This results in a pressure increase in the chilled water loop. Again, the pressure transmitter in the chilled water loop senses this increased pressure and sends the signal back to the DDC system where the DDC system responds by sending a signal to the VFD’s to slow down because of the previous demand for chilled water is no longer needed. This is the perfect energy miser system based on demand.
Variable Frequency Drives VFDs
