Variable Frequency Drives or VFD’s have added a whole new dimension to energy efficiency and control. These drives, or VFD’s, are hooked up to two different YorkVAV air handlers that have two three phase motors a piece. Each air handler has a return fan and a supply fan. Two thirds down stream from the supply fan motor is a static pressure sensor (Pressure Transducer). This is connected back to a DDC controller which will regulate the speed of the fans based on the static pressure set point. Variable frequency drives, or VFD’s, allow the system to use only the amount of air that is needed for the VAV boxes. If demand for air flow is high the drives, VFD’s, ramp up and deliver more air. If the demand for air flow is low the drives, VFD’s, slow down so less air is delivered. This allows the system to use less energy and maintain comfort levels based on temperature set points. Variable frequency drives (VFD’s) have been around for a long time and were used with some success with older pnuematic systems. With the rise of DDC in the last 10 to 15 years more and more variable frequency drives (VFD’s) are being used to modulate air flow and save energy. Variable frequency drives work by changing the frequency or hertz of the power supplied to the electric motor. This change in frequency causes the electric motor to speed up and slow down depending the frequency. 60 hertz and the motor is at full speed and generally (depending on drive parameter settings) 15 or 20 hertz is low speed. The motor can run anywhere in between the minimum and maximum hertz ranges and will respond with the appropiate speed for the hertz. Variable speed compressors are the wave of the future and this will save the industry tons of money when they become more popular and cheaper.