Impact of Motor Slip and Belt Slip on Fan Airflow Station for VAV AHU Systems

Fan airflow measurement is critical for HVAC systems operation and control. The fan airflow station measures the airflow using measured fan speed, fan head, and fan curve. The fan speed can be measured by a tachometer, and the fan head can be measured by a differential pressure transducer. In a typical building automation system, the controller knows the desired fan speed and gives the speed command to the variable frequency drive (VFD). Motor slip occurs in the direct drive fan. Belt slip also occurs in a belt drive fan besides the motor slip. If the motor and belt slip can be calculated by the VFD speed, the control command can be used to represent the actual fan speed. Consequently the tachometer can be eliminated from the fan airflow station. This modification will simplify the system, reduce the cost, and improve the system’s reliability. To validate the theory, theoretical and experimental studies were conducted. The experiments were conducted with a full size air handling unit (AHU) in a 50,000 square foot building. The theoretical analyses and experiments prove that the control command can be used to represent the actual fan speed. No tachometer is needed for the fan airflow station. The experiment also shows that the fan airflow station can maintain positive building pressure at most of time and prevent unstable return air fan operation under all weather conditions.