On the Efficiency Optimization of VFD Drives in HVAC Systems

Nowadays, induction motors in HVAC systems are driven using V/F controlled VFDs, owing to the slow dynamic behaviour of the load. This connection led to an increase in the overall efficiency and decrease in the overall cost of the HVAC system. However, as the load torque is proportional to the square of the rotor speed, the efficiency of the VFD can be further increased by optimizing the magnitude of flux in the machine. In this paper, the field weakening algorithm with MTPA is extended to the VFDs in HVAC systems such that, the efficiency of the drive is increased when the drive is operated in part loading conditions. Further, to reduce the switching loss, the inverter of the VFD is switched using Bus-Clamped PWM technique. Thus, efficiency of the overall VFD is improved at all operating points. The analytical, simulated, and experimental results are found to validate the proposed technique.