Applying energy-efficient motors in the petrochemical industry

Several US and Canadian utilities have implemented demand side management (DSM) programs for retrofitting energy efficient (EE) motors. This article examines some of the practical problems associated with this. When determining the energy savings achievable by retrofitting standard efficient (SE) motors with EE motors, the nominal efficiency difference between the SE and EE motors is often used. However, EE motors often operate at a higher speed or a lower slip in order to achieve the higher efficiency. This has an impact on centrifugal loads where the power consumed goes up as the cube of the speed, but the increased flow goes up only linearly with speed. Since the majority of loads in the petrochemical industry are centrifugal in nature (fans, pumps, and compressors), the impact of this speed differential on the actual efficiency difference is highlighted. Another consideration when retrofitting EE motors is the effect of operating voltage on efficiency. From end use metering it has been observed that most motors are supplied at above their rated voltage. The difference in rated voltage and realistic operating voltages also affects motor efficiency; however, SE and EE motors are affected differently. The impact of this operational difference on energy efficiency is also examined. Manufacturers' data is also presented to show relationships between efficiency, power factor, horsepower, and percentage load. As a consequence of this, the important question of downsizing of motors to operate closer to the smaller motor's maximum efficiency is considered.