Technical FeaTure Fea this particular compressor was installed with a VFD, as well as a solid state starter, that allowed the machine to operate by electrically bypassing the drive to run in a traditional fixed speed operating mode. The installation of this compressor afforded an opportunity to directly compare the performance of the same physical compressor operating as a fixed speed machine using a slide valve for capacity control and variable speed; thereby, eliminating any machine-to-machine variability. With the VFD active, speed was used as the first level of capacity control until the manufacturer's recommended minimum speed of 1,775 rpm was reached. If further capacity reduction was required at the minimum speed, the control system then used the compressor's continuous slide valve. This particular compressor operates with a nominal suction pressure of 0.4 psig (2.7 kPa) and discharges to an intercooler operating at a fixed pressure of 37 psig (255 kPa). The nature of this particular installation gave complete flexibility in varying the load on the compressor and maintaining a given load condition for an extended period of time to ensure stead- state conditions were reached. The capacity of the compressor was determined by measuring the flow rate of refrigerant in the discharge line using a differential pressure flow sensor (Figure 8b) and the refrigerant conditions at the compressor discharge condition. Both true power and apparent power were measured using a watt transducer with inputs of measured voltage and three-phase current supplied to the compressor. Figure 9 (Page 24) shows the measured compressor true power input to the variable frequency drive over a range of part-load conditions while operating in both variable speed and fixed speed modes. Note the higher electrical power required at full-load condition for the variable speed case is attributable to the added penalty of electrical losses for the variable frequency drive itself. Once the machine unloads below 95%, variable speed drive has an increasing power advantage compared to the fixed speed drive operation. From an efficiency perspective, it is advisable to avoid operating a VFD-equipped compressor at or near full load. When one combines this finding with the relatively high capital cost for a large horsepower low voltage (480 www.info.hotims.com/44638-51 N ovem ber 2013 ashrae.org ASHRAE JouRnAl 27