Guide book 1 Guide-Book Last update January RECOMMENDATIONS FOR USING FREQUENCY INVERTERS WITH POSITIVE DISPLACEMENT REFRIGERANT COMPRESSORS Contents Page Scope and purpose General Operation Application ranges Design criteria Selection of Frequency Invert

Guide-Book Last update January RECOMMENDATIONS FOR USING FREQUENCY INVERTERS WITH POSITIVE DISPLACEMENT REFRIGERANT COMPRESSORS Contents Page Scope and purpose General Operation Application ranges Design criteria Selection of Frequency Inverters and electrical design considerations Recommendations for commissioning Scope and purpose These recommendations concern the application of externally-mounted frequency inverters for the speed variation of refrigerant compressors powered by three-phase induction motors This paper is directed to planners and contractors designing and installing such energysaving systems The purpose is to reduce the risk of damage to refrigerant compressors when operated with variable-speed and to provide general design guidelines for reliable energy-saving operation of refrigeration installations General In classical applications without frequency inverters the compressor operates at an approximately constant speed determined by the supply frequency and the number of motor poles A frequency inverter allows the stepless variation of compressor speed to the cooling capacity requirement of the installation Features of operation of a refrigerant compressor with a frequency inverter z Higher system e ?ciency under partial load z Increased compressor operating life due to fewer compressor starts With an optimum system the variable-speed compressor would operate continuously z Integrated soft start function The starting current and torque much lower than with star delta or part- winding starting z Lower risk of liquid slugging due to reduced volumetric capacity during starting z An increase in compressor capacity is often possible by operation above rated speed z A frequency inverter controlled compressor will operate over a wide speed range Under certain operating conditions mechanical resonances and gas resonances at critical pipe lengths can occur In most cases these can be eliminated by good mechanical mounting and design and by making appropriate settings to the inverter control parameters e g setting skip frequencies CGuide-Book Last update January Operation Compressors without frequency inverters for speed control The cooling capacity of a reciprocating compressor can be adapted to the cooling demand of the installation by cylinder-bank o ?-loading by hot-gas by- pass by suction pressure control by starting and stopping compressors in a compressor rack or by other means The nominal compressor speed depends on the supply frequency and on the no of motor poles With a -pole induction motor squirrel-cage motor the following approximate compressor speeds result z -pole min- at Hz or min- at Hz With -pole motors the following corresponding compressor speeds are z -pole min- at Hz or min- at Hz Compressors with frequency inverters for speed control Reciprocating screw and scroll compressors are positive displacement machines The average load torque at the compressor shaft remains approximately constant over a wide range of speed frequency Cooling capacity and electrical power consumption therefore vary approximately proportional to the speed see Fig The cooling capacity can be adapted to the cooling requirement of the installation plant by varying the compressor speed with frequency control Optimum stepless control is possible With reciprocating compressors the relative COP COP Coe ?cient Of Performance Refrigeration Capacity Electrical power input to compressor can vary slightly with the operating frequency

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