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Guide-Book Last update: January 2012 1 / 16 RECOMMENDATIONS FOR USING FREQUENCY

Guide-Book Last update: January 2012 1 / 16 RECOMMENDATIONS FOR USING FREQUENCY INVERTERS WITH POSITIVE DISPLACEMENT REFRIGERANT COMPRESSORS Contents Page 1 Scope and purpose ......................................................................................................................................... 1 2 General ............................................................................................................................................................. 1 3 Operation .......................................................................................................................................................... 2 4 Application ranges .......................................................................................................................................... 5 5 Design criteria ................................................................................................................................................ 11 6 Selection of Frequency Inverters and electrical design considerations .................................................. 14 7 Recommendations for commissioning ........................................................................................................ 16 1. 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 energy- saving 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. 2. 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 efficiency 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). Guide-Book Last update: January 2012 2 / 16 3. Operation 3.1 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 off-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 4-pole induction motor (squirrel-cage motor) the following approximate compressor speeds result: z 4-pole: 1450 min-1 at 50 Hz or 1750 min-1 at 60 Hz With 2-pole motors the following corresponding compressor speeds are: z 2-pole: 2900 min-1 at 50 Hz or 3500 min-1 at 60 Hz 3.2 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 3.2. 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: Coefficient Of Performance: Refrigeration Capacity Electrical power input to compressor) can vary slightly with the operating frequency and speed of the compressor speed, see Fig 3.2. At frequencies lower than rated speed a slightly higher relative COP can result. At frequencies higher than rated speed a lower relative COP usually results. Q: Cooling capacity COP: Coefficient of performance R50 Rated speed at 50 Hz R60 Rated speed at 60 Hz Fig 3.2: Cooling performance of a typical positive-displacement, reciprocating, refrigerant compressor over the permissible frequency range at steady-state operation Guide-Book Last update: January 2012 3 / 16 3.3 Operation below rated frequency Since the torque of the electric motor is proportional to the product of magnetic flux and internal current, it is necessary to ensure that the magnetic flux is constant. This is achieved by selecting a control mode with a constant voltage/frequency (V/f) ratio in the inverter. Ideally the voltage of the motor reaches the rated voltage corresponding to the electrical supply voltage at the rated frequency. For compressors using standard motors this is as follows: − 400 V at 50 Hz − 460/480 V at 60 Hz Operation below the rated frequency is referred to as operation in the “constant field” region, i.e. the magnetic flux in the motor remains approximately constant due to the constant V/f ratio. The minimum permissible frequency is limited by the following considerations: − Lubrication − Refrigerant mass flow necessary for sufficient motor cooling − Reciprocating compressors: − Inertia to ensure no significant speed droop at top-dead-center piston position − Suitability of compressor mounting (a stiffer mounting is often required at lower frequency) − Screw compressors: − Adequate oil sealing − Scroll compressors − Radial centrifugal forces, gas forces, adequate oil sealing. Refer to compressor supplier for the lowest permissible frequency of operation. 3.4 Operation above rated frequency Refer to the compressor supplier for the highest permissible frequency of operation. The following are common connections used on standard motors for compressors: Versorgungsspannung Anschluß − 400 V at 50 Hz / 480 V at 60 Hz: Three terminals (most hermetic compressors). − 400 V at 50 Hz / 480 V at 60 Hz: Six terminals connected in star / 230 V at 50 Hz: Six terminals connected in delta for 230 V supplies. − 400 V at 50 Hz / 480 V at 60 Hz: 2x three terminals each of a part winding connection connected in parallel for inverter operation. − 690 V at 50 Hz / 828 V at 60 Hz: Six terminals connected in star for starting/ 400 V at 50 Hz / 480 V at 60 Hz: Six terminals connected in delta for normal operation. Operation higher than these frequencies at the above supply voltages is referred to as operation in the “field weakening” region”, i.e. the motor flux is less than the constant value up to rated speed. Guide-Book Last update: January 2012 4 / 16 Normal frequency inverters can only supply an output voltage up to the supply voltage of 400 V (or 480 V). Above the rated frequency the output voltage will remain constant at the supply voltage. The voltage/frequency ratio (V/f) will be lower resulting in a corresponding decrease in magnetic flux in the motor. However the average torque of a positive displacement compressor (such as a reciprocating compressor) is usually approximately constant under constant load conditions. The motor current will therefore increase approximately proportional to the increase in frequency. The frequency can be increased until the maximum continuous rms (thermal) motor current is reached. In order to operate safely above rated frequency with medium temperature applications it is recommended that a compressor variant with a larger motor is used. Operation with a compressor using a smaller motor typical for low-temperature operation is not recommended due to the restricted range of frequency. The maximum permissible frequency is limited by the following considerations: − Power reserve of the motor at rated frequency (factor by which current is lower than the maximum current at the target operating point taking refrigerant, evaporating and condensing temperatures into consideration) − Mechanical considerations (e.g. piston velocity) − Gas-flow and valve plate considerations. 3.5 Operation above rated frequency with a special 87 Hz motor connection Refer to the compressor supplier for the highest permissible frequency of operation in this standard motor connection. The following motor connections are considered here: − 230 V at 50 Hz / 400 V at 87 Hz with a 3 terminal motor − 230 V at 50 Hz / 400 V at 87 Hz with a 6 terminal star/delta motor connected in delta. The electrical rated frequency (the so-called base frequency) in these connections is 87 Hz. Note: 87 Hz is √3 * 50 Hz corresponding to the inverse of the factor of voltage when connecting a motor winding from star to delta. Operation below 87 Hz is referred to as operation in the “constant field” region. Operation above 87 Hz is referred to as operation in the “field weakening” region. This connection has the following advantages and disadvantages: Advantages: z Higher cooling capacity can be achieved with a given size of compressor z Wide range of speed variation, i.e. high ratio (Qmax - Qmin)) / Qmax. Disadvantages: z Inverter and current ratings of switchgear and cables are 73 % higher which may result in a corresponding cost penalty. z Emergency operation of compressor directly connected to the electrical supply is only possible with a 6 terminal star/delta motor connected in star which requires 4 electrical power contactors in the electrical control enclosure for activation without re-wiring. Guide-Book Last update: January 2012 5 / 16 4. Application ranges Safe and reliable operation of a speed-controlled compressor requires a careful consideration of the permissible frequency range. In the following the limit of safe operation of the electric motor and the compressor will be considered separately for various examples of semi-hermetic reciprocating uploads/s3/ guide-book 1 .pdf