Challenges and Misconceptions

  • in Implementing Variable-Speed Technology

    Compressor and drive selection, designer skills makes the system overall reliability and efficiency.

    Importance of the inverter drive: The compressor and drive need to be qualified to work together and for dedicated applications. The drive modulates the compressor speed and prevents it from operating out of the compressor operating limits. The inverter frequency drives need to use algorithms developed specifically for heating, ventilation and air conditioning (HVAC) or for refrigeration. They ensure that the system will run within the application constraints. The drive can also manage other devices such as oil injection valves or multiple compressors. As the compressor rotational speed changes, the amount of refrigerant — and oil — flowing through the compressor increases or decreases. The drive ensures that the compressor is optimally lubricated at all compressor speeds.

    OEM manufacturer skills in inverter system integration: not all OEMs are yet prepared to adopt inverter technology due to following reasons:

    1. Competence level and experience engineering OEM in implementing inverter technology. The strong experience in mechanical must be complemented with electrical engineering skills as well as controller development and programming.
    2. Inverter compressors continuously keep changing speed to match the load, which makes the oil management of the system more complex compared to traditional technology. Oil management mastering is a key issue – when not using an oil-free compressor type - where compressor manufacturer experience makes the difference
    3. The application support for OEMs by compressor and inverter drive manufacturer is critical in system integration.
    Misconceptions about variable speed technology

    Inverter technology reacts too slowly to load changes: some inverter scrolls ramp up/down speed range is between <0.1s; 3600s>Full load inverter losses 5-10% at full speed the inverter car lose 3% of efficiency. As full load is not the dominant profile, the loss will be minimal in the overall unit consumption.

    Oil circulation increases at high frequency operation: oil circulation rate electronically controlled by some inverter scrolls can be less than 3% at full speed. Oil management is more complex with inverter compressor systems. Some inverter scrolls control oil circulation at both low and high speed requiring minimal design accommodations.

    Inverters can’t be used where EMC (Electro Magnetic Compatibility) is a concern: some inverter solutions offer 2 levels of EMC filtration which meet the most stringent European standards related to electromagnetic interference.

     

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