Refrigerating capacity and system efficiency

Mechanical capacity control

In medium and large HVAC (heating, ventilation and air conditioning) systems like air cooled liquid chillers, a common method of capacity control are multi-compressor circuits where multiple compressors are switched on and off. The properly sized liquid loop generally provides a damper for changes in fluid temperature caused by switching individual compressors on and off. The system controller ‒ along with various fluid and ambient temperature sensors ‒ provides the logic for compressor staging to maintain the desired fluid temperature.

The compressor is operated at a constant speed, the motor speed correlates directly with the mains supply frequency.For 2-pole asynchronous motors, this results in a nominal speed of

• 2900 min^-1 at 50 Hz and
• 3500 min^-1 at 60 Hz.

For high energy efficiency, high control accuracy of fluid temperature or very low load conditions, frequency inverters can be of advantage for capacity control.

Capacity control with frequency inverter

The average load torque at the compressor shaft mainly depends on operating conditions and refrigerant properties. Thus, it remains approximately constant over a wide range of speed / frequency. Refrigerating capacity and power consumption therefore vary approximately proportional to the speed (see graph below), refrigerating capacity can be steplessly adapted via speed control. Permitted speeds / frequencies for Bitzer compressors are given below (Application range).

Voltage characteristic

For a given operating condition, the motor torque will tend to be relatively constant regardless of its speed. For good motor efficiency and reliability the voltage should be adjusted over the speed range to achieve a constant current (amperage) at a given condition. This is best done by establishing the ratio of the name plate voltage to the name plate frequency and programming the inverter to maintain this ratio over the speed range. This is commonly known as the voltage-frequency (U/f) ratio or the Volt-Hertz ratio.

The frequency inverter cannot deliver voltage above the input voltage (= supply voltage). Therefore, the stator voltage cannot increase any further with higher inverter frequency. The magnetising current in the main inductance drops, the stator rotating field and torque are weakened.

This means that when raising the frequency above the synchronous speed, the voltage-frequency ratio U/f falls. Since the torque required by the compressor remains constant, the current consumption of the motor will increase (see figure below). Therefore, the motor should have adequate reserve (current / power) at supply frequency. The frequency / speed can be increased up to the maximum motor current (RMS – root mean square) (see maximum operating current on the name plate or in the Bitzer Software).