Cooling capacity and system efficiency

Mechanical capacity control

In medium and large HVAC (heating, ventilation and air conditioning) systems like air cooled chillers, a common method of capacity control are multi-compressor circuits where multiple compressors are switched on and off. The properly sized fluid 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

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. Cooling capacity and power consumption therefore vary approximately proportional to the speed (see graph below), cooling capacity can be steplessly adapted via speed control. Permitted speeds / frequencies for Bitzer compressors are given below (Application range).

Typical graph of the cooling capacity Q0 depending on the speed and frequency of an Orbit compressor (E..7 series: max. 65 Hz)
Typical graph of the cooling capacity Q0 depending on the speed and frequency of an Orbit compressor (E..7 series: max. 65 Hz)

Electrical power consumption at full load is slightly higher than when operating the compressor directly on the mains supply. This is due to losses in the frequency inverter – caused by the losses of individual electronic components for power conversion and for cooling the frequency inverter. Another source of motor heat-up and reduced motor efficiency are harmonics: The higher the quality of the frequency inverter and the better it is configured, the lower the harmonic distortion factor in the output signal.

There are several variables involved in the operation of the inverter which affect the running and starting of the compressor:

In general, however, losses caused by the frequency inverter are normally offset by gains in system efficiency by operating at a more efficient cycle through matching compressor capacity to system load requirements. Inverter applications will thus usually increase overall system efficiency under “real world” conditions.

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).

Operating characteristics of an Orbit compressor motor (E..7 series: max. 65 Hz) for operation with frequency inverter (400 V/3/50 Hz) with reserve. P: max. compressor power consumptionM: max. torque of the motor at compressor shaftI: max. compressor current consumptionf: frequency (frequency inverter output)U: voltage (frequency inverter output)
Operating characteristics of an Orbit compressor motor (E..7 series: max. 65 Hz) for operation with frequency inverter (400 V/3/50 Hz) with reserve.
P: max. compressor power consumption
M: max. torque of the motor at compressor shaft
I: max. compressor current consumption
f: frequency (frequency inverter output)
U: voltage (frequency inverter output)