Refrigerating capacity and system efficiency
Mechanical capacity control
The refrigerating capacity of a screw compressor can be mechanically adapted to the cooling demand of the system e.g. by sliders or control pistons – in compound systems also (additionally) by switching individual compressors on and off. The compressor is operated at a constant speed, the motor speed correlates directly with the mains supply frequency. This results in the following nominal speed for 2-pole asynchronous motors:
- 2900 min-1 at 50 Hz and
- 3500 min-1 at 60 Hz.
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).
Compressor and motor damage!
Do not combine a frequency inverter with mechanical capacity control of the compressor! Especially at low speed, adequate motor cooling is not guaranteed because refrigerant mass flow is heavily reduced. Certain exceptions for screw compressors are possible in consultation with Bitzer.
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:
- The voltage characteristic limits and regulates the current supplied to the motor,
- the switching frequency of the converter in the frequency inverter regulates the motor performance and reliability,
- the start sequence and voltage boost control the starting process 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.
In order for the motor to always operate at its nominal operating conditions, a control mode with constant voltage-frequency (U/f) ratio is chosen at the frequency inverter.