Electronic expansion valves and frequency inverters

For refrigeration systems where compressor capacity is controlled by frequency inverters (FI), the alignment of the controllers is essential for problem-free operation. The application of different electronic controllers (e.g. electronic expansion valves or pressure control valves) in the refrigeration system or heat pump with unchanged default factory settings often leads to faults or even to compressor damage:

• Frequency inverters for motor compressors often have factory settings with very short times for ramping the speed up and down. The mechanical parts of the compressor can handle fast speed increases without problems and achieve good lubrication quickly in this way.
• Conversely, electronically controlled expansion valves usually have factory settings with significantly longer times for opening and closing the valve. This effectively avoids pressure fluctuations ("hunting") for most evaporators.

The combination of these two default settings with a simple control scheme by the system controller unfortunately leads to operational problems: If the compressor capacity is reduced much faster than the injected amount into the evaporator, unevaporated (i.e. liquid) refrigerant may reach the compressor. By starting the compressor up again quickly while the expansion valve opens slowly, the suction pressure will drop sharply, and the low pressure switch could cut out - liquid refrigerant in the compressor dilutes the oil and leads to compressor damages.

• One solution would be to set the opening and closing times for the expansion valve and for speeding up and down of the compressor in the system controller program and to set the frequency inverter and the expansion valve on short values. The system controller would control the timing. Thus, different control cases can use different timing for temperature control, pump down, etc.
• Another option is to shorten the timing of the expansion valve as much as possible without risking stable operation of the evaporator and to set the frequency inverter to a longer time for the speed adjustment above the minimum speed. In this way, the evaporator can be controlled well while the compressor speed is changing. However, with this method, no differences can be made for differing operation modes.