From my experience, the required cooling load will be worked out by the main control unit for the chiller/refrigeration machine, from the leaving water/air temperature and the time it took to reduce the temperature.
The control unit will then send a signal to both the compressor and EXV to regulate the refrigerant flow and pressure difference to match the required cooling capacity; if possible.
I hope this helps, though it is a bit vague. I have done a lot of work with Turbocor units, and they generally work on this principle.
The vsd needs a signal to change the frequency of vsd . The signal is either from a pressure transducer mounted on the casing of compressor . In other words, the suction pressure change due to fall in load is sensed by pressure transducer & it is converted in a 4 to 20 milliamp signal .the signal changes the frequency & hence the speed .
You can install a temperature transmitter to achieve the frequency change .the exv will respond on the pressure differential across it`s port
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