GUys managed to get this from LG that shows the control strategy. Notice that it says in heating the unit runs the comp at full capacity even if its at set point. Tried to upload the attachment but its too big, tried compressing it and still too big! So copied and pasted it there were a couple of graphs but these havent pasted.



How does it work?

Cooling Operation





The Multi F and FDX products have quite a complex way of working.

The inverter compressor always starts first.
On compressor start up the compressor speeds up to its target frequency (we will talk more about this later). The refrigerant flows through the 4-way valve (de-energised in cooling) into the condenser where it condenses to a liquid. The liquid refrigerant passes through the electronic expansion valves where it partially expands and leaves the condenser to the fan coil via the field pipe work. The refrigerant continues to expand in the fan coils and returns to the condensing unit as a vapour. The vapour then returns to the compressor via the 4-way valve.
NB solenoid valve 1 is always open on start up and stays open to allow oil return.
As the capacity is increased the fixed speed compressor is needed this will happen when all indoor units are running with all EEV’s fully open.

How this inverter compressor speed controlled?

This unit has a dc compressor (digitally controlled) which is used to vary the cooling or heating capacity to meet the load required. The compressor has several speeds available, it is almost impossible to measure the speed of the compressor with a normal multi meter.

Compressor Target Speed

In normal conditions the compressor is controlled using the indoor units return air thermistor and the capacity. The indoor units measure the room temperature using the return air thermistor; this is compared with the set point temperature on the controller. The fan coils tells the condensing unit what its capacity is and what this delta t figure is. The condenser then calculates what the target compressor speed is.


It can be seen that if the room temperature is more than 3 degrees higher than the set temperature and the outdoor capacity is equal to the capacity of the fan coils then the compressor will be told to run at full speed.

When is the target speed over ridden?

Once the system starts some other parameters are measured to ensure the unit does not operate outside design conditions or operate in a condition which will cause the unit to fail see below:

Ambient temperature

The outdoor units return air sensor is used to protect the compressor in either high or low ambient temperatures, this will over ride the target speed of the compressor.
i.e. if the outdoor temperature is below 24ºC or above 54ºC the compressor target speed will be reduced causing the compressor to slow down.

Indoor unit frost prevention

If the indoor coil temperature falls below 6ºC there will be a risk that the indoor coil will freeze up causing liquid to return to the compressor. The indoor coil thermistor is used to measure this temperature. At 6ºC the maximum operating speed of the compressor is reduced to step 4, if the coil temperature falls to 3ºC the maximum operating speed of the compressor is reduced to step 2 and finally if the coil temperature falls to 0ºC the compressor is stopped. The compressor will not restart or speed up until the coil temperature reaches 7ºC. see below.
To see this in operation you need to have gauges fitted so you can read the suction temperature.
This can also be seen via a temperature probe measured on the indoor coil.
The coil temperature should be above 7ºC in normal operation

How does the fixed speed compressor start?

Once the inverter compressor has reached its maximum speed the fixed speed is called for how this happens is the inverter ramps down and stops and then the fixed speed compressor starts then the inverter will start this will take 4 mins for this operation.

Outdoor Unit fan speed operation

The condenser unit Fan speed is controlled by the outdoor unit coil thermistor, the outdoor ambient thermistor and the speed of the compressor.

In cooling operation once the system starts the outdoor fan will start at full speed (900rpm) and then after (240 Seconds) slow down to a medium speed (650 rpm).

The ambient air temperature affects the fan speed as follows

0ºC fan off
21ºC medium speed 650 rpm
46ºC high speed 900 rpm




Heating how does it work?



On compressor start up the inverter compressor starts up first and the fixed speed starts up shortly after (30 seconds) this is to build up the pressure for the reversing valve.
The compressor will ramp towards its target frequency.

At this time the indoor units fans will not running this is called pre-heat or hot start (we will talk more about this later). The refrigerant flows through the 4-way valve (energised in heating) out to the fan coil via the field pipe work. The refrigerant condenses to a liquid in the indoor fan coil and returns to the outdoor unit as a liquid the liquid passes through the electronic expansion valves where it expands into the outdoor coil (evaporator). Finally it returns to the compressor as a vapour, via the 4-way valve.
NB the solenoid valve 1 will be energised for oil return.
In heating mode All EEV’s are left open 10% to allow oil return to the compressors.












Compressor Target Speed

In normal conditions the compressor is controlled using the indoor units return air thermistor and the capacity. The indoor units measure the room temperature using the return air thermistor; this is compared with the set point temperature on the controller. The fan coils tells the condensing unit what its capacity is and what this delta t figure is. The condenser then calculates what the target compressor speed is.



It can be seen that the units all operate to 4 degrees higher than set temperature so if you set 20 degrees C on the controller the unit will continue to heat to 24 degrees C. If the room temperature is the same as or higher than the set temperature and the outdoor capacity is equal to the capacity of the fan coils then the compressor will be told to run at full speed.


When is the target speed over ridden?

Once the system starts some other parameters are measured to ensure the unit does not operate outside design conditions or operate in a condition which will cause the unit to fail.

Ambient temperature

The outdoor units return air sensor is used to protect the compressor in either high or low ambient temperatures, this will over ride the target speed of the compressor.
i.e. if the outdoor temperature is below -10ºC or above 15ºC the compressor target speed will be reduced causing the compressor to slow down.


Over heat protection

Over heat protection is the discharge temperature which is measured on the pipe sensor at the indoor coilS (due to the unit being in the reverser cycle the indoor unit becomes the condenser rejecting heat and the outdoor becomes the evaporator absorbing heat). If temperature reaches 65c the outdoor fan will begin to slow down this is to reduce the amount of heat generated by the outdoor fan until the temperature drops to 55c.
NB If the indoor coil temperature continues to rise above 65c then the compressor will stop.

De-frost how does it work?

When a unit is in heating mode a small amount of ice may form on the outdoor coil this is normal.
This will give poor heat exchange which will cause the indoor coil to drop in temperature.
NB there is a solenoid valve 2 this is to allow hot gas to enter the liquid line when the ambient temperature is below 0 degrees.
There is a defrost sensor that will measure the condensing temperature.
NB if the liquid temperature is below 15 degrees this will cause the outdoor unit to go into defrost.
NB If the unit is incorrectly charged of refrigerant ie undercharged this will cause unnecessary defrost causing little or no heat.
After defrost the unit will resume normal action starting with pre-heat.

What controls the expansion valve?

The expansion valve is controlled as a function of the speed of the compressor as the compressor speeds up then the expansion valve opens.
NB In heating mode all electronic expansion valves are open weather the indoor units are running or not this is to ensure oil return to the compressor.



Outdoor Unit fan speed operation in heating


The outdoor unit Fan speed in heating is controlled by the outdoor ambient thermistor, indoor pipe thermistor and the speed of the compressor.

In heating operation once the system starts the outdoor fan will start at full speed (900rpm) and then after (240 Seconds) slow down to a medium speed (650 rpm).

The ambient air temperature affects the fan speed as follows

-15 ºC high speed 900 rpm
5 ºC medium speed 650 rpm
25 ºC off
NB On all outdoor units that have a twin fan system one fan will switch off this will be the bottom fan and the top fan will work in the same way as above.