Midea M-Thermal Series Service Manual page 20

Table 3‐4.2: Component control during cooling operation
Component
Inverter compressor
DC fan motor
Electronic expansion valve
Four‐way valve
4.2 Compressor Output Control
The compressor rotation speed is controlled according to the load requirement. Before compressor startup, the
M‐Thermal Split outdoor unit determines the compressor target speed according to outdoor ambient temperature, leaving
water set temperature and actual leaving water temperature and then runs the appropriate compressor startup program.
Refer to Part 3, 3.2 "Compressor Startup Program". Once the startup program is complete, the compressor runs at the
target rotation speed. During operation the compressor speed is controlled according to the rate of change in water
temperature, the refrigerant system pressure and the refrigerant temperature.
4.3 Compressor Step Control
The running speed of six‐pole compressors (used on 4‐10kW models) in rotations per second (rps) is one third of the
frequency (in Hz) of the electrical input to the compressor motor. The frequency of the electrical input to the compressor
motors can be altered at a rate of 1Hz per second.
4.4 Four‐way Valve Control
The four‐way valve is used to change the direction of refrigerant flow through the water side heat exchanger in order to
switch between cooling and heating/DHW operations. Refer to Figures 2‐3.1 and 2‐3.2 in Part 2, 3 "Refrigerant Flow
Diagrams".
During heating and DHW operations, the four‐way valve is on; during cooling and defrosting operations, the four‐way
valve is off.
4.5 Electronic Expansion Valve Control
The position of the electronic expansion valve (EXV) is controlled in steps from 0 (fully closed) to 480 (fully open).
 At power‐on:
 The EXV first closes fully, then moves to the standby position (480 (steps)). After compressor runs at 38Hz for 40
seconds the EXV moves to an initial running position, which is determined according to outdoor ambient
temperature. After a further 180 seconds, the EXV is controlled according to suction superheat and discharge
temperature. Once a further 6 minutes have elapsed, the EXV is then controlled according to suction superheat,
discharge temperature and compressor speed.
 When the outdoor unit is in standby:
 The EXV is at position 480 (steps).
 When the outdoor unit stops:
 The EXV first moves to 478 (steps) and remains for 30 seconds, then closes fully, then moves to the standby
position (480 (steps)).
201909 19
Wiring diagram
4‐10kW
label
COMP ●
FAN
●
EXV
●
4‐WAY
●
Midea CAC Confidential
M‐Thermal Split
Control functions and states
Controlled according to load requirement from hydronic
system
Controlled according to outdoor heat exchanger pipe
temperature
Position (steps) from 0 (fully closed) to 480 (fully open),
controlled according to outdoor ambient temperature,
discharge temperature, suction superheat, compressor
speed and refrigerant system pressure
Off