Purpose

Concise refresher on legacy multi-type wall-mounted inverter systems: architecture, operation, protections, diagnostics, and common service checks.

1) System overview

  • Multi-split with up to four indoor units (A–D) connected to a single outdoor unit via branch electronic expansion valves (EEVs).

  • Typical indoor fan targets (approx., mode dependent): Hi ~1330 rpm, Med ~1170–1260 rpm, Lo ~970–1190 rpm, Quiet/S-Lo ~500–1,050 rpm.

2) Inverter fundamentals (legacy implementation)

  • AC mains → DC bus (rectifier + smoothing) → PWM inverter to a three-phase compressor.

  • Varying the PWM frequency/voltage controls capacity, enabling faster pull-down, tighter temperature control, and shorter defrosts compared with fixed-speed systems.

3) Control & communications

  • Indoor units → Multi-controller → Inverter (forward signal): mode request, capacity code (“F-code”), timer flags, quiet/power-save status.

  • Inverter/Outdoor → Indoors (reverse signal): operating state, defrost status, outdoor temperature, EEV initialisation.

  • First indoor requesting operation sets system mode; typical priority order is A > B > C > D.

  • The multi-controller totals indoor capacity codes to determine compressor frequency.

4) Capacity & airflow control

  • Compressor frequency follows the combined F-code, with reductions in Quiet or Power-save.

  • Outdoor fan speed is scheduled vs ambient and operating zone; special tables apply for cooling/heating and extreme conditions.

  • Indoor fan targets differ by mode. In heating, cool-draft prevention holds very low fan until the coil warms.

5) EEV management (multi)

  • On power-up, each branch EEV re-indexes to zero, then opens to a preset pulse value based on model, mode, and single vs multi-room operation. Heating corrections apply after a short stabilisation period.

6) Defrost & pre-heat

  • Defrost starts by combined time/temperature logic when the outdoor coil is sufficiently cold. Sequence fixes valve pulses, switches the 4-way valve, and drives a fixed compressor frequency until coil temperature rises or a maximum time elapses.

  • Crankcase/Pre-heat may energise after extended OFF time in low ambient to protect the compressor.

7) Protections (high level)

  • Discharge temperature: staged frequency reductions, then stop at excessive temperature with auto-restart after cooldown.

  • Inverter power module (IPM): over-current/over-temperature trips; repeated trips can lead to lockout.

  • Current release: limits outdoor input current; thresholds vary by operating zone and airflow.

  • Additional safeguards: low-ambient cooling, heating overload, pressure switch A/B, CT error, active filter protection, outdoor-fan error.

8) Indoor control highlights

  • Typical setpoint ranges: cooling 18–30 °C, heating 15–30 °C.

  • Capacity code derived from room/setpoint delta at regular intervals.

  • Dry mode uses stepped fan strategy and moderate compressor frequency.

  • Auto changeover selects mode from room/outdoor conditions, with a sampling period at low indoor fan.

  • Quiet mode enforces reduced indoor fan and tighter current limits.

9) Diagnostics & error indication

  • Lamp codes on indoor/outdoor PCBs classify faults: thermistors, serial communication, pressure A/B, discharge over-temp, current trip, CT error, active filter, outdoor fan, and more.

  • Thermistor resistance tables (room/pipe, outdoor coil/ambient, discharge) support bench testing.

  • Reading both “major” and “minor” indications helps isolate sensor vs PCB vs communication issues.

10) Field troubleshooting (quick flow)

Communication not received (outdoor):

  • Verify mains supply, then measure serial rails; use decision logic to discriminate indoor PCB, inverter board, wiring, and fusing.

Outdoor thermistor abnormal:

  • Remove and measure; compare to temperature–resistance tables (allow typical tolerance). Replace if out of range.

Outdoor current trip / suspected IPM:

  • Safely discharge and check DC bus; compare phase outputs (U-V, U-W, V-W). If bus is correct but phases are unbalanced, suspect inverter board; otherwise investigate the compressor.

Active filter abnormal:

  • Confirm supply voltage range, connector seating, and filter module integrity; apply prescribed bypass tests where applicable.

11) Installer/service cautions

  • Respect the 3-minute restart lockout after stops or trips.

  • On mode change (heat ↔ cool), compressor stops before the 4-way valve changes; expect controlled delays.

  • Always de-energise and bleed the DC bus before IPM or capacitor work—stored charge remains after power-off.

  • Record baseline operating data (frequencies, temperatures, currents) after commissioning for future comparison.

Takeaway

Legacy inverter-multi systems coordinate indoor ΔT-based capacity requests into a single compressor frequency while independently managing EEVs and protections. Effective diagnosis relies on serial-signal checks, thermistor verification, current/voltage measurements, and accurate interpretation of lamp codes.