AIRSTAGE Technical Training
Three-Phase Inverter Control and Diagnostics
Power conversion, PFC operation, compressor drive and field testing
Technical reference for qualified service technicians working on Fujitsu three-phase outdoor units
Purpose: This article explains how inverter control operates, the roles of the Power PCB, PFC circuit and Intelligent Power Module, the principal diagnostic measurement points, and common fault conditions.
High-voltage hazard: Three-phase inverter systems contain hazardous AC and DC voltages. Only appropriately qualified personnel should perform live electrical testing. Isolate the supply, prove isolation and discharge the DC bus capacitors before handling PFC, Power PCB or IPM components.
1. System overview
Main components
| Component | Function |
|---|---|
| Rectifier / Converter | Converts incoming 240 V or 415 V AC into DC voltage for the inverter circuit. |
| Inverter / IPM Module | Converts the DC bus voltage into variable-frequency three-phase AC for compressor speed control. |
| Microprocessor / Main PCB | Controls PWM output, feedback monitoring and protection functions. |
| PFC Circuit | Maintains a stable DC bus voltage of approximately 360-380 V DC. |
2. Inverter control principle
- The microcontroller adjusts the frequency and amplitude of the three-phase output to match system load.
- Compressor speed is controlled by the PWM - Pulse Width Modulation signal applied to the IPM.
- The inverter electronically synthesises the output waveform, enabling variable compressor speed and improved efficiency.
Operating relationship
High load → higher frequency → increased compressor speed.
Low load → lower frequency → reduced compressor current draw.
High load → higher frequency → increased compressor speed.
Low load → lower frequency → reduced compressor current draw.
3. Power flow path
- AC supply → EMC filter → rectifier → DC bus capacitors.
- The PFC circuit boosts and stabilises the DC bus voltage.
- The IPM module converts DC into a variable-voltage, variable-frequency three-phase supply.
- The compressor motor receives balanced three-phase power.
4. DC bus and PFC operation
- Normal DC bus voltage is approximately 360-380 V DC.
- The PFC maintains a stable bus voltage while line input and compressor speed vary.
- PFC or IPM waveforms may be evaluated with an appropriate oscilloscope and approved high-voltage probes.
Service note: A faulty PFC board may cause DC bus fluctuation or an over-current trip. The source material references the PFC Error Bulletin for AOTD60LATT, where software versions 51011 and 51028 were replaced by version 51035.
5. Compressor drive - IPM control
The Intelligent Power Module performs the following functions:
- Power switching of compressor phases U, V and W.
- Current feedback sensing.
- Over-current and over-temperature protection.
| Typical phase relationship | PWM signals phase-shifted by 120 degrees |
| Typical carrier frequency | Approximately 15 kHz |
| Typical RMS phase voltage | Approximately 200 V AC, depending on operating frequency and load |
6. Control signal interfaces
| Signal | Function |
|---|---|
| INV-ON | Inverter activation command |
| FAN-ON | Outdoor fan control |
| COMP-SPD | Compressor frequency reference signal |
| TH | Compressor thermistor feedback |
| CT | Current transformer / over-current feedback |
The source material states that these signals are opto-isolated for protection.
7. Diagnostic and measurement guide
| Test point | Expected reading | Comments |
|---|---|---|
| DC bus P-N | 360-380 V DC | PFC output |
| CN-U / V / W | 0-240 V AC variable | Three-phase inverter output to compressor |
| TH sensor | 10-15 kohm at 25 degrees C | Compressor thermistor |
| CN-CT | 0-0.5 V, load dependent | Over-current feedback |
Recommended tools
- True RMS multimeter rated for the circuit being tested.
- Oscilloscope with appropriate probes for phase waveform analysis.
- Insulation resistance tester for compressor winding verification.
8. Common inverter faults and causes
| Error | Description | Probable cause | Service action |
|---|---|---|---|
| Er 33 | PFC board communication fault | Incorrect software version 51011 or 51028 | Replace with version 51035 board, subject to model documentation |
| Er 19 | IPM error | IPM over-temperature or gate short | Replace the Power PCB |
| Er 411 | Compressor lock | Seized compressor or phase loss | Test winding resistance and replace the compressor if confirmed faulty |
| Er J5x | Communication fault between modules | Cable or connector fault | Check harness connections and continuity |
Error-code interpretation and replacement decisions must be confirmed against the applicable model service manual and current technical bulletins.
9. Safety and service recommendations
- Discharge the DC bus capacitors before handling PFC or IPM boards.
- Use insulated tools and approved test equipment when probing live circuits.
- When replacing the Power PCB or PFC board, confirm firmware and model compatibility.
- Confirm earth-bonding integrity before restarting inverter operation.
10. Key learning points
- The inverter converts DC into variable-frequency three-phase AC for precise compressor control.
- The PFC stabilises the DC bus voltage and supports power quality.
- Common inverter faults relate to board communication, PFC instability or IPM protection.
- Accurate voltage and resistance testing at the designated connection points supports effective diagnosis.