Purpose
To provide service technicians with a detailed understanding of:
How inverter control operates within 3-phase Fujitsu systems.
The functions of the Power PCB, PFC circuit, and Compressor Drive Module (IPM).
Typical measurement points for diagnostics.
Common fault conditions and field test procedures.
1. System Overview
Main Components
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 3-phase variable-frequency AC for compressor speed control.
Microprocessor (Main PCB): Controls PWM output, feedback monitoring, and system protection.
PFC (Power Factor Correction) Circuit: Maintains a stable DC bus voltage (~360–380 V DC).
Diagram on page 3 shows the relationship between the main PCB, IPM, and compressor windings.
2. Inverter Control Principle
The microcontroller adjusts the frequency and amplitude of the 3-phase output to match system load.
Compressor speed is controlled by the PWM (Pulse Width Modulation) signal applied to the IPM.
The sine-wave output is synthesized electronically, allowing for variable compressor speed and improved efficiency.
Key relationships:
High load → higher frequency (Hz) → increased compressor speed.
Low load → reduced frequency → lower compressor current draw.
3. Power Flow Path (Page 5 Diagram)
AC Supply → EMC Filter → Rectifier → DC Bus Capacitors.
PFC Circuit boosts and stabilizes DC bus voltage.
IPM Module converts DC back to a 3-phase variable voltage/frequency supply.
Compressor Motor receives balanced 3-phase power.
4. DC Bus & PFC Operation
Normal DC voltage range: ~360 V to 380 V DC.
The PFC maintains constant voltage even with varying line input or compressor speed.
Figure 4 (Page 6) highlights typical oscilloscope waveform patterns across the PFC and IPM stages.
Service Note:
A faulty PFC board may cause DC bus fluctuation or over-current trip.
(Refer to PFC Error Bulletin – AOTD60LATT, where software versions 51011/51028 were replaced by 51035.)
5. Compressor Drive (IPM) Control
The IPM (Intelligent Power Module) performs:
Power switching of the 3 output phases (U, V, W).
Current feedback sensing.
Over-current and over-temperature protection.
Typical IPM output waveform:
120° phase-shifted PWM signals at carrier frequency ~15 kHz.
RMS phase voltage ≈ 200 V AC (depending on frequency and load).
6. Control Signal Interfaces
| Signal | Function |
|---|---|
| INV-ON | Inverter activation command |
| FAN-ON | Outdoor fan control |
| COMP-SPD | Frequency reference signal |
| TH | Compressor thermistor feedback |
| CT | Current transformer (over-current feedback) |
All signals are opto-isolated for protection.
Page 8 block diagram shows microprocessor signal routing and protection logic.
7. Diagnostic and Measurement Guide
| Test Point | Expected Reading | Comments |
|---|---|---|
| DC bus (P-N) | 360–380 V DC | PFC output |
| CN-U/V/W (to compressor) | 0–240 V AC variable | 3-phase inverter output |
| TH sensor | 10–15 kΩ @ 25 °C | Compressor thermistor |
| CN-CT | 0–0.5 V (load-dependent) | Over-current feedback |
Recommended Tools:
True RMS multimeter.
Oscilloscope for phase waveform analysis.
Insulation tester for compressor winding verification.
8. Common Inverter Faults and Causes
| Error Code | Description | Probable Cause | Service Action |
|---|---|---|---|
| Er 33 | PFC board communication fault | Incorrect software (51011/51028) | Replace with 51035 board |
| Er 19 | IPM error | IPM over-temperature or gate short | Replace Power PCB |
| Er 411 | Compressor lock | Seized compressor or phase loss | Test winding resistance, replace compressor |
| Er J5x | Communication fault between modules | Cable or connector fault | Check harness continuity |
9. Safety and Service Recommendations
Always discharge the DC bus capacitors before handling PFC or IPM boards.
Use insulated tools when probing live circuits.
When replacing Power PCB or PFC, confirm firmware version compatibility.
Confirm earth bonding integrity before restarting inverter operation.
10. Summary
The training module provides technicians with a foundational understanding of inverter technology in Fujitsu’s three-phase outdoor units.
Key learning points:
The inverter converts DC to variable 3-phase AC for precise compressor control.
The PFC stabilizes DC voltage, ensuring efficiency and power quality.
Most inverter faults stem from board communication errors, PFC instability, or IPM protection triggers.
Accurate voltage and resistance testing at the designated CN points enables effective diagnosis.