Purpose
To explain the operating principles, control systems, and service functions of DC fan motors used in Fujitsu air conditioning systems.
This module provides technicians with an understanding of how DC fan motors operate under inverter control and how they differ from conventional AC fan motors.
1. Components of a DC Fan Motor System
The DC fan motor system in a Fujitsu indoor or outdoor unit comprises the following key components
DC Fan Motor
:
| Component | Function |
|---|---|
| Fan Motor | Drives airflow through heat exchanger (indoor or outdoor coil). |
| Drive Circuit PCB | Contains inverter circuitry and switching devices for motor control. |
| Control IC | Receives RPM or load commands from the main indoor PCB. |
| Drive IC | Converts command signals into control pulses for the motor coils. |
| Hall IC (Sensor) | Detects rotor position for feedback to the control circuit. |
| Capacitor | Smooths DC bus voltage and suppresses voltage ripple. |
| Thermistor | Monitors motor temperature for overload protection. |
| Diode Bridge | Converts incoming AC power to DC voltage. |
| Switching-Mode Power Supply (SMPS) | Provides regulated DC voltage to the control circuits. |
2. Operating Principle
Step 1: AC to DC Conversion
Incoming AC power is rectified through a diode bridge, creating a DC supply which is filtered by capacitors and fed into the inverter circuit.
Step 2: DC to Variable Frequency AC
The inverter within the drive circuit PCB converts DC voltage to AC voltage of variable frequency and phase, allowing precise speed control of the fan motor.
Step 3: Control Feedback
The Hall IC sensors detect rotor position and send feedback signals to the Control IC, ensuring correct commutation timing and torque control.
Step 4: Command and Regulation
The Indoor Unit PCB sends a fan-speed signal (PWM or analog) to the Drive IC, which adjusts output accordingly to maintain stable airflow and reduced noise.
3. Motor Speed Control Methods
DC fan motors typically use two main control systems
DC Fan Motor
:
| Control System | Definition | Method of Control | Effect on Motor |
|---|---|---|---|
| PWM – Pulse Width Modulation | FGL standard system | The width of the DC voltage pulse is varied while maintaining constant amplitude. | Adjusts the average voltage to control fan speed. |
| PAM – Pulse Amplitude Modulation | Used in high-efficiency systems | The amplitude (voltage level) of the DC pulse is varied between ~140 V–390 V. | Provides higher torque and dynamic speed response. |
Simplified principle:
PWM changes time width, PAM changes voltage height.
4. Efficiency Advantages
DC inverter fan motors provide major efficiency improvements compared to traditional AC motors:
Variable-speed operation: Motor runs only as fast as required for system demand.
Reduced power consumption: Lower average voltage at reduced speeds.
Improved torque response: Stable operation at low frequencies and varying loads.
Quieter performance: Smooth fan speed transitions eliminate mechanical switching noise.
Typical applications include floor/ceiling cassette and ducted units, where air volume modulation is essential for energy efficiency and occupant comfort.
5. Control Architecture
The following diagram illustrates the general control flow
DC Fan Motor
:
Key relationships:
The Control IC and Drive IC coordinate to generate phase-shifted signals for motor rotation.
The Indoor PCB sends target speed data based on system load or thermistor feedback.
The Hall sensors provide positional feedback for closed-loop control, ensuring accurate RPM regulation.
6. Diagnostic and Service Notes
| Test Point | Expected Value | Purpose |
|---|---|---|
| DC Bus Voltage | 140–390 V DC | Verify supply to inverter circuit |
| Hall Sensor Output | Pulsing waveform | Confirms rotor feedback |
| PWM Signal Input | 0–5 V (Duty Variable) | Confirms control signal from main PCB |
| Motor Resistance | Low (balanced across 3 windings) | Confirms motor integrity |
| Thermistor Resistance | 10 kΩ @ 25°C | Monitors overheating protection |
Service Tip:
When a fan fails to start or runs intermittently, check for:
Missing or distorted PWM signal.
Open-circuit Hall sensor.
Faulty DC supply or damaged SMPS section on the Drive PCB.
7. Field Benefits of DC Fan Motor Systems
| Feature | Benefit to Technician or End User |
|---|---|
| Variable-speed inverter | Reduced start-up noise and smoother operation |
| Hall sensor feedback | Improved reliability and fault diagnostics |
| Integrated PCB | Fewer external components; simplified wiring |
| Energy-efficient design | Lower running costs and power draw |
| Self-protection features | Built-in thermal and over-current shutdown |
8. Summary
DC fan motors are electronically commutated motors controlled by an inverter system. Their integration into Fujitsu systems delivers:
Higher efficiency and quieter operation.
Intelligent speed modulation based on thermal load.
Easier maintenance through diagnostic feedback and modular design.
Technicians should understand PWM/PAM operation and feedback control principles to accurately diagnose faults and confirm normal system behavior.