J Series Outdoor Unit Inverter Control
1. What is an inverter?
In this context, an inverter is the equipment used to change DC power into AC power. The source material distinguishes this from a converter:
- Converter / rectifier: transforms AC to DC.
- Inverter: transforms DC to AC.
In air-conditioning systems, the term inverter generally refers to the equipment that varies output frequency and/or voltage to control motor operation.
Inverter block of the outdoor unit
| AC Supply | → | Filter PCB | → | Power Relay + Thermistor | → | Diode Bridge | → | Choke Coil |
| Power is conditioned and rectified before entering the active filter and DC bus. | ||||||||
| Active Filter Module | → | Smoothing Condenser | → | IPM | → | Compressor |
| Main PCB controls the active filter and IPM switching functions. | ||||||
2. Basic circuit of a 3-phase inverter
The inverter uses six switching devices arranged as three upper and three lower switches. By changing the ON/OFF timing of these switching devices, the inverter produces the three output phases U, V and W.
| Upper switching devices | Lower switching devices | Motor outputs |
|---|---|---|
| S1, S3, S5 | S4, S6, S2 | U, V, W |
3. PWM and PAM control
AC inverter – PWM control system
The motor requires a sine-wave input voltage. The inverter varies the pulse width of the DC voltage by switching, creating an average voltage that approximates a sine wave and controls motor rotation.
DC inverter – PWM control system
With a brushless motor, the DC voltage pulse width from the converter is varied by the inverter. Motor speed is controlled by adjusting the pulse width to change the desired average voltage. The pulse pattern differs from the AC compressor example shown in the source material.
PAM control system
With a brushless motor, motor rotation is controlled by applying a variable DC voltage of approximately 140 V through 390 V to the motor winding through the voltage booster circuit, which comprises a reactor and switching component.
4. Circuit description
| Circuit / component | Function |
|---|---|
| Varistor | When abnormal voltage is applied across the varistor terminal (VA101), it shorts and provides circuit protection. |
| Surge absorber | Protects outdoor-unit electronic components against abnormal surge voltage such as lightning-induced surges and discharges the surge energy. |
| Noise filter | Suppresses pulse noise generated by high-speed inverter switching. The source describes the filter as absorbing noise through the coil and bypassing high-frequency harmonic components through the capacitor. |
| Coil | Improves the incoming power-supply waveform and power factor by reducing harmonic current, and also provides insulation protection. |
| Current detection circuit | Continuously detects input current. The detected value is compared with a 5 V reference and supplied to the microcomputer so current does not exceed the configured value and CT-related abnormalities can be detected. |
| Diode bridge | Performs full-wave rectification of the AC voltage after it has passed through the power-factor improvement filter. |
| Thermistor | Detects heatsink temperature rise and helps protect electronic components from excessive temperature. |
| Choke coil | Conditions the current waveform and removes noise. |
| Active filter module | Corrects input current using the module microcomputer and boosting chopper circuit. It improves power factor and controls harmonic current in the power supply. |
| Smoothing condenser | Removes ripple from the DC voltage output of the active filter. |
| IPM – Integrated Power Module | Contains six power transistors and drives the compressor motor by high-speed switching. The microcomputer supplies the drive signal and varies the output frequency using PWM control. |
Active filter DC voltage behaviour
When the compressor is stopped: Input Voltage × √2 (effective value).
When the compressor is operating: approximately 380 V DC.
The target DC voltage is established from the input voltage and compared with the measured DC voltage. Target current and input current are also compared. The resulting comparison signal is used by the PWM comparator to control the switching component ON and OFF.
5. DC motor / DC inverter compressor
The rotor of the DC compressor motor uses permanent magnets. Applying the voltage produced by PAM control to the stator winding creates a rotating magnetic field. Attraction and repulsion between the rotating field and the permanent-magnet rotor causes the rotor to rotate.
The rotor position must be controlled so the permanent magnets remain correctly aligned with the rotating magnetic field. The source material describes two position-detection methods:
- Sensor type: using a built-in position sensor such as a Hall-effect device.
- Sensorless type: determining rotor position from the reverse power/electromotive force generated in the stator winding by the rotating rotor.
Compressor speed is changed by controlling voltage through PAM and/or PWM control.
6. Inverter power-flow summary
| Stage | What happens |
|---|---|
| 1. Input protection & filtering | Varistor, surge absorber, noise filter, relay/thermistor and coil protect and condition the incoming AC supply. |
| 2. Rectification | The diode bridge converts AC into DC. |
| 3. Power-factor correction | The active filter module shapes input current and boosts/controls the DC bus. |
| 4. DC smoothing | The smoothing condenser reduces ripple on the DC bus. |
| 5. Inversion | The IPM switches the DC bus at high speed to generate the U/V/W motor output. |
| 6. Compressor control | PWM/PAM control varies the motor voltage/frequency and therefore compressor speed. |