8-bit MCU solution for air-con units’ outdoor fans

A split type air-conditioner normally consists of one outdoor unit and several indoor fan coils. The outdoor unit contains the compressor, which carries out the heat exchange process. To regulate the amount of heat exchange in an outdoor unit according to variations in outdoor temperature, a fan is used. Normally, the outdoor fan operates within a speed range from 250rpm to 1,000rpm with a power consumption of less than 100W (70W typical). The speed command is given in analog format and a frequency generation (FG) signal is feedback to the main controller board.

One of the requirements of outdoor fan control is that it needs to be started reliably and consistently under different climate conditions. Strong wind facing the outdoor unit will force the fan blade to spin in the reverse direction. Also, a motor would only reach half of the maximum speed in a violent typhoon.

Traditionally, an outdoor fan is a brushless DC (BLDC) motor and is controlled by trapezoidal commutation. Advantages of trapezoidal control include easy control, strong torque and reliable performance. However, due to intrinsic torque ripples, trapezoidal commutation induces audible noise especially when motor is operated at low speed. To reduce the audible noise and to meet all other application requirements, a simplified sinusoidal control is proposed using Infineon Technologies’ 8-bit microcontroller XC866.

Based on the XC800 Core that is compatible with the industry-standard 8051 processor, the 8-bit XC866 microcontroller is equipped with a dedicated three-phase motor control unit – capture compare unit 6 (CCU6), and a 10-bit analog-to-digital converter (ADC) with extended functionalities. These features make XC866 suitable for low-end three-phase motor control, such as for BLDC and induction motor. The XC866 also features one UART, one SPI interface, and three 16-bit timers. The block diagram of XC866 8bit micro-controller is shown in Figure 1.

Figure 2 shows the system block diagram of the fan inverter board. The microcontroller, gate driver and six IGBTs are assembled on the same inverter board. Normally, this board is mounted within the motor casing. A voltage of 310VDC is supplied to the inverter board directly; therefore no rectifier stage is required. Infineon’s IGBT IKD04N60R and gate driver 6ED003L06-F are selected for this application due to their suitable features and reliable quality.

During real operation, the fan motor may run at one of the following states: STOP, CHECK_DIR, BRAKE, RAMP and SINU. Upon power up, a routine (state CHECK_DIR) is called to check the spinning status of fan motor. In case of standstill, the motor will be started using trapezoidal commutation (state RAMP), since this method can provide stronger starting torque. After successful startup, control switches to sinusoidal modulation (state SINU) to reduce the audible noise. However, if the motor spins in the reverse direction upon power up, the software will trigger the motor to brake until it becomes standstill (state BRAKE). Braking force is calculated based on the initial spinning speed. For example, if the fan blade spins at 400rpm, more current will be drawn to stop the motor than the current needed to stop a motor running at 200rpm.

Motor speed and direction information are provided by three Hall sensors, which are of 60° resolution. The XC866 is integrated with hardware Hall input logic (within CCU6 module), which avoids software polling of Hall signals and hence saves CPU overhead. Motor speed is calculated instantly and can be readily used for rotor angle calculation. Within every PWM interrupt, the rotor angle is updated and used as sinusoidal look-up table index. Speed command is sampled via the ADC channel and the conversion result is given as voltage norm. Finally, the CCU6 output six PWM signals based on the sinusoidal look-up table value and voltage norm. Figure 3 shows the control block diagram for a stable operation.

To reduce the switching loss of modulation, a special method is used. Within one electrical cycle, there are six states and each one lasts for 60°. Two out of six states are not modulated—the S5 and S6 for Phase U—this is similar to trapezoidal control. The other four states are modulated by sinusoidal function, that is, S1 and S2 using sin(θ), and S3 and S4 using sin(120-θ) for Phase U.

Tests have been conducted at the customer site with satisfactory results on noise reduction, reliable reverse brake and startup, and good efficiency. Meanwhile, this method is independent of motor parameters, and hence suitable for a range of fan blade models. This shortens the development period and provides great convenience for production.
Captions

Fig. 1: Block diagram of XC866 8bit micro-controller.

Fig. 2: System block diagram of outdoor fan inverter board.

Fig. 3: Control block diagram for a stable operation.