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Abstract: This paper introduces the SPWM principle of air conditioner inverter, and takes the electronic control system composed of Siemens special-purpose single-chip C504 as an example to illustrate the basic structure, implementation method and key technology of the inverter air conditioner electronic control system.Keywords : dedicated microcontroller SPWM frequency conversion
1 Introduction
Air conditioning systems have been widely used in production and life. As energy is declining, air pollution is becoming more serious, and energy conservation is a problem that cannot be ignored. As we all know, inverter air conditioner is a new type of product that combines energy saving, comfort and quiet noise. It has shown great vitality when it was first introduced. It can be expected that the air conditioner in the next century will achieve frequency conversion at a faster pace. The inverter air conditioner structure is shown in Figure 1.
The indoor part receives the control information sent by the remote controller, and according to the indoor air temperature, the heat exchanger temperature, and the state information sent by the outdoor unit, sends fuzzy control information to the outdoor unit, including: the frequency of the inverter compressor, Four-way valve status, etc. The outdoor unit generates an SPWM waveform based on the control information sent from the indoor unit, and drives the compressor to operate at the corresponding frequency. During the operation control process, the operating frequency is automatically adjusted with the change of the outdoor temperature, the change of the exhaust temperature of the compressor and the temperature of the heating device, so that the compressor is always in the optimal operating state. At the same time, the outdoor unit continuously detects changes in current and voltage, detects the occurrence of short-circuit, over-voltage, under-voltage and other faults, and takes protective measures in time to ensure the good operation of the control system.
In the new inverter air conditioner electronic control system developed, the various control functions of the indoor unit and the outdoor unit are completed by the special single-chip microcomputer C504 produced by SIEMENS. In addition to the general functions of the general-purpose single-chip microcomputer, this type of single-chip microcomputer also has a CCU unit specially designed to drive the three-phase AC inverter compressor and the brushless sensorless DC compressor. It has powerful functions, good performance and convenient programming.
2 C504 working principle of CCU
The general inverter air conditioner compressor is divided into three-phase AC frequency conversion and DC frequency conversion. The C504 microcontroller can drive both types of compressors, only programming methods are different.
Figure 2 is a block diagram of the internal structure of C504. It can be seen that the C504 consists of three parts: CPU, CCU and asynchronous communication. The CPU part is fully compatible with the 8051. The CCU part is its most unique stand-alone unit. It includes independent timers, comparators, dividers and registers. It can work independently from the CPU. Its purpose is to generate three-phase sinusoidal AC with variable frequency.
2.1 Calculation of period and offset
Assume that the pulse width modulation frequency is 20 kHz, that is, =20kHz, which means
The compare timer 1 generates an interrupt every 50μs, and a new pulse width value is formed in its interrupt service routine and stored in the compare register. Since the pulse width of the pulse sequence according to time is in accordance with the requirements of the sinusoidal waveform, it is impossible to calculate the pulse width in real time. The most common method is to create a sine table in memory that is periodically read out during the execution of the interrupt service routine and sent to the compare register to form the SPWM waveform. In the design, we set the comparison timer 1 that determines the PWM period to the mode 1 state, the so-called bilateral modulation state. At this time, after the timer 1 is counting forward, it will immediately count backwards, and an interrupt request will be made after the overflow. Insert timer
2.2 Principle of sine wave formation
The CCU's PWM has two kinds of unilateral modulation and bilateral modulation. It can be arbitrarily selected through initial setting. This article only discusses bilateral modulation with relatively small harmonics. The generation of the three-phase SPWM waveform is determined by the software in conjunction with the complex hardware structure inside the CCU. The pulse width depends on the size of the data value stored in the SPWM compare register. These six registers (all 16 bits) are divided into three groups, which are defined as CCL0, CCH0, CCL1, CCH1, CCL2, CCH2, and the three groups are independent of each other. Before each SPWM cycle, it is checked by the CPU through the sine table and stored in these registers. If the count value of internal timer 1 exceeds the value of the register, the corresponding output port is triggered, thereby outputting a set of PWM rectangular waves of opposite polarity (the duty ratio depends on the value in the comparator), see Figure 3. Each time the internal timer reverse count ends, an underflow interrupt request is generated, a new one PWM cycle is started, and corresponding parameters are generated in the interrupt service routine. The size of the dead zone between the upper and lower arms is determined by the offset of the signal. This value is stored in the offset register, defined as CT10FX. The driving waveform of one bridge is formed as shown in Figure 3. Once the cycle The value (input to compare timer 1), the offset, and the values ​​of the three comparators have been input. When the internal timer 1 dedicated to timing the comparator is activated, the CCU does not have CPU intervention. Independent operation, generating 3 independent PWM pulses, the pulse width value depends on the value of the comparator.
For three-phase inverter compressors, a three-phase sinusoidal current must be supplied. The CPU of the C504 must periodically feed three different sets of data to the CCU compare register. These data come from a sine table set internally. The period of each transmission is the PWM period, which is determined by the overflow time of the timer 1. A pulse chain composed of several pulses of different pulse widths can form a sine wave period, as shown in Fig. 4. Obviously, the output AC cycle is determined by the PWM cycle and the variation of the pulse.
3 Basic hardware structure of the motor drive part
For the air conditioning electronic control system, both the indoor unit and the outdoor unit must have a motor-driven function. Especially for the new inverter air conditioner, in order to achieve the noise suppression effect, the indoor fan is often a DC brushless motor type. Therefore, the room The focus of internal and outdoor unit design is still the design of different types of variable frequency speed control motor drives. Figure 5 shows the basic motor drive block diagram.
Figure 5 (a) shows the general three-phase asynchronous motor drive block diagram, the three sets of bridge arms of the full bridge are driven by CCO, COUTO; CC1, COUT1; CC2, COUT2, the current value is detected by the sensor, sent to A / of C504 The D conversion unit is tested. Fig. 5(b) and (c) show the driving method of the brushless DC motor, wherein Fig. 5(b) is a built-in sensor type, and Fig. 5(c) shows the operation without incremental built-in sensor. .
4 Design of the drive unit of the inverter air conditioner outdoor unit
Figure 6 shows the electrical schematic of the outdoor unit drive part of the BPK-II inverter air conditioner electronic control system.
From the figure, it can be seen that the driving part is divided into three parts: the rectifying part, the inverting part and the control part. The rectification part includes circuits such as AC filtering, peak absorption, current detection, rectification, and filtering. The inverter part includes an IPM module (PM20CTM060), a working power supply section, an opto-isolated section, and a driving section. The control part is mainly composed of C504. C504 selects the appropriate operating frequency and related control information according to the control information sent by the indoor unit, as well as the compressor temperature, the outdoor temperature and the temperature of the heat exchanger, and sends the corresponding register and timer 1 of the CCU through the CPU, and starts timing. 1, the SPWM pulse train passes CCO, COUTO; CC1, COUT1; CC2, COUT2 output, through the 74AC04 drive circuit, drives the optocoupler PS2501, and finally sends into the IPM module to generate a three-phase sine wave to drive the compressor motor.
In the driving part, it can detect the AC voltage and current, and perform over-voltage, under-voltage and over-current protection. At the same time, according to the change of compressor temperature, heat exchanger temperature and outdoor temperature, the operating frequency can be adjusted to disengage the compressor. Dangerous operation area to avoid the occurrence of fault shutdown. When the compressor is operating in a dangerous state, such as severe overcurrent and compressor overheating (over 120 °C), it can be shut down quickly to protect the safety of the system. PM20CTM060 is an IPM module with complete internal protection measures. Once it enters the fault state, F0 outputs a low level. In addition to turning off the external 5V power supply, it also informs the CPU to enter the fault state through the C5049 pin.
5 Conclusion
For the current general-purpose split-type inverter air conditioner control system, C504 is used as the indoor unit and outdoor unit control chip, which not only can meet the needs of functions, but also improve the reliability. The chip is suitable for a wide temperature range, including SAB-C504, :0°C～70°C; SAF-C504,
:-40°C～85°C; SAH-C504,
:-40°C～110°C; SAK-C504,
: -40 ° C ~ 125 ° C. Its operating frequency is 12MHz, 24MHz and 40MHz. Therefore, in the case where the IPM module chopping frequency and heat allow, you can make
More than 10 kHz, the range of adaptation is wide. If you want to develop a "one-to-many" split machine, you can use the same type of C508 as the control chip.
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