Development of sensorless overmodulation technology for variable frequency air conditioner based on PE-PRO/V850IA4

0 Introduction
In recent years, permanent magnet synchronous motors with better control performance have been gradually used in air conditioning compressor systems to replace brushless DC motors for driving. This permanent magnet synchronous motor is in a high-temperature sealed compressor and is filled with highly corrosive high-pressure refrigerant. It is impossible to install a rotor position sensor, so a sensorless control method must be used. On the other hand, most air conditioners operate in the medium and high speed range. In the compressor speed control system, the operating range and load capacity of the motor directly depend on the range and quality of the inverter output voltage. Therefore, in order to improve the performance of the motor and obtain the maximum output electromagnetic torque, the voltage utilization of the inverter must be increased as much as possible. In order to make full use of the DC bus voltage to achieve the maximum output voltage, overmodulation technology must be used in inverter control, and its upper limit is the six-step wave condition (voltage utilization 0.78).
Based on the requirements and characteristics of the permanent magnet synchronous motor-compressor system operating in the high-speed zone, this paper introduces the basic principles of sensorless overmodulation technology, namely the single-mode overmodulation algorithm and the MRAS speed identification algorithm, and applies this method to the high-speed operation control of the permanent magnet synchronous motor, and conducts experimental verification on the MyWay development system.

1 Control strategy
(1) Single-mode overmodulation algorithm
This paper adopts two strategies: SVPWM linear modulation and single-mode overmodulation proposed in literature [2]. The basic principle of the single-mode overmodulation technology is as follows: Take the first sector of the space vector hexagon as an example to illustrate this single-mode overmodulation algorithm, as shown in Figure 1. Assume that the amplitude and phase angle of the reference voltage vector ur are |ur| and θr respectively. When |ur| is less than the radius of the inscribed circle of the hexagon, the inverter is in the SVPWM linear modulation area; as |ur| further increases, the system enters the overmodulation area. At this time, the reference voltage vector ur needs to be adjusted so that the actual voltage vector output by the inverter falls within the hexagon after adjustment. The method of adjusting the reference vector is to map the arc trajectory of the vector to the arc part within the hexagon in equal proportion (as shown by the thick black line in Figure 1). When |ur| is equal to the radius of the circumscribed circle of the hexagon, the inverter enters the six-step wave working state, and the corresponding voltage utilization rate also reaches the theoretical maximum value of 0.78.

(2) MRAS speed identification algorithm
This paper adopts a motor low speed/position identification algorithm based on MRAS proposed in the literature [3]. The permanent magnet synchronous motor itself is selected as the reference model, and the current equation of the permanent magnet synchronous motor is selected as the adjustable model. The operation block diagram of the entire identification algorithm is shown in Figure 2.

2 PE-PRO／V850IA4 Experimental System Platform
This experiment is implemented on the motor control development system PE-PRO／V850IA4 developed by Myway. The development system consists of the control board PE-PRO／V850IA4, the integrated development environment PE-VIEW8 and the PEOS for power electronics. Figure 3 shows the structure diagram of the PE-PRO／V850IA4 system. The whole system can be divided into three parts according to its function: the main circuit unit, the DSP control unit and the microcomputer editing and display unit.

The main circuit realizes the detection of feedback signals, the driving of switch devices and the protection of the system. The switch devices in the main circuit of the PE-PRO/V850IA4 system adopt the intelligent module IPM, and the main circuit has hardware protection for undervoltage, overvoltage, overcurrent and overheating. In addition, the main circuit also detects the DC bus voltage of the inverter and the two-phase current of the motor stator for control. The detection of voltage and current is realized by LEM Hall elements.
The CPU used in the PE-PRO/V850IA4 system is a single DSP chip V805IA4 produced by NEC. This CPU has the advantages of flexible instruction system, flexible internal operation and parallel structure, high-speed performance and low power consumption. The main function of the DSP control unit is to realize the data calculation of the entire control system. The sampling signal of the main circuit (motor stator phase current, DC bus voltage, etc.) is converted into a digital signal through AD conversion, which is processed in the DSP control unit to realize the estimation of rotor position and speed and the closed-loop control of speed loop and current loop. Finally, the calculated FDWM switch signal is transmitted to the switch device of the inverter through the inverter control board. In addition, the control unit can also realize data transmission between DSP and microcomputer and main circuit. The analog signals sampled by current and voltage sensors on the main circuit are converted into digital signals through A/D conversion. The relevant variable data can be displayed on the microcomputer screen through the DSP control unit. At the same time, the control system instructions can be passed from the microcomputer to the DSP.