摘要
无刷直流电机具有高效率和高功率密度的特点,适用于水下推进装置。为了检测关键的转子位置信号,一般需在电机内安装霍尔传感器。水下推进装置中空间有限,且对可靠性要求高,而安装霍尔传感器不仅占用体积,而且需要从电机端部引出导线,带来密封和可靠性问题。故无位置传感器控制具有重要意义。水下推进无刷直流电机系统一般采用蓄电池供电。受蓄电池输出电压限制,本文研究的无刷直流电机具有反电势低和电流大的特点,运行时续流时间较长,存在反电势过零淹没现象,基于反电势过零检测的常规位置估算方法不能应用。故研究减小续流时间的方法以及续流时间较长时的无位置传感器控制具有重要意义。
首先,基于无刷直流电机系统的动态电路模型推导了电流的解析表达式,计算和分析了影响续流导通角的因素,研究了减小续流导通角的方法。为了精确计算续流导通角,对三相六态驱动方式下的动态电路拓扑结构分析,推导了续流状态和两相导通状态下的电流解析表达式,获得了求解续流导通角的方程。结合电流表达式中的参数,分析了影响续流导通角的主要因素。提出了两种减小续流导通角的方法,仿真结果表明利用改进的变流器拓扑能够有效减小续流导通时间。
其次,针对续流时间较长造成无法直接检测反电势过零点的问题,提出了一种基于转子磁链模型的磁链过零点估算方法。推导的磁链计算模型中采用了以虚拟中点(由电阻网络构造)为参考点的绕组端电压,无需引出电机中线,而且计算磁链中不含三次谐波。分析了利用估算的三相磁链过零点控制逆变器的换相逻辑。为了获得磁链模型中的电感参数,利用有限元软件计算了电机空载时的视在电感。通过Simulink仿真对本文提出的无位置传感器控制方法进行了验证。
第三,结合转子磁链的实际计算过程,研究了磁链过零点的估算误差。对磁链计算误差的数学模型进行了分析,结果表明电机电感变化是造成磁链计算误差的主要原因。结合电机电枢反应磁场的空间分布,分析了各电感分量受饱和影响的程度。针对本文研究的样机,利用Flux-Simulink联合仿真研究了电感误差对磁链过零点估算的影响。
最后,设计并实现了基于转子磁链过零点估算的无刷直流电机无位置传感器控制系统。通过合理选择电压电流传感器、驱动模块和数字控制芯片,搭建了硬件系统。推导了磁链计算模型的离散化形式,设计了以实际电机转速为参数的滤波环节,利用加权平均思想设计了动态响应较快的平均值测速模块。实验结果证明了本文提出的无位置传感器控制方法的有效性。
The brushless DC motors is increasingly being used in underwater propulsions because of its high torque, compactness, and high efficiency. Its self-synchronous operation needs six discrete rotor position information, which is usually generated by hall effective sensors. When sensors are used, extra space and wires are needed. However, in underwater propulsions space is limited and reliability,which will be reduced by too much wiring, is very important. So sensorless control is much more attractive. Moreover, underwater propulsions are always powered by batteries. Because of the relative low output voltage of batteries, the brushless DC motors are designed with low rated back emf and high rated current. So the freewheeling time of the brushless DC motor system, which is studied in this paper, is significantly long. The sensorless control technique based on back-emf detection from terminal voltages can’t be used any more. This paper studies the methods to restrain freewheeling and the sensorless control technique when freewheeling time is long.
In the first part of this paper, based on the dynamic circuit model of brushless DC motor drive system, the analytical formula of phase currents is derived,which is then used to calculate the conduction angle of freewheel diodes.The influence of each parameter in current formula on conduction angle is discussed in detail before the method restraining freewheeling procedure is presented. The simulation result demonstates that improved converter topology can restrain the freewheeling procedure significantly.
In the second part,a rotor position estimation technique,which is based on rotor flux linkage zero-crossing detection and not effected by freewheeling procedure, is presented. The flux linkage model in this paper utilizs virtual neutral point,which is generated by resistor network, as the referance point of the terminal voltage. So the real neutral point is not required and flux linkage without the third harmonic is obtained. How flux linkage zero-crossing is used for inverter commutation control is also analyzed. As a necessary parameter in flux linkage model, the no-load apparent inductance of the studied brushless DC motor is calculated using FEA software. Using Simulink software, the presented sensorless control technique is verified.
In the third part, zero-crossing detection error of flux linkage in practical systerm is analyzed. Error model of calculated flux linkage is derived and the analysis implies that delta inductance is the main reason of flux linkage error. Distribution of the armature reaction magnetic field ,then the influnce of saturation on each inductance components is discussed. For the studied brushless DC motor,the effect of inductance fluctuation on zero-crossing detection is studied by co-simulation of FLUX and Simulink.
At last, a brushless DC motor system is established using the presented sensorless control technique. Hardware system is constructed with accurate current and voltage sensors, power driving module and high performance control chip. Discrete flux linkage calculation model, filters using actual rotor speed as parameter and weighted average speed measurement block are designed to gain correct commutation instants. To verify the validity of the system, experimental results are presented.
引文
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