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永磁无刷直流电动机的最新进展

BHIM SINGH

电气工程技术学院,印度理工大学,Hauz Khas ,新德里110016,印度

电子邮箱:bsingh @ ee.iitd.ernet.in

摘要 本文论述了永磁无刷直流(PMBLDC )电机驱动的最新发展成果,并作出对先进水平的各类电机的建设、闭环控制器的位置、速度和电流/转矩的控制以及最近趋势的逆变器、传感器等的综合考量。详细讨论无机械传感器技术,和减少转矩脉动,噪音和振动的特殊方法,叙述在永磁无刷直流电机驱动控制中使用集成芯片的微电子影响。由于驱动器性能的改善和成本的减少,其应用范围在日益扩大。

关键词 永磁无刷直流电机;传感器;控制器;无位置传感器运行;转矩脉动。

1.引言

永磁无刷直流(PMBLDC )电机的应用范围越来越广,如家用电器、汽车、信息技术

设备、工业设备、公共生活设施、交通运输、航空航天、国防设备、电动工具、玩具、视

觉音响设备、医疗健康器材,这些设备从微瓦至兆瓦不等。其性能的优越性体现在:效率高、响应速度快、重量轻、控制精准、可靠性高、免维护运行、无电刷、功率密度高和体积小。最新的无刷直流电动机采用高性能稀土材料制成,应用不同的电机结构,如轴向场、径向场、封装类型、矩形反馈、正弦反馈来提高传感器技术,加快半导体模块反应速度,降低成本,提高微电子器件性能。最新的控制理念,如拥有稳定性好,适应性强的模糊神经网络控制器,对于其在转速为每分钟几转到几千转的范围内的广泛使用是一个福音。他们已被证明在机床、机器人技术和高精度伺服系统,以及各种工业和过程控制中的速度控制和转矩控制的应用上最适用于位置控制。

尽管永磁无刷直流电机是电机中的佼佼者,但它在目前看来依旧面临着许多障碍,如大量零部件带来的成本、转矩脉动、噪音、振动而降低了可靠性的问题,还有操作上的限制,如温度升高等。为了克服这些问题,已经在驱动器的各个方面做了坚持不懈的努力。永磁无刷直流电机本身无疑有着相当大的使命;本文集中讨论从电机结构、闭环控制器、

半导体功率模块、传感器和无传感器、转矩脉动最小化、微电子影响、降低成本和潜在的应用这些方面来讲,永磁无刷直流电机的最新进展。

2.永磁无刷直流电机的最新发展

在各种电机中永磁激励已代替了直流励磁,如直流电机、同步电机和新型永磁无刷电机如永磁步进电机、混合步进电机和永磁无刷直流电机。高成本的永磁材料是电机使用和发展的主要瓶颈。更好的永磁材料、日益提高的制造工艺、电机为了满足特定的应用而设计的不同的构造使其成为目前最好的电机。永磁电机的范围很广,本文仅限永磁无刷直流电机。

目前永磁无刷直流电机中的永磁材料可以分为以下三种:铝镍钴(A1-Ni-Co-Fe),陶瓷包括铁氧体和稀土材料,如钐-钴(Sm-Co)、钕-铁-硼(Nd-Fe-B)。铝镍钴和铁氧体早已在永磁电机中使用,因为它们便宜且容易获得。稀土永磁材料,即钐钴,由于其高剩余磁通密度、抗磁力和低温度系数造成的高能量密度,目前也投入使用。钕铁硼被认为是目前最好的永磁材料之一,因为它提供了更高的剩余磁通密度和抗磁力,然而,它唯一的缺点是其温度界限,目前正在不断努力克服这一点,这也将使得永磁无刷直流电机拥有更高的效率和更小的规模等优点。

永磁无刷直流电机可分为不同的类别,例如相数、径向或轴向场、无保持架或带保持架杆,表面安装永磁铁或埋入磁铁,正弦波或矩形馈电机等,其中一部分在本章会作简要讨论。

2.1相数

永磁无刷直流电机的管轴流风机设计为低功率单相(<50W),用于冷却电子设备。它在太阳能光伏反馈制冷系统,伺服控制等家用电器中为两相结构。大部分的中,高功率的电机被设计成类似于传统的交流电机三相结构。在一些电动车和潜艇推进器等兆瓦等级电动机中,设计师为了降低每相的功率处理要求将相数增加至五个,六个或更多。

2.2径向和轴向场电机

市面上大多数电机都是径向场类型(圆柱形或凸极构造)。然而,轴向场电机在功率密度,转矩惯量比,峰值扭力,磁铁重量少,电感低,绕组匝数少,设计紧凑等方面比传统的径向构造更有优势。轴向场电机设计为包裹,磁盘和三明治型结构,并且转子中没有

铁,这使得惯性降低。在轴向场电机中,轴向磁场是由转子磁铁与径向电流的相互作用产生的,磁铁被封装在树脂或塑料壳内。由于它们的性能,它们被认为最适用于机器人技术,计算机设备和机床等。径向场电机的设计也有不同的磁链波形,如正弦波或梯形波,不同的形状和位置的转子磁铁,例如深埋或表面安装等。因为定子设计类似于常规交流同步或异步电动机,它们被广泛使用。图1显示了这两种类型的流行永磁无刷直流电动机的典型横截面。

2.3转子内永磁铁的形状和位置

在永磁无刷直流电机中永磁铁放置于转子内。在轴向场电机中,磁体被封装在圆盘状的树脂或塑料内,如图所示,这些磁体是为了诱发正弦波或梯形波的反电动势而被这样放置。在径向场电机中,永磁体的放置方式不同,例如在低速电机中表面贴装,而在高速永磁无刷电机中放置在内部径向方向或切向方向,图2显示出了这种转子的几何形状。根据应用程序,它们还能实现正弦或梯形反电动势。

2.4正弦和矩形反馈电机

永磁无刷直流电机设计成有正弦或梯形波(激发)引起反电动势。正弦电机需通入多相交流电流,类似于传统同步电动机,能够通过频率控制恒转矩运行于基速之下,并拥有相同的无纹波转矩和功率因数,在恒功率运行条件下能够通过电流影响磁场的削弱。运行速度限制为会由电枢反应和机械结构引起的退磁的最大速度。转子的磁性特点为磁阻转矩有助于加宽恒功率运行速度范围。梯形激发电机需要具有120度电宽度和可调节大小和方向的多相位均衡矩形电流。磁与恒定振幅多相电流不断相互作用形成类似于电子换相的传统直流电机的无纹波转矩。因为这些矩形电流,他们也被称为开关永磁电机、无刷直流电机和电子换向永磁无刷直流电动机。

图3为这两种类型的电机的理想电流波形。位置传感器是根据在电机绕组中的自同步控制模式来实现这些理想的电流波形的要求而变化。

3.闭环控制器

不论正弦或梯形激励,永磁无刷直流电机在运动控制应用中用于位置控制,速度控制和转矩控制。图4示出了一个典型的位置闭环控制内的速度和电流回路。速度控制系统外位置环是不需要的,且速度给定是命令信号。转矩控制与高性能运动控制结合,其通过同

步相电流和轴的位置反馈进行闭环调节。在大多数永磁无刷直流电机中,转矩与电流是线性相关的,并且转矩指令只用一个简单的比例常数即可映射到当前指令。在特殊情况下,转矩和电流指令之间为非线性映射。永磁无刷直流电机驱动的恒功率运行是通过弱磁控制技术,这也要求转矩与电流映射的另一个命令信号。永磁无刷直流电机相绕组的电流调节无论是正弦波还是矩形波方式都是利用电流控制电压源逆变器(VSI)实现的。脉宽调制是用于发出开关信号给逆变器从而实现绕组电流与命令电流一致的滞后和预测电流的控制器。速度控制一般是使用速度反馈和速度指令实现的,速度指令是通过速度控制器给转矩控制器输出的指令信号。位置控制的实现是通过位置控制器的位置反馈和位置命令实现的,位置控制器的输出是内部速度环的速度指令。位置和速度闭环控制器都是使用不同宽度的闭环控制器,比如PI(比例-积分)、PID(比例-积分-微分)、SMC(滑膜控制器)、自适应控制器、模糊控制器和神经控制器。

这些经典(PI或PID)控制器或先进的闭环控制器,如SMC、模糊和神经网络控制器都是采用DSP、微处理器和专用集成芯片(ASIC)来实现速度和位置控制。许多制造商已经开发ASIC用于永磁无刷直流电动机的典型应用。

4.逆变器和转换器的最近发展

永磁无刷直流电机均是由变频逆变器提供电子换向。在小功率的变频器中,基于场效应晶体管的虚拟交换接口用来实现理想的电流控制和合理的高开关频率。在中等功率的变频器中,基于场效应晶体管的虚拟交换接口用来服务于永磁无刷直流电机。由于GTO型逆变器的自整流功能和电流控制的改进,它被用于高功率的永磁无刷直流电机中。在中、高额定功率额永磁无刷直流电机中,输入功率因数整流器PWM电流控制是用来实现稳压直流环节和永磁无刷直流电机驱动的再生功能。经典的PI/ PID和前馈闭环控制器用于前端转换器的控制,它提高了交流电源的电能质量,在电源流动的各个方向上降低了谐波和单位功率因数。

最近的MOSFET/IGBT为基于VSI使用模块的形式实现紧凑驱动器的设计。功率MOS 集成电路用于控制小功率的永磁无刷直流电机,该电机为了控制和功率放大使用集成微电子去激励电机。此外,它们的门驱动器也以模块的形式用在了所有的保护和智能控制功能中。在一些典型的应用(住宅商业鼓风机)中,它们安装非常紧凑以致全部安装在电机外壳内。

5.传感器的最新趋势

在永磁无刷直流电机的控制中,位置、速度和电流传感器基本上都需要调节相电流与转子位置同步。此外,有时端电压传感器也须估计任一位置或速度,电压传感器还需要在制动或前端转换器的控制下来调节直流母线电压。在一些典型情况下,磁通传感器和扭矩传感器也使用于电机的精确控制中。这些传感器的基础性作用已经在第三章永磁无刷直流电机驱动器的闭环控制中作了讨论。在下面的部分中,会对传感器的最新趋势及其功能进行简要讨论。

5.1位置传感器

在永磁无刷直流电机中,转子磁通位置是由旋转机械角度定义的,这是转子位置传感器以某种形式实现的。转子磁通位置所需的相电流与转子位置同步,还需要进行位置控制。转子位置使用位置传感器直接感知或间接地使用各种测量参数估计。因此,转子位置检测是必不可少的电流控制永磁无刷直流电机驱动器。

转子位置的感应采用旋转变压器、感应模块绝对系统(IMAS)、霍尔效应传感器、磁敏电阻、电子和光学编码器、同步器和角度传感器。角度传感器是一个具有永磁场和梯形输出波形的气隙磁敏三相交流发电机,它用于位置和速度检测和模拟信号输出,可配置有4、6和8极。该编码器的特点是每转的脉冲数(PPR),现在他们应用于几千PPR。接口芯片也可用于将这些传感器信号转换成数字形式进给数字处理器以用于永磁无刷直流电机驱动的智能控制。

间接位置检测是通过运用其他测量参数,如电流和电压来估计转子位置实现的。在下一节中将会详细描述转子磁连位置估计的相关技术。

5.2速度/转速传感器

在永磁直流电机驱动中,速度和速度信号基本上需要位置控制驱动器的速度控制回路和速度控制驱动器的速度反馈。速度测量由速度传感器直接得出或用转子位置信号估计间接得出。

一般的直流测速发电机和无刷测速发电机用于检测电机的速度,它们提供了一个模拟直流电压信号,该信号正比于轴速度。在各种类型的测速发电机上电压的极性信号反映出旋转的方向。如今通过使用高分辨率位置传感器或估量转子磁通位置能够更准确地估计出转子的速度/转速。有时,这些传感器不同于用于电子换向的位置传感器。

5.3电流传感器

高性能永磁无刷直流电机驱动的快速转矩控制是通过与转子磁通位置信息同步的相绕组电流的闭环调节来实现的,绕组电流的闭环调节是通过CC-VSI的PWM或超过参考所需的感应电流或电流控制器的滞后电流控制器实现的。因此,绕组电流检测在永磁无刷直流电机的驱动中变得不可缺少。

电流检测一般使用霍尔效应电流传感器,它检测电流的大小和方向,并整合以提供灵敏和精确的电流检测。非常快的响应(小于1微秒)和精确的电流传感器在大范围电流检测(安培至千安的分数)的不同厂家(ABB、LEM等)都有现货。这些霍尔效应电流传感器有几千伏的电气隔离,这是在高水平的驱动器中非常理想的要求。通常在三相电机中需要两个电流传感器,第三相电流由与电动机连接的星型结构的其他两相电流估计得出。这些电流调节矩形永磁无刷直流电机的电流检测要求通常减少到在逆变器的直流链路的单个电流传感器。电流分流电阻器具有低功耗,通常为了成本效益在低功率驱动器中做电流传感器。在调制解调器的电源设备中,例如MOSFET / IGBT的电流检测功能是许多制造商在逆变器/转换器供给永磁无刷直流电机的控制中省去了使用额外的电流传感器。

5.4电压传感器

在现代先进永磁无刷直流电机驱动器中,端电压传感技术需要估算转子的位置和速度,故选用机械传感器驱动以减小尺寸、成本、维护和增强可靠性。电压传感器还需要在制动过程中调节直流母线电压,或在高规格的变频器中用于再生功能的前端转换器控制。端电压检测是通过使用电子隔离放大器(ADI公司制造的AD202等)和霍尔效应电压传感器(ABB制造等)的电隔离。在小规格的变频器中,电压检测采用高阻值电阻分压器来降低驱动器的成本。有时,在电机绕组的感应电压使用特殊绕组诸如探测线圈等来实现。然而,前端变换器的控制中交流电源电压用电压互感器感测。

6.传感器的消除和减少

传感器最新趋势,它们的要求和可用类型的传感器已经在第5章讨论过。然而,这些传感器在无刷直流电机驱动中可以从尺寸、成本、维护和可靠性的角度降低。通常机械转子位置和速度传感器的缺点是增大了电机和控制器之间的连接数量,增加了干扰,由于环境因素如温度,湿度,震动等限制了传感器的精度,增加了摩擦和惯性及电机外壳额外的空间。由于这些问题,在去除机械转子位置/速度传感器而代以利用感应电流和电压估算

转子位置和速度的技术上有很大的兴趣和发展。而且,电压和电流传感器的数目可以通过在逆变器馈送永磁无刷直流电机的控制中使用智能处理器来减少。这些传感器的消除和减少的各种技术将在下面的部分作简要讨论。

6.1无机械传感器

无转子角位置传感器技术是在最新发展的永磁无刷直流电机驱动中最迅速发展的新技术。在许多应用中,无轴装式位置传感器是非常理想的特性,因为该传感器是在驱动器中最昂贵和脆弱的部件之一。以下为一些无位置传感器方案的分类简述。

6.2反电动势位置估计法

转子磁通位置检测最常用的方法是根据推导的反电动势信号。下面对基于反电动势的转子位置检测的各种方法作简要介绍。

6.2a 直接反电动势检测:这个方法对矩形馈永磁无刷直流电机很适用。在这些永磁电机中,特别是相绕组被激励为每个电周期的2/3,且最好始终有一相是不被激励的。直接按顺序检测未被激励相的反电动势,用来产生与转子磁通同步的离散的转子位置信号电流。它已被应用于许多工业应用,包括磁盘驱动器、紧凑的立体声播放机和室内空调机。

6.2b 反电动势的估算:此方法同时适用于正弦馈和矩形馈永磁无刷直流电机。此方法基于用电动机的电压方程式重建反电动势)

e-

=。反电动势的重建既涉及模拟公

v

-

iR

Ldi

/

(dt

式与运算放大器,也涉及运用通常用于控制的在线数字处理器来解这个方程。在这种方法中,端电压和线电流均是直接测量,且上述等式是用于实现反电动势和转子位置的估算。

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b

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a

图1. 两种永磁无刷直流电机横截面(a) 轴向电机(b) 径向电机

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图2 永磁无刷直流电机的转子几何形状(a) 表面安装(b) 内部径向取向(c) 内部磁体切向

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图3 (a) 正弦电机和(b) 梯形波电机的理想电流激励波形

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图4 永磁无刷直流电机速度和电流回路的闭环控制模块图

英文原著

Recent advances in permanent magnet brushless DC motors

BHIM SINGH

Department of Electrical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

e-mail: bsingh @ ee.iitd.ernet.in

Abstract. This paper deals with the latest developments in Permanent Magnet Brushless DC (PMBLDC) motor drives. A comprehensive account of the state of the art on types of construction of the motor, closed loop controllers in position, speed and current/torque control and recent trends in inverters, sensors etc. are given. Techniques for mechanical sensors elimination are discussed in detail. Special efforts made to reduce torque ripples, noise and vibrations are described. The impact of microelectronics through integrated chips used in the control of PMBLDC motor drives is given. The increasing applications of this drive due to improved performance and its cost reduction are also enlisted.

Keywords. PMBLDC motor; sensors; controllers; sensorless operation; torque pulsations.

1. Introduction

Permanent Magnet Brushless DC (PMBLDC) motors are increasingly being used in a wide spectrum of applications such as domestic equipments, automobiles, information technology equipment, industries, public life appliances, transportation, aerospace, defence equipment, power tools, toys, vision and sound equipment and medical and health care equipment ranging from microwatts to megawatts . It has become possible because of their superior performance in terms of high efficiency, fast response, light weight, precise and accurate control, high reliability, maintenance free operation, brushless construction, high power density and reduced size. Recent developments in PMBLDC motor technology in terms of availability of high performance rare earth PM materials, varying motor constructions such as axial field, radial field, package type, rectangular fed, sine fed motors, improved sensor technology, fast semiconductor modules, low cost high performance microelectronics devices, new control philosophy such as robust, adaptive, fuzzy, neural AI based controllers, have been a boon to their widespread use in the large speed ranges from few revolutions to several thousand revolutions per minute (rpm). They have been proven most suitable for position control in machine tools, robotics and high precision servos, speed control and torque control in various industry and process control applications.

Inspite of being one of the best, the PMBLDC motor has faced many hurdles to come to its present stage in terms of cost, torque ripples, noise, vibrations, reduced reliability due to the large number of components, operational constraints such as temperature rise etc. Continuous efforts have been made to overcome these problems on different aspects of this drive. The PMBLDC motor drive is

undoubtedly quite a big mission in itself; this paper concentrates on the recent advances in PMBLDC motors in terms of motor construction, closed loop controllers, semiconductor power modules, sensors and their reduction, torque ripple minimization, impact of microelectronics, cost reduction and potential applications.

2. Latest developments in the PMBLDC motors

Permanent magnet (PM) excitation has been used in place of dc excitation in different electric machines such as dc machines, synchronous machines and new PMBL machines such as PM stepper motors, hybrid stepper motors and PMBLDC motors. High cost of PM materials has been a major bottleneck for use and development of these electric machines. Gradual growth of better PM materials, improved manufacturing technology, varying nature of construction of these motors to suit specific applications have brought them at a level where they are considered one of the best motors available nowadays. PM machines have a wide spectrum but this paper is restricted to PMBLDC motors.

Presently PM materials used in PMBLDC motors are classified in the following three broad categories, namely Alnico (A1-Ni-Co-Fe), Ceramics also include ferrites and rare-earth materials such as samarium-cobalt (Sm-Co), neodymium-iron-boron (Nd-Fe-B). Alnico and ferrites have long been used in the development of PM motors as they are cheap and easily available. Rare-earth PM materials, namely SmCo, are used nowadays because of the high energy density

caused by its high residual flux density, coercive force and low temperature coefficient. NdFeB is considered one of the best PM materials presently since it offers much higher residual flux density and coercive force. However, its only drawback is the temperature limit. Continuous efforts are being made to overcome this and it is hoped that this will enable PMBLDC motors to attain higher efficiencies and lower sizes along with other advantages.

PMBLDC motors may be classified into different categories such as number of phases, radial or axial field, cageless or with cage bars, surface mounted PMs or buried magnets, sinusoidal or rectangular fed motors etc. Some of them are briefly discussed in this section.

2.1 Number of phases

PMBLDC motors are developed in single phase in low power (< 50W) for tube axial fans to cool electronics equipments. They are manufactured in two phase construction for home appliances such as solar PV fed refrigeration system, servo control etc. Most of the medium and high power rating motors are designed in three-phase construction similar to conventional ac motors. In some electric vehicles and megawatt rating motors for submarine propulsion etc., designers have compelling reasons to increase the number of phases to five, six or more in order to reduce the per phase power handling requirements.

2.2 Radial and axial field motors

Most of the motors in the market are radial field type (cylindrical or salient pole construction). However, the axial field motors have some advantages over the conventional radial field construction in terms of power density, torque to inertia ratio, peak torque, less magnet weight, low inductance, short winding turns, compact design etc. Axial field motors are designed in package, disk and sandwich type construction and have no iron in the rotor, resulting in low inertia. Axially directed magnetic field from rotor magnet interacts with radially directed currents in these axial field motors. The magnets are encapsulated in resin or plastic. Because of their construction, they are considered most suitable for robotics, computer equipments, machine tools etc.

Radial field motors are also designed with varying desired flux linkage waveforms such as sinusoidal or trapezoidal, different shapes and positions of magnets in the rotor such as buried or surface mounted etc. They are widely used since stator design is similar to conventional ac synchronous or induction motors. Figure 1 shows the typical cross-sections of these two types of popular PMBLDC motors.

2.3 Shape and location of PM in rotors

Permanent magnets are placed in the rotor in PMBLDC motors. In axial field type of motors, the magnets are encapsulated in resin or plastic in disc form as shown in figure la. These magnets are placed in such a manner that induced back emf are either sinusoidal or trapezoidal waveforms. In radial field motors, the

magnets are placed in different form such as surface mounted for low speed motors and interior radially oriented or interior tangentially oriented in high speed PMBL motors. Figure 2 shows such rotor geometries. They are also designed to achieve sinusoidal or trapezoidal back emfs depending upon applications.

2.4 Sinusoidal and rectangular fed motors

PMBLDC motors are designed to have either sinusoidal or trapezoidal (excited) induced back emfs. Sinusoidal excited motors are fed with sinusoidal polyphase currents similar to conventional synchronous motors for ripple-free torque with unity power factor for constant torque operation below base speed with frequency control and having leading currents to affect field weakening for constant power operation. Maximum speed of operation is restricted with demagnetization caused by armature reaction and mechanical construction. Magnetic saliency on rotor with reluctance torque helps to achieve wide speed range of constant power operation. Trapezoidal excited motors need polyphase balanced rectangular currents with 120 electrical degree width and adjustable magnitude and direction. Constant flux interaction with constant amplitude polyphase currents develops ripple free torque similar to conventional dc motor with electronic commutation. Because of these rectangular currents they are also called switched PM motors, brushless dc motors and electronic commutated PMBLDC motors.

Figure 3 shows the ideal current waveforms for these two types of motors. Position sensors requirement is accordingly changed to realize these ideal current

waveforms in the motor windings in self synchronous control mode.

3. Closed loop controllers

Irrespective of sinusoidal or trapezoidal excitation, PMBLDC motors are used for position control, speed control and torque control in motion control applications. Figure 4 shows a typical position closed loop control with inner speed and current loops. For speed control system outer position loop is not required and speed reference is the command signal. Torque control is incorporated in high performance motion control through closed loop regulation of phase currents in synchronization with shaft position feedback. In the majority of PMBLDC motors, torque is linearly related to currents and torque command maps into current commands with only a simple proportionality constant. In some typical cases, a nonlinear mapping is required between torque and current commands. The constant power operation of PMBLDC motor drive is extended through field weakening control techniques which also requires another command signal for torque to current mapping. Current regulation in the phase windings of PMBLDC motors either in sinusoidal or rectangular manner is carried out using current controlled voltage source inverter(VSI). PWM, hysteresis and predictive current controllers are used to issue the switching signals to the devices of the inverter to realize winding currents close to command currents. Speed control is generally achieved by using a speed feedback and speed command through speed controller which outputs a command signal for torque controller. Position control is implemented through position

feedback and position command using position controller. The output of the position controller is the speed command for inner speed loop. Both position and speed closed loop controllers are realized using wide varying closed loop controllers such as PI (proportional-integral), PID (proportional-integral-derivative), SMC (sliding mode controller), adaptive controllers, fuzzy based control and neural based controllers.

These classical (PI or PID) controllers or advanced closed loop controllers such as SMC, fuzzy and neural network-based ones are implemented using DSP, microcontroller and specific application integrated chip (ASIC) for speed and/or position control. Many manufacturers have developed ASICs for typical applications of PMBLDC motors.

4. Recent developments in inverters and converters

PMBLDC motors are invariably fed from variable frequency inverters to provide electronic commutation. At small ratings, MOSFET-based VSIs are used to achieve ideal current control with reasonable high switching frequency. In medium rating drives, IGBT-based VSIs are used to feed PMBLDC motors. GTO-based inverters are used in high power rating PMz BLDCM drives because of their self-commutating feature and improved current control. In medium and high power rating PMBLDC motors, an input unity power factor rectifier with PWM current control is used to achieve regulated dc link and regenerative feature of the PMBLDC motor drive. Classical PI/PID and feedforward closed loop controllers are used for

the control of front end converters. It improves the power quality of the ac mains in terms of reduced harmonics and unity power-factor in both the directions of power flow.

Recently MOSFET/IGBT, based VSIs are used in module forms to achieve compact design of the drive. Power MOS ICs are used to control small rating PMBLDC motors which has integrated microelectronics for control and power amplifier to excite the motor. Moreover, their gate drives are also used in module form with all protective and intelligent control features. In some typical applications (residential commercial blower), they are made so compact that these are fitted inside the motor housing.

5. Latest trends in sensors

In the control of PMBLDC motors, position, speed and current sensors are essentially required to regulate the phase currents in synchronization with rotor position. Moreover, sometimes, terminal voltage sensors are also required to estimate either position or speed. V oltage sensors are also needed to regulate dc bus voltage during braking or for front end converter control. In some typical attempts, flux sensors and torque sensors are also used in the precise control of these motors. Basic role of these sensors are already discussed in closed loop control of PMBLDCM drive in §3. In the following section, the recent trends in sensors and their function are briefly discussed.

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