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Speed Control of DC MotorAbstract Conditioning system is characterized in that output power to maintain stability. Different speed control system can use a different brake system, high starting and braking torque, quick response and quick adjustment range of degree requirements of DC drive system, the use of the electric braking mode. Depends on the speed control of DC motor armature voltage and flux. To zero speed, or U = 0 or = . The latter is impossible, it only changes through the armature voltage to reduce speed. To speed to a higher value can increase or decrease the U .Keyword DC Speed Feedback BrakeRegulator SystemsA regulator system is one which normally provides output power in its steady-state operation.For example, a motor speed regulator maintains the motor speed at a constant value despite variations in load torque. Even if the load torque is removed, the motor must provide sufficient torque to overcome the viscous friction effect of the bearings. Other forms of regulator also provide output power; A temperature regulator must maintain the temperature of, say, an oven constant despite the heat loss in the oven. A voltage regulator must also maintain the output voltage constant despite variation in the load current. For any system to provide an output, e.g., speed, temperature, voltage, etc., an error signal must exist under steady-state conditions. Electrical BrakingIn many speed control systems, e.g., rolling mills, mine winders, etc., the load has to be frequently brought to a standstill and reversed. The rate at which the speed reduces following a reduced speed demand is dependent on the stored energy and the braking system used. A small speed control system (sometimes known as a velodyne) can employ mechanical braking, but this is not feasible with large speed controllers since it is difficult and costly to remove the heat generated.The various methods of electrical braking available are:(1) Regenerative braking.(2) Eddy current braking.(3) Dynamic braking.(4) Reverse current braking(plugging)Regenerative braking is the best method, though not necessarily the most economic. The stored energy in the load is converted into electrical energy by the work motor (acting temporarily as a generator) and is returned to the power supply system. The supply system thus acts as a”sink”into which the unwanted energy is delivered. Providing the supply system has adequate capacity, the consequent rise in terminal voltage will be small during the short periods of regeneration. In the Ward-Leonard method of speed control of DC motors, regenerative braking is inherent, but thyristor drives have to be arranged to invert to regenerate. Induction motor drives can regenerate if the rotor shaft is driven faster than speed of the rotating field. The advent of low-cost variable-frequency supplies from thyristor inverters have brought about considerable changes in the use of induction motors in variable speed drives.Eddy current braking can be applied to any machine, simply by mounting a copper or aluminum disc on the shaft and rotating it in a magnetic field. The problem of removing the heat generated is severe in large system as the temperature of the shaft, bearings, and motor will be raised if prolonged braking is applied.In dynamic braking, the stored energy is dissipated in a resistor in the circuit. When applied to small DC machines, the armature supply is disconnected and a resistor is connected across the armature (usually by a relay, contactor, or thyristor).The field voltage is maintained, and braking is applied down to the lowest speed. Induction motors require a somewhat more complex arrangement, the stator windings being disconnected from the AC supply and reconnected to a DC supply. The electrical energy generated is then dissipated in the rotor circuit. Dynamic braking is applied to many large AC hoist systems where the braking duty is both severe and prolonged.DC Motor Speed ControlThe basis of all methods of DC motor speed control is derived from the equations:the terms having their usual meanings. If the IaRa drop is small, the equations approximate to or 。Thus, control of armature voltage and field flux influences the motor speed. To reduce the speed to zero, either U=0 or=.The latter is inadmissible; hence control at low speed is by armature voltage variation. To increase the speed to a high value, either U is made very large or is reduced. The latter is the most practical way and is known as field weakening. Combinations of the two are used where a wide range of speed is required.A Single-Quadrant Speed Control System Using ThyristorsA single-quadrant thyristor converter system is shown in Fig.1.For the moment the reader should ignore the rectifier BR2 and its associated circuitry (including resistor R in the AC circuit), since this is needed only as a protective feature and is described in next section.Fig.1 Thyristor speed control system with current limitation on the AC sideSince the circuit is a single-quadrant converter, the speed of the motor shaft (which is the output from the system) can be controlled in one direction of rotation only. Moreover, regenerative braking cannot be applied to the motor; in this type of system, the motor armature can suddenly be brought to rest by dynamic braking (i.e. when the thyristor gate pulses are phased back to 180o, a resister can be connected across the armature by a relay or some other means).Rectifier BR1 provides a constant voltage across the shunt field winding, giving a constant field flux. The armature current is controlled by a thyristor which is, in turn, controlled by the pulses applied to its gate. The armature speed increases as the pulses are phased forward (which reduces the delay angle of firing), and the armature speed reduces as the gate pulses are phased back.The speed reference signal is derived from a manually operated potentiometer (shown at the right-hand side of Fig.23.1), and the feedback signal or output speed signal is derived from the resistor chain R1 R2, which is connected across the armature. (Strictly speaking, the feedback signal in the system in Fig.23.1 is proportional to the armature voltage, which is proportional to the shaft speed only if the armature resistance drop, IaRa, is small. Methods used to compensate for the IaRa drop are discussed in Reading Material.)Since the armature voltage is obtained from a thyristor, the voltage consists of a series of pulses; these pulses are smoothed by capacitor C. The speed reference signal is of the opposite polarity to the armature voltage signal to ensure that overall negative feedback is applied.A feature of DC motor drives is that the load presented to the supply is a mixture of resistance, inductance, and back EMF Diode D in Fig.1 ensures that the thyristor current commutates to zero when its anode potential falls below the potential of the upper armature connection, in the manner outlined before. In the drive shown, the potential of the thyristor cathode is equal to the back EMF of the motor while it is in a blocking state. Conduction can only take place during the time interval when the instantaneous supply voltage is greater than the back EMF.Inspection of Fig.2 shows that when the motor is running, the peak inverse voltage applied to the thyristor is mush greater than the peak forward voltage. By connecting a diode in series with the thyristor, as shown, the reverse blocking capability of the circuit is increased to allow low-voltage thyristor to be used.References:Fig.2 Illustrating the effect of motor back EMF on thePeak inverse voltage applied to the thyristorFig.3 Armature voltage waveformsThe waveforms shown in Fig.2 are idealized waveforms as much as they ignore the effects of armature inductance,commutator ripple,etc.Typical armature voltage waveforms are shown in Fig.3.In this waveform the thyristor is triggered at point A, and conduction continues to point B when the supply voltage falls below the armature back EMF.The effect of armature inductance is to force the thyristor to continue to conduct until point C,when the fly-wheel diode prevents the armature voltage from reversing. When the inductive energy has dissipated (point D), the armature current is zero and the voltage returns to its normal level, the transients having settled out by point E.The undulations on the waveform between E and F are due to commentator ripple.References1.Landau ID(1999)From robust control to adaptive control.Control Eng Prac 7:111311242.Forssell U,Ljung L(1999)Closed-loop identification revisited. Automatica 35:121512413.Soderstrom T,Stoica P(1989)System identification.Prentice Hall,Cambridge,UK4.Horng JH(1999)Neural adaptive tracking control of a DC motor.Information Sci 118:1135.Lyshevski SE(1999)Nonlinear control of mechatronic systems with permanent-magnet DC motors.Mechatronics 9:5395526.Yavin Y,Kemp PD(2000)Modeling and control of the motion of a rolling disk:e?ect of the motor dynamics on the dynamical model.Comput Meth Appl Mech Eng 188:6136247.Mummadi VC(2000)Steady-state and dynamic performance analysis of PV supplied DC motors fed from intermediate power converter.Solar Energy Mater Solar Cells 61:3653818.Jang JO,Jeon GJ(2000)A parallel neuro-controller for DC motors containing nonlinear friction.Neurocomputing 30:2332489.Nordin M,Gutman P(2002)Controlling mechanical systems with backlasha survey.Automatica 38:1633164910.Wu R-H,Tung P-C(2002)Studies of stick-slip friction,pre-sliding displacement,and hunting.J Dyn Syst 124:11111711.Ogata K(1990)Modern control engineering.Prentice Hall,Englewood Cli?s,NJ12.Slotine E,Li W(1991)Applied nonlinear control.Prentice Hall,Englewood Cli?s,NJ13.Lee PL(1993)Nonlinear process control:applications of gen-eric model control.Springer,Berlin Heidelberg New York直流电动机调速控制摘要 调节系统的特征在于能保持输出功率的稳定。不同的速度控制系统可以使用不同的制动系统，在有高起、制动转矩，快速响应和快速度调节范围要求的直流调速系统中，采用的是电气制动的方式。直流电机的速度控制取决于电枢电压和磁通。要将转速降为零，或者U=0或=。后者是不可能的，因此只可通过电枢电压的变化来降低转速。要将转速增加到较高值，可以增大U或减小。关键词 直流调速 反馈 制动调节系统调节系统是一类通常能提供稳定输出功率的系统。例如，电机速度调节器要能在负载转矩变化时仍能保持电机转速为恒定值。即使负载转矩为零，电机也必须提供足够的转矩来克服轴承的粘滞摩擦影响。其他类型的调节器也提供输出功率，温度调节器必须保持炉内的温度恒定，也就是说，即使炉内的温度散失也必须保持炉温不变。一个电压调节其也必须保持负载电流值变化时输出电压值恒定。对于任何一个提供一个输出，例如，速度、温度、电压等的系统，在稳态下必须存在一个误差信号。电气制动在许多速度控制系统中，例如轧钢机、矿坑卷扬机等这些负载要求频繁地停顿和反向运动的系统。随着减速要求，速度减小的比率取决于存储的能量和所使用的制动系统。一个小型速度控制系统（例如所知的伺服积分器）可以采用机械制动，但这对大型速度控制器并不可行，因为散热很难而且很昂贵。可行的各种电气制动方法有：（1） 回馈制动。（2） 涡流制动。（3） 能耗制动（4） 反接制动回馈制动虽然并不一定是最经济的方式，但却是最好的方式。负载中存储的能量通过工作电机（暂时以发电机模式运行）被转化成电能并返回到电源系统中。这样电源就充当了一个收容不想要的能量的角色。假如电源系统具有足够的容量，在短时回馈过程中最终引起的端电压升高会很少。在直流电机速度控制渥特-勒奥那多法中，回馈制动是固有的，但可控硅传动装置必须被排布的可以反馈。如果转轴速度快于旋转磁场的速度，感应电机传动装置可以反馈。由晶闸管换流器而来的廉价变频电源的出现在变速装置感应电机应用中引起了巨大的变化。涡流制动可用于任何机器，只要在轴上安装一个铜条或铝盘并在磁场中旋转它即可。在大型系统中，散热问题是很重要的，因为如果长时间制动，轴、轴承和电机的温度就会升高。在能耗制动中，存储的能量消耗在回路电阻器上。用在小型直流电机上时，电枢供电被断开，接入一个电阻器（通常是一个继电器、接触器或晶闸管）。保持磁场电压，施加制动降到最低速。感应电机要求稍微复杂一点的排布，定子绕组被从交流电源上断开，接到直流电源上。产生的电能继而消耗在转子回路中。能耗制动应用在许多大型交流升降系统中，制动的职责是反向和延长。任何电机都可以通过突然反接电源以提供反向的旋转方向（反接制动）来停机。在可控情况下，这种制动方法对所传动装置都是使用的。它主要的缺点就是当制动等于负载存储的能量时，电能被机器消耗了。这在大型装置中就大大增加了运行成本。直流电机速度控制所有直流电机速度控制的基本关系都可由下式得出：各项就是她们通常所指的含义。如果IaRa很小，等式近似为或。这样，控制电枢电压和磁通就可影响电机转速。要将转速降为零，或者U=0或=。后者是不可能的，因此只可通过电枢电压的变化来降低转速。要将转速增加到较高值，可以增大U或减小。后者是最可行的方法，就是我们通常所知道的弱磁场。在要求速度调节范围宽的场合可综合使用这两种方法。使用晶闸管的单向速度控制系统一个单相晶闸管逆变器系统如图1所示。读者应该先忽略整流器BR2和它的相关电路（包括交流回路中的电阻器R），因为这部分只有在具有保护功能时才需要，将在下一节介绍。图1 单向晶闸管逆变器系统因为该电路是一个单向转换器，只能在一个旋转方向控制电机轴（系统的输出）的速度。而且，回馈制动不能用于电机；在这种系统类型中，电机电枢可以通过电气制动静止（例如，当晶闸管门极脉冲反向时，电阻可通过一个继电器或其他装置连接到电枢上）。整流器BR1给并联励磁绕组提供一个稳定电压，产生稳定的磁通。电枢电流由一个晶闸管控制，该晶闸管又由加在它们极上的脉冲控制。脉冲正向时（减小起动延时角）电枢转速增加，门极脉冲反相时电枢转速减小。速度参考信号可从人工操作的电位器（如图1右侧所示）上获得，反馈信号或输出转速信号可从连接在电枢上的电阻器链上获得。（严格的讲，图1系统中反馈信号只有当电枢电组的压降很小时，才与轴转速成正比的电枢电压成正比。用于补偿IaRa压降的方法将在阅读材料中讨论。）因为电枢电压是从一个晶闸管上获得的，该电压包括一系列由电容器C滤波的脉冲。速度参考信号与电枢电压信号极性相反，以确保施加的都是负反馈。直流电机装置的一个特征就是需要供电的负载时电阻、电导的混合，并且在图1中反电动势二极管D确保当晶闸管阳极电势低于前面叙述的电枢连接方式的上限时，晶闸管电流应换向为零。在所示拖动系统中，当晶闸管处于断开状态时，其阳极电势等于电机反电动势。只有在瞬时电源电压大于反向电势的间隔时它才会导通。图2所示的检测表明电机运行时晶闸管上峰值反向电压大于峰值正向电压。如图所示，在晶闸管上串联一个二级管，电路的反向关断能力就会增强，所以允许使用低压晶闸管。图2晶闸管对电机反电动势的影响图3电枢电压波形图2所示的波形是理想的波形，因为忽略了电枢电感、换向器纹波等因素的影响。典型的电枢电压波形如图3所示。在该波形中，晶闸管在A点触发，一直到B点电源电压低于电枢反电动势时导通。电枢电感的作用使晶闸管保持到C点飞轮二极管使电枢电压反向之前导通。当电感能量消失（D点），电枢电流为零，电压恢复到它的正常水平，这个暂态过程最后稳定在E点。点E、F之前的纹波是由换向器引起的纹波。
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