AC MachinesIntroductionThe electrical machine that converts electrical energy into mechanical energy, and vice versa, is the workhorse in a drive system. A machine is a complex structure electrically, mechanically, and thermally.Although machines were introduced more than one hundred years ago, the research and development in this area appears to be never-ending. However, the evolution of machines has been slow compared to that of power semiconductor devices and power electronic converters.Traditionally, AC machines with a constant frequency sinusoidal power supply have been used in constant-speed applications, whereas DC machines were preferred for variable-speed drives. But in the last two or three decades, we have seen extensive research and development efforts for variable-frequency, variable-speed AC machine drive technology, and they will progressively replace DC drives. In most cases, new applications use AC drives.AC machines can generally be classified as follows:Induction machines: Cage or wound rotor (doubly-fed), Rotating or linear;Synchronous machines: Rotating or linear, Reluctance, Wound field or permanent magnet, Radial or axial gap (disk), Surface magnet or interior (buried) magnet, Sinusoidal or trapezoidal;Variable reluctance machines: Switched reluctance, Stepper.Induction MachinesAmong all types of AC machines, the induction machine, particularly the cage type, is most commonly used in industry. These machines are very economical, rugged, and reliable, and are available in the ranges of fractional horse power (FHP) to multi-megawatt capacity. Low-power FHP machines are available in single-phase, but poly-phase (three-phase) machines are used, most often in variable-speed drives. Fig. 1-6A-1 shows an idealized three-phase, two-pole induction motor where each phase winding in the stator and rotor is represented by a concentrated coil. The three-phase windings are distributed sinusoidally and embedded in slots. In a wound-rotor machine, the rotor winding is similar to that of the stator, but in a cage machine, the rotor has a squirrel cage-like structure with shorted end rings. Basically, the machine can be looked upon as a three-phase transformer with a rotating and short-circuited secondary. Both stator and rotor cores are made with laminated ferromagnetic steel sheets. The air gap in the machine is practically uniform(non-salient pole).Pf N ee 120=One of the most fundamental principles of induction machines is the creation of a rotating and sinusoidally distributed magnetic field in the air gap. Neglecting the effect of slots and space harmonics due to non-ideal winding distribution, it can be shown that a sinusoidal three-phase balanced power supply in the three-phase stator winding creates a synchronously rotating magnetic field. The rotational speed can be given as equation (1-6A-1). Ne is called synchronous speed in rpm andπ2/e e w f =is the stator frequency in Hz. P is the pole numbers of a machine.The rotor winding will be subjected to a sweeping magnetic field, and have inducing current in the short-circuited rotor.The interaction of air gap flux and rotor mmf produces torque, make the rotor rotate. But the speed of the rotor is less than synchronous speed. So it called induction machine or asynchronous machine. To meet the various starting and running requirements of a variety of industrial applications, several standard designs of squirrel-cage mot ors are available from manufacturers’ stock. The torque-speed characteristics of the most common designs, readily available and standardized in accordance with the criteria established by the National ElectricalbsStatoras axis cs b's Rotor as Stator c's c'r br a'r cr b'rar Rotorar axisa'sFig. 1-6A-1 idealized three-phase, two-pole induction motor (1-6A-1)Manufacturers’ Association (NEMA), are shown in Fig. 1-6A-2. The most significant design variable in these motors is the effective resistance of the rotor cage circuits.DCBA T /N ⋅m200 300 100n sn /r ⋅m in -1Class A Motors These machines are suitable for applications where the loadtorque is low at start (such as fan or pump loads) so that full speed is achieved rapidly, thereby eliminating the problem of overheating during starting. In large machines, low-voltage starting is required to limit the starting current.Class B Motors Motors of this class are good general-purpose motors and have a wide variety of industrial applications. They are particularly suitable for constant-speed drives, where the demand for starting torque is not severe. Examples are drives for fans, pumps, blowers, and motor-generator sets.Class C Motors Class C motors are suitable for driving compressors, conveyors, and so forth.Class D Motors These motors are suitable for driving intermittent loads requiring rapid acceleration and high-impact loads such as punch presses or shears. In the case of impact loads, a flywheel is fitted to the system. As the motor speed falls appreciably with load impact, the flywheel delivers some of its kinetic energy during the impact.Fig. 1-6A-2 torque-speed characteristics for different class of induction motorsSynchronous MachinesA synchronous machine, as the name indicates, must rotate at synchronous speed, that is, the speed is uniquely related to the supply frequency, as indicated in Equation (1-6A-1). It is a serious competitor to the induction machine invariable-speed drive applications.Fig. 1-6A-3 shows an idealized three-phase, two-pole wound field synchronousmachine.The stator winding of the machine is identical to that of the induction machine, but the rotor has a winding that carries DC current and produces flux in the air gap that helps the stator-induced rotating magnetic field to drag the rotor along with it. The DC field current is supplied to the rotor from a static rectifier through slip rings and brushes, or by brushless excitation.Since the rotor always moves at synchronous speed, the synchronously rotating de-qe axes are fixed with the rotor, where the de axis corresponds to the north pole, as shown. There is no stator-induced induction in the rotor, and therefore, the rotor mmf is supplied exclusively by the fieldcsb'sasc'sbsRotorq e axisSNFig. 1-6A-3 idealized three-phase,two-pole synchronous machineRotorStatora'sStatoras axisRotord e axisωewinding.This permits the machine to run at an arbitrary power factor at the stator terminal, that is, leading, lagging, or unity. On the other hand, in an induction machine, the stator supplies the rotor excitation that makes the machine power factor always lagging.The mechanism of torque production is somewhat similar to that of an induction machine. The machine shown is characterized as a salient pole because of the nonuniform air gap around the rotor, which contributes to asymmetrical magnetic reluctance in the d and q axes. This is in contrast to a machine with a cylindrical rotor structure having a uniform air gap (such as an induction motor), defined as a nonsalient pole machine.For example, low-speed synchronous generators in hydro-electric power stations use salient pole machines, whereas high-speed generators in steam-power stations use nonsalient pole machines. In addition to field winding, the rotor usually contains an amortisseur, or damper winding, which is like short-circuited squirrel cage bar in an induction motor. The machine is more expensive but efficiency is somewhat higher.Wound field machines are normally used for high-power (multi-megawatt) drives.Variable Reluctance Machine (VRM)A variable or double reluctance machine (VRM), as the name indicates, has double saliency, meaning it has saliency in the stator as well as in the rotor.As mentioned before, the VRM has two classifications: switched reluctance machine (SRM) and stepper motor. The stepper motor is basically a digital motor, i.e., it moves by a fixed step or angle with a digital pulse. Small stepper motors are widely used for computer peripheral-type applications. However, since the machine is not suitable for variable-speed applications, there will not be any further discussion of it.There has been interest in switched reluctance motor drives in the literature, and recently, great effort has been made to commercialize them in competition with induction motors.Fig. 1-6A-4 shows the cross-section of a four-phase machine with four stator-pole pairs and three rotor-pole pairs (8/6 motor). The machine rotor does not have any winding or PM. The stator poles have concentrated winding (instead of sinusoidal winding), and each stator-pole pair winding, as shown in the figure, is excited by a converter phase. For example, the stator-pole pair A-A' is energized when the rotor pole-pair a-a' approaches it to produce the torque by magnetic pull, but is de-energized when pole alignment occurs.All four machine phases are excited sequentially and synchronously with the help of a rotor position encoder to getunidirectional torque.The magnitude of torque can be given as:221miT e .Where m = inductance slope and i = instantaneous current. The current i can be maintained constantly by during the inductance slope. At high speeds, the rotor-induced CEMF is high.The favorable attributes of this electronic motor are simplicity and robustness of construction; potentially, it is somewhat cheaper than other classes of machines. However, the torque generation is pulsating in nature and there are serious acoustic noise problems.Fig. 1-6A-4 Construction of switched reluctance machineABD'C'B'A'DCS 1S 2S 3S 4 Forwardr18°18°42°交流机简介将电能转换成机械能或将机械能转换成电能的电机是传动系统中的主要组成部分。
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