* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Stator
Survey
Document related concepts
Power engineering wikipedia , lookup
Voltage optimisation wikipedia , lookup
Three-phase electric power wikipedia , lookup
Alternating current wikipedia , lookup
Dynamometer wikipedia , lookup
Electrification wikipedia , lookup
Commutator (electric) wikipedia , lookup
Brushed DC electric motor wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Electric motor wikipedia , lookup
Stepper motor wikipedia , lookup
Brushless DC electric motor wikipedia , lookup
Transcript
电气技术专业英语 朱一纶 主编 中国电力出版社 Unit 8 Electric motor 课件制作: 吴岱曦 Index • Text – 1. Induction Motor – 2. DC motor • Reading materials – 1. Stepping motor – 2. Servo controllers – 3. Linear motor • Exercises – 1. – 2. – 3. – 4. Put the Phrases into English Put the Phrases into Chinese Sentence Translation Translation Text • An electric motor (Fig 8.1) uses electrical energy to produce mechanical energy. The reverse process, which using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. • Electric motors are found in household appliances such as fans, refrigerators, washing machines, pool pumps, floor vacuum , and fan-forced ovens, and many industrial applications are dependent upon motors (or machines)too, which range from the size of one's thumb to the size of a railroad locomotive. • All loads moved by electric motors are really moved by magnetism. The purpose of every component in a motor is to help harness, control, and use magnetic force. To move a load fast does not require more magnets, you just move the magnets fast. • To move a heavier load or to decrease acceleration time (accelerate faster), however, more magnets (more torque) are needed. This is the basis for all motor applications. Fig 8.1 Various Motors 1. Induction Motor • The induction motor is a three phase AC motor and is the most widely used machine. Its characteristic features are: – Simple and rugged construction – Low cost and minimum maintenance – High reliability and sufficiently high efficiency • An induction motor has basically two parts : – An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field; – An inside rotor attached to the output shaft that is given a torque by the rotating field. Stator (Fig 8.2) • The stator is made up of a number of stampings with slots to carry three phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart. Fig 8.2 Stator • Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. • Each grouping of coils, together with the steel core it surrounds, form an electromagnet. Electromagnetism is the principle behind motor operation. The stator windings are connected directly to the power source. Rotor (Fig 8.3) • Two types of rotors are used in Induction motors - squirrel-cage rotor and wound rotor. Fig 8.3 rotor of an induction motor • (a) lamination - left • (b) The construction of the squirrel cage rotor - right • The construction of the squirrel cage rotor is reminiscent of rotating exercise wheels found in cages of pet squirrel. The rotor consists of a stack of steel laminations with evenly spaced conductor bars around the circumference. • The laminations are stacked together to form a rotor core. Aluminum is die cast in the slots of the rotor core to form a series of conductors around the perimeter of the rotor. Current flow through the conductors forms the electromagnet. • The conductor bars are mechanically and electrically connected with end rings. The rotor core mounts on a steel shaft to form a rotor assembly. • The enclosure (Fig 8.4) consists of a frame and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotor fits inside the stator with a slight air gap separating it from the stator. • There is no direct physical connection between the rotor and the stator. The enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates. • Bearings, mounted on the shaft, support the rotor and allow it to turn. A fan, also mounted on the shaft, is used on the motor for cooling. Fig 8.4 Induction Motor • The wound rotor motor or slip ring motor is an induction machine where the rotor comprises a set of coils that are terminated in slip rings to which external impedances can be connected. The stator is the same as is used with a standard squirrel cage motor. • By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered. • The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. • By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low current from zero speed to full speed. • A secondary use of the slip ring motor is to provide a means of speed control. Because the torque curve of the motor is effectively modified by the resistance connected to the rotor circuit, the speed of the motor can be altered(Fig 8.5). Fig 8.5 torque curve modified by R2 • Increasing the value of resistance on the rotor circuit will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced. 2. DC motor • By far the most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to create an oscillating AC current from the DC source so they are not purely DC machines in a strict sense. • The classic DC motor design generates an in a wound rotor with a split ring commutator (Fig 8.3), and either a wound or permanent magnet stator, a rotor consists of a coil wound around a rotor which is then powered by any type of DC voltage source. Fig 8.6 DC motor • Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. • Brushes may bounce off the irregularities in the commutator surface, creating sparks. This limits the maximum speed of the machine. The current density per unit area of the brushes limits the output of the motor. The imperfect electric contact also causes electrical noise. • Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance. The commutator assembly on a large machine is a costly element, requiring precision assembly of many parts. • Some of the problems of the brushless DC motor are eliminated in the brushless design. In this motor, the mechanical "rotating switch" or commutator / brushgear assembly is replaced by an external electronic switch synchronized to the rotor's position. • Brushless motors are typically 85-90% efficient, whereas DC motors with brush gear are typically 75-80% efficient. • Brushless DC motors are commonly used where precise speed control is necessary, as in computer disk drives or in video cassette recorders, the spindles within CD drives, and mechanisms within office products such as fans, laser printers and photocopiers. • Modern DC brushless motors range in power from a fraction of a watt to many kilowatts. Larger brushless motors up to about 100 kW rating are used in electric vehicles. This is the End of the Text Reading materials 1. Stepping motor • Stepping motors can be viewed as electric motors without commutators. Typically, all windings in the motor are part of the stator, and the rotor is either a permanent magnet or, in the case of variable reluctance motors, a toothed block of some magnetically soft material. • For example, a variable reluctance stepping motor has three windings, typically connected as shown in the schematic diagram in Figure 8.7, with one terminal common to all windings. In use, the common wire typically goes to the positive supply and the windings are energized in sequence. • The rotor in this motor has 4 teeth and the stator has 6 poles, with each winding wrapped around two opposite poles. With winding number 1 energized, the rotor teeth marked X are attracted to this winding's poles. • If the current through winding 1 is turned off and winding 2 is turned on, the rotor will rotate 30 degrees clockwise so that the poles marked Y line up with the poles marked 2. To rotate this motor continuously, we just apply power to the 3 windings in sequence. Fig8.7 variable reluctance motor • All of the commutation must be handled externally by the motor controller, and typically, the motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other. • Stepping motors come in a wide range of angular resolution. The coarsest motors typically turn 90 degrees per step, while high resolution permanent magnet motors are commonly able to handle 1.8 or even 0.72 degrees per step. • With an appropriate controller, most permanent magnet motors can be run in half-steps, and some controllers can handle smaller fractional steps. • Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. 2. Servo controllers • Servo controller is a wide category of motor control. Common features are: – precise closed loop position control – fast acceleration rates – precise speed control • Servo motors may be made from several motor types, the most common being – brushed DC motor – brushless DC motors – AC servo motors • Servo controllers use position feedback to close the control loop. This is commonly implemented with encoders, resolvers, and Hall effect sensors to directly measure the rotor's position. • Others measure the back electromotive force (EMF) in the undriven coils to infer the rotor position, and therefore are often called "sensorless" controllers. • A servo may be controlled using pulse- width modulation (PWM). How long the pulse remains high (typically between 1 and 2 milliseconds) determines where the motor will try to position itself. • For some applications, there is a choice between using servomotors and stepping motors. Both types of motors offer similar opportunities for precise positioning, but they differ in a number of ways. Servomotors require analog feedback control systems of some type. • Typically, this involves a potentiometer to provide feedback about the rotor position, and some mix of circuitry to drive a current through the motor inversely proportional to the difference between the desired position and the current position. 3. Linear motor • A linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing a torque (rotation), it produces a linear force along its length by setting up a traveling electromagnetic field. • Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground, and in many rollercoasters[1] where the rapid motion of the motorless railcar is controlled by the rail. • Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and highacceleration linear motors. Lowacceleration linear motors are suitable for maglev trains and other ground-based transportation applications. • The Shanghai Maglev Train connects the rapid transit network 30.5 km to the Shanghai Pudong International Airport (Fig 8.8). High-acceleration linear motors are normally quite short, and are designed to accelerate an object up to a very high speed and then release the object, like roller coasters. Fig 8.8 The Shanghai Maglev Train • They are usually used for studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion. • The high-acceleration motors are usually of the linear induction design (LIM) with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. • The low-acceleration, high speed and high power motors are usually of the linear synchronous design (LSM), with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent magnets or energized magnets. This is the End of the Reading materials Exercises • Ready to go? 1. Put the Phrases into English • • • • • 1) 2) 3) 4) 5) 异步电动机 三相交流电动机 定子绕组 旋转(磁)场 鼠笼式转子 Show the Answer • • • • • 6) 7) 8) 9) 10) 绕线式电动机 最大转矩 在严格意义上 直流电机 速度控制 Show the Answer 2. Put the Phrases into Chinese • 1)to produce mechanical energy • 2)floor vacuum • 3)decrease acceleration time • 4)produce a rotating magnetic field • 5)the output shaft Show the Answer • 6)geometrically spaced 120 degrees apart • 7)speed/torque curve • 8)a means of speed control • 9)oscillating current • 10)wear out and require replacement Show the Answer 3. Sentence Translation • 1)Electric motors are found in household appliances such as fans, refrigerators, washing machines. • 1)很多家用电器里都有电动机,例如电风 扇,冰箱,洗衣机。 • 2) To move a load fast does not require more magnets, you just move the magnets fast. • 2)要让负载转的更快并不需要增加磁场强 度,你只要增加磁场的转速就可以了。 • 3) Two types of rotors are used in induction motors - squirrel-cage rotor and wound rotor. • 3)异步电动机的转子分成两种 – 鼠笼式转 子和绕线式转子。 • 4) By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered. • 4)改变转子电路的阻抗,可以改变速度/电 流曲线和速度/转矩曲线。 • 5) The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. • 5)绕线式电动机最常用的场合是拖动惯性 比较大的负载或者需要高起动转矩的负载 起动。 • 6) The imperfect electric contact also causes electrical noise. • 6)不完善的电接触还会产生电噪声。 • 7) At higher speeds, brushes have increasing difficulty in maintaining contact. • 7)在更高的速度下,电刷更难保持接触状 态。 • 8) In brushless DC motor, the mechanical "rotating switch" or commutator assembly is replaced by an external electronic switch synchronized to the rotor's position. • 8)在无刷直流电动机中,机械“旋转开关” 或换向器被和转子位置同步的外置电子开 关所取代。 4.Translation • Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. • Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers. Show the Answer This is the End of the Exercises Good work everyone !!! (习题答案仅供参考)