1、This is one of the most rugged and most widely used machines in industry. Its stator is composed of laminations of high-grade sheet steel. The inner surface is slotted to accommodate a three-phase winding. In Fig.5.2 （a） the three-phase winding is represented by three coils, the axes of which are 12
2、0 electrical degrees apart. Coil aa represents all the coils assigned to phase a for one pair of poles. Similarly coil bb represents phase b coils, and coil cc represents phase c coils. When one end of each phase is tied together, as depicted in Fig.5.2 （b）, the three-phase stator winding is said to
3、 be Y-connected. Such a winding is called a three-phase winding because the voltage induced in each of the three phases by a revolving flux density field are out of phase by 120 electrical degreesa distinguishing characteristic of a balanced three-phase system.The rotor also consists of laminations
4、of slotted ferromagnetic material, but the rotor winding may be either the squirrel-cage type or the wound-rotor type. The latter is of a form similar to that of the stator winding. The winding terminals are brought out to three slip rings. This allows an external three-phase resistor to be connecte
5、d to the rotor winding for the purpose of providing speed control. As a matter of fact, it is the need for speed control which in large measure accounts for the use of the wound-rotor type induction motor. Otherwise the squirrel-cage induction motor would be used. The squirrel-cage winding consists
6、merely of a number of copper bars embedded in the rotor slots and connected at both ends by means of copper end rings. （In some of the smaller sizes aluminum is used.） The squirrel-cage construction is not only simpler and more economical than the wound-rotor type but more rugged as well. There are
7、no slip rings or carbon brushes to be bothered with.In normal operation a three-phase voltage is applied to the stator winding at points a-b-c in Fig.5.2.Magnetizing currents flow in each phase which together create a revolving magnetic field having two poles. The speed of field is fixed by the freq
8、uency of the magnetizing currents and the number of poles for which the stator winding is designed. Fig.5.2 shows the configuration for two poles. If the pattern a-c-b-a-c-b is made to span only 180 mechanical degrees and then is repeated over the remaining 180 mechanical degrees, a machine having a
9、 four-pole field distribution results. For a p-pole machine the basic winding pattern must be repeated p/2 times within the circumference of the inner surface of the stator.The revolving field produced by stator winding cuts the rotor conductors, thereby inducing voltages. Since the rotor winding is
10、 short-circuited by the end rings, the induced voltages cause currents to flow which in turn react with the field to produce electromagnetic torqueand so motor action results.Accordingly, on the basis of the foregoing description, it should be clear that for the three-phase induction motor the field
11、 winding is located on the stator and the armature winding on the rotor. Another point worth noting is that this machine is singly excited, i.e., electrical power is applied only to the stator winding. Current flows through the rotor winding by induction. As a consequence both the magnetizing curren
12、t, which sets up the magnetic field, and the power current which allows energy to be delivered to the shaft load, flow through the stator winding. For this reason, and in the interest of keeping the magnetizing current as small as possible in order that the power component may be correspondingly lar
13、ge for a given current rating, the air gap of induction motors is made as small as mechanical clearance will allow. The air-gap lengths vary form about 0.02in for smaller machines to 0.05in. for machines of higher rating and speed.2.Analog Sensors for Motion MeasurementMeasurement of plant output, i
14、s essential for feedback control. Output measurement are also useful in performance evaluation of a process. Furthermore, in learning systems （ e.g. teach-repeat operation of robotic manipulators ）, measurements are made stored in the computer for subsequent use in operating the system,. Input measu
15、rements are needed in feed forward control. It is evident, therefore, that the measurement subsystem is an important part of a control system.The measurement subsystem in a control system contains sensors and transducers that detect measurand and convert them into acceptable signals-typically, volta
16、ges. These voltage signals are then appropriately modified using signal-conditioning hardware such as filters, amplifiers, demodulators, and analog-to-digital converters. Impedance matching might be necessary to connect sensors and transducers to signal-conditioning hardware.Accuracy of sensors, tra
17、nsducers, and associated signal-conditioning devices is important in control system applications for two main reasons. The measurement system in a feedback control system is situated in the feedback path of the control system. Even though measurements are used to compensate for the poor performance
18、in the open-loop system, any errors in measurements themselves will enter directly into the system and cannot be corrected if they are unknown. Furthermore, it can be shown that sensitivity of a control system to parameter changes in the measurement system is direct. This sensitivity cannot be reduc
19、ed by increasing the loop gain, unlike in the case of sensitivity to the open-loop components. Accordingly, the design strategy for closed-loop（feedback） control is to make the measurements very accurate and to employ a suitable controller to reduce other types of errors.Most sensor-transducer devic
20、es used in feedback control applications are analog components that generate analog output signals. This is the case even in real-time direct digital control systems. When analog transducers are used in digital control applications, however, some type of analog-to-digital conversion （ADC） is needed
21、to obtain a digital representation of the measured signal. The resulting digital signal is subsequently conditioned and processed using digital means.In the sensor stage, the signal being measured is felt as the “response of the sensor element.” This is converted by the transducer into the transmitt
22、ed （ or measured ） quantity. In this respect, the output of a measuring device can be interpreted as the “ response of the transducer.” In control system applications, this output is typically （ and preferably ） an electrical signal. Note that it is somewhat redundant to consider electrical-to-elect
23、rical sensors-transducers as measuring devices, particularly in control system studies, because electrical signals need conditioning only before they are fed into a controller or to a drive system. In this sense, electrical-to-electrical transduction should be considered a “conditing” task rather th
24、an a “measuring” function.By motion, we mean the four kinematic variables:Displacement （ including position, distance, proximity, and size or gage）Velocity AccelerationJerkNote that each variable is the time derivative of the preceding one. Motion measurement are extremely useful in controlling mech
25、anical responses and interactions in dynamic systems. Numerous examples can be cited of situations in which motion measurements are used for control purposes. The rotating speed of a work piece and the feed rate of a tool are measured in controlling machining operations. Displacements and speeds （ b
26、oth angular and translatory ） at joints （ revolute and prismatic ） of robotic manipulators or kinematic linkages are used in controlling manipulator trajectory. In high-speed ground transit vehicles, acceleration and jerk measurements can be used for active suspension control to obtain improved ride
27、 quality. Angular speed is a crucial measurement that is used in the control of rotating machinery, such as turbines, pumps, compressors, motors, and generators in power-generating plants. Proximity sensors （ to measure displacement ） and accelerometes （ to measure acceleration ） are the two most co
28、mmon types of measuring devices used in machine protection systems for condition monitoring, fault detection, diagnosis, and on-line （ often real-time ） control of large and complex machinery.3. Applications of General-Purpose InvertersThe applications for inverters and their associated AC induction
29、 drive motors fall generally into three basic categories: 1） in place of or as retrofits for DC motor drives,2）instead of or as retrofits for mechanical speed changers, and 3）to regulate or control fluid flow（e.g., air or liquids）by speed instead of restrictive devices such as vanes, dampers, and mo
30、dulating valves. Each of these is discussed in the following sections.Many existing DC drives are being used because of historic preference and performance. Users were （and are） willing to put up with the space, cost and, maintenance in a DC drive because it was, so to speak, the only game in town .
31、The great majority of stand-alone single-motor DC drive applications can be supplied or replaced by an inverter and an AC induction motor.This is not to say, however, that all DC systems are replaceable with general-purpose inverters. We will discuss some of the pitfalls that can be encountered when
32、 DC drives are indiscriminately replaced by inverters and AC motors.Which DC drives are candidates for replacement by general-purpose invert? Typical examples would be those applications with performance requirements that were available only from DC drives, now available with general-purpose inverters, such as :（1） Operation of several motors and inverters from a master reference, with individually adjustable vernier speeds among the motors.（2） Individual start-stop control of each motor.（3） Independently adjustable acceleration and decel
