1、英文翻译 第 1 页 共 8 页 Construction Features of Electrical Machines The energy-conversion process usually involves the presence of two important features in a given electromechanical device. These are the field winding, which produces the flux density, and the armature winding, in which the “working emf i
2、s induced. In this section the salient construction features of the principal types of electric machines are described to show the location of these windings, as well as to demonstrate the general composition of such machines. 1.Three-Phase Induction Motor This is one of the most rugged and most wid
3、ely 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 120 electrical degrees apart. Coil aa represe
4、nts 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 be Y-connected. Such a winding is called a
5、 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 of slotted ferromagnetic material, but the
6、 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 connected to the rotor winding for the purpose of
7、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 merely of a number of copper 英文翻译 第 2 页 共
8、8 页 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 no slip rings or carbon b
9、rushes 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 frequency of the magnetizing
10、 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 four-pole field distrib
11、ution 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 short-circuited by the
12、 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 winding is located on
13、 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 current, which sets up the m
14、agnetic 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 large for a given current
15、 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. 英文翻译 第 3 页 共 8 页 2.Synchronous Machines The essential construction features of the synchro
16、nous machine are depicted in Fig.5.3. The stator consists of a stator frame, a slotted stator core, which provides a low-reluctance path for the magnetic flux, and a three-phase winding imbedded in the slots. Note that the basic two-pole pattern of Fig.5.2 (a) is repeated twice, indicating that the
17、three-phase winding is designed for four poles. The rotor either is cylindrical and equipped with a distributed winding or else has salient poles with a coil wound on each leg as depicted in FIg.5.3.The cylindrical construction is used exclusively for turbo-generators, which operate at high speeds.
18、On the other hand the salient-pole construction is used exclusively for synchronous motors operating at speeds of 1800 r.min-1 or less. When operated as a generator the synchronous machine receives mechanical energy from a prime mover such as a steam turbine and is driven at some fixed speed. Also,
19、the rotor winding is energized from a DC source, thereby furnishing a field distribution along the air gap. When the rotor is at standstill and DC flows through the rotor winding, no voltage is induced in the stator winding because the flux is not cutting the stator coils. However, when the rotor is
20、 being driven at full speed,voltage is induced in the stator winding and upon application of a suitable load electrical energy may be delivered to it. For the synchronous machine the field winding is located on the rotor； the armature windings is located on the stator. This statement is valid even w
21、hen the synchronous machine operates as a motor. In this mode AC power is applied to the stator winding and DC power is applied to the rotor winding for the purpose of energizing the field poles. Mechanical energy is then taken from the shaft. Note, too, that unlike the induction motor, the synchron
22、ous motor is a doubly excited machine； i.e., energy is applied to the rotor as well as the stator winding. In fact it is this characteristic which enables this machine to develop a nonzero torque at only one speed hence the name synchronous. Because the magnetizing current for the synchronous machine originates from a
