外文翻译-直流电动机的计算.doc

1、外文文献1：DC Motor CalculationsOverviewNow that we have a good understanding of dc generators, we can begin our study of dc motors. Direct-current motors transform electrical energy into mechanical energy. They drive devices such as hoists, fans, pumps, calendars, punch-presses, and cars. These devices

2、may have a definite torque-speed characteristic (such as a pump or fan) or a highly variable one (such as a hoist or automobile). The torque-speed characteristic of the motor must be adapted to the type of the load it has to drive, and this requirement has given rise to three basic types of motors:

3、1. Shunt motors 2. Series motors 3. Compound motors Direct-current motors are seldom used in ordinary industrial applications because all electric utility systems furnish alternating current. However, for special applications such as in steel mills, mines, and electric trains, it is sometimes advant

4、ageous to transform the alternating current into direct current in order to use dc motors. The reason is that the torque-speed characteristics of dc motors can be varied over a wide range while retaining high efficiency. Today, this general statement can be challenged because the availability of sop

5、histicated electronic drives has made it possible to use alternating current motors for variable speed applications. Nevertheless, there are millions of dc motors still in service and thousands more are being produced every year.Counter-electromotive force (cemf)Direct-current motors are built the s

6、ame way as generators are; consequently, a dc machine can operate either as a motor or as a generator. To illustrate, consider a dc generator in which the armature, initially at rest, is connected to a dc source Es by means of a switch (Fig. 5.1). The armature has a resistance R, and the magnetic fi

7、eld is created by a set of permanent magnets. As soon as the switch is closed, a large current flows in the armature because its resistance is very low. The individual armature conductors are immediately subjected to a force because they are immersed in the magnetic field created by the permanent ma

8、gnets. These forces add up to produce a powerful torque, causing the armature to rotate.Figure 5.1 Starting a dc motor across the line.On the other hand, as soon as the armature begins to turn, a second phenomenon takes place: the generator effect. We know that a voltage Eo is induced in the armatur

9、e conductors as soon as they cut a magnetic field (Fig. 5.2). This is always true, no matter what causes the rotation. The value and polarity of the induced voltage are the same as those obtained when the machine operates as a generator. The induced voltage Eo is therefore proportional to the speed

10、of rotation n of the motor and to the flux F per pole, as previously given by Eq. 5.1:Eo = ZnF/60 (5.1)As in the case of a generator, Z is a constant that depends upon the number of turns on the armature and the type of winding. For lap windings Z is equal to the number of armature conductors. In th

11、e case of a motor, the induced voltage Eo is called counter-electromotive force (cemf) because its polarity always acts against the source voltage Es. It acts against the voltage in the sense that the net voltage acting in the series circuit of Fig. 5.2 is equal to (Es - Eo) volts and not (Es + Eo)

12、volts.Figure 5.2 Counter-electromotive force (cemf) in a dc motor. Acceleration of the motorThe net voltage acting in the armature circuit in Fig. 5.2 is (Es - Eo) volts. The resulting armature current /is limited only by the armature resistance R, and soI = (Es - Eo)IR (5.2)When the motor is at res

13、t, the induced voltage Eo = 0, and so the starting current isI = Es/RThe starting current may be 20 to 30 times greater than the nominal full-load current of the motor. In practice, this would cause the fuses to blow or the circuit-breakers to trip. However, if they are absent, the large forces acti

14、ng on the armature conductors produce a powerful starting torque and a consequent rapid acceleration of the armature. As the speed increases, the counter-emf Eo increases, with the result that the value of (Es Eo) diminishes. It follows from Eq. 5.1 that the armature current / drops progressively as

15、 the speed increases. Although the armature current decreases, the motor continues to accelerate until it reaches a definite, maximum speed. At no-load this speed produces a counter-emf Eo slightly less than the source voltage Es. In effect, if Eo were equal to Es the net voltage (Es Eo) would becom

16、e zero and so, too, would the current /. The driving forces would cease to act on the armature conductors, and the mechanical drag imposed by the fan and the bearings would immediately cause the motor to slow down. As the speed decreases the net voltage (Es Eo) increases and so does the current /. The speed will cease to fall as soon as the torque developed by the armature current is equal to the load torque. Thus, when a motor run