(電氣工程與自動(dòng)化專業(yè)英語)第6章DC-Motor-Drives課件

1、 DC Motor Drives DC Motor Drives DC Motor Drives 2 Study of the d.c. drive is valuable for several reasons: the structure and operation of the d.c. drive are reflected in almost all other drives; the d.c. drive tends to remain the yardstick by which other drives are judged. The first and major part

2、of this chapter is devoted to thyristor-fed(可控硅供電) drives, after which we will look briefly at control arrangements for DC drives. DC Motor Drives 2 Study of Text A Thyristor DC Drivers 3 For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase

3、mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier(二極管整流器) is used to supply the field of the motor: the power is much less than the armature power, so the supply is often single-phase, as shown in Figure 6. 1. Text A Thyristor DC Drivers(電氣工程與自動(dòng)化專業(yè)英語)第6

4、章DC-Motor-Drives課件(電氣工程與自動(dòng)化專業(yè)英語)第6章DC-Motor-Drives課件 Text A Thyristor DC Drivers 6 Low power control circuits are used to monitor the principal variables(主變量) of interest (usually motor current and speed), and to generate appropriate firing pulses so that the motor maintains constant speed despite v

5、ariations in the load. The speed reference (Figure 6. 1) is typically an analogue voltage varying from 0 to 10 V, and obtained from a manual speed-setting potentiometer or from elsewhere in the plant. Text A Thyristor DC Drivers Text A Thyristor DC Drivers 7 The combination of power, control, and pr

6、otective circuits constitutes the converter. Standard modular converters are available as off-the-shelf items in sizes from 0.5 kW up to several hundred kW, while larger drives will be tailored to individual requirements. Individual converters may be mounted in enclosures with isolators, fuses etc.,

7、 or groups of converters may be mounted together to form a multi-motor drive. Text A Thyristor DC Drivers 6.1 Motor operation with converter supplyText A Thyristor DC Drivers1 6.2 Motor current waveforms 82Outline3 6.3 Discontinuous current 6.4 Converter output impedance: overlap45 6.5 Four-quadrant

8、 operation and inversion 6.1 Motor operation with con6.1 Motor operation with converter supply9 The basic operation of the rectifying bridge has been discussed, and we now turn to the matter of how the d.c. motor behaves when supplied with d.c. from a controlled rectifier.Basic IntroductionBy no str

9、etch of imagination could the waveforms of armature voltage be thought of as good d.c., and it would not be unreasonable to question the wisdom of feeding such an unpleasant looking waveform to a d.c. motor.6.1 Motor operation with conve6.1 Motor operation with converter supply10 Firstly, the armatu

10、re inductance of the motor causes the waveform of armature current to be much smoother than the waveform of armature voltage, which in turn means that the torque ripple is much less than might have been feared.Basic Introduction In fact it turns out that the motor works almost as well as it would if

11、 fed with pure d.c., for two main reasons. Secondly, the inertia of the armature is sufficiently large for the speed to remain almost steady despite the torque ripple. 6.1 Motor operation with conve 6.1 Motor operation with converter supplyText A Thyristor DC Drivers1 6.2 Motor current waveforms 112

12、Outline3 6.3 Discontinuous current 6.4 Converter output impedance: overlap45 6.5 Four-quadrant operation and inversion 6.1 Motor operation with con6.2 Motor current waveforms12 1For the sake of simplicity we will look at operation from a single-phase(2-pulse) converter, but similar conclusions apply

13、 to the 6-pulse converter. The voltage (Va) applied to the motor armature is typically as shown in Figure 6. 2(a): it consists of rectified chunks of the incoming mains voltage, the precise shape and average value (平均值)depending on the firing angle(觸發(fā)角).armature current6.2 Motor current waveforms12

14、6.2 Motor current waveforms13 1The voltage waveform can be considered to consist of a mean d.c. level(Vdc), and a superimposed pulsating or ripple component which we can denote loosely as Vac.The mean voltage can be altered by varying the firing angle, which also incidentally alters the ripple (i.e.

15、 Vac).armature current6.2 Motor current waveforms13 6.2 Motor current waveforms14 2The ripple voltage (紋波電壓)causes a ripple current to flow in the armature, but because of the armature inductance, the amplitude of the ripple current is small. In other words, the armature presents a high impedance to

16、 a.c. voltages. This smoothing effect of the armature inductance is shown in Figure 6.2(b),Armature voltage6.2 Motor current waveforms14 6.2 Motor current waveforms15 2It can be seen that the current ripple is relatively small in comparison with the corresponding voltage ripple. The average value of

17、 the ripple current is of course zero, so it has no effect on the average torque of the motor. There is nevertheless a variation in torque every half-cycle of the mains, but because it is of small amplitude and high frequency the variation in speed ) will not usually be noticeable.Armature voltage6.

18、2 Motor current waveforms15 6.2 Motor current waveforms16 3The current at the end of each pulse is the same as at the beginning, so it follows that the average voltage across the armature inductance (L) is zero. We can therefore equate the average applied voltage to the sum of the back e.m.f. (assum

19、ed pure d.c. because we are ignoring speed fluctuations)and the average voltage across the armature resistance, to yield:Basic IntroductionIt underlines the fact that we can control the mean motor voltage, and hence the speed, simply by varying the converter delay angle.6.2 Motor current waveforms16

20、 6.2 Motor current waveforms17 3The smoothing effect of the armature(電樞) inductance is important in achieving successful motor operation: the armature acts as a low-pass filter, blocking most of the ripple, and leading to a more or less constant armature current. Basic IntroductionFor the smoothing

21、to be effective, the armature time-constant needs to be long compared with the pulse duration (half a cycle with a 2-pulse drive, but only one sixth of a cycle in a 6-pulse drive). This condition is met in all 6-pulse drives, and in many 2-pulse ones. 6.2 Motor current waveforms17 6.2 Motor current

22、waveforms18 3The no-load speed is determined by the applied voltage (which depends on the firing angle of the converter); there is a small drop in speed with load and as we have previously noted, the average current is determined by the load.Basic IntroductionWe should note that the current ripple（紋

23、波） remains the same only the average current changes with load. The speed is determined by the converter firing angle, which represents a very satisfactory state because we can control the firing angle by low-power control circuits and thereby regulate the speed of the drive.6.2 Motor current wavefo

24、rms18 6.2 Motor current waveforms19 3Basic IntroductionThe current waveforms in Figure 6.2(b) are referred to as continuous, because there is never any time during which the current is not flowing. This continuous current condition is the norm in most drives, and it is highly desirable because it is

25、 only under continuous current conditions that the average voltage from the converter is determined solely by the firing angle, and is independent of the load current. 6.2 Motor current waveforms19 6.1 Motor operation with converter supplyText A Thyristor DC Drivers1 6.2 Motor current waveforms 202O

26、utline3 6.3 Discontinuous current 6.4 Converter output impedance: overlap45 6.5 Four-quadrant operation and inversion 6.1 Motor operation with con6.3 Discontinuous current21 1We can see from Figure 6. 2 that as the load torque is reduced, there will come a point where the minima of the current rippl

27、e touches the zero-current line, i.e. the current reaches the boundary between continuous and discontinuous current. The load at which this occurs will also depend on the armature inductance, because the higher the inductance the smoother the current (i.e. the less the ripple). Basic Introduction6.3

28、 Discontinuous current21 16.3 Discontinuous current22 2Discontinuous current mode is therefore most likely to be encountered in small machines with low inductance (particularly when fed from two-pulse converters) and under light-load or no-load conditions.Discontinuous current6.3 Discontinuous curre

29、nt22 26.3 Discontinuous current23 2Discontinuous currentTypical armature voltage and current waveforms in the discontinuous mode are shown in Figure 6. 3, the armature current consisting of discrete pulses of current that occur only while the armature is connected to the supply, with zero current fo

30、r the period . 6.3 Discontinuous current23 26.3 Discontinuous current24 2The shape of the current waveform can be understood by noting that with resistance neglected, equation can be rearranged as :which shows that the rate of change of current ( the gradient of the lower graph in Figure 6. 3) is de

31、termined by the instantaneous difference between the applied voltage V and the motional E. Discontinuous current6.3 Discontinuous current24 26.3 Discontinuous current25 2Values of(V- E) are shown by the vertical hatchings in Figure 6. 3, from which it can be seen that if V E, the current is increasi

32、ng, while if V E, the current is falling. The peak current is thus determined by the area of the upper or lower shaded areas of the upper graph.Discontinuous current6.3 Discontinuous current25 26.3 Discontinuous current26 2It should be clear by comparing these figures that the armature voltage wavef

33、orms (solid lines) differ because, in Figure 6. 3, the current falls to zero before the next firing pulse arrives and during the period shown as u the motor floats free, its terminal voltage during this time being simply the motional e.m.f. (E) (動(dòng)生電動(dòng)勢(shì)).Discontinuous current6.3 Discontinuous current2

34、6 26.3 Discontinuous current27 2 To simplify Figure 6. 3 it has been assumed that the armature resistance is small and that the corresponding volt-drop(IaRa) (電壓降) can be ignored. In this case, the average armature voltage (Vdc) must be equal to the motional e.m.f., because there can be no average v

35、oltage across the armature inductance(電樞電感) when there is no net change in the current over one pulse: the hatched areas representing the volt-seconds in the inductor are therefore equal.Discontinuous current6.3 Discontinuous current27 26.3 Discontinuous current28 2The most important difference betw

36、een Figure 6.2 and Figure 6.3 is that the average voltage is higher when the current is discontinuous,and hence the speed corresponding to the conditions in Figure 6. 3 is higher than in Figure 6.2 despite both having the same firing angle.Discontinuous current6.3 Discontinuous current28 26.3 Discon

37、tinuous current29 2And whereas in continuous mode a load increase can be met by an increased armature current without affecting the voltage (and hence speed), the situation is very different when the current is discontinuous. In the latter case, the only way that the average current can increase is

38、when speed (and hence E)falls so that the shaded areas in Figure 6. 3 become larger.From the users viewpoint the behavior of the motor in discontinuous mode is much worse than in the continuous current mode, because as the load torque is increased, there is a serious drop in speed.Discontinuous curr

39、ent6.3 Discontinuous current29 26.3 Discontinuous current30 2Under very light or no-load conditions, the pulses of current become virtually non-existent, the shaded areas in Figure 6. 3 become very small and the motor speed reaches a point at which the back e.m.f. is equal to the peak of the supply

40、voltage.Discontinuous current6.3 Discontinuous current30 26.3 Discontinuous current31 3 It is easy to see that inherent torquespeed curves with sudden discontinuities of the form shown in Figure 6. 4 are very undesirable. If for example the firing angle is set to zero and the motor is fully loaded,

41、its speed will settle at point A, its averagetorquespeed curves6.3 Discontinuous current31 36.3 Discontinuous current32 3 It is easy to see that inherent torquespeed curves with sudden discontinuities of the form shown in Figure 6. 4 are very undesirable. If for example the firing angle is set to ze

42、ro and the motor is fully loaded, its speed will settle at point A, its averagetorquespeed curves6.3 Discontinuous current32 36.3 Discontinuous current33 3 Armature voltage and current having their full (rated) values. As the load is reduced, current remaining continuous, there is the expected sligh

43、t rise in speed, until point B is reached. This is the point at which the current is about to enter the discontinuous phase. Any further reduction in the load torque then produces a wholly disproportionate not to say frightening increase in speed, especially if the load is reduced to zero when the s

44、peed reaches point C.torquespeed curves6.3 Discontinuous current33 36.3 Discontinuous current34 3 There are two ways by which we can improve these inherently poor characteristics. Firstly, we can add extra inductance in series with the armature to further smooth the current waveform and lessen the l

45、ikelihood of discontinuous current. The effect of adding inductance is shown by the dotted lines in Figure 6. 4. Secondly, we can switch from a single-phase converter to a 3-phase converter which produces smoother voltage and current waveforms.torquespeed curves6.3 Discontinuous current34 36.3 Disco

46、ntinuous current35 3When the converter and motor are incorporated in a closed-loop control the user should be unaware of any shortcomings in the inherent motor/converter characteristics because the control system automatic ally alters the firing angle to achieve the target speed at all loads.torques

47、peed curves6.3 Discontinuous current35 3 6.1 Motor operation with converter supplyText A Thyristor DC Drivers1 6.2 Motor current waveforms 362Outline3 6.3 Discontinuous current 6.4 Converter output impedance: overlap45 6.5 Four-quadrant operation and inversion 6.1 Motor operation with con6.4 Convert

48、er output impedance: overlap37 We have treated the converter as an ideal voltage source.The output voltage from the converter as independent of the current drawn by the motor, and depended only on the delay angle1In practice the supply has a finite impedance, and we must therefore expect a volt-drop

49、 which depends on the current being drawn by the motor. Perhaps surprisingly, the supply impedance(which is mainly due to inductive leakage reactance(漏電抗) in transformers）manifests itself at the output stage of the converter as a supply resistance, so the supply volt-drop (or regulation) is directly

50、 proportional to the motor armature current.Basic Introduction6.4 Converter output impedance6.4 Converter output impedance: overlap38 Overlap（重疊） :we should note that the effect of the inductive reactance of the supply is to delay the transfer (or commutation) of the current between thyristors; a ph

51、enomenon known as overlap.2The consequence of overlap is that instead of the output voltage making an abrupt jump at the start of each pulse, there is a short period when two thyristors are conducting simultaneously（同時(shí)地）.Overlap6.4 Converter output impedance6.4 Converter output impedance: overlap39

52、During this interval the output voltage is the mean of the voltages of the incoming and outgoing voltages, as shown typically in Figure 6. 5. It is important for users to be aware that overlap is to be expected, as otherwise they may be alarmed the first time they connect an oscilloscope to the moto

53、r terminals. 2Overlap6.4 Converter output impedance6.4 Converter output impedance: overlap40 2When the drive is connected to a stiff (i.e. low impedance) industrial supply the overlap will only last for perhaps a few microseconds, so the notch shown in Figure 6. 5 would be barely visible on an oscil

54、loscopeOverlap Figure 6. 5: with a 50 or 60 Hz supply, if the overlap lasts for more than say 1ms, the implication is that the supply system impedance is too high for the size of converter in question, or conversely, the converter is too big for the supply.6.4 Converter output impedance6.4 Converter

55、 output impedance: overlap41 2Returning to the practical consequences of supply impedance, we simply have to allow for the presence of an extra source resistance in series with the output voltage of the converter. This source resistance is in series with the motor armature resistance, and hence the

56、motor torquespeed curves for each value of a have a somewhat steeper droop than they would if the supply impedance was zero.Overlap6.4 Converter output impedance 6.1 Motor operation with converter supplyText A Thyristor DC Drivers1 6.2 Motor current waveforms 422Outline3 6.3 Discontinuous current 6.

57、4 Converter output impedance: overlap45 6.5 Four-quadrant operation and inversion 6.1 Motor operation with con6.5 Four-quadrant operation and inversion43 1Machine running in the positive direction and acting as a motor. This is known as one-quadrant operation, by reference to quadrant （象限）1 of the c

58、omplete torquespeed plane.Four-quadrant operation 6.5 Four-quadrant operation an6.5 Four-quadrant operation and inversion44 1 Machine is inherently a bidirectional(雙向的) energy converter.Four-quadrant operation If we apply a positive voltage V greater than E, a current flows into the armature and the

59、 machine runs as a motor. If we reduce V so that it is less than E, the current, torque and power automatically reverse direction, and the machine acts as a generator, converting mechanical energy (its own kinetic energy in the case of regenerative braking) into electrical energy. 6.5 Four-quadrant

60、operation an6.5 Four-quadrant operation and inversion45 1Four-quadrant operation And if we want to motor or generate with the reverse direction of rotation, all we have to do is to reverse the polarity of the armature supply. The d.c. machine is inherently a four-quadrant device, but needs a supply