1、Add Your Company Slogan(电气工程与自动化专业英语电气工程与自动化专业英语)第第6 6章章DC Motor DrivesDC Motor Drives DC Motor Drives DC Motor Drives2 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 yards
2、tick by which other drives are judged.The first and major part of this chapter is devoted to thyristor-fed(可控硅供电可控硅供电)drives,after which we will look briefly at control arrangements for DC drives.Text A Thyristor DC Drivers3 For motors up to a few kilowatts the armature converter can be supplied fro
3、m either single-phase or three-phase mains,but for larger motors three-phase is always used.A separate thyristor or diode rectifier(二极二极管整流器管整流器)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 T
4、hyristor DC Drivers4 The main power circuit consists of a six-thyristor bridge circuit(桥式电桥式电路路),which rectifies the incoming a.c.supply to produce a d.c.supply to the motor armature.The assembly of thyristors,mounted on a heat sink(散热器散热器),is usually referred to as the stack.By altering the firing
5、angle of the thyristors the mean value of the rectified voltage can be varied,thereby allowing the motor speed to be controlled.Text A Thyristor DC Drivers5 The main power circuit consists of a six-thyristor bridge circuit(桥式电桥式电路路),which rectifies the incoming a.c.supply to produce a d.c.supply to
6、the motor armature.The assembly of thyristors,mounted on a heat sink(散热器散热器),is usually referred to as the stack.By altering the firing angle of the thyristors the mean value of the rectified voltage can be varied,thereby allowing the motor speed to be controlled.Text A Thyristor DC Drivers6 Low pow
7、er control circuits are used to monitor the principal variables(主变量主变量)of interest(usually motor current and speed),and to generate appropriate firing pulses so that the motor maintains constant speed despite variations in the load.The speed reference(Figure 6.1)is typically an analogue voltage vary
8、ing from 0 to 10 V,and obtained from a manual speed-setting potentiometer or from elsewhere in the plant.Text A Thyristor DC Drivers7 The combination of power,control,and protective circuits constitutes the converter.Standard modular converters are available as off-the-shelf items in sizes from 0.5
9、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.,or groups of converters may be mounted together to form a multi-motor drive.Text A Thyristor DC Drivers1 82Outline3 45 9 The basic
10、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 stretch of imagination could the waveforms of armature voltage be thought of as good d.c.,and it would not be un
11、reasonable to question the wisdom of feeding such an unpleasant looking waveform to a d.c.motor.10 Firstly,the armature 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 th
12、an might have been feared.Basic Introduction In fact it turns out that the motor works almost as well as it would if 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.Text A Thyristor DC
13、Drivers1 112Outline3 45 12 1For the sake of simplicity we will look at operation from a single-phase(2-pulse)converter,but similar conclusions apply 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 inc
14、oming mains voltage,the precise shape and average value(平均值平均值)depending on the firing angle(触发角触发角).armature current13 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
15、 be altered by varying the firing angle,which also incidentally alters the ripple(i.e.Vac).armature current14 2The ripple voltage(纹波电压纹波电压)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 p
16、resents a high impedance to a.c.voltages.This smoothing effect of the armature inductance is shown in Figure 6.2(b),Armature voltage15 2It can be seen that the current ripple is relatively small in comparison with the corresponding voltage ripple.The average value of the ripple current is of course
17、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 voltage16 3The current at the end of each pulse
18、 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.(assumed pure d.c.because we are ignoring speed fluctuations)and the average voltage across the armature r
19、esistance,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.17 3The smoothing effect of the armature(电枢电枢)inductance is important in achieving successful motor operation:the armature acts as a
20、low-pass filter,blocking most of the ripple,and leading to a more or less constant armature current.Basic IntroductionFor the smoothing 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-
21、pulse drive).This condition is met in all 6-pulse drives,and in many 2-pulse ones.18 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
22、by the load.Basic IntroductionWe should note that the current ripple（纹波）（纹波）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 circui
23、ts and thereby regulate the speed of the drive.19 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 bec
24、ause it is 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.Text A Thyristor DC Drivers1 202Outline3 45 21 1We can see from Figure 6.2 that as the load torque is reduced,there will com
25、e a point where the minima of the current ripple 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.th
26、e less the ripple).Basic Introduction22 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.23 2Typical armature voltage and current waveforms in the di
27、scontinuous 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 for the period.24 2The shape of the current waveform can be understood by noting that with resistance neglected,equat
28、ion can be rearranged as:which shows that the rate of change of current(the gradient of the lower graph in Figure 6.3)is determined by the instantaneous difference between the applied voltage V and the motional E.25 2Values of(V-E)are shown by the vertical hatchings in Figure 6.3,from which it can b
29、e seen that if V E,the current is increasing,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.26 2It should be clear by comparing these figures that the armature voltage waveforms(solid lines)differ because,in F
30、igure 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)(动生电动势动生电动势).27 2 To simplify Figure 6.3 it has been assumed that the armature resistance is small
31、and that the corresponding volt-drop(IaRa)(电压降电压降)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 voltage across the armature inductance(电枢电感电枢电感)when there is no net change in the current over one pulse:the hatched ar
32、eas representing the volt-seconds in the inductor are therefore equal.28 2The most important difference between 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
33、 6.2 despite both having the same firing angle.29 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 aver
34、age current can increase is 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
35、speed.30 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 voltage.31 3 It is easy to see that inherent torquespee
36、d 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,its speed will settle at point A,its averagetorquespeed curves32 3 It is easy to see that inherent torquespeed curves with sudden dis
37、continuities 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,its speed will settle at point A,its averagetorquespeed curves33 3 Armature voltage and current having their full(rated)values.As the load is reduced,current
38、 remaining continuous,there is the expected slight 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,especia
39、lly if the load is reduced to zero when the speed reaches point C.torquespeed curves34 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 likelihood o
40、f 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 curves35 3When the converter and motor are incorporated in
41、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.torquespeed curves Text A Thyristor DC Drivers1 362Outline3 45 37We have tre
42、ated 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 which depends on the current being drawn by
43、 the motor.Perhaps surprisingly,the supply impedance(which is mainly due to inductive leakage reactance(漏电抗漏电抗)in transformers）manifests itself at the output stage of the converter as a supply resistance,so the supply volt-drop(or regulation)is directly proportional to the motor armature current.Bas
44、ic Introduction38Overlap（重叠）（重叠）: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 phenomenon known as overlap.2The consequence of overlap is that instead of the output voltage making an abrupt jump at
45、the start of each pulse,there is a short period when two thyristors are conducting simultaneously（同时地）（同时地）.Overlap39During 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 o
46、verlap is to be expected,as otherwise they may be alarmed the first time they connect an oscilloscope to the motor terminals.2Overlap402When 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.
47、5 would be barely visible on an oscilloscopeOverlap 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.412Returning
48、 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 motor torquespeed curves for each value of
49、 a have a somewhat steeper droop than they would if the supply impedance was zero.Overlap Text A Thyristor DC Drivers1 422Outline3 45 431Machine 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 complete torquespeed p
50、lane.441 Machine is inherently a bidirectional(双向的双向的)energy converter.If we apply a positive voltage V greater than E,a current flows into the armature and the 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 machi
