WO2006056235A1

WO2006056235A1 - Overshoot filter

Info

Publication number: WO2006056235A1
Application number: PCT/EP2004/053087
Authority: WO
Inventors: Mika Sippola
Original assignee: Schaffner Emv Ag
Priority date: 2004-11-24
Filing date: 2004-11-24
Publication date: 2006-06-01
Also published as: DE112004002990T5; DE112004002990B4

Abstract

Filter (6) for avoiding voltage overshoots on a transmission line (73) between a motor drive (70) and a motor (71) or between a motor drive (70) and an electrical distribution network, said filter (6) comprising: an input (60), an output (61), at least one inductive component (64) connected between the input (60) and the output (61) and at least one parallel capacitor (66) connected between the input (60) and the output (61) in parallel to the inductive component (64), such that, when the voltage at the input (60) quickly switches from a first voltage value to a second voltage value, the voltage at the output (61) quickly switches in a first step to an intermediate value (12) comprised between the first voltage value and the second voltage value and then more slowly reaches in a second step an asymptotic voltage value. As the output of the filter (6) of the invention switches in two steps from the first voltage value to the second voltage value, the maximal voltage overshoot at the motor end of the transmission line (73) is significantly reduced. According to the invention, this two-level output scheme is achieved with a simple and relatively inexpensive filter (6).

Description

Overshoot filter

The present invention relatesto an overshoot filter for avoiding voltage overshoots due to transmission line effects and/or impedance discontinuities at the inputsof a motor connected with a cable to a motor drive and/or between a motor drive and an electrical distribution network.

Frequency converters such asfor example adjustable speed motor drives (ASD) are widely used today for controlling motors such asfor example induction or permanent magnet motors In most cases, pulse width modulation (PWM) is used for adjusting the desired motor operation: a DC voltage is chopped by power semiconductors such as Insulated Gate Bipolar Transistors (IGBT) in order to generate a seriesof pulsesof different widths, the average of which correspondsto the required voltage. In such a motor drive, the power semiconductors are thusused as ON/OFF-sw itches

In practice, these switches are not ideal: they have power losses during their ON-state and they cannot be switched from their OFFto their ON-state and back with an infinite speed, thusresulting into switching losses. In order to reduce the switching losses, the usual solution isto use faster switching power semiconductors. Today'sfast power semiconductors typically switch the drive's output voltage at a speed around 3000V/μs. Such a high switching speed results, in the typical case of a DC voltage of 560V, into switching times of 200nsor less between two discrete voltage values.

A typical waveform for the output voltage of an ASD is illustrated in Figure 1 representing a DC voltage pulse 11 delivered by a PWM-based motor drive. The horizontal axis representsthe time while the vertical axis representsthe voltage. The line 1 thus representsthe drive's output voltage over time. The drive'soutput voltage 1 typically switches between a low DC voltage value and a high DC volt age value, where the low DC voltage value isfor example zero voltsand the high DC voltage value is represented by the dotted line 10. In this example, the voltage switching speed, which can be expressed asthe value of the voltage derivative du/dt, is approximately 3000V/μs. Although high speed switching allowsthe reduction of switching losses, it has some drawbackswhen considering a complete installation comprising a motor drive, electrical cables and a motor.

Firstly, high speed voltage switching stressesthe motor and the electrical cables by driving capacitive currentsinto their insulationsand bearings. Capacitive currents may also contribute to the electromagnetic interference (EM I) of the installation, thusgenerating electromagnetic perturbations for the neighboring devices

Secondly, and sometimes more importantly, high speed switching can give rise to significant overvoltage oscillations at the motor end of the cable connecting the motor to the drive, particularly in the case of a long cable, which can lead to motor damages and/or to the breakdown of the cable's insulation.

The reason for these overvoltage oscillations are the so called transmission line effects: in a long cable, the voltage during power switching cannot be considered as constant over the entire cable. The voltage must be considered asa signal needing some time to travel from the cable'sinput to the cable's output, typically at the speed of 0.8^* light speed. Under such conditions, impedance discontinuities within the transmission line will cause voltage reflections leading to overvoltagesas they add to the originally transmitted waveform.

It can be approximated that, if the travel time of the voltage within the cable is longer than half of the voltage's rise time at the cable's input, the voltage at the motor end of the cable can oscillate up to twice the value of the voltage at the cable's input. For example, with a typical rise time of 200ns at the motor drive's output, the value of the overvoltage at the motor end of a 24 meters long cable is approximately twice the value of the DC-link voltage: 2* 560V = 1120V. This overvoltage problem is even more severe with a 690V mains network system where the DC voltage may be 1100V, potentially resulting into overvoltage oscillations of 2200V while the maximum permissible motor voltage in many applicationsistypically only 1500V.

Rgure 2 repreεentsfor example the voltage 2 measured at the input of a motor connected with a 200 meters long cable to an ASD having the output voltage illustrated in Rgure 1. As in figure 1 , the horizontal axis in figure 2 repressntsthe time while the vertical axisrepresentsthe voltage. In this example, the overvoltage oscillations21 due to the transmission line effectsaswell asto the impedance discontinuity between the cable and the motor, reach a maximum voltage 20, for example 1000V, which is approximately equal to twice the drive's high DC voltage value 10.

A solution to avoid voltage overshootsat the motor end would be to keep the switching speed du/dt of the drive's output voltage below a critical value depending on the characteristics of the entire system. From the approximation given above, it can be calculated for example that with a 200 meter long cable between the drive and the motor and with a high DC voltage value of 560 Volts, significant overvo It ages would already appear with a du/dt-value of 336V/μs. The switching speed of the motor drive's power semiconductorsshould thusbe lower than thisvalue in order to avoid overvoltagesduring system operation. Thissolution however has the drawback that significant switching losses are present. Moreover, setting the characteristics of the drive'selementsis not always possible and/or desired.

A solution to prevent and/or reduce voltage overshoots isto use a so-called du/dt -filter, usually placed between the drive'soutputsand the cable's input for reducing the voltage switching speed du/dt in the cable.

In itssimplest form, such a du/dt-filter consistsof one inductor, also known asa choke, connected between each of the drive'soutput phases and the corresponding cable phase. Capacitors are often added on the cable'sside, connecting the chokes' outputsto each other or to the ground or a neutral conductor, in order to adjust the du/dt-value and thus stabilize the operation of the filter for example with different cable lengths Such du/dt-filters effectively reduce the du/dt-value and at the same time reduce the currents responsible for electromagnetic interference.

A schematic of a typical prior art du/dt -filter is illustrated in figure 3. For the sake of simplicity, only one of the possibly two, three or more phases is shown. The drive isto be connected to the left side of the choke 30, while the cable will be connected to the right side of the choke 30, to the capacitor 31 and choke 30 connection point. The filter is therefore a LC-low passfilter.

Although such simple filters made only of chokes or of chokes and capacitorsare able, in most applications, to bring the du/dt-value into a suitable range in order to avoid voltage overshootsdue to transmission line effectsand/or to reflections, they have an important drawback: the chokesthemselves generate voltage overshoot at their output, which is then propagated up to the motor along the cable.

The voltage overshoot of the choke 30 can be explained in two different ways:

1) When the motor drive'soutput voltage quickly switches, the du/dt -filter'soutput voltage first remains unchanged and then changes relatively slowly towards a new drive output voltage value, thusslowing down the voltage switching speed. During thisswitching time, there is therefore a voltage difference acrossthe choke 30 and, according to basic circuit theory, a voltage across an inductor builds up a current to the choke according to the formula:

where I isthe current, U isthe voltage difference over the choke 30, t isthe time and L isthe inductance of the choke 30. On the other hand, when the filter'soutput voltage level finally reachesthe desired output voltage level, i.e. the drive's output voltage, considerable current hasbeen built up in the choke, in other words energy hasbeen stored in the inductor 30, which can be calculated with

E = Iu²,

where Eisthe energy, L isthe inductance of the choke 30 and I isthe current. The current in the inductor 30, i.e. the energy stored in it, cannot disappear instantaneously. The current thushasto continue flowing from the motor drive through the choke 30 to the filter'soutput capacitance 31 and/or to the cable's capacitance. Thiscurrent flow will then cause the voltage overshoot by charging the output capacitances When the output voltage of the filter reachesthe drive's output voltage, a voltage gradually builds up acrossthe choke 30 in the reverse direction to the direction of the voltage difference during the previoustime period. This voltage difference gradually reducesthe inductor'scurrent toward zero. The inductor 30 is thus" reset" .

2) The voltage overshoot can also be explained by the fact that the du/dt -filter is operated below the LC-filters' natural resonant f requency. In other words, the switching frequency of the drive is lower than the resonant frequency of the LC- filter:

F_res = ^πVLC.

An alternative solution to the problem would be to set the filter's resonant frequency well below the motor drive'sswitching frequency. In thiscase, the voltage waveformswould not have the time to go through all extremes, including the maximum voltage overshoot point. Thissolution is well known and called "sinus-filter" , because it also smoothensthe pulse width modulated voltage into a sin us waveform. However, for many applications a sinus-filter would be too expensive and bulky, and by filtering much more than a du/dt-filter it also negatively impactsthe system's dynamic response. The typical waveform of the output voltage of the du/dt-filter of figure 3 when connected to a drive having the output voltage of figure 1 is shown in figure 4. It can be seen that the du/dt -value has been reduced in that the voltage rise time is longer than that of figure 1 or figure 2, but the peak level of the voltage oscillations41 iscomparable with that of figure 2, due to the voltage overshoot of the choke 30 itself. These voltage oscillations 41 will propagate through the cable and create a similar voltage waveform at the motor's input.

In most cases, thisvoltage overshoot can be reduced by so called damping resistors An example of a prior art du/dt-filter using a damping resistor isillustrated in figure 5. ft>r the sake of simplicity, the schematics illustrate one phase only of the possibly two, three or more phases of the filter. In the prior art du/dt-filter of figure 5, the damping resistor 33 is connected in parallel with the choke 30. The voltage overshoot at the filter'soutput is reduced because part of the energy which would otherwise charge the cable and/or the capacitor 31 isdissipated by the resistor 33 and because the damping resistor 33 also conductsthe current from the choke'soutput back to the drive'soutput when the cable and/or the capacitor 31 discharge and the choke'soutput voltage rises higher than itsinput voltage. A typical waveform for the output voltage 5 of the filter of figure 5 is illustrated in figure 6 in the case where a series of pulsesis inputted into the filter. The output voltage haslow switching speed with low overvoltage oscillations51.

However, while the discharge current in the filter of figure 5 is conducted by the damping resistor 33, the energy isturned into losses which can typically reach several hundredsof watts when such a damped du/dt-filter is operated with a 200 meters long cable and with switching frequenciesaround 1OkHz. Thusalthough a damping resistor 33 in parallel with the choke 30 effectively attenuates voltage overshootsat the filter's output, the power dissipation, or losses, in the damping resistor 33 is excessive asthe cable and/or the capacitor 31 isactually charged and discharged through it. In another embodiment of prior art du/dt filters (not represented), the damping resistor is connected in serieswith the filter's choke. This solution however also usually implies excessive damping losses.

A further solution for preventing high voltage overshootsat the motor end of the cable isto use one or more intermediate switching steps in the motor drive between the low DC voltage value and the high DC voltage value. If the low DC voltage value iszero volts, for example, the drive'soutput voltage switches for example in a first step from zero voltsto half of the high DC voltage value and in a second step f rom this intermediate value to the high DC voltage value. In conditionswhere the voltage overshoot at the motor end of the cable isabout twice the high DC voltage value if the drive'soutput voltage switches in one step from zero voltsto the high DC voltage value, the voltage overshoot after the first switching step isthen reduced to about twice half of the high DC voltage value, thusto about the high DC voltage value. After the second step in which the drive's output voltage switches from the intermediate voltage value to the high DC voltage value, the voltage overshoot isalso approximately equal to the high DC voltage value, which, added to the intermediate value, bringsthe maximal overshoot voltage to one and a half timesthe maximal DC voltage, thus representing a voltage overshoot of 50%. Such a value is usually acceptable for most industrial applications, because the componentssuch asthe insulationsof the motor and of the cable are usually dimensoned to accept such peak values.

A major drawback of motor driveswith intermediate switching steps is that they imply a significantly more complex and thus more expensive construction than that of motor drives without intermediate switching steps In particular, a motor drive with only one intermediate step requires two separate inverters or a three-level inverter for delivering three different DC voltages: a low DC voltage, an intermediate DC voltage and a high DC voltage. Moreover, the power semiconductors for switching the drive'soutput voltage also need to be doubled. An aim of the present invention isto propose an overshoot filter and a method for reducing the voltage overshoots at one end of an electrical cable when the voltage at the other end of the cable very quickly switchesf rom a low DC voltage value to a high DC voltage value, without the drawbacks of prior art filters and methods.

Another aim of the present invention isto propose an overshoot filter and a method allowing for a significant reduction of the voltage overshoot at a low cost and with a low complexity.

These aims are achieved with a voltage overshoot filter having the characteristics of independent claim 1 , advantageous embodiments being given by the dependent claims.

These aims are achieved in particular with a filter for avoiding voltage overshootson a transmission line between a motor drive and a motor or between a motor drive and an electrical distribution network, said filter comprising: an input, an output, at least one inductive compo¬ nent connected between the input and the output and at least one parallel capacitor connected between the input and the output in parallel to the inductive component, such that, when the voltage at the input quickly switches from a first voltage value to a second voltage value, the voltage at the output quickly switches in a first step to an intermediate value compri¬ sed between said first voltage value and said second voltage value and then more slowly tendsin a second step to an asymptotic voltage value.

These aims are achieved in particular with a method for avoiding voltage overshootson a transmission line between a motor drive and a motor or between a motor drive and an electrical distribution network with a filter, the method comprising:

- quickly switching the voltage at an input of the filter from a first voltage value to a second voltage value,

- quickly switching in a first step the voltage at an output of the filter from the first voltage value to an intermediate value,

- more slowly reaching in a second step an asymptotic volt age value. Asthe output of the filter of the invention switchesin two steps from the first voltage value to the second voltage value, the maximal voltage overshoot at the motor end of the transmission line issignificantly reduced. According to the invention, thistwo-level output scheme is achieved with a simple and relatively inexpensive filter.

The present invention will be better understood by reading the description of a preferred embodiment, illustrated by the figures 1 to 8, where:

Previously discussed Figure 1 illustrates a typical ASD drive's output voltage waveform,

Previously discussed Figure 2 isthe measured voltage at the motor's input, using a long cable from the drive to the motor,

Previously discussed Figure 3 isa prior art du/dt-filter,

Previously discussed Figure 4 isthe typical non-damped output voltage of a prior art du/dt-filter,

Previously discussed Figure 5 isa prior art du/dt-filter with a damping resistor parallel to the choke,

Previously discussed Figure 6 showsthe output voltage's waveform of the prior art du/dt-filter of Figure 5,

Figure 7 isa schematic view of a system with an overshoot filter according to a preferred embodiment of the invention,

Figure 8 showsthe voltage's waveform at the output of the filter of Figure 7 and at the motor end of the cable of Figure 7.

A preferred embodiment of an overshoot filter 6 according to the invention is schematically illustrated in figure 7. For the sake of simplicity, only one of the possibly two, three or more phasesof the system isshown. The overshoot filter 6 comprises a choke 64, a damping resistor 65 and a parallel capacitor 66. The choke 64 and the damping resistor 65 are connected in series while the parallel capacitor 66 is connected in parallel to them. In an embodiment of the invention, the damping resistor 65 isa discrete component physically distinct f rom the choke 64. The damping resistor 65 however can also illustrate the electrical resistance of the non ideal choke 64 or the resulting resistance of the choke'sinternal resistance and an additional discrete damping resistor in series. According to an embodiment of the invention, the resistance of the damping resistor 65 is frequency-dependent, whereasthe resistance preferably increaseswith the frequency of the electrical signal. The high f requency voltage oscillations are then efficiently damped while lossesdue to the working current fed to the motor are minimized.

A grounding capacitor 72 illustrated in figure 7 representsthe capacitance of the cable and/or of an optional filter output capacitor. A filter output capacitor is sometimes used for example in order to achieve a desired resulting capacitance for the electrical transmission line. In thiscase, the grounding capacitor 72 representsthis resulting capacitance. When present, the optional filter output capacitor ispreferably part of the filter 6.

When used for avoiding voltage overshoots between a motor drive 70 and a motor 71 , the input 60 of the overshoot filter 6 isconnected to the output of the motor drive 70 and itsoutput 61 isconnected to the drive end of an electrical power cable 73 whose other end isconnected to the input of the motor 71.

The motor drive 70 isfor example a PWM motor drive comprising a standard two-level inverter. The output voltage of the motor drive 70 thus switches between a low DC-voltage value and a high DC-voltage value, whereasthe switching speed isfor example approximately 3000V/μs. The waveform of the output voltage of the motor drive 70 isthen for example a seriesof periodically initiated pulses similar to the pulse 11 of figure 1 , whereasthe width or duty cycle of the pulsesdeterminesthe rotation speed of the motor 71. For the sake of simplicity, the low DC-voltage value in the illustrated examples is zero volts DC.

In a preferred embodiment, the filter 6 is preferably embedded in a casing, for example a box, allowing itstransportation, handling and installation as a single electronic component aswell asits use in safe conditions. The box ispreferably a metallic box which isgrounded when the filter isin use. The filter'scomponents64, 65, 66 aswell asthe optional filter capacitor are then preferably attached within the box and electrically connected to each other according to the schematicsof Figure 7. Electrical connectors are for example provided through the wallsof the box in order to connect each phase of the filter to the corresponding phase of the drive 70 and of the cable 73. The connectors are thus electrically isolated from the grounded box and in electrical contact with the filter circuit's corresponding contact point inside the box. When in use, the filter isthen for example placed, preferably fastened, beside or on the box of the motor drive 70 or it isdirectly integrated within the box of the drive 70. In an alternative embodiment, the elements of the filtersare directly integrated within the motor drive, without being first placed in a separate box. The filter 6 schematically illustrated in Figure 7 isthen for example the output stage of the motor drive's circuit.

According to a preferred embodiment of the invention, the capacitance of parallel capacitor 66 isapproximately equal to the capacitance of grounding capacitor 72, i.e. to the resulting capacitance of the cable 73 and the optional filter output capacitor.

Figure 8 illustratesthe effect of the overshoot filter 6 of the invention and showsthe filter'soutput voltage 8 and the motor'sinput voltage 9 when the filter's input voltage switches from the low DC-voltage value to the high DC- volt age value 10. In the illustrated example, the filter'sinput voltage (not represented in figure 8), which is essentially equal to the drive's output voltage, switches very fast from the low DC-voltage value, for example zero volts, to the high DC-voltage value 10. In a first step, the high DC voltage applied at the filter'sinput 60 isalmost instantly applied acrossthe capadtors66 and 72, causing the voltage at the filter's output 61 to rise very fast, at a speed close to the switching speed of the drive's output voltage. The capacitance of the parallel capacitor 66 being preferably equal to the capacitance of the grounding capacitor 72, the capacitive voltage division causesthe voltage 8 at the filter'soutput 61 to rise in this first step to a first level 12 which is approximately half of the high DC-voltage value 10.

Because of the previously discussed transmission line effects and/or reflections, depending for example on the switching speed of the voltage at the filter'soutput 61 and on the characteristics of the cable 73 connecting the filter'soutput 61 to the motor 71 , the voltage 9 at the motor'sinput oscillates. In this first step, the voltage oscillationsat the motor'sinput rise for example up to around twice the value of the first level 12, i.e. approximately to the high DC-voltage value 10. In this example, the voltage oscillationsat the motor'sinput during thisfirst step thus rise up to the normal operating voltage 10 of the motor 71. The peak voltage of these oscillationsthusshould not be able to damage the motor 71 or the cable 73.

In a second step, the voltage 8 at the filter's output 61 charges more slowly to the high DC-voltage value 10 applied at the filter'sinput 60, plussome overshoot depending on the level of the damping, thus depending essentially on the value of the damping resistor 65. Because the voltage at the filter'soutput 61 during thischarging period hasa much slower rising speed du/dt, the transmission line effects arising in the cable 73 are significantly attenuated. The rising voltage at the filter'soutput 61 during the charging period thusdoes not cause consderable additional voltage overshoot at the motor end of the cable 73. The maximal voltage overshoot at the motor'sinput isthusapproximately equal to the voltage overshoot at the filter'soutput 61 which can be tuned, if necessary, by tuning the value of the damping resistor 65. By using the overshoot filter of the invention, the voltage spikes are thus considerably reduced at the motor end of the cable, making it possible to keep them within tolerable values preventing damage the motor, for example damage to its insulation.

In other words, the functioning of the overshoot filter of the invention can be summarized asfollows.

In a first step, when the voltage at the filter's input 60 switches from a low DC-voltage value to a high DC-vo It age value 10, the parallel capacitor 66 and the grounding capacitor 72 quickly charge. If the capa¬ citance value of both capacitors66, 72 isequal, the voltage at their contact point, which correspondsto the voltage at the filter'soutput 61 , isapprox- mately equal to half of the difference between the high DC voltage value 10 and the low DC voltage value, plusthe low DC-voltage value. I.e., if the low DC voltage value isequal to zero volts, the voltage at the filter'sout- put in this first step isapproximately equal to half of the high DC voltage value. The filter'soutput voltage thustakeson a first output level 12.

In thisfirst step, the rising speed du/dt of the voltage at the filter'soutput 61 is not significantly reduced compared to the rising speed of the voltage at the filter'sinput 60. Transmission line effectsare thus not attenuated, which can generate voltage overshootsat the motor end of the cable. However, asthe first level 12 of the filter'soutput voltage is preferably significantly lower than the motor's normal operating voltage 10, the voltage oscillations are kept in a range where they cannot damage the motor and/or the cable.

In a second step, the choke 64 charges and part of the charging current is being damped by the damping resistor 65. The choke 64 charges up to the high DC-voltage value 10, plusa certain voltage overshoot whose amplitude dependson the amount of damping occurring in the filter. After thisvoltage overshoot, the voltage at the filter's output 61 stabilizes around a second level which is practically equal to the high DCvoltage value 10. The higher the value of the damping resistor 65, the lower the amplitude of the voltage overshoot at the filter'soutput 61. Higher damping resistors however generate higher damping losses. A compromise must thus be found between keeping the overshoot voltage below a determined level and minimizing the lossesin the filter 6. In the theoretical case where no damping occurs in the filter 6, i.e. where the value of the damping resistor 65 isequal to zero ohms, the amplitude of the voltage overshoot is equal to the difference between the first level 12 and the high DC-vo It age value 10. In the case where the low DC-voltage value is equal to zero and both capacitors 66, 72 have the same capacitance, the filter's output voltage thus rises up to one and a half timesthe high DC-voltage value 10, which isin most practical casesstill within acceptable limitsfor the safety of the system.

In the second step, the rising speed du/dt of the voltage 8 at the filter'soutput 61 issignificantly reduced compared to the rising speed of the voltage at the filter's input 60 because of the time needed by the choke 64 for charging. In most configurations, transmission line effects can thus be avoided. The peak voltage at the motor'sinput isthus not significantly higher than the peak voltage at the filter'soutput 61.

A notable benefit of the overshoot filter 6 of the invention compared to prior art overshoot filters, such asf or example the one represented in figure 5, isthat losses are considerably reduced because the damping resistor 65 can be chosen smaller in order to keep the peak voltage below a given value at the motor'sinput. For identical input voltage conditions, the voltage overshoot after the charging of the choke 64 (figure 7) isindeed significantly smaller, typically half, of the voltage overshoot after the charging of the choke 30 (f igures3 and 5).

The overshoot filter 6 of the invention isa multilevel overshoot filter transmitting the high DC-vo It age value 10 which is output by the motor drive for example in two steps having two distinct voltage levels 12, 10. In a preferred embodiment, the capacitance of the parallel capacitor 66 isequal to the grounding capacitance 72 so that the first level corresponds to about half of the high DC-voltage value when the low DC-voltage value isequal to zero volts DC. Because the cable's length and type, and thusits capacitance, vary from one installation to another, a difficulty isto propose a filter 6 according to the invention whose first voltage level would be the same in any system comprising a drive 70, a cable 73 and a motor 71.

According to a preferred embodiment, the parallel capacitor 66 is for example a variable capacitor, i.e. a capacitor whose impedance can be varied, for example mechanically and/or electronically. The parallel capaci¬ tor 66 comprisesfor example an array of capacitors whose configuration can be modified, for example by mechanical and/or electronic switches, in order to achieve the desired capacitance. The capacitance of the parallel capacitor 66 being variable, it isthen preferably adjusted to the characteris¬ tics of each system, in particular to the cable's length and/or capacitance.

According to a preferred embodiment, asthe overshoot filter 6 of the invention is installed in a new system, the capacitance of the parallel capacitor 66 isfor example set to a predetermined value. The motor drive 70 isswitched on and the peak voltage at the filter'soutput 61 is measured. The capacitance value of the parallel capacitor 66 isthen adjusted until the peak voltage stays within predetermined limits. In a variant embodiment, the first level 12 of the voltage at the filter's output 61 isdetermined, for example with the help of an oscilloscope. Again, the capacitance value of the parallel capacitor 66 isthen adjusted until the desired first voltage level 12 is reached. A drawback of this last variant of the adjusting method is that it requiresthe use of more sophisticated and expensive equipment than just sensing the value of the peak voltage at the filter'soutput 61.

When the capacitance of the parallel capacitor 66 isthe same as the capacitance of the grounding capacitor 72, the first voltage level 12 at the filter'soutput 61 is approximately equal to half of the high DC-voltage value 10 and the voltage at the filter'soutput 61 peaks at 150% of the high DC-voltage value 10 when a lossless choke 64 isconsidered and when the low DC-voltage value is equal to zero volts DC. If the capacitance of the parallel capacitor 66 is larger than the capacitance of the grounding capacitor 72, the high DC voltage applied at the filter'sinput 60 is not equally divided between both capacitors 66 and 72. In this particular configuration, the first voltage level 12 at the filter'soutput 61 is higher than half of the high DC-voltage value 10, thusinducing higher voltage oscillations at the motor'sinput during the first step, but the voltage overshoot at the filter'soutput 61 during the second step is lower than 150% of the high DC-voltage value 10. If the capacitance of the parallel capacitor 66 is smaller than the capacitance of the grounding capacitor 72, the first voltage level 12 at the filter's output 61 is lower than half of the high DC-voltage value 10 thus resulting in lower voltage oscillationsat the motor'sinput during the first step, but the voltage overshoot at the filter's output 61 during the second step is higher than 150% of the high DC- voltage value 10. The capacitance of the parallel capacitor 66 isthus preferably determined in order to obtain the desired behavior of the voltage at the filter'soutput 61.

In figure 7, the damping resistor 65 is represented asa discrete distinct component placed in seriesto the choke 64. As explained before, the damping resistor 65 however possibly comprisesthe resistive component of the choke 64 itself. The damping resistor 65 thus preferably representsthe resulting resistance of the choke 64 and of an optional additional discrete damping resistor which is placed in seriesor in parallel with the choke 64.

In an embodiment of the invention, the overshoot filter 6 is directly attached to the motor drive 70, for example integrated in its enclosure, and the electrical outputsof thisenclosure are already filtered outputswhich can be directly connected to the cable 73. The peak voltage at the filter'soutput 61 and/or the first voltage level 12 are then preferably measured through an analog interface in the enclosure of the motor drive 70 and the capacitance value of the parallel capacitor 66 isfor example adjustable through selection relays or other types of switches placed within the enclosure of the drive 70 and accessible through four digital interfaces or other types of interfaces. Preferably, the overshoot filter 6 of the invention is placed on the drive side of the cable 73 connecting the motor 71 to the drive 70.

In the description above, the overshoot filter of the invention isa two-level overshoot filter. When the voltage at the filter's input 60 switches from a low voltage value to a high voltage value 10, the voltage at its output 61 quickly switches in a first step f rom the low voltage value to an intermediate voltage value 12 and then rises more slowly in a second step from the intermediate voltage value 12 to an asymptotic voltage value resp. to the high voltage value 10. The level of the intermediate voltage value 12 is essential Iy determined by the ratio between the capacitance value between the filter'soutput 61 and the filter'sinput 60 on one side and the capacitance value between the filter'soutput 61 and the electrical ground or a neutral conductor on the other side.

It is however possible, within the framework of the invention, to imagine an overshoot filter with more than two levels, i.e. an overshoot filter whose voltage at its output takeson more than one intermediate value when the voltage at itsinput switchesfrom a low voltage value to a high voltage value. Thiscan be done for example by connecting a plurality of capacitors in series between the filter's input 60 and the filter'soutput 61. The actual output voltage isthen switched from one point of connection between two capacitorsto the next, thuscreating a stepwise rise of the output voltage, until the filter'soutput isswitched to the choke and damping resistor'soutput.

The description above describesthe effectsof the filter 6 of the invention when the voltage at the filter's input 60 switchesf rom a low voltage value, for example zero volts, to a high voltage value 10. The effectsof the filter 6 of the invention are of course similar when the voltage at the filter'sinput 60 switchesfrom a high voltage value 10 to a low voltage value, for example zero volts, the voltage at the filter's output 61 then quickly switching in a first step from the high voltage value 10 to an intermediate voltage value 12 and then reducing more slowly in a second step from the intermediate voltage value 12 to fhe low voltage value.

In the description above, the filter 6 of the invention is placed between a drive 70 and a motor 71 in order to avoid voltage overshoots at the motor'sinput. The filter 6 of the invention can however also be connected at the input of a motor drive, between the motor drive and a power distribution network, in order to attenuate perturbationsgenerated by the drive and limit their propagation within the power distribution network.

For the sake of simplicity, the description above describesa filter 6 in a single-phase system without neutral conductor. The filter of the invention can however be adapted to work in a system having a different number of phases, for example in a two-phase system, a three-phase system, etc., with or without neutral conductor. The above described structure is preferably applied to each phase, with the output capacitor 72 representing either the resulting capacitance between the filter output 61 of the corresponding phase and the ground or the resulting capacitance between the filter output 61 of the corresponding phase and the neutral conductor, depending on the system'sconfiguration.

Claims

1. Rlter (6) for avoiding voltage overshoots on a transmission line (73) between a motor drive (70) and a motor (71) or between a motor drive (70) and an electrical distribution network, said filter (6) comprising: an input (60), an output (61), at least one inductive component (64) connected between said input (60) and said output (61), characterized in that it further comprises at least one parallel capacitor (66) connected between said input (60) and said output (61 ) in parallel to said at least one inductive component (64), such that, when the voltage at said input (60) quickly switches from a first voltage value to a second voltage value, the voltage at said output (61) quickly switches in a first step to an intermediate value (12) comprised between said first voltage value and said second voltage value and then more slowly tendsin a second step to an asymptotic voltage value.

2. Rlter (6) according to claim 1 , comprising meansto modify the capacitance of said at least one parallel capacitor (66) in order to modify said intermediate value (12).

3. Rlter (6) according to claim 2, said at least one parallel capacitor (66) comprising an array of capacitors whose configuration can be modified in order to achieve the desired capacitance.

4. Rlter (6) according to claim 3, wherein said configuration can be modified by mechanical and/or electronic switches.

5. Rlter (6) according to one of the claims 1 to 4, further comprising a damping resistor (65) for reducing the voltage peak at said output (61) during said second step.

6. Rlter (6) according to one of the claims 1 to 5, the capacitance of said at least one parallel capacitor (66) being approximately equal to the capacitance of a discrete or parasite grounding capacitor (72) between said output (61) and the ground or a neutral conductor.

7. Rlter (6) according to claim 6, the capacitance of said grounding capacitor (72) comprising the capacitance resulting from the capacitances of said transmission line (73) and/or of an additional filter output capacitor.

8. Rlter (6) according to one of the claims 1 to 7, including a casing.

9. Rlter (6) according to claim 8, said casing comprising manually or electrically operable control elementsfor modifying the capacitance of said at least one parallel capacitor (66).

10. Rlter (6) according to one of the claimsδ or 9, said casing comprising interfacesfor sensing the voltage at said output (61).

11. Rlter (6) according to one of the claims 1 to 10 for use in a three-phase electrical system, said filter comprising three said inputs (60) and three said outputs (61).

12. Motor drive comprising an enclosure and a filter (6) according to one of the preceding claimswithin said enclosure, said input (60) of said filter (6) being directly connected to an output of said motor drive (70), said output (61) of said filter (6) being accessible through a connector and/or terminalson said enclosure.

13. Motor drive according to claim 12, being a Pulse Width Modulation (FWM) motor drive.

14. Motor drive according to one of the claims7 or 8, said motor drive comprising an interface for sensing the voltage at said output of said filter (6).

15. Motor drive according to one of the claims 7 to 9, said motor drive comprising at least one interface for modifying the capacitance of said at least one parallel capacitor (66).

16. System with: a motor (71 ), a motor drive (70) for driving said motor (71), a transmission line (73) connecting said motor (71) to said motor drive (70), a filter according to one of the claims 1 to 11 for avoiding voltage overshoots on said transmission line (73) and/or between said motor drive (70) and an electrical distribution network.

17. System according to claim 16, said motor drive (73) being a Pulse Width Modulation (PWM) motor drive.

18. System according to one of the claims 16 or 17, said motor (70) being an induction or permanent magnet motor.

19. Method for adjusting a filter according to claim 3 or 9, said method comprising the stepsof :

- connecting said input (60) of said filter (6) to the output of a motor drive (70),

- connecting said output (61) of said filter (6) to a transmission line (73) connecting said motor drive (70) to a motor (71) or said motor drive (70) to an electrical distribution network,

- switching on said motor drive (70),

- measuring the peak value of the voltage at said output (61) of said filter (6) and/or determining the value of said intermediate value (12) when the voltage at said input of said filter (6) switchesfrom said first voltage value to said second voltage value,

- modifying the capacitance value of said at least one parallel capacitor (66) in order to achieve a desired value for said peak value and/or for said intermediate value (12).

20. Method for avoiding voltage overshootson a transmission line (73) between a motor drive (70) and a motor (71) or between a motor drive (70) and an electrical distribution network with a filter (6), said method (6) comprising:

- quickly switching the voltage at an input (60) of said filter (6) from a first voltage value to a second voltage value,

- quickly switching in a first step the voltage at an output (61) of said filter (6) from said first voltage value to an intermediate value (12),

- more slowly reaching in a second step an asymptotic voltage value.