GB2120868A

GB2120868A - Drive circuit for a three-phase inverter

Info

Publication number: GB2120868A
Application number: GB08314403A
Authority: GB
Inventors: Arne Jensen
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1982-05-26
Filing date: 1983-05-25
Publication date: 1983-12-07
Also published as: DE3219751C2; JPS58215974A; FR2527860A1; DK220583D0; GB8314403D0; DE3219751A1; FI831824L; FI831824A0; FR2527860B1; DK220583A; GB2120868B

Abstract

A drive circuit for a three-phase inverter, in which each square halfwave of the output voltage is, for the purpose of harmonic wave damping, interrupted by two gaps which are symmetrically disposed in the first and last 30 DEG section and bounded by the angular spacings alpha 1 and alpha 2 from the ends of the halfwave, particularly for operating a speed regulated A.C. motor. The drive circuit is characterised in that alpha 1 is less than 13.85 DEG (the first zero crossing of the 13th harmonic), the angular spacing alpha 2 lies between 12.86 DEG (the first maximum of the 7th harmonic) and 18 DEG (the first maximum of the 5th harmonic), and the gap width ( alpha 2- alpha 1) lies between 3 DEG and 7 DEG inclusive. Particularly effective combinations, for reducing oscillating torque due to 5th, 7th, 11th and 13th harmonics, are alpha 2 = 15 DEG with alpha 1 = 9 DEG or 10 DEG . <IMAGE>

Description

SPECIFICATION Drive circuit for a three-phase inverter The invention relates to a drive or control circuit for a three-phase inverter, wherein each square halfwave of the output voltage is, for the purpose of harmonic wave damping, interrupted by two gaps which are symmetrically disposed in the first and last 30 section and bounded by the angular spacings a1 and a2 from the ends of the halfwave, particularly for operating a speed regulated A.C. motor.

In a known control circuit of this kind (IEEE-Transactions on Industry Applications,-1973, pages 310-317), the 3rd and 9th harmonics are absent because of the interlinkage of the three-phase output voltage whereas the 5th and 7th harmonic are eliminated in that two gaps are provided in the square halfwave, of which the first is present between 16.24 and 22.06 and the second between 180 minus 22.06 and 180 minus 16.24 .

The accurate maintenance of these values presents considerable difficulties from a circuitry point of view, especially if they are to be maintained over the entire frequency range of the A.C. inverter. In addition, the 11th and 13th harmonics have such a high amplitude that the oscillatory torque of an A.C. motor operated with this output voltage, particularly an asynchronous motor, is roughly 50% of the torque for a pure square halfwave without gaps. It is noticeable by irregular running, particularly at a low rotary speed. If one also wishes to eliminate the 11th and 13th harmonic, a total of four gaps have to be provided in the square halfwave. This higher switching frequency makes the construction of the control circuit even more complicated and the switching losses are increased.

The invention is based on the problem of providing a control circuit of the aforementioned kind wherein on the one hand the frequency of switching is low but on the other hand the oscillatory torque arising because of the harmonic waves can be kept small.

The present invention provides: A drive circuit for a three-phase inverter, wherein each square halfwave of the output voltage is, for the purpose of harmonic wave damping, interrupted by two gaps which are symmetrically disposed in the first and last 30 section and bounded by the angular spacings a1 and a2 from the ends of the halfwave, particularly for operating a speed regulated A.C. motor, characterised inthata1 is less than 13.85 (the first zero crossing of the 13th harmonic), the angular spacing a2 lies between 12.86 (the first maximum ofthe7th harmonic) and 18 (the first maximum of the 5th harmonic), and the gap width (anal) lies between 3" and 7" inclusive.

The above-mentioned problem is solved according to the invention in that a1 is less than 13.85 , the first zero passing of the 13th harmonic, the angular spacing a2 lies between 12.86', the first maximum of the 7th harmonic, and 18, the first maximum of the 5th harmonic, and the gap width (a2-a1) lies between 3" and 7 .

In this construction, only two gaps in each square halfwave enable the proportion of harmonic waves in the square halfwave to be kept so low that the oscillatory torque becomes smaller or even very much smaller than hitherto. This occurs in that no attempt is made to eliminate individual harmonic waves completely but the aim is to keep the amplitudes of the individual harmonic waves so low that the sum of the oscillatory torques of the 5th, 7th, 11th and 13th harmonics is low. It is sufficient to consider these four harmonic waves because the evenly numbered harmonics and those divisible by three are in any case negligible and the harmonic waves of the 17th order and higher are insignificant for the oscillatory torque because they enter the calculation only with the reciprocal value of the square of their order number.The thus fixed switching pattern is maintained independently of the inverter frequency. Changes in the amplitude of the fundamental wave can be achieved by altering the D.C. voltage supply, for example by using a variable intermediate circuit voltage.

To damp the 5th and 7th harmonic waves as much as possible, one requires gaps with a width of from 3" to 7" preferably from 4" to 6 , disposed in a zone where the 5th and 7th harmonics have a comparatively large amplitude. Whereas the angular spacings were hitherto selected so that the first gap was disposed predominently in the second halfwave of the 11th and 13th harmonics, whereby these were considerably increased, according to the invention the gaps are displaced toward the ends of the square halfwave so that at least parts of the first halfwave of the 11th and 13th harmonic lie in the zone of the gap.Since the effects of the portions symmetrically arranged in the gap zone about the zero passage of these harmonics balance each other out, the influence of the 11th and 13th harmonics is substantially less. One can even achieve a situation where in the gap zone there is a larger portion of the first than of the second halfwave of the 11th and 13th harmonics, so that there is a very considerable reduction in the proportion of these harmonics. All this is achieved without having to accept too large a proportion of the 5th and 7th harmonics.

Advantageously, the angular spacings a1 and a2 have a common divisor equal to 30 : m, where m is a whole number less than or equal to 10. This utilises the discovery that the advantages of the invention are valid not for particular points but for particular zones so that, within these zones, there is a further possibility of selection for the angular spacings. The use of a common divider permits a store to be used with a limited number of storage cells for describing the condition of a three-phase A.C. inverter with respectively equally long time intervals.

In one embodiment, there is a pulse generator delivering timing pulses (b1) with a frequency of fr = 12 . m fw where fw is the frequency of the output voltage and m a whole number between 3 and 10, and there is a counter which is operated by these timing pulses and which gives timing signals at predetermined timer outlets for controlling the A.C. inverter. By changing the frequency of the pulse generator, one can change the frequency of the inverter but with the position of the gaps in the square halfwaves being maintained.

It is preferred to select the angular spacings a1 and a2 so that the oscillatory torque

forn=5,7,11 and 13 is a maximum of 0.025 K, namely a third of the torque M for a pure square halfwave. In practice, the oscillatory torque can even be reduced to a value below one quarter of the torque for a pure square halfwave.

It is also advisable for the angular spacings a1 and a2 for a given m to be selected so that the oscillatory torque M is a minimum. This gives an additional criterion for selection if different pairs of angular spacings can be selected for a given m.

It has proved favourable in practice if a1 is less than or equal to 12 . The smaller the value for a1, the greater will be the damping of the 11th and 13th harmonics.

It is particularly advantageous in practice if a1 = 10 and a2 = 15 . This results in an extraordinarily low oscillatory torque M = 0.0175 K. This is achieved by sub-dividing the 30 section into six parts of 5" each.

A like good result is obtained with a1 = 9" and a2 = 15 . The oscillatory torque M will be 0.0183 K, achieved with a sub-division of the 30 section into 10 parts of 3" each.

A drive circuit, for a three-phase inverter, constructed in accordance with the invention will now be described by way of example only with reference to the accompanying tables and drawings, in which: Table 1 gives the amplitudes of the 1st, 5th, 7th, 11th and 13th harmonics in relation to the angular spacings a1 and a2 at various divisor conditions 30: Table 2 gives the oscillatory torque in relation to the 5th, 7th, 11th and 13th harmonics at various divisor conditions 30: m and the respective sums of the harmonics, Figure lisa diagram of the drive circuit according to the invention, Figure2 illustrates the output wave of the drive circuit, and Figure 3 shows the first part of a halfwave, the 1st, 5th, 7th, 11th and 13th harmonics being shown for an uninterrupted square wave.

Figure 1 shows a three-phase A.C. inverter 10 fed by a D.C. voltage U from a controllable D.C. voltage source 11 and operating a three-phase asynchronous motor 12 with three-phase A.C. voltage. The three output leads 13, 14 and 15 branch off between respective pairs of series-connected switching elements 16, 17 and 18, 19 and 20, 21. The switching elements are formed for example by controlled thyristors, transistors or the like. A drive signal generator 22 serves to bring the switching elements of each pair into opposed conductive and blocking conditions corresponding to the shape of the output voltage halfwave. With thyristors, the control signals can consist of ignition and extinction pulses and in the case of transistors of steady signals to be applied during the conductive condition.

The drive circuit also comprises a pulse generator 23 which gives timing pulses P1 through a lead 24 to a counter 25. The pulse generator 23 is a voltage-controlled oscillator fed with a voltage u1 substantially proportional to the D.C. voltage U. The counter 25 will then give a transfer pulse P2to a second counter 27 by way of a lead 26. At the respective outputs 28 and 29 of these counters, successive output voltages are given as timing signals to a logic circuit 30 in which the signals are logically combined by storage elements and logic elements. By actuating the drive signal generator 22, output signals are given at the outputs 31 of the logic portion 30.

It is assumed that the two counters 25 and 27 are modulo 6 counters and that the frequency fr of the timing pulses p1 is equal to 72 times the desired A.C. inverter frequency fw. The spacing between two timing pulses will then correspond to an angle of 5 . An output voltage will therefore occur at the output 2 of counter 25 when the angular spacing is 15 . Running of the counter 25 up to delivery of the transfer pulse p2 therefore corresponds to a 30 section. After each 30 section, the output voltage changes from one to the next output of the counter 27.These output voltages therefore characterise the start of each 30 section of a halfwave on the one hand and the start of each halfwave of the three phases on the other hand.

By appropriately combining these signals, one obtains output voltages displaced by 120 and each having the shape of Figure 2. The voltage of each halfwave has substantially the shape of a rectangle extending from 0 to 180 . It is, however, provided with two gaps 33 and 34 symmetrically disposed at equal angular spacings from the ends of the rectangular halfwave 32. In the example of Figure 1, the gap 33 extends from the angular spacing a1 to the angular spacing a2 and the gap 34 from the angular spacing (180 - ap) to the angular spacing (180 - a1). For the Figure 1 embodiment, this means that the gap 33 extends from 10" to 15 and the gap 34 from 165" to 175 . The gap is obtained with the aid of output signals at the outputs 2 and 3 of counter 25 and the gap 34 with the aid of output signals at the outputs 3 and 4 of counter 25. The conditions are maintained if the frequency of the timing pulses p1 alters through a change in the control voltage u1.

Figure 3 shows the first 30 section of such a halfwave in which the gap 33 extends from 9" to 15 . This is achieved if one uses a modulo 10 counter as the counter 25 and employs the outputs 3 and 5 for fixing the limits of the gap 33. Figure 3 also shows the fundamental wave W1, the 5th harmonicW4, the 7th harmonic W7, the 11th harmonicW11 and the 13th harmonic W13 as they would occur in a pure square halfwave. It will be evident that gap 33 already starts in front of the first zero passage 35 of the 13th harmonic and terminates between the first maximum 36 of the 7th harmonic and the first maximum 37 of the 5th harmonic.The width is here 6 . Since in all of the four stated harmonics W5, W7, W, and W13 substantial portions of the first halfwave are intersected by the gap 34, these harmonic waves are strongly damped.

Just how good this damping is will be evident from the compilation of Table 1 which gives the amplitudes of the fundamental wave and the four harmonics for a pure square halfwave (column 1) and for a square halfwave provided with two gaps (columns 2 to 8) according to examples of the invention, each of columns 2 to 8 being one example of the invention.

In Table 2, the oscillatory torque for the invidivual harmonics and the sum of these oscillatory torques are shown for the same conditions. It follows that, in comparison with the sum in the first column, all the examples of the invention represented by columns 2 to 8 give an improvement of considerably better than 50%, all except the example of column 7 give an improvement of better than 2/3, and particularly good results can be achieved with angular spacings of 10 and 15 as well as 9" and 15 .

Claims

1. A drive circuit for a three-phase inverter, wherein each square halfwave of the output voltage is, for the purpose of harmonic wave damping, interrupted by two gaps which are symmetrically disposed in the first and last 30 section and bounded by the angular spacings a1 and a2 from the ends of the halfwave, particularly for operating a speed regulated A.C. motor, characterised in that a1 is less than 13.85 (the first zero crossing of the 13th harmonic), the angular spacing a2 lies between 12.86 (the first maximum of the 7th harmonic) and 18 (the first maximum of the 5th harmonic), and the gap width (a2-(1) lies between 3" and 7" inclusive.

2. A drive circuit according to claim 1, characterised in that the angular spacings a1 and a2 have a common divisor equal to 30: m, where m is a whole number less than or equal to 10.

3. A drive circuit according to claim 1 or 2, characterised by a pulse generator (23) arranged to deliver timing pulses (b1) with a frequency of fr = 12 . m where fw is the frequency of the output voltage and m is a whole number between 3 and 10 inclusive, and characterised by a counter arranged to be operated by these time pulses and to give timing signals at predetermined timer outlets for driving an A.C. inverter.

4. A drive circuit according to any one of claims 1 to 3, characterised in that the angular spacings a1 and a2 are selected so that the oscillatory torque

forn = 5,7,11 and 13.

is a maximum of 0.025 K, namely a third of the torque M for one pure square halfwave.

5. A drive circuit according to claim 4, characterised in that the angular spacings a1 and a2 for a given m are selected so that the oscillatory torque M is a minimum.

6. A drive circuit according to any of claims 1 to 5, characterised in that a1 is less than or equal to 12 .

7. A drive circuit according to any one of claims 1 to 6, characterised in that a1 = 10 and a2 = 15 .

8. A drive circuit according to any one of claims 1 to 6, characterised in that a1 = 9" and o2 = 15 .

9. A drive circuit substantially as herein described with reference to, and as illustrated by, the accompanying drawings.

10. A drive circuit according to any preceding claim substantially as herein described with reference to any one of columns 2 to 8 of Table 1 of the accompanying tables.

11. A three-phase inverter having a drive circuit as claimed in any preceding claim.