FI104394B - Connector structure between drive and its load - Google Patents

Description

, 104394 CONNECTOR STRUCTURE BETWEEN A FREQUENCY CONVERTER AND ITS LOAD * Introduction 5 Frequency converters are nowadays increasingly used to control the speed of alternating current motors, and the efficiency of the frequency converters is that of fast components, e.g. Thanks to IGBT type transistors, it got quite high. At the same time, the rise and / or fall speeds of the drive start-up and shut-down pulses have to be made very high, even up to the order of 20,000 V / ps. As a result, 10 sharp voltage pulses entering the motor winding cause a variety of voltage level rises: higher than the cable impedance, even its multiple.

As it progresses to the motor winding, the voltage pulse is known to cause an increase in voltage between the turns of the conductor strands of the winding. The shorter the rise or fall of the voltage pulse, the greater this voltage rise.

- For low voltage drives (less than 1000 V), motors often use n.s. made of 20 round copper wires. breeze winding, characterized by, ·. the wire turns are located on the machine coil in an indefinite stiffness. In this case, n.s. Voltage Voltage: The voltage difference between the first and last conductor turns of the coil, or the voltage of the coil pair. These voltages rise in a similar way to the rotational voltage.

25 The drive may also be loaded with an electrical device other than a motor, such as a τ v transformer, but the problems are similar.

The introduction of new types of frequency inverters has led in some applications to over-voltage of the wire, and in particular to the voltage of the coil and the coil voltage, for conventional winding and insulating structures and may have resulted in damage to the windings. Technical solutions for damping voltage pulses have been proposed, e.g. by means of inductors, but these solutions have either been too complex or have impaired the properties of the drives, e.g. the efficiency. Strengthening the insulation of windings has often remained the only option, which in turn results in, for example, a special motor and a significant increase in costs.

The present invention is characterized in that the rise in the voltage pulse of the frequency converter entering the winding of a loaded electrical device is extended so that said critical voltage stresses are kept within the limits allowed for conventional windings. The invention is made particularly advantageous by the fact that the efficiency of the frequency converter remains high and no additional losses occur.

The economic importance of the invention is very high, since this way windings for normal operation can be used in most AC drives, for example, expensive special motors are not needed and users do not have to maintain a large number of different spare machines.

DISCLOSURE OF THE INVENTION At the heart of this invention is the realization that if a voltage pulse of a frequency converter is forced to enter the winding of an electrical appliance 'stepwise' but without increasing the maximum voltage previously, the result is a decrease in the average voltage rise time. This stepwise arrival of the pulse is tracked in accordance with an embodiment of the invention by dividing the cable or other interconnector 20 between the inverter and the electrical device into two or more successive portions of different wave impedances. In one characteristic of the invention, this is achieved by dividing the cable or other part of the interconnector into two or more parallel cables. Another characteristic of the invention to change the wave impedance of a cable or other interconnector is to change the dielectric constant of its insulation (known to have a wave impedance of almost inversely proportional to the square root of the dielectric constant, for more details see below). A third characteristic way to change the wave impedance is to "change the inductance of a cable or other conductor per unit length, for example by varying the outer diameter of the conductor portion. Further, the invention is characterized can be done by using different lengths of interconnectors, or so that the propagation speed of these interconnectors is different.The propagation speed depends on the same parameters as the wave impedance.

(Propagation speed v = V (1 / Ic), wave impedance Z = V (l / c) where I = inductance and c capacitance per unit length), 104394 3 H-

Parallel cables have been used in these and in general electric drives in the past 5, but the cables are always made of substantially the same length and material over the entire distance of the frequency converter - electrical device, so they do not undergo the voltage pulse weakening of this invention.

One consequence of dividing the cable or other interconnector according to the invention into parallel conductors is that the conductors do not have to be identical or balanced so that the current densities become the same. This can be done because only one cable or other interconnector can be made into the actual power cord, while others act as a 'pulse lightener'. An embodiment of the invention is characterized in that the longer or longer cables are made of a cable of substantially smaller cross-section, whereby the additional loops required by the longer cables, together with their bends, fit on the same cable shelf as the actual power cable.

Although only the term 'cable' has sometimes been used hereinbefore and hereinafter to describe the interconnector between the frequency converter and the electrical device, any practically possible conductor may be used as the interconnector, without affecting the invention in any way.

Description of the invention by Examples

The following describes a simple example of voltage pulse smoothing when *. the load is the motor winding. For the sake of illustration, the interconnector is referred to as a 'cable', which in practice is the most common structure of a interconnector. Figure A initially shows the current conventional cabling between the drive (1) and the motor (2) 25. Fig. B shows how the voltage pulse (4) (propagation speed is known to be in the order of 100-150 m / ps) propagated from the frequency converter (1) through the cable (3) is reflected in the motor winding hubs (2) up to twice (5) large compared to the cable wave impedance (the machine is relatively low power and the cable is longer than the so-called critical cable length - this is half the distance that 30 pulses travel during the rise time). At the motor terminals, the ascent rate increases relative to the magnitude of the voltage reflection.

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Figure C shows a cabling according to the invention. The voltage pulse (voltage u, rise time e.g. 200 ns) propagated by a single cable (6) first meets the cable junction (two cable branches in this example), and then proceeds to a voltage pulse of 2/3 u in each cable (7) and (8). 200 ns

5 The pulse propagating along the shorter cable (7) first meets the motor winding and the second junction of the cable (8). The reflection occurring here is not significant if we assume the wave impedance of the motor (2) to be high, as in Fig. B. Thus, a wave with a rise time of 200 ns and a magnitude of about 2/3 is introduced into the motor winding.

If we assume that the cable (8) is just as long as the cable (7) that the wave 10 enters the motor 200 ns later than the cable (7), the voltage at the motor terminals will continue to rise at the same speed and the same voltage step.

The result is initially a voltage rise to about 4/3 u, for a total of 400 ns. Although the peak value of the voltage may rise to nearly the same level as in Fig. B due to the reflections still occurring at various nodes, the result is a doubling of the rise time of the voltage 15, i.e. a reduction of the rise rate to less than half. If the cable length difference is made larger, the voltage rise becomes more steep and the overall rise time even longer. The same happens if multiple parallel cables are used or if successive ducts of parallel cables have different numbers of parallel cables.

20 Assuming that the parallel cables are the same length and the pulse propagation time is the same in both, a voltage pulse of 2/3 u arrives simultaneously on each cable. rise time 200 ns. As a result, the voltage at the start of the motor winding is 4/3 u in 200 ns. Subsequent new reflections from Node 1 eventually raise the voltage to 2 u, but this occurs at least in two 25 steps, so that the total rise time again increases considerably. It is likely that the first step of the pulse at 4/3 u in 200 ns is the dominant rotation; so that these voltages are reduced by 4/3: 2.

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The exemplary mode of operation of the invention does not change if, instead of parallel cables, the wave impedance of the cable or other interconnector at the node 30 changes appropriately, or if instead of cables of different lengths, the pulse propagation speed differs from other cables of the same group.

Claims (10)

1. Connection line structure (3, 6, 7, 8), composed of cables and / or other conductor types, between drive (1) and its load (2) forming motor or other electrical device. characterized in that the connecting line is composed of two or more series-connected parts (6, 7, 8) such that road impedances of two successive parts are different at least at one place. 2. Connection line structure (7, 8) between drive (1) and its load (2) forming motor or other electrical device, composed of parallel conductors, which may be of one or more conductor types, characterized in that at least one of the The parallel conductors (7, 8) are different from the others for the duration of the voltage pulse. 3. Connection line structure (3, 6, 7, 8) between drive (1) and its load (2) forming motor or other electrical apparatus, characterized in that the connection line structure contains series connected parts according to both claims 1 and claim 2. Connection line construction (3, 6, 7, 8) according to claim 1 or 3, characterized in that the first part (6) of the connection line which likes from the drive ψ · (1) has a higher road impedance than the following part (7, 8). 30 Connection line construction (3, 6, 7, 8) according to claim 1 or 3, characterized in that the different road impedances of the different parts (6, 7, 8) have been achieved by using one or more places in the connection line two. or more parallel wires (7, 8). Connection line construction (7, 8, 6) according to claim 2 or 3, characterized in that the deviating throughput time has been achieved by using conductors of different length (7) than in the other parallel connected conductors in the same group having different or different throughput time (s) (8). Connection line construction (7, 8, 6) according to claim 2 or 3, characterized in that the deviating throughput time has been achieved by using conductors (7) at different pulse rates than in the other parallel connected conductors in the same group having different or different pulse rate (s) (8). Connection line construction (3, 6, 7, 8) according to claim 1 or 3, characterized in that the different path impedances of parts in the connection line have been achieved by using in one or more parts conductors (6), wherein inductance and / or capacitance per unit length differs from inductance or capacitance per unit length in other parts (7, 8) of the connecting line structure. Connection line construction (6, 7, 8) according to claim 7, characterized in that the different pulse rate has been achieved by using conductors (7), where the inductance and / or capacitance per unit length differs from the inductance and / or capacitance per unit length in others. portions (8) of the connecting conduit structure. 20 Connection line structure (7, 8,6) according to claim 2 or 3, characterized in that in a group of parallel conductors only such conductors (8) having the same throughput of the - - pulse are dimensioned together to carry that load current. which «is required, and that the other conductors (7) show substantially smaller cross sections. ..