Method for Measuring DC resistance of aluminum stranded conductors
Many companies have also done a lot of research on the measurement of DC resistance of aluminum stranded conductors.
In GB3048, the distance between the voltage terminal and the current terminal during resistance measurement and the magnitude of the measured current are specified.
For the wire core of qualified single wire stranded with normal process, there are still problems that can be discussed in actual measurement, such as the influence of the shape of the potential end on the measurement results, the key points of the four-terminal method for measuring the resistance, and how to realize the uniformity of the measurement current. I would like to make the following comments on the above issues.
First, the wire core resistance twisted by qualified single wire and normal twisting process has a certain margin of insulation between each single wire in the stranded wire core, and the current flows in a spiral shape along the single wire, and all single wires of the wire core are reliably connected to the current introducing terminal.
Calculate the unfolded length of each layer of 1m long stranded wire, according to the calculation formula of parallel resistance, calculate the DC resistance formula of stranded wire, such as: there is still a certain conductance between the single wire of the actual stranded wire core, the size of conductance is related to the surface condition of monofilament.
The existence of conductance makes the DC in the wire core flow incompletely along the single wire spiral, and a part of the current flows along the single wire into an axial direction.
At this time, the DC resistance of the wire core is smaller than that when the current flows completely along the monofilament into a spiral.
Since there is no obvious change in the resistance of the stranded aluminum single wire, under the qualification of the single wire, even if the length of the wire core is increased by 1%after the cable is formed, the stranded wire core resistance still has a certain margin.
2. The reason for the unqualified measured value and the unqualified resistance of the stranded wire are all due to the fact that the measured current fails to flow evenly through the sample in the resistance measurement.
All monofilaments of The Wire core are reliably connected with the current introduction terminal, and the insulation current between monofilaments in The Wire core flows spirally along the monofilament.
Then under the condition of the same diameter and uniform texture of monofilament, the cross section perpendicular to the wire core on the sample should be an equipotential plane, and the potential difference between monofilaments on this cross section is zero.
Under the same condition, the potential difference between the voltage terminals will not change due to the shape of the voltage fixture, the way of contact with the sample and the change surface of the pressing force on the sample.
At present, the shape of the clamp for measuring the resistance of the twisted wire is mostly knife-shaped and enclosed, and the pressure on the sample is generated by tightening the bolt.
Due to the existence of the oxide film on the surface of the aluminum monofilament, when the measurement current is introduced into the sample through this form of fixture, the current distribution is often uneven, and the current density of the outer monofilament of the stranded wire. This phenomenon is serious with the increase of stranded wire section.
Although the V-shaped current fixture has been greatly improved compared with the knife-shaped and annular current fixture, it cannot make the current reach the ideal uniform state.
The uneven distribution of the measurement current makes the voltage difference between the two voltage terminals 1m apart greater than the voltage difference between the two voltage terminals under the uniform measurement current.
In this case, the voltage end of the fixture shape, the pressure for further improvement is often not effective.
Therefore, the key to solve the problem of resistance measurement is how to measure the current uniformity and the requirements of the voltage end fixture, which can make the measured current uniform and drop to the bottom.
3. Solution, in order to make the internal current of the wire core uniform, the sample end can be treated.
The treatment method is to use the end welding method, and try to make the current end close to the welding of the wire core. There are still some problems in adopting this method.
In addition to the welding method, there is also the end cold pressing aluminum nose method, which can achieve a simple and effective method to measure the current evenly.
In order to make the current uniform, the contact resistance between the lead-in end and the sample must be reduced.
The method of tightening bolts with compression type should be small.
The contact resistance of compression type ranges from several microohms to more than ten microohms, while the contact resistance of clamping type current end and sample ranges from tens to two hundred microohms.
Using the cold crimping method, the measured current is directly introduced from the aluminum nose, which can better overcome the phenomenon of uneven current distribution due to the oxide film between monofilaments.
The solution of the resistance measurement method of aluminum stranded conductor can make a fair evaluation of power cable products. The accurate measurement of resistance provides an important basis for enterprises to reduce the diameter of monofilament, save raw materials and improve the process, in this respect, it is of more practical significance.