Development of a precision trimming process for manganin shunt resistors

Misti, Siti Nabilah (2016) Development of a precision trimming process for manganin shunt resistors. Doctoral thesis, Northumbria University.

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Abstract

As electrical energy prices continue to rise, accurate energy consumption monitoring is becoming increasingly important and the introduction of smart energy meters is well-known in this capacity. One of the key components in the smart energy meter is the current sensing shunt resistor and its resistance must be as low as possible, typically in the range 100 μΩ to 10 mΩ, to minimise energy usage. Although this low resistance requirement reduces power consumption, it is inherently difficult to manufacture shunt resistors in this micro-ohm range to the required precision and at reasonable cost. Typical resistance accuracy of commercially available shunts suitable in this application is 100 μΩ ±5%, which can in turn lead to ±5% errors in power measurement within the smart meter.

This research presents a novel automated electro-mechanical trimming process to improve the accuracy and performance of 100 μΩ Manganin shunt resistors for use in smart energy meters. Theoretical and experimental investigations were carried out to determine the optimum technique and design geometry to remove the resistive material from the shunt. More specifically, the work focuses on the development of the precision trimming process to improve the resistance tolerance of the shunt resistors. A novel laboratory prototype of an automated concurrent trimming system is developed which combines the mechanical cutting process and electrical measurement system to remove controlled amounts of material from the Manganin shunt. Design of Experiments (DoE) is then conducted in order to find the optimum feed rate and cutting speed for the trimming process.

The effects of under a varying the trimming geometry on the key performance crietria of the shunt resistors was investigated by simulating the conditions that the shunts will exposed to when positioned within a high current circuit and encapsulated within a smart energy meter. Tests have examined changes in physical structural conditions as well as electrical properties under a varying environmental conditions. By using concurrent trimming, the tolerance of the Manganin shunt resistors has been reduced from ±5% to less than ±1% within 5 seconds. Trimming does not have a significant effect on the key properties of the shunt resistors and the results obtained can be used to inform production processes for large scale manufacture of precision shunt resistors.

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