ALGOR FEA Used to Optimize Siemens Residential Electric Meter Component

Consumers expect that the electric usage reported on their monthly utility bills is correct, so a major design consideration for any electric meter component is that it be accurate. In addition to accuracy, utility companies demand that metering products be able to be upgraded easily and inexpensively. Since 1996, Siemens Metering has been meeting these needs with its adaptable metering system. The adaptable metering system uses plug-in modules to provide for easy and less expensive upgrades in a market where obsolescence happens quickly due to changing utility standards. Modules can also provide different functions, such as communications for automatic meter-reading.

Recently, new British short-circuit protection requirements led to the complete redesign of the electric meter’s switched shunt. This meter measures the power consumption by monitoring the voltage drop across the shunt, which is designed to prevent overheating and excessive power consumption. Siemens’ supplier, Samuel Taylor, Ltd., contracted Neil Rothwell, a freelance mechanical design engineer, to analyze and optimize the components. Rothwell used ALGOR’s electrostatic analysis software to predict the performance of the shunt and optimize its geometry.

The adaptive meter’s function is to measure kilowatt-hour power consumption. To achieve this, an accurate means of measuring current is needed. In the past, this has been done using relatively complex and expensive devices such as a current transformer that measures the magnetic field.In the adaptive meter, a simple shunt is used. It is simply a conductive element of known resistance placed in the current circuit for which the voltage drop is measured. From this voltage and known resistance, the current can be calculated using Ohm’s law (I = V/R). Power consumption can then be calculated by integrating the current and voltage over time.

In order to accurately measure the current, the shunt cannot change resistance as it heats up. Manganin, a copper manganese nickel alloy, is used for the element because its resistance remains virtually constant as the current load and temperature change, and it therefore maintains its accuracy at a variety of load currents. Another design consideration is that the total power loss in the circuit must be less than 8 watts at the full load current of 100 amps to prevent overheating and excessive power consumption. Finally, the geometry of the shunt must be optimized to meet a precise resistance value specified by Siemens. 

In the past, Rothwell used a combination of hand calculations, 2-D current flow programs and laboratory testing to optimize shunt designs and determine their resistance values. “ALGOR’s 3-D current flow capabilities are a great improvement over methods used in the past such as hand calculations and 2-D current flow programs,” said Rothwell. “With the 2-D methods, my clients and I were either restricted to simple, constant-thickness shapes or limited in how well we could predict the current flow of a 3-D design. Now, we are able to use more complex 3-D shapes and achieve an accurate design with the first prototype.”

Rothwell began with a 3-D design based on hand calculations, which was modeled in I-DEAS. He imported the model as an IGES file to create a 3-D finite element model. 100 amps of current was applied as a load to one end of the shunt. A current sink was placed at the other end to make the current flow through the shunt. 

	ALGOR’s electrostatic analysis software was used to predict voltage and current density. Based on the results, Rothwell changed the thickness and height of the manganin element (highlighted) to optimize the design’s resistance to the value specified by Siemens.
	Neil Rothwell is shown here holding a Siemens adaptive meter with the ALGOR electrostatic analysis results in the background.

The electrostatic analysis was performed to predict voltage and current density. Based on the results, Rothwell varied the thickness and height of the manganin element to optimize the design’s resistance to the value specified by Siemens. 

Laboratory tests correlated with the electrostatic analysis results within 3%, well within the manufacturing tolerances for this type of part. “My clients and I were happy that the laboratory tests correlated so well with the analysis results,” said Rothwell. “Shunt prototypes are expensive to produce and take about four weeks to manufacture. Getting the results right the first time saved considerable time and money.”

The upgraded adaptive meter has been in production for 12 months and is the main Siemens residential electric metering product being used in the United Kingdom.

Rothwell is moving on to integrate ALGOR software into his analysis process to a greater degree. “Currently, I use hand calculations and experience to decide if thermal effects from Joule heating will be an issue,” said Rothwell. “In the future, I plan to integrate ALGOR’s heat transfer capabilities, specifically the multiphysics Joule heating capability for calculating a temperature profile based on the results of an electrostatic analysis.”

Siemens Metering is part of the global Siemens group. Samuel Taylor, Ltd. is a world-class manufacturer of precious metal contacts located in the U.K. and a major supplier to Siemens Metering. Neil Rothwell is a freelance mechanical design engineer in Cheshire, U.K. He has 20 years experience in the electric utility industry, both in his freelance career and working for Siemens Metering and other electric utility suppliers.