MEASUREMENT
measurement matters
Following the flow:
using shunts to
measure high current
Ohm’s law allows the determination of current by
measuring the voltage across a series resistance of known
value (I=V/R). Specially-designed resistors, called current
shunts for historical reasons, are used to measure high dc
currents (say above 1 A).
This article describes the key design features of these
devices and how to make good measurements with them.
The central problem with measuring large current is
dealing with the power dissipated by Joule heating, which
increases in proportion to the square of the current. To scope
the problem, a current of 1000 A flowing through a small
resistance of 1 mΩ generates 1 kW of heat. The resulting
temperature rise changes the value of the resistance and in
extreme cases may damage the resistor and its surroundings.
Traditionally current shunts are designed to produce
a voltage of 50 mV to 100 mV at the rated current. The
equivalent resistance of a 100 mV shunt rated for 1000 A is
100 µΩ and the power dissipation is then a more acceptable
100 W.
With such small resistances it is practically impossible to
avoid having the connections interfere with the measurement
either through heating or difficulty in accurately defining
the shunt resistance. These same concerns apply to the
encapsulated current sensing resistors used in modern dc
power supplies, for example.
The physical construction of current shunts rated for
greater than 10 A is typically like that shown in the
photograph of a 500 A, 50 mV shunt. The essential features
of a good shunt, either such a physically large shunt or an
encapsulated current sensing resistor, are:
•
Use of a resistance alloy with a low temperature coefficient.
Manganin is commonly used and has a temperature
coefficient over 100 times less than that of copper. This
minimises the change in resistance with heating.
•
Well separated terminals for the voltage and current.The
voltage isn’t then sensitive to the details of how the current
connection is made.
•
Substantial current terminals to minimise resistance and
therefore heating at the connection. With encapsulated
shunts you need to strive for a low resistance current
connection.
•
Voltage terminals that accurately define the shunt resistance.
The shunt shown illustrates the principles but is not ideal.
There is a voltage drop across the width of the screw
terminal that leads to a 0.2 percent variability in
the defined resistance. A better shunt
design makes the voltage connection at
the end of a wire that is permanently attached to the shunt.
Encapsulated current sensing resistors generally use this
remote voltage connection approach.
•
Capacity to dissipate the heat – the shunt shown in the
photograph has a fin structure to increase the surface area
for improved dissipation. Encapsulated current sensing
resistors may have a metal tab that is designed to be
bonded to a heat sink.
When we calibrate a current shunt we state the orientation
and environment of the shunt since this affects the airflow
across the resistance fins. We also describe the cables used for
the current connections since these will affect the magnitude
of the temperature rise.
With care the measurement uncertainty for large dc
currents can be reduced one or more orders of magnitude to
100 µA/A. As an alternative to resistive shunts there are now
current transducers that offer measurement uncertainties of
much better than 100 µA/A for currents up to thousands of
amperes. These “dc current transformers” use electronics to
produce a secondary current that is a scaled-down version of
the primary current.
Article by Laurie Christian, Measurement Standards
Laboratory of New Zealand, PO Box 31310, Lower Hutt.
Laurie can be contacted at l.christian@irl.cri.nz
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