VOLTECH AT3600 USER MANUAL KELVIN CONNECTIONS
3. KELVIN CONNECTIONS
In testing many transformer parameters, such as winding resistance or inductance, it
is necessary to measure electrical impedance.
The normal method of measuring impedance is to pass a test current through the
unknown component, and to measure the resulting voltage produced across it.
Dividing the voltage by the current gives the required value of impedance.
In making such measurements, great care must be taken not to include the impedance
of the measuring leads in the result. A connection system that avoids such problems
is shown below; it uses four wires and is often referred to as a Kelvin connection.
In this arrangement, the test current passes through the two ‘power’ leads, and the
voltage is measured using the two ‘sense’ leads.
Provided that the sense leads are connected as close as possible to the body of the
device under test, any additional voltage drop produced by the test current flowing
through the impedance of the power leads is not measured.
The Kelvin connection therefore provides the most accurate means of sensing the
voltage, and hence the impedance of the winding.
Ideally, all impedance measurements would be made using Kelvin connections.
However, many terminals do not permit the use of four wires all the way to the body
of the component under test.
In such cases, separate power and sense leads are used up to the base of the terminal,
and the length of ‘common’ lead (from the junction of the power and sense leads,
through the terminal and the component lead to the body of the component) should be
kept to a minimum.
FIXTURES VPN 98 - 024 V07 PAGE 5.3.1.
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