Making Fixture Connections - Kelvin Connections - Voltech AT5600 User Manual

C H A P T E R 1 3 – T E S T
13.4. Making Fixture Connections – 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.
The 'common' lead length is shown as AA' and BB' in the diagram below.
AT5600 User Manual 98-119 issue 14
F I X T U R E S
Page 262