What is it used for?
The winding resistance test is used to find open windings, shorts to ground, wrong turn count, wrong wire gauge, resistive connections, round wires in hand that are not connected in a coil, some connection mistakes, the resistance balance between phases, and in some cases shorted turns.
Winding resistance is an important measurement because other tests and measurements will not find some of the problems a resistance measurement will find. Some of these are the wire gauge issues, resistive connections, and blown or disconnected magnet wires in hand.
How it works:
Winding resistance tests are a measurement of the applied DC voltage and current to the device under test -DUT. Using Ohm’s law the resistance is calculated in µΩ (micro Ohms) or mΩ (milli Ohms) by the winding analyzer.
For a 3-phase motor, 3 resistance measurements are normally done between the phases, and the balance or imbalance between the 3 measurements is calculated and displayed along with the measurement values.
For single coils and sometimes for motors, the resistances measured may be compared to a resistance value instead of having a balance calculated. The difference in percent to the target resistance value is then calculated.
A temperature correction factor can automatically be applied to correct the measurement to a standard temperature so results can be more accurately tracked over time with the winding analyzer.
2-wire vs 4-wire resistance measurements
Resistance can be measured with two leads connected to a DUT. This type of resistance measurement will include the tester’s leads in the winding resistance test’s measurement. A four lead measurement is better. It eliminates the lead resistance and is typically more accurate.
All winding resistance measurements done with Electrom Instruments motor testers are done with a four lead system. For the iTIG III Model D the leads are the high voltage output leads so the micro Ohm winding resistance measurement can be part of an automatic sequence of motor tests including high voltage tests.