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Measuring Motor Winding Resistance

Measuring Motor Winding Resistance

Saturday, August 6, 2022 4 Wire Resistance testing motors

4 wire Probes vs Clips, which is better?

Resistance Meter comparison with Clips versus Probes

I have purchased a set of 4 wire Resistance Probes because using Clips is impossible to use on Screw terminals while in place. Also the Clips are very difficult to use on smaller motors while trying to get the clips to sit flat. Often they will fall off at the crucial time.

So is there a Difference?

Measurement with small clips 2.9167 Ohms

Measurement with Probes 2.9170 Ohms

As you can see I didn't measure in the exactly the same position but both were on the terminal posts.

The difference of 0.3 milli Ohm for this motor is 0.01% difference.

I'm happy with that. Problem solved for measuring Motor Winding Resistances and Circuit Breakers.

Note: When a Motor Size exceeds 75kW the winding resistance is typically below  10 milli Ohm, therefore in these cases a Micro Ohm meter will detect an imbalance in resistance.

I have measured with Probes New Contactors & Isolators with surprising results. The resistance imbalance from New are quite large, more than 100% imbalance.

As these components age with switching contacts decay and resistances increases. A recommended Electrical Maintenance task is to check these resistances.

Isolator High Resistance- Tab broken Internally

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