Winding resistance, the resistance of a length of copper wires or bars from one end to the other, is a measure of DC voltage and current and the application of Ohm’s law as follows:
where R is resistance in Ohms, V is voltage applied in Volts and I is the resulting current in Amperes.
2-wire vs 4-wire measurement
The winding resistance can be measured with 2 wires from the measurement device connected to each end of the DUT. In this case the resistance measured will include the resistance of the leads from the measurement device to the DUT.
With a 4-wire measurement 4 leads come from the measurement device and are connected in pairs to the ends of the DUT using what is called Kelvin clamps. Each pair has a drive lead and a sense lead, and the resistance is “sensed” or measured from one Kelvin clamp to the other. In other words, only the DUT resistance is measured, the resistance in the leads from the measurement device to the DUT is eliminated. Consequently the measurement of DUT resistance is more accurate.
A 4-wire resistance measurement uses a Kelvin bridge or Wheatstone bridge to eliminate the lead resistance in the measurement device.
What Electrom Instruments does
Electrom’s iTIG III series of motor testers and winding analyzers use highly accurate 4-wire winding resistance measurements. Models come with measurements done through a separate Kelvin clamp lead set, or through Kelvin clamps connected to the high voltage output leads used for DC hipot and surge tests. The measurements can be in milli Ohms or micro Ohms from a few µΩ to 2kΩ.
Why winding resistance is an important measurement to make
Winding resistance measurements can find problems not found with other tests and measurements (other than impedance measurements), and is therefore very important. Following are problems that can be found.
A common misunderstanding is that a surge test can always find a blowout in a random wound motor. It does if there is a turn to turn short, short between coils, or short to ground. But, with a situation like the one below a fault will not be found with a surge test because there is no change in the winding inductance, little if any in the winding capacitance, and a surge test is independent of winding resistance. See: What causes differences in surge test waves?
Partial Blowout example: Four in-hand (or 4 magnet wires in parallel per coil), two blown out, no turn to turn short and no short to ground. Two wires are still intact, so the inductance in the coil is not changed.
Winding resistance can be a comparison to an absolute number of Ohms or fractions of Ohms if the target resistance is known. It can also be a comparison of phase to phase resistances in a 3-phase motor or generator with a calculation of balance (or imbalance).
The balance is calculated as the percentage of the maximum difference between the three resistance measurements divided by the average of the three measurements made phase to phase: (R_max – R_min)/R_avg %.
If winding resistance measurements are to be compared and tracked over time, the measurements need to be compensated for temperature unless the temperature is the same every time. Copper for example has a temperature coefficient of about 0.0039 per degree C for moderate temperatures. This means if the temperature changes by 10°C, the resistance changes by close to 4%.
If what is important is the resistance balance in the phases, then temperature compensation is not necessary since the balance calculation is a ratio and the compensation factor falls out.