# Winding Resistance Measurement Method

## Winding Resistance Measurement

### Winding Resistance

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. Ohm’s law can be written as:

{R}=\frac{V}{I}

where

• R is resistance in ohms
• V is voltage applied in volts
• I is the resulting current in amperes

### Why Winding Resistance Is an Important Measurement

Because winding resistance measurements can find problems not found using other tests and measurements (except by using impedance measurements), they are very important. See the list on this page for which types of problems can be found by measuring winding resistance.

### Problems That Can Be Found with a Winding Resistance Measurement

• Resistive connections, internally or externally
• All strands not connected with several in hand
• Partial blowout of multi-strand random wound coils
• Wrong gauge wire in coil or feeder cable
• Open or shorted coils
• Hard shorts in the windings to other turns or coils
• Wrong turn count
• Incorrect coil or group connections
• Can be found only with DC resistance tests
• Can be found with DC resistance and IR measurement/surge tests
• Can be found with DC resistance and surge tests

## Measuring Winding Resistance

### 2-Wire versus 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.

The winding resistance can also be measured with 4 wires. A 4-wire resistance measurement uses a Kelvin bridge or Wheatstone bridge to eliminate the lead resistance in the measurement device.

For 4-wire resistance measurement, 4 leads coming from the measurement device are connected in pairs to the ends of the DUT using Kelvin clamps. Each pair of leads has a drive lead and a sense lead. The resistance from one Kelvin clamp to the other is sensed or measured. Because only the DUT resistance is measured, while the resistance in the leads from the measurement device to the DUT is eliminated, the measurement of DUT resistance is more accurate.

### What Electrom Instruments Does

The Electrom iTIG motor tester and winding analyzer uses highly accurate 4-wire winding resistance measurements. iTIG models measure using a separate Kelvin clamp lead set or the same Kelvin clamps that are connected to the high voltage output leads that are used for DC hipot and surge tests. The measurements can be taken in milliohms or microohms, from a few µΩ to 2kΩ.

Image: Kelvin clamps from an iTIG being used in a 4-wire winding resistance measurement. A 4-wire measurement is more accurate than a 2-wire measurement.

### Measuring Winding Resistance

#### 2

###### “A common misunderstanding is that a surge test can always find a blowout in a random wound motor.”
Winding resistance standards
StandardAR100-2010CSAIEEE 1415-2006Commonly used
Balancenone0.023–5%2% or 3%
Percent deviation from the average of three phase measurements

#### Clearing Up a Common Misunderstanding

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 example 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

Illustrated at top, left: 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.

#### Standards

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:

\left( \frac {R\_max-R\_min}{R\_avg}  \right) \%

#### Temperature Compensation

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 Celsius 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.

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