Motor Winding Resistance Imbalance Calculator

A motor winding resistance imbalance calculation compares the measured DC resistance of the three stator phase windings. The primary result is Resistance imbalance, expressed as a percentage of the average phase resistance.

Unequal phase winding resistance can point to a connection problem, unequal conductor condition, winding damage, or a measurement condition that needs further investigation. The calculation organizes preliminary motor-test readings; it does not by itself establish motor condition, identify a fault location, or replace the acceptance criteria specified by the motor manufacturer or test procedure.

This calculation is used during motor commissioning, maintenance troubleshooting, rewind evaluation, and comparative trending. It is commonly reviewed before interpreting related motor-test results such as insulation-resistance testing, surge testing, current balance, operating voltage imbalance, vibration analysis, and temperature performance.

Phase Resistance Readings

Enter the asset identification in Motor or asset label and record each measured winding resistance:

Input	Purpose
Motor or asset label	Identifies the motor, asset, or test context associated with the readings
Phase A resistance (ohm)	Measured resistance of the Phase A winding circuit
Phase B resistance (ohm)	Measured resistance of the Phase B winding circuit
Phase C resistance (ohm)	Measured resistance of the Phase C winding circuit
Entered reference imbalance (%)	Optional manufacturer, maintenance-program, or test-procedure reference value

Resistance readings should be taken using a suitable low-resistance test method for the motor and resistance range involved. Small motors may have readings high enough for an ordinary ohmmeter to resolve adequately, while larger motors often require a low-resistance instrument and proper lead-compensation method.

The three readings must represent comparable test conditions. Record the same winding connection condition, test-lead configuration, approximate winding temperature, and measurement method for all phases. A resistance difference created by poor probe contact, oxidized terminals, loose lugs, or unequal lead resistance can appear as winding imbalance.

Resistance Imbalance Formula

The calculator first determines the average of the three entered phase resistances:

\(\displaystyle R_{\text{avg}} = \frac{R_A + R_B + R_C}{3}\)

It then determines the smallest and largest measured values:

\(\displaystyle R_{\text{min}} = \min(R_A, R_B, R_C\)

\(\displaystyle R_{\text{max}} = \max(R_A, R_B, R_C\)

The reported Maximum deviation is the greatest absolute difference between an individual phase resistance and the average resistance:

\(\displaystyle \Delta R_{\text{max}} = \max \left( |R_A - R_{\text{avg}}|, |R_B - R_{\text{avg}}|, |R_C - R_{\text{avg}}| \right)\)

The calculator reports Resistance imbalance as:

\(\displaystyle \text{Resistance imbalance (\%)} = \frac{\Delta R_{\text{max}}}{R_{\text{avg}}} \times 100\)

This method expresses the furthest phase-resistance departure as a percentage of the three-phase average. It is not the same as simply reporting the total spread between the highest and lowest readings.

Calculation Example

For Motor M-1, the preliminary motor-test record is:

Reading	Value
Phase A resistance (ohm)	0.42 ohm
Phase B resistance (ohm)	0.43 ohm
Phase C resistance (ohm)	0.41 ohm
Entered reference imbalance (%)	0%

The average resistance is:

\(\displaystyle R_{\text{avg}} = \frac{0.42 + 0.43 + 0.41}{3} = 0.42\ \text{ohm}\)

The minimum phase resistance is 0.41 ohm, and the maximum phase resistance is 0.43 ohm.

The greatest departure from the 0.42-ohm average is 0.01 ohm:

\(\displaystyle \Delta R_{\text{max}} = 0.01\ \text{ohm}\)

The resulting winding resistance imbalance is:

\(\displaystyle \frac{0.01}{0.42} \times 100 = 2.381\%\)

The resulting record is:

Result	Value
Average resistance	0.42 ohm
Minimum phase resistance	0.41 ohm
Maximum phase resistance	0.43 ohm
Maximum deviation	0.01 ohm
Resistance imbalance	2.381%
Margin vs entered reference	0%

With Entered reference imbalance (%) left at 0, the displayed Margin vs entered reference is 0%. Enter a documented manufacturer or procedure reference when a defined comparison value is required.

Field Verification

A winding-resistance imbalance result must be evaluated alongside the motor’s test conditions and applicable acceptance criteria.

• Verify that all three readings were obtained at substantially the same winding temperature. Copper resistance changes with temperature, so readings from different thermal conditions are not directly comparable.
• Confirm the motor was isolated and tested in the same terminal configuration for every phase. Series-parallel connections, wye or delta lead arrangements, and external circuit paths can change the resistance observed at the terminals.
• Inspect motor leads, splices, terminal lugs, contactors, disconnects, and test points when one phase is elevated. A high-resistance connection can produce an abnormal reading without proving internal winding damage.
• Use an instrument, test current, lead-compensation method, and measurement resolution appropriate for low-ohm winding testing.
• Compare the result with the motor manufacturer’s documented limits, maintenance procedure, or qualified test standard. The calculator does not provide a pass/fail threshold.
• Investigate a meaningful imbalance with additional testing as appropriate, including connection inspection, phase-to-phase voltage review under load, current balance, insulation-resistance testing, or motor-shop diagnostic testing.

Motor winding resistance is a diagnostic measurement, not an ampacity calculation. It does not determine branch-circuit conductor AWG or kcmil, feeder ampacity, adjustment factor, correction factor, terminal rating, insulation temperature rating, raceway fill, or voltage-drop compliance. Those installation and code decisions require the applicable equipment data, conductor information, load conditions, and AHJ requirements.