Motor Ohms Calculator
Calculate motor winding resistance & impedance for single-phase and three-phase AC motors (Star and Delta layouts) using Ohm's Law. Troubleshoot insulation unbalance and winding health.
Motor Ohms Calculator
How to Use Motor Ohms Calculator
Using this calculator for troubleshooting or diagnostics is straightforward. Whether checking stator coils, rotor windings, or verifying system voltage, the tool helps you analyze electrical systems safely. Follow this standard diagnostic checklist to compute motor values:
- 1Select Motor Phase Configuration: Choose from Single-Phase AC Motor, 3-Phase Star (Wye), or 3-Phase Delta connected winding depending on your motor nameplate layout.
- 2Select Test Method: Choose DC Resistance Test (Offline) for standard multimeter diagnostic checks, or AC Impedance Test (Running Load Test) to analyze electrical parameters under active voltage.
- 3Enter known voltage: Input the measured voltage in Volts (V) applied across the terminals.
- 4Enter known current: Input the measured current in Amperes (A) flowing through the circuit.
- 5Click calculate: Click the Calculate button to run the configuration-specific Ohm's Law formulas.
- 6Review results: Verify calculated line resistance/impedance, target winding resistance/impedance, and the exact formula applied.
How to Calculate Motor Ohms
Sizing and diagnosing motor windings is performed using classic electrical engineering formulas derived from Ohm's Law. In pure DC environments, or when performing offline resistance tests with a low-voltage DC source, current flow is restricted only by copper conductor resistance. Winding layout geometry determines how measured line resistance relates to actual internal winding resistance:
1. Single-Phase AC Motors
In single-phase motors, windings are tested independently. Line-to-line resistance measured directly represents the specific winding under test (Main/Run or Auxiliary/Start):
2. Three-Phase Star (Wye) Motors
In a Star-connected stator, measuring terminal resistance across any two line terminals tests two phase windings connected in series. The neutral node floats, meaning winding resistance is exactly half the measured line value:
3. Three-Phase Delta Motors
In a Delta-connected stator, measuring terminal resistance tests one phase winding in parallel with the other two phase windings in series. Solving this parallel network reveals that phase winding resistance is 1.5 times the measured line resistance:
Practical Engineering Example (DC Offline Test)
Assume you are diagnosing a 3-Phase Delta connected induction motor. You apply a DC test voltage of 12 V across terminal leads T1 and T2, measuring a current of 2 A.
Given Parameters:
- Motor Configuration: 3-Phase Delta
- DC Test Voltage (V): 12 V
- DC Current (I): 2 A
Step 1 — Calculate Measured Line Resistance
Divide Voltage by Current using Ohm's Law:
Step 2 — Calculate Phase Winding Resistance
Apply Delta parallel correction multiplier (1.5):
Running AC Load Test vs Offline DC Winding Test
Under running AC conditions, motor winding current is restricted by both resistance and inductive reactance (called Impedance, Z). For three-phase systems under load, line impedance relates to winding impedance based on the active AC formulas: Wye phase impedance is Zline / √3, while Delta phase impedance is Zline × √3. Direct DC testing bypasses inductive reactance, allowing engineers to isolate and compare pure copper resistance to locate short-circuited turns or unbalanced winding paths.
Motor Ohms Calculator Chart
This reference chart displays verified mathematical values for voltage, current, line resistance, and calculated phase winding resistance across typical test scenarios for single-phase and three-phase AC motor configurations (DC Offline Resistance Test).
| Motor Configuration | Voltage (V) | Current (A) | Line Resistance (Ω) | Phase Winding Resistance (Ω) |
|---|---|---|---|---|
| Single-Phase AC | 12 V | 2 A | 6.00 Ω | 6.00 Ω |
| 3-Phase Star (Wye) | 12 V | 2 A | 6.00 Ω | 3.00 Ω |
| 3-Phase Delta | 12 V | 2 A | 6.00 Ω | 9.00 Ω |
| Single-Phase AC | 24 V | 4 A | 6.00 Ω | 6.00 Ω |
| 3-Phase Star (Wye) | 24 V | 4 A | 6.00 Ω | 3.00 Ω |
| 3-Phase Delta | 24 V | 4 A | 6.00 Ω | 9.00 Ω |
| 3-Phase Star (Wye) | 120 V | 10 A | 12.00 Ω | 6.00 Ω |
| 3-Phase Delta | 230 V | 10 A | 23.00 Ω | 34.50 Ω |
Note: Phase winding resistance calculations isolate individual stator winding values from combined terminal readings, allowing accurate diagnostics of winding symmetry and health.
VFD Harmonic Heating and Shaft Currents in Motor Ohms
Variable Frequency Drives (VFDs) are excellent for adjusting the speed of motors in Motor Ohms setups, but they output pulse-width modulated (PWM) voltage waves instead of pure sine waves. These fast voltage transients cause harmonic currents, which increase core heating and stator insulation stress.
Additionally, high-frequency voltage spikes cause capacitive common-mode currents to build up on the motor shaft, discharging through the bearings and causing micro-pitting. Installing shaft grounding rings and dV/dt output filters protects motors from VFD-induced damage.
Starting Currents and Voltage Sag Control in Motor Ohms Motors
Electric motors used in Motor Ohms systems draw high inrush currents during startup, typically 5 to 8 times the normal full-load current (FLA). This transient surge can trigger voltage drops across local feeders, disrupting nearby electronics. Sizing starting devices properly is key to system stability:
To mitigate voltage sags, engineers use VFDs (Variable Frequency Drives), soft starters, or Star-Delta starting configurations. VFD starting is highly recommended for Motor Ohms because it limits the starting current to 1.5 times FLA while maintaining high starting torque.
Motor Ohms Calculator Frequently Asked Questions
Motor resistance, specifically winding resistance, refers to the electrical resistance of the copper conductors that make up the motor windings. Measured in Ohms (Ω), it determines the current flow for a given voltage and is a critical factor in determining efficiency and identifying internal faults such as shorted or broken coils.
For single-phase motors, winding resistance is calculated directly using Ohm's Law (R = V / I). For three-phase Star (Wye) connected motors, the phase winding resistance is half of the measured line-to-line resistance (R_phase = R_line / 2). For Delta connected motors, it is 1.5 times the measured line-to-line resistance (R_phase = 1.5 × R_line).
Motor winding resistance is important because even minor deviations from specs point to insulation degradation, phase unbalance, loose connections, or short circuits, which cause overheating and motor failure. Winding checks help prevent costly downtime.
Yes, Ohm's Law (V = I × R) can be used on motor windings to calculate DC winding resistance. However, when the motor is running on AC power, inductive reactance is active. The total opposition to current is AC impedance (Z), which is calculated using AC line parameters: Z = V / I.
A good winding resistance depends on motor size. Three-phase windings must be balanced, with line-to-line readings within 1% to 3% of each other. High-horsepower motors have extremely low resistance (fractions of an ohm), while small motors have higher values (tens of ohms).
Yes, copper resistance increases with temperature. Electrical resistance rises by roughly 0.393% per degree Celsius. To compare measurements accurately with nameplate specs, values must be corrected to a standard reference temperature, typically 20°C or 25°C.
In a Delta motor, measuring line-to-line terminal resistance tests one winding in parallel with the other two windings in series: R_line = R × (2R) / (R + 2R) = (2/3)R. Solving for the individual phase winding resistance (R), we get: R = 1.5 × R_line.
In a Star motor, the neutral node is not connected to the test leads. Measuring line-to-line terminal resistance tests two phase windings connected in series: R_line = R + R = 2R. Therefore, the resistance of a single phase winding is R = R_line / 2.