Motor winding resistance is the electrical resistance of the copper coils inside a motor, and testing it is one of the most reliable ways to detect faults before they cause failure. A multimeter and five minutes are all you need for the basic test. This guide covers what winding resistance is, why it matters, the tools required, and a step-by-step testing procedure for the three most common faults.
What Is Motor Winding Resistance?
Motor winding resistance is the resistance of the wire coils that make up the stator (and rotor, in some motor types) of an electric motor. These coils carry the current that generates the magnetic field responsible for producing torque and rotation.
Resistance in the windings is determined by the wire’s length, cross-sectional area (gauge), material resistivity, and number of turns. Temperature also plays a role — as a motor heats up, winding resistance increases due to the increased movement of metal ions in the conductor. This is why winding resistance measurements should always be taken at a known, stable temperature and compared against manufacturer specifications at the same reference temperature.
Every motor has a specified winding resistance value in its datasheet. Deviations from this value — either too high or too low — indicate a problem in the winding or associated circuitry.
Why Does Winding Resistance Matter?
Winding resistance directly affects motor efficiency, thermal behavior, and reliability:
- Too high: Increased resistance means more electrical energy is converted to heat rather than mechanical output. The motor runs hotter, draws more current than expected, and operates less efficiently. Over time, excess heat degrades winding insulation and shortens motor life.
- Too low: Abnormally low resistance indicates a short circuit between winding turns or phases. A shorted winding draws excessive current, generates concentrated heat, and can cause rapid motor failure if not identified and addressed.
- Imbalanced phases: On a three-phase motor, resistance should be equal across all three phases. An imbalance greater than 3–5% causes uneven current distribution, vibration, reduced torque, and accelerated bearing wear.
For drone motors and other high-performance brushless systems, monitoring winding resistance is particularly important — these motors operate at high current densities where even small deviations from spec can affect thrust output and thermal management.
What Tools Do You Need?
For basic winding resistance testing, you need:
- Digital multimeter (DMM) — set to the resistance (ohms) function. Adequate for most diagnostic checks where resistance values are in the range of 0.5–10 Ω.
- Micro-ohmmeter or precision bridge — required for accurate measurements on low-resistance windings (milliohm range), common in large industrial motors and high-performance brushless motors. Standard multimeters introduce lead resistance error at these values.
- Insulation resistance tester (Megger) — needed for insulation testing between windings and ground. Applies 500V DC or higher; a standard multimeter cannot perform this test.
- Motor datasheet — essential for knowing the manufacturer’s specified resistance values and acceptable tolerance ranges.
For most multi rotor motors and small brushless motors, a good-quality digital multimeter with 0.1 Ω resolution is sufficient for phase resistance checks and basic fault diagnosis.
How to Test Motor Winding Resistance
Before testing, disconnect the motor from all power sources and allow it to cool to ambient temperature. Isolate the motor from its drive or ESC by disconnecting all wiring at the motor terminals.
Testing for a Short to Ground
A short to ground occurs when a winding wire contacts the motor frame or housing, creating a current path to earth. Symptoms include tripping circuit breakers, reduced power output, and shock risk.
- Set the multimeter to the highest resistance range (or use an insulation tester for accurate measurement).
- Place one probe on a winding terminal (any phase).
- Place the other probe on the motor casing or frame — a bare metal surface, free of paint or corrosion.
- Record the reading.
Expected result: Infinite resistance (OL on the display) — no connection between winding and frame.
Fault indicated: Any finite resistance reading, especially near zero, confirms a short to ground. The motor requires rewinding or replacement.
Repeat for each phase terminal. A single grounded phase is enough to take the motor out of service.
Testing for Shorts Between Windings (Phase-to-Phase)
A phase-to-phase short occurs when insulation fails between winding turns or between phases, creating an unintended low-resistance path. The affected phase draws more current than the others, generating localized heat.
- Set the multimeter to the ohms function on an appropriate range.
- Measure resistance between each pair of phase terminals: T1–T2, T2–T3, T1–T3.
- Record all three readings.
Expected result: All three readings are equal, matching the manufacturer’s specified phase resistance within 3–5%.
Fault indicated: One pair showing significantly lower resistance than the others indicates a short between those phases. A reading near zero confirms a direct short circuit.
Testing for an Open Circuit
An open circuit means the winding wire is broken — no current can flow through the affected phase. Causes include broken conductors, burnt windings, and corroded or loose terminal connections.
- Set the multimeter to the ohms function.
- Measure resistance between each pair of phase terminals.
- Record the readings.
Expected result: Low, consistent resistance across all phase pairs matching the datasheet specification.
Fault indicated: Infinite resistance (OL) on any phase pair confirms an open circuit in that winding. No current can pass, and the motor will not run or will run erratically on the remaining phases.
How to Interpret Your Results
Once you have measurements from all three tests, compare them against the motor’s datasheet specifications:
| Result | Likely Cause | Action |
| All phases balanced, match spec | Windings healthy | No action required |
| One phase higher than others | Partial open or loose connection | Check terminals; retest |
| One phase lower than others | Partial short between turns | Rewinding likely required |
| Any phase reads zero (short) | Direct winding short | Remove from service immediately |
| Any phase reads OL (open) | Broken wire or burnt winding | Rewinding or replacement required |
| Any phase reads near zero to ground | Short to ground | Remove from service immediately |
| All phases elevated above spec | Overheating damage or degraded wire | Check thermal history; monitor closely |
A single abnormal reading is grounds for investigation. Two or more abnormal readings usually indicate the motor requires rewinding or replacement rather than repair.
For ongoing health monitoring, record baseline resistance values for each motor when new and compare them at regular intervals. A gradual increase in resistance over time indicates insulation degradation; a sudden change indicates an acute fault. This approach is particularly useful for drone components operating in high-vibration or thermally demanding environments where winding stress accumulates over many flight cycles.
Frequently Asked Questions
What is a normal winding resistance value for a brushless motor?
It depends on the motor’s size and KV rating. Small high-KV FPV motors typically measure 0.05–0.3 Ω per phase; larger, lower-KV multi-rotor motors range from 0.1–1.0 Ω. Always compare against the manufacturer’s datasheet rather than a generic figure.
Can I test winding resistance with the motor connected to the ESC?
No. Always disconnect the motor from the ESC and all power sources before testing. Measuring through a connected ESC gives inaccurate readings and risks damaging the ESC’s MOSFETs.
How often should I test motor winding resistance?
For industrial motors, annual testing is standard under normal conditions; quarterly for motors under continuous heavy load. For drone motors, test after any crash, overheating event, or unusual performance change.
What causes winding resistance to increase over time?
Heat is the primary cause — sustained high temperatures degrade copper conductors and increase resistivity. Moisture ingress, vibration damage, and corrosion also contribute to resistance increases over the motor’s service life.
Is a megger test the same as a multimeter resistance test?
No. A multimeter measures DC resistance in ohms and detects shorts and opens in the windings. A megger applies high DC voltage (typically 500V or 1000V) to measure insulation resistance in megaohms — it tests the integrity of the insulation between the windings and ground, which a standard multimeter cannot assess.
Conclusion
Testing motor winding resistance is a straightforward diagnostic process that reveals shorts, open circuits, and insulation degradation before they cause operational failure. A basic multimeter handles most checks; a micro-ohmmeter or Megger is required for precision industrial testing. Record baseline values when the motor is new, test regularly, and act on any deviation from specification before it becomes a failure. YAHREE manufactures brushless motors and drone components built to consistent winding specifications — contact us if you need datasheet values or technical support for your application.
