Brushless motors usually have three phase wires

Brushless motors are widely used in drones, electric bikes, RC vehicles, power tools, and industrial equipment because they are efficient, durable, and low-maintenance. However, like any electrical component, a brushless motor can fail due to overheating, damaged windings, or wiring issues. Testing a brushless motor with a multimeter is a simple and effective way to diagnose basic problems before replacing or repairing it. This article explains how to check bldc motor using a multimeter.

Understanding Brushless Motor Basics

A brushless DC (BLDC) motor typically has three wires, often colored yellow, blue, and green. These wires connect to the motor’s internal windings, known as phases. Unlike brushed motors, brushless motors do not have physical brushes or commutators, so testing focuses mainly on checking winding resistance and electrical continuity.

Tools and Safety Precautions

Before starting, gather the following tools:

• A digital multimeter

• Safety gloves (optional but recommended)

• Clean working surface

Always disconnect the motor from its power source or controller before testing. Never test a motor while it is connected to a battery or power supply, as this can damage the multimeter and pose a safety risk.

Step 1: Set the Multimeter Correctly

Turn your multimeter to the resistance (Ω) mode. If your multimeter is not auto-ranging, select the lowest resistance range (usually 200 ohms). Brushless motor windings have very low resistance, so accuracy at low values is important.

Step 2: Identify the Motor Wires

Brushless motors usually have three phase wires. If the wires are not labeled, simply identify them as Wire A, Wire B, and Wire C. You will be measuring resistance between each pair of wires.

Step 3: Measure Phase-to-Phase Resistance

Place one multimeter probe on Wire A and the other probe on Wire B. Note the resistance reading. Repeat this process for:

• Wire B to Wire C

• Wire C to Wire A

All three readings should be very close to each other, typically between 0.1 and 1 ohm depending on the motor size. Minor differences are normal, but a large difference indicates a problem.

Step 4: Interpret the Results

• Equal resistance readings: This usually means the motor windings are healthy.

• Infinite resistance (OL or no reading): This indicates a broken or disconnected winding.

• Zero resistance: This suggests a short circuit in the motor winding.

• One reading significantly different: This may indicate partial winding damage or overheating.

If any abnormal readings appear, the motor may need repair or replacement.

Step 5: Check for Short to Ground

Set the multimeter to continuity or resistance mode. Place one probe on the motor’s metal casing and the other probe on each phase wire, one at a time. The multimeter should show no continuity or infinite resistance.

If continuity is detected between a phase wire and the motor casing, it means the winding is shorted to ground, and the motor is faulty.

Step 6: Spin Test (Optional)

While not a multimeter test, you can gently rotate the motor shaft by hand. It should rotate smoothly with slight magnetic resistance. Grinding, stiffness, or uneven movement may indicate bearing damage rather than an electrical issue.

Limitations of Multimeter Testing

A multimeter test can identify basic electrical faults, but it cannot detect issues such as:

• Demagnetized rotor

• Controller problems

• Performance issues under load

For advanced diagnostics, specialized equipment or a motor controller test may be required.

Conclusion

Testing a brushless motor with a multimeter is a quick and effective way to check for common electrical faults such as open windings, short circuits, and grounding issues. By following proper safety steps and comparing resistance readings carefully, you can determine whether your motor is in good condition or needs further attention. This simple procedure can save time, money, and unnecessary replacements.