Brushless DC (BLDC) motors are widely used in electric vehicles, drones, industrial machines, and home appliances because of their high efficiency, long life, and low maintenance. When a BLDC motor fails to run properly, a multimeter is one of the most useful tools for basic troubleshooting. Although a BLDC motor also relies on an electronic controller, you can still identify many common motor faults using a multimeter. This article explains how to check bldc motor with multimeter step by step.
Tools and Safety Precautions
Before starting, you will need:
- A digital multimeter (with resistance and continuity modes)
- Screwdrivers or wrenches to access motor terminals
- Insulated gloves (recommended)
Always disconnect the motor from the power supply and controller before testing. Never test resistance or continuity on a powered circuit, as this can damage the multimeter and cause injury.
Step 1: Visual Inspection
Begin with a visual check of the motor and wiring. Look for:
- Burnt or discolored wires
- Melted insulation
- Loose or broken connectors
- Signs of overheating or physical damage
If you notice obvious damage, the motor may already be faulty, but electrical testing will help confirm the issue.
Step 2: Identify Motor Wires
Most BLDC motors have three main phase wires, usually colored yellow, blue, and green (colors may vary). These wires connect the motor to the controller. Some BLDC motors also have thinner Hall sensor wires, but this article focuses on checking the motor windings.
Label the three phase wires as U, V, and W for easier reference.
Step 3: Set the Multimeter to Resistance Mode
Turn the multimeter dial to the ohms (Ω) setting. If your multimeter is not auto-ranging, select the lowest resistance range (usually 200 Ω).
Touch the multimeter probes together to ensure it reads near zero. This confirms the multimeter is working properly.
Step 4: Measure Phase-to-Phase Resistance
Measure resistance between each pair of motor phase wires:
- U to V
- V to W
- W to U
Record the readings. In a healthy BLDC motor:
- All three resistance values should be very close to each other
- Typical resistance is low, often between 0.1 Ω and 10 Ω, depending on motor size
If one pair shows significantly higher resistance or “OL” (open loop), it indicates a broken winding. If one pair shows near zero resistance compared to others, it may indicate a shorted winding.
Step 5: Check for Short to Ground
Next, test whether any phase wire is shorted to the motor body.
- Set the multimeter to continuity or resistance mode.
- Place one probe on the motor’s metal housing.
- Place the other probe on each phase wire, one at a time.
A good motor should show no continuity and very high resistance (often OL). If the multimeter beeps or shows low resistance, the winding insulation has failed, and the motor is shorted to ground.
Step 6: Spin Test with Resistance Observation (Optional)
While measuring resistance between two phase wires, gently rotate the motor shaft by hand. The resistance may fluctuate slightly due to magnetic effects, but it should remain generally stable. Sudden drops to zero or erratic readings may suggest internal damage.
Also, the motor should spin smoothly without grinding or locking. Mechanical resistance can indicate bearing failure or rotor damage.
Step 7: Interpreting the Results
- Equal resistance across all phases: Motor windings are likely healthy.
- One phase open or shorted: Motor winding failure.
- Short to motor housing: Insulation breakdown; motor replacement is recommended.
- Electrical readings normal but motor still not working: The issue may be with the controller, Hall sensors, or power supply rather than the motor itself.
Conclusion
Checking a BLDC motor with a multimeter is a simple but effective way to diagnose common electrical problems. By measuring phase resistance, checking for shorts to ground, and observing mechanical behavior, you can quickly determine whether the motor is healthy or faulty. While this method does not test the electronic controller or sensors, it provides a solid foundation for troubleshooting and can save time and repair costs.
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