BLDC Motor Weak But Not Burned Out? How to Diagnose Magnet Demagnetization
If the motor isn't burned out and the electricals check out, the culprit is often something else: magnet demagnetization – a common but often overlooked failure mode in brushless DC motors.
If the motor isn't burned out and the electricals check out, the culprit is often something else: magnet demagnetization.
Permanent magnets in BLDC motors
Here's how to spot it, test for it, and decide what to do next.
How to Tell Demagnetization from Electrical or Mechanical Failure
Demagnetization can be tricky to identify because its symptoms overlap with other failures. But there are telltale signs.
Typical symptoms of magnet demagnetization:
Reduced torque output under load (motor feels "weak")
Increased no load speed (KV increases as flux drops)
Higher running current without corresponding torque increase
Poor speed stability, especially at low RPM
Abnormal temperature rise under normal load
No phase-to-phase shorts or winding continuity issues
Demagnetization faults in PM machines significantly reduce output torque and spoil motor characteristics. When irreversible demagnetization occurs, motor performance is extremely degraded because the back EMF and output torque are reduced.
Quick exclusion checklist
Symptom | Likely Demag? | Likely Other Cause |
Torque loss, windings OK | Yes | — |
Torque loss, phase short | No | Winding short |
Speed high, low torque | Yes | — |
Grinding noise, torque OK | No | Bearing failure |
Over current, no torque | Maybe | Demag or controller issue |
On-site technical personnel should prioritize the following steps:
1. Check winding resistance (all three phases balanced)
2. Check for mechanical binding (spin shaft by hand – should be smooth)
3. Check controller parameters (current limits, hall sensors)
4. If all above check out → suspect demagnetization
How to Test Magnet Strength with a Gaussmeter and KV Test
Once you suspect demagnetization, you need to confirm it. Here's how.
Method 1 – Gaussmeter (Surface Field Measurement)
A gaussmeter measures magnetic flux density in Gauss (G) or Tesla (T). The Rotor Check 400 Gauss Meter is a practical testing device for checking the magnetic field strength curve of a rotor.
Step-by-step procedure:
1. Zero the gaussmeter – Move at least 300mm away from any steel surface or strong magnet
2. Set units – Select Gauss or Tesla
3. Position the probe – Place the probe flat against the pole face of the magnet, perpendicular to the surface
4. Take readings – Record surface field at each pole (measure all magnets)
5. Compare to baseline – If you don't have a baseline, compare readings across poles – significant variation indicates partial demagnetization
Tip: Keep the probe steady until the reading stops drifting. Avoid pressing the probe against the magnet – it can damage sensitive tips.
Method 2-KV (Back-EMF) Test (No Disassembly Required)
1. Spin the motor at a known speed (using a drill or another motor)
2. Measure the AC voltage between two phases with a multimeter
3. Calculate KV = RPM / Voltage (peak or RMS, be consistent)
4. Compare to the motor's rated KV – if it's significantly higher, flux has decreased
A simulated reduction in magnet coercivity by 30% results in increased current draw and failure to generate net torque. KV drift is one of the most reliable indicators of flux loss.
Method 3 – Load Test (Field-Practical)
Run the motor at a known load (e.g., a fan or pump at fixed speed). Record current draw and RPM. Compare to historical data or a known-good motor of the same model. Higher current for the same output = demagnetization.
What Causes Magnet Demagnetization?
Demagnetization falls into two categories: reversible and irreversible. Reversible is temporary – flux returns when the field is restored. Irreversible is permanent.
Temperature – The No.1 Killer
NdFeB (Neodymium):
Curie temperature: approximately 312°C
Maximum working temperature for high-coercivity grades: 200°C
Irreversible demagnetization typically begins at 80–140°C depending on grade
Magnetism drops significantly above 150°C
Heat resistance of different grades of NdFeB magnets
SmCo (Samarium Cobalt):
Superior thermal stability with maximum operating temperature up to 350°C
Higher intrinsic coercivity at elevated temperatures
The practical takeaway: If your motor runs above 100°C continuously and uses standard NdFeB magnets, you're at risk.
Other Causes
Armature reaction / inverse magnetic fields from high stator currents
Heavy inrush current during startup
Aging and micro-cracks over time
Mechanical shock and vibration
Strong external magnetic fields
How Much Performance Do You Lose?
Demagnetization doesn't just make the motor "weak" – it changes the entire operating behavior.
Quantitative relationships (approximate, load-dependent):
Flux Loss | Torque Loss (approx) | KV Increase (approx) |
5% | 5–8% | 3–5% |
10% | 10–15% | 6–10% |
20% | 20–25% | 12–18% |
30% | 30%+ | 20%+ |
These are approximations – actual values depend on load type, air gap, and winding parameters. But the pattern is clear: flux loss directly translates to torque loss and KV drift.
How the controller reacts:
To maintain torque, the controller increases phase current – which shows up as higher current draw with no performance gain
The controller may hit current limits and appear to "cap" performance
FOC controllers may struggle with parameter estimation if flux is wrong
Diagnostic check: Review controller logs for current draw vs commanded torque. If current is high but torque feels low, suspect demagnetization.
How Volcaomotor Can Help
At Volcaomotor, we eliminate the risk of premature demagnetization at the design stage. By utilizing premium SH, UH, or EH grade NdFeB magnets (and SmCo for extreme environments up to 350°C), our customized BLDC motors retain their flux stability under continuous, heavy‑duty industrial loads.
We also help you diagnose existing motors:
Download our Field Diagnostic Checklist and Gaussmeter Reading Template – includes step‑by‑step test procedures, baseline recording sheet, and decision flowchart
Schedule a remote diagnostic session – our engineers will review your controller logs, current waveforms, and test results
Send us your rotor for professional flux testing and remagnetization assessment – quick turnaround with detailed report
Ready to upgrade your system reliability?
Contact us for a free engineering consultation: info@volcaomotor.com
FAQ
Q1: What symptoms indicate magnet demagnetization rather than an electrical fault?
A1: Torque loss without winding shorts, increased no load speed (KV drift), higher running current without performance gain, and normal winding resistance. If the motor spins freely but feels weak under load, suspect demagnetization.
Q2: How do I measure magnetic flux with a gaussmeter?
A2: Zero the meter away from steel, place the probe flat against each magnet pole face perpendicular to the surface, hold steady, and record readings. Compare across all poles – significant variation indicates partial demagnetization.
Q3: How much torque or KV change should I expect from flux loss?
A3: Approximately proportional: 10% flux loss typically causes 10–15% torque reduction and 6–10% KV increase.
Q4: At what temperatures do common magnet materials begin to demagnetize?
A4: NdFeB irreversible demagnetization typically begins at 80–140°C depending on grade. SmCo withstands up to 350°C.
Q5: What field tests can I run using controller logs to prove demagnetization?
A5: Compare commanded torque vs actual current draw – if current is high but torque feels low, suspect demagnetization. Review harmonic components of stator current as a diagnostic mean