A roughly 1.5 kW brushless motor installation developed a key that appeared welded between a gearbox and transmission shaft. The evidence does not establish electrical discharge as the cause. Because the damage appeared only at the key-to-gearbox interface while the shaft remained clean, first distinguish mechanical galling from electrical arcing before changing protection or grounding.
Separate Mechanical Galling From Electrical Damage
Localized adhesion can result from seizure, fretting, excessive contact pressure, poor fit, or inadequate lubrication. Electrical damage remains a hypothesis until the interface shows arc craters, localized melting, or resolidified metal. Preserve and inspect the key and both keyways rather than relying on the extraction force or a welded appearance alone.
| Observation | Engineering interpretation | Next check |
|---|---|---|
| Damage only at the gearbox side | Supports a localized interface problem; it does not prove leakage current | Check fit, deformation, lubrication, fretting, and seizure marks |
| Shaft surface remains clean | A continuous current path through the shaft is not demonstrated | Look for corresponding electrical marks at bearings, couplings, and grounding points |
| Bearings and protective conductor show no damage | Weakens, but does not independently eliminate, the high-current explanation | Inspect them for pitting, heating, or discoloration |
| Machine may have been welded previously | Welding current could have crossed an unintended mechanical interface | Review repair history and welding return-clamp placement |
Interpret the 300 mA Differential Device Carefully
The machine supply includes a 300 mA differential device upstream of the brushless drive. Its failure to trip does not prove that no motor-side current occurred. The evidence does not identify the drive topology, differential-device type, leakage waveform, current path, or duration, so it cannot establish whether a particular downstream event would be detected.
Conversely, a current large enough to fuse metal should prompt a search for a complete return path and collateral evidence. Inspect the protective conductor, motor and gearbox bonding, bearings, couplings, and drive fault history. Do not increase or otherwise alter the 300 mA protection setting merely to address an unconfirmed shaft-current theory.
Use a Controlled Diagnostic Sequence
- Photograph and retain the key, then inspect both mating surfaces for galling, fretting, arc craters, and resolidified metal.
- Check key and keyway dimensions, surface condition, alignment, lubrication, transmitted load, and evidence of relative motion.
- Review whether welding was performed on the machine and where the welding return clamp was attached. An unsuitable return path can force welding current through shafts, bearings, keys, or couplings.
- Inspect bearings, the protective conductor, bonding connections, and adjacent interfaces for pitting, heating, or discoloration that would support an electrical-current path.
- Review drive alarms or trip history and verify the installed differential device without assuming that its 300 mA marking alone defines response to every drive-output fault.
Verify Motor Insulation Without Exposing the Drive
An insulation test can help detect winding-to-frame insulation failure, but the supplied evidence gives no approved test voltage, acceptance limit, winding connection, or drive-specific procedure. Disconnect and isolate the motor from the electronic drive before applying an insulation-test instrument, then use the motor and drive manufacturers' instructions for terminal preparation, test conditions, and limits.
A tester used for machinery verification under 60204-1 should not be connected through the drive unless the drive documentation explicitly permits it. Record each winding-to-frame result under stable conditions and compare it with manufacturer criteria; without those criteria, the measurement can identify imbalance or deterioration but cannot establish a definitive pass threshold.
FAQ
Can leakage current weld a brushless motor shaft key?
The localized damage alone does not prove it. Confirm arc craters or resolidified metal and a credible return path; otherwise investigate mechanical galling, fit, lubrication, alignment, and fretting first.
Why did the 300 mA differential device not trip?
The evidence does not specify the device type, current waveform, path, or duration. Therefore, no conclusion about detection of a drive-output event can be made from the 300 mA rating alone.
Can I insulation-test a brushless motor with a 60204-1 tester?
Test only after disconnecting and isolating the motor from the drive. Obtain the approved voltage, connections, duration, and acceptance criteria from the motor and drive documentation; the evidence provides none of those values.
