Rotary Encoder Failure Triggers VFD Overcurrent Trip on 160 kW Steel Mill Vector-Control Drive

Fault Symptom

At 10:14 a.m. the drive suddenly tripped on “E-01” (acceleration overcurrent). The mill operator noticed the motor was not blocked and the steel temperature was normal. Two attempts to restart failed at the same instant, forcing the line to stop for troubleshooting.

Root-Cause Analysis

1. Power circuit check – Megger and bridge tests showed no phase-to-phase or phase-to-ground short circuit; motor insulation > 500 MΩ.
2. Load inspection – Roll gap and bearing temperature were within specification; no mechanical blockage.
3. Control circuit check – Oscilloscope on the encoder feedback board revealed pulse dropout and random pulse doubling; feedback jitter reached ±18 % of nominal frequency.
4. Mechanical inspection – The encoder coupling was finger-tight only; axial play measured 0.7 mm (spec ≤ 0.1 mm). Removing the encoder revealed a worn shaft bushing and loose set-screw, confirming rotary encoder failure due to mechanical wear.

Why a Bad Encoder Causes VFD Overcurrent

In vector-control mode the VFD continuously compares the calculated rotor flux angle with the encoder position. When the rotary encoder feedback becomes unreliable, the de-coupled d-q axis current control collapses. The drive “thinks” the motor is running slower than ordered and keeps raising voltage and frequency to compensate. Torque current (Iq) spikes, often exceeding 150 % of rated within milliseconds, and the VFD overcurrent protection trips.

Solution & Results

• Replaced the damaged encoder with an identical, IP65-rated unit.
• Re-aligned the shaft using a laser coupling tool; axial run-out reduced to 0.04 mm.
• Re-tensioned the coupling to 2.5 Nm and applied Loctite 243.
• Executed the drive’s auto-tune routine for encoder offset and motor parameters.

After recommissioning the mill accelerated smoothly to base speed; overcurrent fault has not re-appeared during 4 weeks of 24/7 operation.

Key Take-aways

1. Always suspect rotary encoder failure when a vector-control VFD trips on overcurrent without signs of mechanical overload.
2. Verify mechanical integrity of the encoder coupling first; electrical tests come second.
3. After any encoder replacement, run the manufacturer’s motor-identification or encoder-offset routine before returning the line to production.

Recommended Preventive Actions

• Add a weekly visual check of encoder coupling and shaft run-out to the PM sheet.
• Use shielded twisted-pair encoder cables with 360° clamp glands to minimise interference.
• Store one spare encoder and coupling in the plant warehouse to cut MTTR below 30 min.