2–3 Hz Overload Trip at Startup: VFD Fault Analysis & Solution

Background

A 7.5 kW/380 V induced-draft fan in a Saudi cement plant was retro-fitted with an AnyHz FST-650L-7R5G/011PT4 VFD.
After power-on the drive ramps from 0 Hz in linear S-curve; every time the reference reached 2–3 Hz the unit tripped on “Err10” (inverter overload). Motor name-plate data had been entered correctly and acceleration time was set to 20 s

Symptoms captured on AnyHz PC2M debugging pad

Trip always occurred at 2.4 Hz
Output current immediately before shutdown: 29 A (≈ 190 % of motor rated current)
DC-bus voltage stable at 542 V
Ambient cabinet temperature 42 °C
Trip code: Err10 (inverse-time overload)

Root-Cause Analysis

Step 1 – Isolate the motor
Motor cables removed → VFD freely accelerated to 10 Hz without trip. Fault therefore located downstream of the terminals.
Step 2 – Insulation check
500 V Megger readings: phase-phase > 200 MΩ, phase-PE > 100 MΩ. No short circuit present.
Step 3 – Mechanical inspection
Fan could not be rotated by hand; bearing housing measured 85 °C. After dismantling, rollers were found brinelled and grease baked.
Step 4 – V/F curve audit
Factory torque-boost = 8 %. At 2 Hz output voltage ≈ 22 V – too low to overcome high break-away torque, forcing the inverter to supply 190 % current and eventually Err10.

Solution implemente

Mechanical: bearings replaced, pulley realigned, labyrinth seals re-greased. Break-away torque fell from 55 N·m to 18 N·m.
Electrical:
Torque boost reduced to 2 % (FST-650L parameter A0-03)
Starting frequency raised from 0 Hz → 3 Hz to avoid deep-saturation zone
Auto-torque-boost (sensor-less vector) enabled after 30-s static auto-tune
Protection: motor overload class set to Class 10, Err10 threshold 150 % × 60 s; output du/dt reactor (2 % Z) added to suppress current spikes.

Results after 30-day logging

Start current at 2–3 Hz dropped from 29 A to 14 A (≈ 95 % I rated)
Drive now completes 100 % of ramps to 50 Hz without tripping
Bearing housing temperature rise reduced to 12 °C (was 45 °C)
Average energy saving at 35 Hz operation: 11 % compared to valve-throttling method

Key Take-aways

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