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75 kW VFD Driving a Fan Trips on Overcurrent at Higher Frequencies

Views: 8 Author: Site Editor Publish Time: 2025-10-29 Origin: Site

Industry Background: The customer is a large-scale chemical plant. A newly-built waste-gas treatment line uses a 55 kW centrifugal fan to convey dusty exhaust from reactors to a scrubber. Stable fan operation is critical for both environmental compliance and plant safety.

Application

Motor: 55 kW / 380 V / 103 A induction motor
Load: quadratic-torque (fan)
VFD: FST-650L-075G/090PT4, 75 kW / 150 A, V/F control
Purpose: soft-start, flow control, energy saving

centrifugal fan

Problem Description

During commissioning the drive ran unloaded without alarm, but as soon as the fan was coupled and the frequency rose above 30 Hz the VFD tripped on “OC” (over-current). Restart attempts failed; the waste-gas line could not be brought on-line.

Root-Cause Investigation

1、Load, motor insulation and cabling were checked—no fault found.
2、Clamp-meter and voltmeter readings on the input side were used to calculate output power; the calculated output current was far below the VFD’s trip level, indicating a false over-current.
3、Maintenance records showed the 75 kW unit had received a replacement control board taken from a 45 kW spare drive of the same model family.
4、Current-sensing circuits (Hall sensors, burden resistors, amplifier gain) are scaled differently for each power rating. The 45 kW board interpreted 75 kW current levels as already exceeding the 45 kW threshold, so it issued an OC alarm long before the real limit was reached.

Solution

Re-installed the original 75 kW control board (shipped overnight from factory stock).
Re-entered motor name-plate data and extended acceleration time from 10 s to 30 s to suit the high-inertia fan.
Step-wise run-up to 50 Hz: current stabilised at 98 A (well within rating); no further trips.

Customer Feedback

“We almost ordered a bigger VFD, thinking the fan was too heavy. Your engineer found the wrong control board in minutes. After the correct board went in, the drive has run perfectly—even at full speed. We learned never to swap boards across power ratings. Thanks for the fast response and solid support—your products will be our first choice for the next expansion.”

Lessons Learned

1、Control boards are power-specific; same model number does NOT guarantee compatibility across kW ratings.
2、Always verify real current with external meters when an over-current alarm looks suspicious.
3、Quadratic-torque loads need longer acceleration ramps; keep starting current below 110 % of motor FLA.
4、Maintain a strict spare-part registry (kW, serial no., firmware rev.) and consult the factory before any board exchange.