VFD tripping on overcurrent on a pipe extrusion line

Mechanism. During acceleration the drive must supply load torque plus accelerating torque (T = T_load + J dω/dt). With a 2 s ramp to 50 Hz the accelerating torque is large, and an extruder screw has a high, nearly constant load torque even at low speed. The drive therefore reaches its current limit and trips - typically at the end of the ramp, around 40 Hz, when both load torque and magnetising demand are high.

A second, quieter suspect: the drive is marginal. The nominal 17 A is measured at a default carrier frequency. At 8 kHz the drive must be derated according to its manual's carrier-frequency table, often to about 80–90% - which can bring 17 A down to 13.5–15 A. Add the motor's magnetising current and the 14.5 A nameplate is already at or beyond the derated rating. A 7.5 kW drive on a 7.5 kW motor with a heavy duty is under-matched.

Separating the causes.

1. Mechanical first, and cheapest. Disconnect the screw or the belt and run the motor free. If the drive accelerates to 50 Hz smoothly with no load, the problem is mechanical or load-related: gearbox binding, a worn screw, dried PVC, a jammed die head, or product build-up.
2. Motor. With the drive isolated, measure winding resistance phase-to-phase (balanced within about 2%), insulation resistance at 500 V (1 MΩ or more), and check for an earth fault. Smell and look for overheating at the terminal box. A motor with one weak phase or a shorted turn will trip only under load.
3. Drive and parameters. Confirm the motor nameplate data are actually entered (rated current, voltage, frequency, speed), check the current limit setting and the V/f or boost setting, and read the stored fault log rather than relying on the operator's description. If the same motor runs correctly from another drive, the drive is implicated.
4. Supply. Check the incoming voltage during starting; a sag on start raises current.

Parameters.

• Ramp time. Accelerating torque is inversely proportional to ramp time. Going from 2 s to about 8–10 s cuts the acceleration torque by roughly four to five times, which is usually enough to keep within a 150% current limit. This is the first change to try, and it costs nothing.
• Current limit. Set around 150% of motor rated current for a short time; do not set it so high that the drive protects nothing.
• Torque boost / control mode. Set a modest boost or use sensorless vector mode if available, so the drive produces torque at low speed without excessive current.
• Nameplate data and auto-tune. Enter the correct motor data and run a static auto-tune so the drive's model of the motor is right.
• Carrier frequency. Reduce from 8 kHz to 4 kHz to recover rating and reduce switching loss; this also reduces leakage currents into the earth system.
• Rating. If the duty is genuinely heavy, move to an 11 kW drive rather than running a 7.5 kW unit at its margin.

Installation defects.

• Motor cable in the signal tray. Separate the motor cable from the 4–20 mA and proximity-switch cables by at least 300 mm, or use a shielded motor cable with the shield bonded at both the VFD and motor ends. Use a twisted pair for the analogue signal and ground the shield at the panel end only.
• Earthing. The 1.5 mm² looped earth is inadequate and the conduit is not an earth conductor. Run a proper protective conductor of at least the motor cable's cross-section, bonded to a clean bare metal surface at the VFD's earth bar, and earth the motor frame directly. High-frequency leakage needs a low-impedance, short path - long thin earth wires make the problem worse, not better.
• Display flicker. This is a supply dip or an EMI event when the heater contactor operates. Check the incoming voltage with a meter while switching the heater; fit an RC snubber or surge suppressor across the contactor coil, consider a line reactor or DC choke on the drive, and confirm the control supply is not sharing a noisy earth.

Proving the fix.

1. No-load run: motor uncoupled, accelerate through 0–50 Hz and check the drive output current stays near magnetising current, typically a few amps.
2. Loaded run: restore the load, run a full production cycle, and log the drive's output current against frequency. It should stay below the drive's derated rating with a margin - not hovering at the current limit.
3. Fault log: confirm no new overcurrent events after a shift, and record the previous fault codes, frequency and time stamps.
4. Thermal check: measure the drive heatsink temperature and the motor frame temperature after an hour; a correctly loaded drive should be comfortably inside its heatsink limit.
5. Signal integrity: verify the 4–20 mA reading against a gauge with the drive running at full load and the heater cycling - the reading should not jump.
6. Power quality: check the supply dip at the drive input when the heater contactor closes, before and after the snubber.

Records to leave. The motor nameplate data as entered, the ramp times, current limit, carrier frequency, control mode, the derating figure taken from the manual, the cable routes used, the earth conductor size and its termination points, the supply voltage measured at the drive terminals under load, and the measured current at 40 Hz and 50 Hz. A short note explaining why the carrier was lowered and why the motor cable was moved saves the next technician from starting the same investigation again.