VFD Tripping on Overcurrent at 45 Hz and DC Bus Overvoltage on Every Stop
A 480 V, 60 hp, 77 A rated VFD drives a 60 hp, 1770 rpm TEFC motor on a high-inertia conveyor through 130 m of shielded three-conductor VFD cable in tray. Acceleration is 5 s, deceleration 3 s, carrier frequency 8 kHz, and the drive is running simple V/Hz with default torque boost; motor nameplate data has not been entered. Two symptoms: the drive trips on overcurrent at about 45 Hz during acceleration whenever the conveyor is loaded, and it trips on DC bus overvoltage on every stop. The overvoltage trip threshold is 800 V DC and the measured bus at rest is 650 V; the drive's DC bus capacitance is 2200 µF. Ambient inside the drive enclosure reaches 45 °C.
1. Diagnose the overcurrent trip at 45 Hz and give the corrective actions in order of priority.
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The trip frequency is the clue: 45 Hz is near the point where the V/Hz curve is approaching rated voltage, so the drive is running out of both voltage and torque margin.
Probable causes, in order:
- Torque boost left at default. Fixed boost adds volts at low frequency but over-fluxes the motor as frequency rises, driving magnetizing current up and saturating the iron - a classic 40–50 Hz overcurrent. Correct action: enter the actual nameplate data (480 V, 72 A, 60 Hz, 1770 rpm) and run an autotune, then either remove the manual boost or move to sensorless vector control.
- Acceleration ramp too short for the inertia. 5 s to 60 Hz on a high-inertia conveyor demands current well above the drive's current limit. Extend the ramp (often 20–30 s) and confirm the current-limit setting is at the drive's rating rather than a reduced default.
- Cable charging current. 130 m of shielded cable adds capacitance that the drive must charge every PWM cycle; at 8 kHz this becomes a meaningful component of output current. Dropping the carrier to 4 kHz reduces both charging current and drive heating.
- Thermal derating. The drive is in 45 °C air; most drives are rated at 40 °C and must be derated (often 1.5–2.5% per °C above 40 °C), so a 77 A drive may effectively be a 70 A drive in that enclosure. Verify with the manufacturer's derating table and improve enclosure cooling.
Sequence: nameplate data and autotune → lengthen accel → confirm current limit and carrier → verify ambient derating → only then suspect the motor or mechanical load.
2. Explain the DC bus overvoltage on stop and quantify how much regenerated energy the bus capacitance can absorb.
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Mechanism. On a 480 V system the rectified bus at rest is about 650 V DC. When the drive decelerates a high-inertia load, the motor becomes a generator; if there is no braking chopper or resistor, that energy has nowhere to go except into the DC bus capacitors, and the bus voltage rises until it hits the 800 V trip.
Energy the capacitance alone can absorb: ΔE = ½ C (V_trip² − V_rest²) = ½ × 2200 × 10⁻⁶ F × (800² − 650²) = 0.0011 × (640 000 − 422 500) = 0.0011 × 217 500 ≈ 239 J
That is about the energy of a 20 kg mass dropping 1.2 m - trivial compared with a loaded conveyor. It shows how quickly a few hundred joules of regeneration will trip the drive, and why 'it only happens on the stop' is diagnostic of regen, not of a supply problem.
Corrective options:
- Lengthen the deceleration ramp (3 s → 30 s or more) so the energy is dissipated in the load and motor losses;
- Enable coast-to-stop if controlled stopping is not required;
- Add a braking chopper and resistor sized from the drive's braking duty cycle, with the resistor mounted outside the enclosure and thermally protected;
- Where regeneration is continuous (declines, unwind stands), use a regenerative front-end drive.
Do not raise the overvoltage trip point: 800 V is already close to the capacitor and IGBT ratings.
3. What is your position on the 130 m motor cable, and what installation details would you verify?
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Position: 130 m of shielded cable at an 8 kHz carrier is at or beyond the point where most drives need mitigation. The long cable produces reflected-wave voltage peaks at the motor terminals that can approach twice the DC bus voltage (roughly 1300 V peak), stressing motor insulation, and it increases common-mode current through the motor bearings.
Verify/implement:
- Motor insulation class - inverter-duty rated motor, or accept reduced life; if the motor is an older standard unit, an output reactor or dv/dt filter is strongly advisable.
- Drive's published maximum cable length for shielded and unshielded cable at the selected carrier; reduce carrier to 4 kHz to extend the permitted length and cut charging current.
- Shield termination - 360° termination at the drive enclosure with a gland or clamp, and at the motor end terminated to the motor frame; not a 'pigtail' drain wire. Do not lift the shield to cure noise; fix the grounding instead.
- Separation - VFD cable should not share tray with signal or instrumentation cable, and should cross power feeders at right angles.
- Grounding - dedicated drive/SECTION grounding conductor bonded to the enclosure and the motor frame, single-point reference to avoid common-mode loops.
- Reflected wave check - if a scope is available, measure line-to-line voltage at the motor terminals at minimum and maximum speed; peaks above about 1000 V indicate a filter is needed.