No display and no activity means no control power. Work through the power path from the utility to the keypad:

1. Measure input voltage at L1/L2/L3 on the drive's input terminals. Missing or low input is the most common cause — check upstream disconnects, fuses, and contactors.
2. If input power is healthy, measure DC bus voltage across DC+ and DC− (typically 320VDC on a 240V drive, 650VDC on a 480V drive). Zero DC bus with good input means a failed rectifier, blown DC bus fuse, or open precharge resistor.
3. If DC bus is healthy but the display stays dark, the control board switching power supply or the keypad itself has failed. Try a known-good keypad. If still dark, the drive needs repair.
4. On smaller drives, check that the keypad ribbon cable is fully seated — a surprisingly common oversight after PM work.

Instantaneous overcurrent on start, before the motor even moves, points to a short circuit or failed output stage rather than a load problem:

1. Disconnect the motor leads from U, V, and W. Try to start the drive with no motor connected. If it still trips — the IGBT output module is shorted. Drive needs repair.
2. If it runs fine with no motor, megger each motor lead to ground. Anything under 1 MΩ indicates motor insulation failure. Also check phase-to-phase resistance for balance — should be within 5%.
3. Inspect the motor cable end-to-end for damaged insulation, pinched spots at the conduit entry, or water in the motor junction box.
4. If motor and cable are good, check for parameter errors: wrong motor FLA, wrong V/Hz curve, boost voltage set too high, or minimum frequency above zero causing instant high current.

Reversed rotation is a wiring issue — loud noise at start can be wiring, parameters, or a mechanical problem:

1. For wrong rotation: with the drive locked out, swap any two of the three motor leads at the drive's U, V, W terminals. Never swap leads on the input side — it won't fix the problem and can damage the drive.
2. Alternatively, most drives have a phase rotation or output phase sequence parameter that reverses rotation electronically.
3. For loud startup noise: check that boost voltage (manual torque boost / V/Hz boost) isn't set too high — excessive boost at low speed causes magnetic saturation and growling.
4. A loose coupling, bad bearings, or worn belt can also make noise only at specific frequencies. Try running the motor uncoupled to isolate.

Time-delayed thermal tripping almost always points to an airflow or environmental issue rather than a drive fault:

1. With the drive running at normal load, feel the airflow at the top of the heatsink (most drives exhaust out the top). Weak or no airflow means the fans are slow, dirty, or failed.
2. Shut down, lockout, and remove the fan covers. Blow out the heatsink fins with compressed air from the top down. Dust mats bond tightly between the fins and insulate the heatsink completely.
3. Check ambient temperature inside the enclosure. Most drives derate above 104°F (40°C). A hot control panel may need a filtered ventilation fan, an AC unit, or a heat exchanger.
4. Verify switching frequency isn't set higher than necessary. Every kHz above the default adds heat. Drop it to 2–4 kHz if acoustic noise isn't a concern.

Nuisance tripping is the hardest fault to diagnose because it's intermittent by definition. Work systematically:

1. Log every fault code with timestamp, load condition, and ambient temperature. A pattern usually emerges within a day or two.
2. Check for loose control wiring on Stop, Enable, Safety, and E-Stop inputs. A single intermittent wire can trip the drive unpredictably. Tug-test every conductor.
3. Verify VFD-rated shielded cable is used between drive and motor, with shield bonded 360° at both ends. Unshielded or poorly terminated cable induces noise on control signals.
4. Look for adjacent contactors, relays, or other drives switching on the same bus. Consider an input line reactor (3–5% impedance) to buffer incoming transients.
5. Check grounding: the drive, motor frame, and panel must all tie to a single clean ground point. Ground loops cause phantom faults.

Unstable speed control is usually an analog signal problem or a PID tuning issue:

1. If using a 4–20mA or 0–10V speed reference, scope the signal at the drive input. Any noise or dips will cause the drive to follow them. Shield and twist the signal pair, ground the shield at the drive end only.
2. Check for deadband or input filter parameters that may be set too aggressively or not enough.
3. For PID hunting, reduce proportional gain first, then reduce integral gain. Add a small derivative term only if the process demands it. Most hunting comes from too-high integral gain on slow processes.
4. Verify the process feedback sensor (pressure transducer, flow meter, tach) is reading steadily on its own — a noisy sensor will send a noisy drive chasing ghosts.

Repeated input fuse failure is serious — it always indicates a hard fault in the drive's power section, not a sizing problem:

1. Do not install larger fuses. The fuses are sized to protect the drive and upstream wiring. Use exactly the type and rating specified in the manual.
2. With the drive disconnected from power, check each input diode in the rectifier with a DMM diode test. A shorted diode will blow fuses the instant power is applied.
3. Check the DC bus capacitors for swelling, bulging, or leaked electrolyte. A shorted capacitor will destroy the rectifier and blow fuses.
4. Test the IGBT output module — a shorted IGBT can crowbar the DC bus and pull excessive input current.
5. If any of the above shows damage, the drive needs factory-level repair. Wistex offers component-level VFD repair with MTBF analysis on every unit.