VFD startup & commissioning
Parameterize, tune, and verify a variable frequency drive on a screw compressor package, including min-speed and PID setup.
This guide is general field reference for trained, qualified service technicians. It is not a substitute for the original equipment manufacturer's service manual, applicable codes (NEC, OSHA, ASME, local), or the judgment of a licensed professional. Procedures, torque values, pressures, refrigerants, and safety requirements vary by make, model, jurisdiction, and revision. Always verify against current OEM documentation and follow your employer's safety program. Working on compressed-air, electrical, pressure-vessel, and refrigerant systems can cause serious injury or death. By using this guide you accept all risk; AirCompDX and its authors disclaim all warranties and are not liable for any damage, injury, loss, or code violation arising from its use.
The DC bus holds a lethal charge after power-off. Wait the drive's stated discharge time (typically 5–15 min) and measure bus volts before touching terminals. Never megger a motor with the drive connected — you will destroy the output IGBTs.
- Drive keypad or laptop with OEM configuration software
- True-RMS clamp meter rated for drive output (or the drive's own current readout)
- Insulation tester (motor disconnected from the drive)
- Calibrated pressure gauge for transducer verification
- Torque screwdriver for control terminals
- Shielded VFD cable with 360° gland terminations
- Line reactor or dV/dt filter for long motor leads
- Shaft grounding ring for non-inverter-duty motors
- Spare cooling fan and heatsink filter
- 1Verify the motor is suitable
Confirm inverter-duty rating or NEMA MG1 Part 31 insulation. A standard motor on a VFD with long leads sees reflected-wave spikes near twice the bus voltage and fails early.
- 2Check the installation
Shielded cable bonded 360° at both ends, motor cable kept under ~50 ft unshielded or ~100 ft shielded without a reactor, and a dedicated equipment ground back to the drive — not a conduit-only path.
- 3Enter nameplate data
Motor voltage, FLA, base frequency, rated RPM, and power factor go in first. Every autotune and current limit calculation derives from these.
- 4Run an autotune
Perform a static or rotating autotune per the drive manual so the drive learns stator resistance and leakage inductance. Skipping this is the usual cause of poor low-speed torque.
- 5Set minimum speed
Screw airends need roughly 20–30% minimum speed to maintain oil pressure and cooling flow. Setting min-speed too low starves bearings — confirm the OEM's floor, do not guess.
- 6Set accel/decel
Ramp long enough to avoid overcurrent on accel and overvoltage on decel. Typical 10–30 s accel, 20–60 s decel. If OU faults persist on decel, extend the ramp or add a brake resistor.
- 7Configure the PID
Set target pressure, feedback scaling to match the transducer range, and proportional gain low to start. Increase gain until pressure recovers quickly without hunting, then add just enough integral to remove steady-state offset.
- 8Verify the transducer
Compare the controller's pressure reading against a calibrated gauge at two points. A 1 psi scaling error at the setpoint costs about 0.5% in energy for every psi of unnecessary pressure.
- 9Commission under real load
Run through the plant's full demand swing. Log amps, speed, discharge pressure, and discharge temperature at low, mid, and full demand. Confirm the drive stays below rated current at 100% speed.
- 10Document and back up
Save the parameter set to the keypad, to a file, and to a printed copy in the panel. An unbacked-up drive replacement turns a 2-hour job into a full day.