Variable Frequency Drives (VFD)

How a drilling drive works

A supply unit rectifies AC from the switchboard, usually through a phase-shifting transformer, into a DC bus. Inverter units switch that DC with IGBTs into variable-frequency, variable-voltage AC for each motor. Drilling lineups typically share one DC bus across several inverters, so energy from a braking drawworks motor can feed a mud pump motor on the same bus. Surplus energy goes to a braking chopper and resistor bank, or back to the board through an active front end. Direct torque control or vector control gives full torque at zero speed, which the drawworks needs to hold a load.

Configurations

• Diode front end with braking chopper. Simple and robust, but it needs a resistor bank and a phase-shifting transformer to control harmonics.
• Active front end (AFE). It regenerates to the board and keeps harmonics low, at the cost of more power electronics and cooling.
• Air-cooled cabinets suit small lineups and spares. Liquid-cooled lineups pack more power per cabinet and need a deionised water cooling unit.
• Medium-voltage drives at 3.3 to 6.6 kV run large thrusters. Drilling drives are mostly 690 V.

Key specifications

Standards and approvals

IEC 60092-304 and IEC 61892 cover semiconductor converters on offshore units. IEC 61800-5-1 sets drive safety, and IEC 61800-3 sets EMC requirements. Class type approval from DNV, ABS or another society applies to the drive family. The installation is then surveyed. IACS UR E24 limits harmonic voltage distortion on the busbars, so a harmonic study is part of any new or added lineup. Cabinets in hazardous areas are rare. Drives sit in pressurised or safe-area rooms.

Failure modes and inspection

• DC link capacitors. Electrolytic capacitors lose their oxide layer when stored without voltage. OEM reforming procedures typically apply after about a year unpowered. Skipping them can destroy capacitors on first energising.
• IGBT modules and gate drivers. Look for burn marks, cracked housings and signs of previous module swaps.
• Liquid cooling. Check coolant conductivity, the deioniser cartridge, pump, heat exchanger and hoses for leaks or corrosion.
• Busbars and connections. Check torque marks, discolouration and dust tracking.
• Braking resistors. Look for overheated elements and damaged insulators.
• Control boards and fans. Check fan bearings and corrosion on connectors.

A bench or load test after reforming should prove each inverter on a motor or test load. Record DC bus voltage ripple, output current balance and any fault log entries.

Documentation

• Part numbers, hardware revisions and firmware versions for every board and module
• Parameter backups and application software
• Class type approval certificate for the drive family
• Single-line diagram and cabinet layout drawings
• Storage history and the date last energised
• Fault logs and service reports

Integration and compatibility

Drive firmware and parameters link the drive to the motor's data and the drilling control system. A spare module must accept your lineup's firmware. Check the fieldbus to drilling control, the motor encoder or sensorless setup, and the safe torque off and emergency stop circuits. Motors fed by a VFD need insulation suited to fast voltage rise and an insulated non-drive-end bearing to block bearing currents. The cooling unit and transformer must match the drive's ratings.

Refurbishment and lifecycle

An OEM refurbishment replaces capacitors, fans and suspect boards and tests the module on a load. Lifecycle status decides how long you can keep a lineup running. ABB drilling drives moved from the ACS800 multidrive to the ACS880 family, and Siemens SINAMICS is common, so check each family's status with the OEM. Older rigs still run GE 752 DC motors on SCR systems, and converting them is a drive and motor project together. The motors sit under Drawworks and Mud pumps, and power management under Electrical control systems.