Crane electrics and VFD drive care

A crane vibrates continuously in operation, so every electrical connection loosens more readily than on fixed equipment. Shorten the terminal tightening interval to quarterly, focusing on bridge conductor connections, motor terminal boxes and control-cabinet main-circuit terminals.

Check cables for vibration-chafed insulation, especially at conduit entries, bends and festoon cables (drum cables, drag-chain cables). Festoon cables are a crane wear item — look for kinks, bulges and cracked outer sheaths.

Cable drums and drag chains must guide correctly. A drum paying out and reeling up out of sync loads the cable until the cores snap; cables in a drag chain must keep the specified bend radius and slack.

Terminal-box seals must be intact. In dusty shops and on outdoor cranes, water and dust getting into a box cause earth faults and short circuits.

Check the conductor bars (enclosed or bare) for straightness, for flush joints, and for oxidation or burn points. An uneven joint makes the collector arc, throwing sparks — damaging the equipment and a fire hazard.

Replace collectors (carbon brushes) at the wear limit mark. Insufficient spring pressure causes poor contact, arcing and phase-loss running. A motor running on lost phase burns out fast — especially common on cranes.

Check conductor-bar insulators and support brackets; a loose bracket lets the bar distort, which in turn worsens arcing.

On outdoor cranes check the conductor bars for standing water, ice and heavy rust — all of these affect conduction and insulation.

When a VFD hoist lowers a load, the motor regenerates, feeding energy back to the DC bus where the braking resistor dissipates it. Resistor cooling is critical — it runs hot itself, and in a poorly ventilated spot it overheats easily, tripping protection or burning out.

Inspect braking resistors regularly: element deformation or breakage, insulation decline, overheated or discoloured terminals, blocked cooling paths. Confirm the fan runs where one is fitted.

Braking-unit faults have a classic signature: over-voltage trips when lowering loads, or runaway lowering speed. On seeing these, check the braking resistor and unit first — do not blindly adjust parameters.

Care for the drive itself follows the general VFD requirements: filters, cooling, fan life, parameter backups. Crane duty vibrates heavily, so pay extra attention to drive mounting fastenings and cable fixing.

Confirm item by item: overcurrent, overload, short-circuit, phase-loss, undervoltage and zero-position protection. Zero-position protection matters greatly — if power comes on with the controller off zero, the motion starts suddenly; this protection guards against operator error.

Test the emergency stop before every shift — both the cab e-stop and the ground-control (remote or pendant) e-stop.

Check the radio remote (ground control): buttons not sticking, housing not damaged, battery compartment sound, e-stop mushroom head reliable. On radio-controlled cranes also confirm exclusive control (only one remote live at a time).

Interlock trial: with no load, run through each motion's operation, limits, braking and protection to confirm the logic is correct. This is mandatory after maintenance — never go straight to load.

Most newly installed cranes are VFD driven. Compared with conventional contactor-and-rotor-resistance control, a VFD drive is clearly better at speed control, shock limitation and energy saving - but the maintenance requirements change with it. The first difference is environmental sensitivity: an inverter tolerates dust, moisture and temperature far worse than a contactor, so air conditioning and sealing of the electrical room, plus cabinet filtration and cooling, become mandatory check items. On a conventional crane most electrical faults are burnt contacts; on a VFD crane most are modules damaged by overheating and dust - the maintenance focus shifts from 'checking contacts' to 'managing the environment'.

The second difference is the importance of parameters: ramp times, torque boost, brake control logic, and overspeed and overload protection settings on the inverter all bear directly on the safe operation of the crane. Brake control is especially critical - on an inverter-driven hoist the opening and closing timing of the holding brake must be matched to the inverter output; wrong timing causes load slip (a serious safety incident) or accelerated brake wear. Parameter changes need approval and records, brake timing adjustment must be done by someone who understands it, and after adjustment both no-load and loaded verification are required.

The third difference is diagnostics: the inverter keeps fault records and running data, so after a trip the first step is reading the fault code and the history, which points straight at the direction (overcurrent, overvoltage, overheating, earth fault, communication) - much faster than point-by-point tracing on conventional control. This requires maintenance staff who can read fault codes and retrieve records, and it requires the parameter and fault-code reference tables to be filed with the equipment dossier - without the tables a fault code is just a string of numbers.

Earthing is the basis of crane electrical safety: the metal structure, equipment enclosures and cable metal sheaths must all be reliably earthed, and earth resistance must be measured periodically against the specified limit. A crane runs along rails, so its earth path goes through the rail and the wheels, and unstable contact resistance is a common problem - which is why, besides measuring total earth resistance, the bonding jumpers on the rail earth must be checked. The consequence of poor earthing is that protective devices fail to operate during an earth leakage and the metal structure becomes live - a major safety hazard.

Insulation checking has two parts: circuit insulation (main and control circuits meggered at the specified interval, more often in damp conditions) and hook insulation (for particular duties such as handling molten metal or working in electrolysis shops, the hook and rope system need insulating protection, and the condition of the insulating parts must be checked as a dedicated item). In ordinary shops insulation checking follows the annual electrical test programme, with data on file for comparison; in special duties the intervals and standards of the relevant safety code apply.

Cranes working in the open (gantry and tower cranes) need lightning protection considered: where the height exceeds the specified value or the area carries a lightning risk, install lightning protection and test earth resistance periodically. The dedicated check before thunderstorm season should cover the condition of the lightning protection, the earth path, and moisture sealing of electrical equipment. Cases of lightning destroying inverters and control systems are not rare, while the cost of lightning protection testing is small. All three of these are protections that 'look useless in normal times and become a major event when something happens' - put them in the annual inspection plan and they will not be missed.