Maintenance and modernization
In short: An inverter controls crane motor speed steplessly, giving smooth starts and stops, precise positioning, less mechanical wear and energy savings. Correct brake control on the hoist and handling regenerated energy during lowering (braking resistor) are the most critical points.
From contactor control to inverter drives
On older cranes motors are switched directly with contactors: motion starts at full speed or switches between two fixed speeds. This jolts the load, stresses brakes and gearboxes and makes precise positioning difficult. An inverter (variable frequency drive) varies frequency and voltage to control speed steplessly.
- Smooth starting and stopping with less load swing
- Precise positioning for dies, machine parts and assembly
- Less mechanical wear: the inverter stops the motion, so brake linings last much longer
- Lower energy use and starting current
Hoist vs travel
In travel motions the load moves horizontally and the inverter manages acceleration and deceleration. In hoisting the load is held against gravity: the motor must produce enough torque before the brake opens and hold the load at zero speed before the brake closes, so hoist inverters are selected and set up with more care.
Control methods
| Method | How it works | Suitable for |
|---|---|---|
| Scalar (V/f) | Voltage proportional to frequency, no speed measurement | Travel motions, simple applications |
| Sensorless vector | Calculates torque from a motor model; better low-speed torque | Travel and medium hoisting |
| Closed loop (encoder) | Measures speed with an encoder; full torque at zero speed | Heavy hoisting, precise positioning, high safety |
Brake control sequence on the hoist
- Run command; the inverter magnetises the motor and builds torque.
- When current and torque are sufficient to hold the load, the brake release signal is given.
- A short delay allows the brake to open mechanically, then acceleration starts.
- When stopping, the inverter ramps to zero speed and holds the load.
- The brake closes; after it has seated, motor torque is removed.
The brake must be supplied from a separate source controlled by an inverter output, not from the inverter’s motor output. A wrong sequence or supply shows up as the load “dropping and catching” at start or creeping at stop.
Energy during lowering: braking resistor
When lowering, the motor acts as a generator and energy flows back to the inverter’s DC bus. If it is not converted to heat in a braking resistor, the DC bus voltage rises and the inverter trips on overvoltage. A brake chopper and a properly rated resistor are therefore essential on hoists; on large, frequently used cranes regenerative units can return the energy to the grid.
Example: braking resistor power
Lowering 10 t at 5 m/min releases roughly weight × speed: 10,000 kg × 9.81 m/s² × (5 ÷ 60) m/s ≈ 8.2 kW. After losses, most of this goes to the resistor. Its continuous rating is chosen by how long and how often lowering occurs; on intensive cranes resistor temperature should be monitored. This shows why an undersized resistor leads to overvoltage trips.
Sizing the inverter
- Current: inverter rated current must not be below the motor nameplate current; short-time overload capacity must cover hoist starting.
- Heavy duty: for hoists with frequent starts choose the heavy duty rating, if necessary one size up.
- One inverter, two motors: on bridge travel two motors can be fed from one inverter sized for their total current, each motor with its own thermal protection.
- Braking resistor: resistance not below the inverter minimum, power according to the lowering energy.
Motor, cables and panel
- Motor cooling: at low speed for long periods the motor fan may be insufficient; monitor with thermistors and add forced cooling if needed.
- Long motor cables: fast voltage changes stress windings; use output filters and shielded cable.
- Earthing and EMC: correctly earthed shields prevent interference on signal lines.
- Panel ventilation: inverter losses become heat; keep panel temperature within limits.
Key parameters
| Setting | Purpose | Caution |
|---|---|---|
| Acceleration / deceleration ramps | Smoothness of starts and stops | Too short: swing, overcurrent; too long: long stopping distance |
| Min / max frequency | Speed range | Stay within motor and gearbox limits |
| Current limit | Protects motor and mechanism | High enough to lift the load on the hoist |
| Brake open / close conditions | Safe holding of the load | Always test under load |
| Motor thermal protection | Prevents overheating | Thermistor connected and active |
Common inverter faults and first checks
| Symptom / fault | Possible cause | First check |
|---|---|---|
| Overvoltage when lowering | Braking resistor missing, open or undersized | Resistor value, connection, temperature |
| Overcurrent at start | Brake not releasing, mechanical binding, short ramp | Brake release, ramp times, free movement |
| Earth fault | Damaged cable or motor insulation | Insulation test of cable and motor |
| Motor overheating | Long low-speed operation, poor cooling | Thermistor, fan, duty profile |
| Load slips at start | Wrong brake sequence or torque setting | Brake release conditions and delays |
| Encoder fault | Cable, connector or encoder failure | Continuity, shield, connection |
Recording the fault code, the motion and the load condition speeds up diagnosis a lot. See insulation test.
Anti-sway and precise positioning
Inverter drives are also the basis of anti-sway. Because the load behaves like a pendulum, matching acceleration and deceleration profiles to its swing period greatly reduces swing when stopping. Advanced systems measure rope length and update the profile continuously. See crane automation.
Retrofitting inverters to an old crane
- Inspect motor nameplates, brake type and supply, cable condition.
- Select rating and control method for each motion; size the braking resistor for the hoist.
- Build a new control panel or revise the existing one.
- Set ramps, speed steps and brake sequence; test under load.
- Readjust limits to the new speeds and stopping distances.
This conversion is usually part of a crane modernization. See also our Vacon NXP product page.
Need support?
If you need help on site with the topics in this guide, the Aylen team is available 24/7. We inspect your crane, take measurements and provide recommendations in a written report. Quotation and service request form
FAQ
Is an inverter mandatory on cranes?
No, but it provides smooth starts and stops, precise positioning, less brake and gearbox wear and energy savings, so it is widely used on new cranes and modernizations.
Why does the inverter trip when lowering?
During lowering the motor returns energy; without a properly rated braking resistor the DC bus voltage rises and the inverter trips on overvoltage.
Is an encoder needed on the hoist?
Closed-loop control is recommended for heavy loads, precise positioning and high safety; for light duty sensorless vector control may be sufficient.