We have recently built a third crane control panel for the same customer, replacing the old slip ring control panels but retaining the original slip ring motors for a 120T beam crane.
All three axis are controlled from the new control system, long travel at 15KW, cross travel at 7.5kW and hoist at 75KW, the main hoist motor being open loop control, using Delta Sensorless Vector drives and regenerative braking.
We fitted line reactors to all the inverters, as the slip ring motors are over 50 years old, this ensures that dVdt is limited by the IGBT system and does not degrade further the windings of the already very old motors.
The mechanical brake control is controlled using the inverter inbuilt hoist / brake release function, this makes sure there is sufficient current / torque available so when the brake is released the motor takes off smoothly with no rollback, in open loop mode full torque at zero speed is possible.
An encoder can be added to open up speed range to 1000:1 if required, in this instance the slow speed for the crane in raise / lower is only 6Hz so the open loop mode is sufficient.
Although an encoder is not used for the inverter closed loop mode, sufficient protection is in place with a secondary safety circuit to make sure the load does not drop if there is no inverter control.