Crane drive for near continuous long-term operation

The new drive system was installed because the original slip ring motor was nearing the end of its life, creating an opportunity to upgrade the entire operation, improving energy efficiency, operational control, reliability and longevity.

Drives engineer George Newton of Newton Tesla, who commissioned the drive, says: "This is a hard working crane drive system. It is part of a 24×7 production process, so has to be reliable day in and day out. Repeatability is also critical, as it operating cycle runs up to 45,000 times a year."

In fact the crane and its Mitsubishi Electric FR-A741 drive is delivering 25 ton loads of steel feedstock to a furnace five or six times an hour, over a quarter of a million tons a year.

George explains: "The crane is manually controlled by a driver, and with the old slip ring set up speed and torque control was achieved by varying the load through a resistor bank. It was a fairly rudimentary level of control and the crane drivers had to work hard to keep everything operating smoothly. The new drive, in contrast, gives precision control making the operation safer and smoother."

In addition, the drive removes the direct-on-line starting of the motor, which prevents voltage spikes and other mains-borne disturbances as well as mechanical shock loads.

Mitsubishi's FR-A700 drive range is designed for demanding applications like cranes and lifting gear, high-bay warehousing, extruders, centrifuges and winding machines. Usually used in open-loop configuration, powerful on-board processors continuously calculate and re-calculate the optimum magnetic flux for the moment, whatever the operating conditions. This leads to performance levels previously the domain of closed-loop vector drives or DC systems.

The drive's advanced autotuning function allowed George to obtain all the necessary specifications from the motor in less than a minute - he did not even have to run the motor - and complete commissioning in comfortably less than a day. Its self-diagnostics mean unexpected down time is greatly reduced, with no problems reported at all during the drives first six months of operation.

The FR-A741 drive is particularly well suited for tasks involving frequent cyclical acceleration and deceleration and in all applications where braking resistors are normally used. The combination of the integrated AC reactor and no need for an external braking unit eliminate a large amount of additional wiring. These inverters need up to 60 per cent less main circuit wiring and occupy up to 40 per cent less space than conventional systems, depending on the output capacity.

To find out more about the FR-A741 drive from Mitsubishi Electric, visit the website at www.automationsolutions.mitsubishielectric.co.uk.