Crane control with no rock and roll

The electrolytic manufacture of aluminium is dependent on a regular supply of carbon anodes so that the aluminium oxide (alumina) stock refined from bauxite ore, can be split into its component elements.

Such are the volumes of metal to be manufactured at Anglesey Aluminium, typically130,000 tonnes a year, that the company produces these one tonne carbon block anodes on site. The company is 51:49 owned by RTZ/Kaiser who purchase and sell all its manufactured pig, sow, rolling ingot and billet aluminium. Initially, carbon and pitch are mixed and then baked in a gas furnace to around 1200 deg C to produce the anode. After cooling, the blocks are mounted on rods ready for fitting in the pot (electrolysis chamber).

The manoeuvre of the anodes to and from the baking furnaces is dependent on two 150 tonne NKM overhead gantry cranes which pick and place three blocks a time. Once in the furnaces, the blocks are covered with coke and heated. At the end of this process the coke is sucked up by a vacuum attached to, and moved by, one of these gantry cranes. The vacuuming operation involves the crane driver making hundreds of small traverses, stops and starts.

Operating in these axes, the cranes have been controlled by a system created and built over ten years ago. The drive system consisted of an array of relays and potentiometers together with electronics which control two slip ring induction motors - one at each end of the crane gantry - which were fitted with rotor resistances and tacho-generator feedback.

The cranes are crucial to the aluminium manufacturing process, yet are high maintenance items which have regularly suffered downtime due to the stress of their work. The large rotor resistances in the motors were a major contributor to the maintenance headache, while frequent braking created wear and the need to regularly maintain the motors high up on the crane's tracks.

One of the most time consuming problems was resetting drives after they had tripped out. "Setting up the cranes had become something of a black art with the existing control system," explains Anglesey's senior electrical control systems engineer, Colin Webb. "With little information to work from, you could spend hours trying to fault find. At the end of the day faults weren't removed from the system, they were transferred within it."

Crane control was set-up with acceleration and deceleration ramps, but these were extremely difficult to set. Often the stopping and starting appeared to be effected almost the instant the driver moved the joystick. This maximised stress on the crane's mechanics, and on long rapid traversing slowing and stopping increased wear on the brakes.

Indeed, to adjust the previous drive involved laboriously setting up and measuring voltages, adjusting potentiometers and making often imprecise assessments of parameters. There was no recording of faults or diagnostics to assist the engineer setting or resetting the control. Moreover, the control equipment was a complex labyrinth of wires and components housed in cramped conditions in panels above the crane driver's cab.

Solving the twin-track problems of a high occurrence of downtime and lengthy set-up periods once failure had occurred, became a priority.

Adds Webb: "A drive system that records operating parameters, indicates faults before critical, provides diagnostic information and can be set up to effect less stressful crane operation; looked attractive provided the technology to do so was available and at a realistic price. The Hitachi sensorless flux vector inverter met these criteria."

To effect motor control, sensorless flux vector technology relies on measuring motor currents directly rather than using external sensors or tacho-generators. These currents are compared, within the inverter, with a software model for the motor being driven. Adjustments are made instantaneously to ensure the flux vector is maintained at a 90 deg. Angle, thereby maximising torque even at low frequencies. Hitachi sensorless flux vector inverters achieve 150% torque even at 1Hz and could therefore provide Anglesey Aluminium with the muscle needed to start the cranes smoothly - something a conventional voltage/frequency inverter could not achieve.

As a trial, Anglesey Aluminium has installed a 37kW sensorless flux vector drive on the long travel of one of the cranes, leaving the cross travel and hoist as they were. The brake is now redundant - apart from the emergency stopping and parking - due to smooth deceleration and stopping from the inverter.

"Employing a standard inverter drive on a crane of such large mass was to a certain extent charting new applications territory," comments Webb. "Factors we had to consider included getting the brake resistor correctly sized. We worked with HID to quickly assess this, and the system has worked perfectly since it was installed earlier this year. Because maintenance access is such a headache with these cranes, it is vital to us that downtime is minimised. The drive has come up trumps on this score."

The sensorless flux vector inverter fits in a neat single box with a simple operator keypad by which to program the drive. A record of diagnostics is provided by the drive, so that in the event of a trip, the drive setter knows exactly what caused the problem. To date, there have been very few trips on the crane since fitting the new inverter. Now, unless there is a system fault, resetting is by the press of a button. Programming the drive involves merely pressing a few keys to recall the relevant program from the control's memory.

Another common problem which has been overcome by using the new inverter is that of crabbing. Crabbing occurs where the two long travel motors start out of synchronisation, causing the gantry to move out of 90 deg alignment with the long travel. Under the control of the sensorless flux vector inverter, the load is inherently shared by the two motors which ensures they start in synchronisation. The fact that acceleration and deceleration ramps are able to be accurately set has also removed the tendency for the crane to shake when manoeuvring heavy loads.

At the moment the sensorless flux vector inverter controls the original two slip ring motors which have had the slip ring shorted out so the motors behave as standard induction machines. Squirrel cage motors will replace these modified units when the maintenance schedule dictates.

Anglesey Aluminium plans to fit further Hitachi drives on the cross travel and hoist of this first crane, and on all three motions of the second. "The Hitachi sensorless flux vector drive delivers high torque at low frequencies and gives good speed control across the entire speed range irrespective of loads. The cranes are picking and placing very heavy blocks to tight tolerances. We needed good stability and repeatability during this process and got if from this drive," concludes Webb.

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