Smooth and safe

Celsa got to have its cake and eat it too when its meltshop crane was fitted with frequency inverter drives. The company produces 1.2m tonnes of corrugated steel, bars, and structural steel profiles a year.
With the drives, the Barcelona, Spain-based steelmaker acquired much greater control of the hoisting and travel motions on its crane. And in case of a fault with the drives, the crane can still be operated manually. In an emergency, the crane uses its traditional contactor circuits to lower a ladleful of molten steel before it solidifies, preventing a costly production outage.
The installation, carried out in early 2004, was also unusual because the crane’s hoisting and trolleying motions are driven by slip-ring motors. Slip-ring motors are difficult for drives to handle, and so are rarely fitted with frequency inverters.
Slip-ring motors have high rotor resistance relative to normal motors, and that resistance changes to control the speed of the hoist, and also changes with temperature, according to Pami Vuorio, marketing manager for cranes at ABB low-voltage drives. He said that the parameter tuning capabilities of its ACS 800 drives enabled them to handle the job.
The 27m span crane transports molten steel from the furnace to the casting machine. The main hoist’s maximum capacity is 250t. Lifting height is 17m and maximum hoisting speed of 7m/min. Hoisting is driven by two 225 kW eight pole slip-ring motors with pulse encoder feedback. To control these motors, local crane builder Grúas Jaso installed two free-standing 315 kW ACS 800 02 industrial drives with brake chopper and RTAC module to read the pulse encoder.
The main hoist’s trolley has a maximum travel speed of 30m/min and is powered by two 18.5 kW six pole slip-ring motors. To control the trolley motors, Jaso installed two 30 kW ABB ACS 800 01 drives with choppers. The auxiliary trolley also can travel up to 30m/min and is powered by a 4.8 kW six pole slip-ring motor. To the auxiliary trolley motor Jaso fitted another 7.5 kW ABB ACS 800 01 drive with internal chopper.
Four 45kw six pole induction motors control gantry travel, which can reach speeds of 60m/min. To control these, Jaso fitted a total of two 110 kW ABB ACS 800 01 drives, each connected to two motors.
The 50t capacity auxiliary hoist, which is driven by an 88 kW 6 pole slip-ring motor, was not fitted with a drive, because it is rarely used.
All seven drives were mounted in an air-conditioned control room fitted inside the steel gantry.
For normal operation, the rotor is short-circuited in a connection box inside the crane. A control panel switches the crane to emergency operation with contactors.
Grúas Jaso has around 200 employees in its industrial cranes divisions and a turnover of Euro 45 million per year.