9 June 2015

Tata Steel cuts billet mill saw stoppages with variable-speed drive

Written by  Halcyon Drives

id135 billet mill variable speed drive

Less blade wear and greater uptime of a billet mill saw are among the benefits from retrofitting an ABB motor and variable-speed drive (VSD) at Tata Steel’s Stocksbridge plant.

The motor and drive improves control of the saw, preventing trips and stoppages that can damage the blade.

Tata Steel Speciality Steels produces high-value special steels for the aerospace and oil and gas industries. Hot rolled steel is presented to the billet saw from the No.2 finishing mill, in sizes ranging from 180 mm to 300 mm at a temperature of about 800 degrees centigrade. The steel is run to a stop and measure system, which allows the operator to position each piece to achieve the required size of finished billet.

However, the original system would frequently trip, requiring the mill electrician to reset the saw protection, leading to delays and lost production. A trip occurred when the traverse speed feed rate was incorrect, or if the loading increased rapidly. In both cases the saw jams in the hot steel, in some instances becoming welded to the billet.

The saw has an 1,800 mm circular blade, which was driven by a 185 kW 6-pole motor started by an autotransformer, providing a soft start facility. Originally, the saw ran at the fixed pole speed and the cutting cross traverse was manually controlled via a joystick, connected through a proportional valve in the hydraulic system. The operator checked the motor load via an ammeter monitoring the load on the saw motor, increasing or decreasing the traverse speed to maintain the optimum load on the motor.

Tata Steel asked ABB authorised value provider, Halcyon Drives, to look at the application and design and install a solution that would solve the tripping problems.
Bernard Sidway, Electrical Development Engineer for Tata Steel, says: “We were asked to provide better remote control of speed for the circular saw, by monitoring the motor load and so providing a more accurate rate of traverse speed.”

This would prevent the motor being stalled or overloaded, which could damage the saw.

Halcyon Drives supplied, installed and commissioned a new ABB 250 kW high efficiency IE2 cast iron, 6-pole motor fitted with an insulated non-drive end bearing. This is controlled by a 200 kW ABB industrial drive, ACS880, housed in an IP54 enclosure.

The VSD is programmed to run at an energy saving speed or full speed, either of which can be selected remotely. When a cut has been completed, the saw motor speed is reduced to 75 percent and initiates a timer. If no further cutting has started for two minutes, the speed is reduced to 50 percent.

The VSD is equipped with a braking system, including a brake chopper and resistors to dissipate heat from the drive while slowing the circular saw. The drive is programmed to monitor overcurrent, with torque monitoring giving a 4-20 mA analogue signal to allow automatic control.

The control system is modified to incorporate this monitoring – although the operator can still control the saw manually, the torque and current monitoring prevent an action that would trip or stall the motor.

An automatic one time overload reset is incorporated into the new drive, to reduce waiting time. Should a trip occur, a second trip within 10 minutes requires an electrician to reset the system.

In the two months since the torque and current protection system was introduced, there have been no reported instances of motor trips. Trials are currently under way to find the optimum traverse speed to optimise the blade life.

ABB (www.abb.com) is a leader in power and automation technologies that enable utility, industry, and transport and infrastructure customers to improve their performance while lowering environmental impact. The ABB Group of companies operates in roughly 100 countries and employs about 140,000 people.