Torque control function gives a lift to hoisting

Rockwell has developed a torque control function that co-ordinates the control of mechanical brakes with the operation of a drive. The company says that the function, called TorqProve, will simplify lifting and hoisting applications and cut installation times and costs by automatically co-ordinating a drive`s starting and stopping functions with the setting and releasing actions of mechanical brakes.

The function is being incorporated into Rockwell`s Allen-Bradley PowerFlex 700 range of AC drives. The co-ordination is achieved by checking the current flowing through all three motor phases. Only if enough torque is available to take control of the load, does it allow the mechanical brake to be released.

TorqProve works with other drive functions such as "brake proving", which is activated when a motor reaches a standstill. The drive then sets the mechanical brake and monitors the motor movement to ensure that the brake is holding the load before releasing control.

If movement exceeds a set limit, an alarm is triggered and the drive starts a brake-slip procedure. The motor is allowed to travel a set distance before a second slip test is performed. This is repeated until the brake holds and the load stops slipping or reaches the ground. According to Rockwell, control of the load is maintained at all times, ensuring that it descends in a controlled manner if the mechanical brake fails.