Overview

The connection of DC motor shafts by the gearbox, when they powered from a common electric source, is often accompanied by a difference in the electromagnetic torques on the shafts, due to a discrepancy in the electromechanical characteristics of the DC motors. This entails increasing loads in the gearbox and other mechanical parts of the drive, their rapid wear, additional heating of one and underutilization of the power of another DC motor.

Such a problem existed in the vertical stand of the Slabbing-1150 mill of a large metallurgical factory. Each stand roll is driven by two DC motors the shafts of which are mechanically joined through a common gearbox. Armature circuits of a pair of DC motors are connected in parallel and are supplied from a common power thyristor converter. DC motors have independent field windings.

To solve this problem, a control system was developed for the automatic equalizing of electromagnetic torques by regulating field currents of DC motors. The reference value of the field current is formed according to a special algorithm, which calculates the deviation of the torques and controls the field current for two DC motors.

The developed algorithm was simulated using plant input data in all modes, which made it possible to improve the algorithm at the development stage.

The developed algorithm was implemented in two power thyristor converters (exciters 63Amp, 220V) which provide individual power supplying to the field windings of each DC motor. 

 

Achievements

The developed control system provides:

• high accuracy of equalizing the torques is satisfied for all modes of a vertical stand of the Slabbing-1150 mill.

• the error when equalizing the torques does not exceed 5% of the nominal motor torque, which with a margin satisfies the technical requirements of a vertical stand of the Slabbing-1150 mill operation.

• the control system does not require manual adjustment of the parameters when replacing the motor, all the necessary parameters are determined automatically.

The implemented control system was highly appreciated by the operating personnel and it showed high quality of work, satisfying all the technological requirements for work as part of the Slabbying-1150 mill.
 

Stages of project

• Creating plant model and parameter calculation 

• Providing tests in plant, gathering data

• Developing control system

• Controllers tuning

• Simulation with input plant data

• Validating the simulation results