Question

As a design engineer, you are asked to analyze and suggest a drive controller for a medium power rated
rolling mill industry in Salalah. Analyze and compare different converters and state the reasons for suitable
converter that can be used for this application. Explicate the electric motor suitable for this industry with
necessary reasons.

Answer 1

DC drives have been used successfully in rolling mill applications for many years. They have an excellent performance record meeting rolling mill drive requirements including; speed control accuracy, rapid speed control response, fast load recovery, and more.Squirrel cage rotor motor applied to medium power requirements of reels and stands.

• Extremely robust construction
• Excellent torque characteristics on ultra-low speed rolling applications
• Constant power output throughout field weakening range
• Meets NEMA MG1 and JEM 1157 standards.

These motors have:

• Superior efficiency
• Robust frame and shaft construction
• Less demand for maintenance
• High degree of control and responsiveness
• Great capacity for acceleration and deceleration.
• Greater flexibility for extreme operating conditions

High strength coil head bracing and connections will withstand magnetic forces under frequent reversing direction of rotation. On squirrel cage motors, an optimal brazing between rotor bars and end rings will permit the motor to withstand the repeated acceleration, deceleration and reversal of rotation that is typical in this steel processing.High reliability and longevity with no unplanned production. Detailed analysis of the electromagnetic field patterns and ventilating airflows yields an optimized motor design with maximum cooling and efficiency.
Speed Accuracy
Many drives in a rolling mill run in a coordinated fashion using a reference from a PLC or earlier analog forms of master control. High quality products require speed control accuracy to the master control reference from all drives in order to maintain the proper forces and tensions.
Speed Response
Drive speeds change during the rolling process for a variety of reasons. The command may come from the operator, an automation function adjusting gauge, or the need to thread the mill at a low speed and then accelerate to a higher rolling speed. Regardless of the cause, rapid speed control response means a smaller offset from the desired speed during the speed change and therefore a higher quality product.
Load Response
When metal impacts the roll face, a very large load is rapidly applied to the motor. The result is that speed drops until the motor output torque can adjust to the required rolling torque. Then the motor speed slowdown is stopped and accelerated to the speed reference. Rapid response to load change reduces the speed drop and improves quality.

Current Feedback Speed Reference Speed Control + Current Control Voltage Control Phase Control ☆ 4 Armature Thyristor Voltage Feedback Voltage Isolation DC Field Field Control Current Control Phase Control 1 Current Feedback Field Thyristor Speed Feedback Speed Sensor Figure 1 Configuration of a de drive system