Choosing the Best Rolling Mill Main Drive

The drive system – comprising all parts from the spindles to the feeding line – is very complex and covers much more than the motor itself. The entire spindle system, including the gearbox, must be taken into account when searching for the optimal drive solutions.
The primary element of the main drive system is in fact the motor. Further components of the drive system are the main converter circuit and the excitation circuit for DC and synchronous motors. A control system – at best, a digital control system – must also be included. Optimized communication links help to meet specific customer requirements. In addition to the motor and the converter, converter transformers are determining factors when developing the optimal drive solution.

MAC drive system selection

Considering the drive speed and power required for mill applications, rolling mill drives can be found in the middle of the spectrum depicted in Figure 1.

Fig. 1 Typical types of applications

Building on a market success story that began some two decades ago, AC drives have now superceded DC drives in rolling mill applications. Comparative advantages include the fact that the AC motor is less maintenance- intensive than a DC motor. It is also more robust and can be built to higher power ratings. Figure 2 gives an overview of the selection criteria of an AC system over a DC system.

Fig. 2 Selection of drive systems

Other aspects must be considered when selecting the right converter system for the rolling mill drive. For combinations of several drives where one or more drives have a regenerating mode, the DC link optimizes the flow of energy, making a voltage source inverter necessary. High quality line reaction requirements also make the voltage source inverter necessary for single drives operating individually. On the other hand, in single motor applications, where the maximum speed is below 790 rpm and the power line quality requirements are less restrictive, the cycloconverter is the optimal solution. Figure 3 provides an overview of the selection process.

Fig. 3 Classification of AC drive systems

Because the majority of rolling mill drive applications use either cycloconverters (CC) or voltage source inverters (VSI), these two types of drive systems merit closer attention.

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Advanced voltage source inverters

Most progress in the development and optimization of AC drive systems has been achieved by using new-generation, high-performance frequency converters featuring excellent dynamic response. The DC links of these drives lend themselves to connection over a common DC bus. Advantages of this arrangement include enhanced system efficiencies and lower costs for the transformer and lineside converters. In a one- or two-stand cold rolling mill, for example, the unwinding and rewinding energy flow can be transferred during the braking of one inverter to the other motoring inverter through a common DC bus.

In addition, the high output frequencies of voltage source inverters make them suitable for high-speed applications such as cold rolling mills. The higher achievable motor speeds used in conjunction with higher gearbox ratios decrease the motor torque required, reducing motor and other costs.

With increasingly powerful GTO (gate turn-off thyristor) devices in the VSI lineside sections as well as in the motorside sections, power in the megawatt range can be achieved and made available to rolling mill main drive applications. Additional thyristor inverters for regenerative braking are not required because of the GTO lineside converter.

In a VSI drive system, the multi- level drive uses a three-level topology. This technology multiplies the converter output power and efficiency in comparison to conventional VSI two-level technology. Since rolling mill main drives must feed braking energy back into the supply system while maintaining their dynamic response, identical GTO inverters are used at both the lineside and the motorside converters.

The most common cycloconverter in rolling mills uses an open circuit or non-circulating current configuration. Its counterpart, the circulating current mode cycloconverter, has an advantage of producing a higher output frequency, but also has several disadvantages. It needs, for example, six transformers and additional reactors, which leads to additional losses and voltage drops. In consequence, the efficiency is less and the power factor is worse than the values of the circulating free configuration.

The CC utilizes DC component techniques. Three anti-parallel bridges are connected via commutating reactors to a single secondary transformer system. Each of these bridges feeds one motor phase that is galvanically isolated from the other phases. The galvanic isolation in the motor minimizes the torque ripple and current ripple, making it possible to have only one main transformer for the drive.

The output phase voltage is formed from sequences of the line voltage by sinusoidal modulation. In effect, three symmetrical voltage systems are applied to the motor winding. Due to the line-commutated converter and torque ripple requirements, the output frequency is limited to 45 % of the line frequency.

Each disruption in continuous current flow causes a small torque ripple. This makes it essential to minimize the current zero time periods as much as possible. With the help of special thyristor voltage- sensing modules, it is possible with most advanced control systems to reduce these periods to less than 1 ms and to keep torque ripple significantly below 1%.

The CC is a highly efficient drive system. Also, the robust design of the cycloconverter makes it a good solution for drives with maximum speed requirements below 790 rpm, such as roughing mill drives and over 90 % of finishing mill drives.