Everything you always wanted to know about drive selection, but were afraid to ask.
Long Product Rolling Mill Process
Long product rolling mills are one of the most demanding applications for motor drives. The rolling process consists of passing a hot steel billet through a "rolling mill", also called a "rolling train". The modern rolling train consists of multiple rolling stands arranged in an in-line configuration. Each rolling stand consists of a top and a bottom roll, driven through a gearbox by an electric motor. The rolls of the stands have contours or “grooves” machined into the rolls, so that the hot billet passing between the grooves is reduced in size and shaped by each subsequent stand. Typical motor sizes for modern mills is 600kW to 1200kW for each stand. Typically 15 to 21 stands are used depending on the size of the feed billet and the finished product. Finishing speeds of 10-15 m/sec. are common today.
The tension between each stand must be accurately controlled, as the slightest change in tension will affect the shape of the product. Additionally as the billet head end enters each subsequent rolling stand, the speed drop must recover very fast, so as not affect the tension control. The motor drives are controlled by a sophisticated cascade / tension / loop control system, with must take into consideration the design reduction of each stand, and the effective roll groove diameter which is constantly changing due to roll wear and temperature changes.
As the hot billet passes through the rolling train it is shaped, reduced in size, and lengthened by the mill stands. The product is then transferred to a walking beam cooling bed (typically 60-90 m long), via a high speed transfer system (braking slide/aprons). Shears in the rolling train make head and tail crops, as well as divide the material to fit the cooling bed.
Because of the impact loads involved, the motors and drives must be selected to allow for momentary high overloads. NEMA standard MG-1 specifies the momentary (1 minute) overloads of at least 200%. In practice the actual requirements may be different. Whenever the actual load duty cycle is known, the overload dimensioning of the motor and drive should be checked by experienced rolling mill applications specialists using dimensioning software tools offered by most drive and motor manufacturers.
From experience it has been shown that in order to meet the tension control requirements the motor speed must be controlled to 0.01%. Fortunately many modern AC and DC digital drives can meet this static accuracy rating. However, the more important criteria is the dynamic performance rating of the drive which is necessary to minimize the speed drop from the bar head entering each stand. The speed drop is affected by the inertia of the stand / gearbox / motor combination as well as the dynamic performance of the drive. The maximum speed drop must be limited to no more than 0.25 %s, as defined above. Actual impact speed drop recording showing 0.100 percent-second impact drop.
DC vs. AC
Most existing rolling mills have DC drives; because until recently, AC drives did not have the performance necessary. Since all drives in the rolling train must have similar performance, early attempts at installing some AC stands in a DC rolling train required the detuning of existing DC drives. Today the opposite is true, the above mentioned rolling mill has a total of 17 stands, with stands 1 to 13 being powered by modern DC digital drives. In this case the four new AC drives were "de-tuned" to match the performance of the existing DC drives.
Even though AC motors are more efficient and require less maintenance, the expense of changing mill stand motors from AC to DC cannot always be reasonably justified. Besides the cost of replacement of power cables, and new motor mounting modifications; many times it is necessary to replace the gearboxes. Rolling mill DC motors are best at low speed, high torque applications, while AC motors typically need to run above 800rpm to be cost effective. Also the case for higher efficiency of AC motors is many times negated by the wasteful braking methods of some AC drive systems. All this being said, for motors below 200-300kW, or when new products necessitate the changing of existing DC motors, the AC alternative should be carefully studied.
Many stand drives in the rolling train can be run with two quadrant drives, but some braking is necessary for rapid controlled stopping for product changes, or E-stop situations. However, some edger-stand drives, shear drives, as well as many smaller drives in the finishing area require full four quadrant operation.
In DC drive systems the braking power is typically regenerated either via a reverse armature bridge (4 quadrant). When only controlled stopping is necessary a controlled reversing field supply is all that is required. For DC drive systems the cost of the regeneration is very inexpensive. Modern DC drive modules can go up to about 5000 amps with only six thyristors. The regenerative four quadrant option is, at worst only 6 additional thyristors. Also new Bi-Directional Controlled Thyristors (BCT’s) are now available, so that four quadrant DC drives up to 5000A with only six power BCT’s are common place.
However, braking in an AC drive system is not as straight forward. Low voltage AC drives consist of a rectifier (AC to DC) section, feeding an inverter (DC to AC) section. During braking the inverters are regenerative to the DC bus as standard, but the standard rectifier section cannot transfer this energy back to the incoming line. To handle this, most drive manufacturers offer several different types of rectifiers sections.
The rectifiers section types are:
This is the most common and most robust type. In most cases this rectifier also includes some thyristors or other switching devices to soft charge the capacitors in the inverter sections. During braking the DC bus voltage rises, and at a certain level a controlled chopper dumps the power into a large resistor. This resistor wastes the power which often negates the efficiency advantage of AC over DC motors.
These are one of the older designs. These are similar to a four quadrant DC drives. During braking, the DC bus voltage rises. At a certain level the forward set of thyristors gate signals are removed and the reverse set is gated. However, while a DC drive is feeding an inductor (motor), the rectifier in a AC drive is feeding a capacitor bank. In the event of an incoming line voltage dip at the same time as the reverse thyristors is gated, the forward set of thyristors may not turn off. This results in short circuit across the DC bus blowing all the fuses. Although this solution is a little less expensive, this type of rectifier is not recommended for most rolling mill applications.
Different drive manufacturers have different names for this type. Basically this is an IBGT inverter unit acting as a rectifier unit. This provides the true four quadrant operation of a DC drive, while also providing near unity power factor or better, and even the limited ability to ride through some power dips. However this is the most expensive type, typically costing more than twice as much as the Diode /Chopper/Resistor type. The standard diode unit provides better than 0.95 power factor so the unity power factor feature is normally not a great benefit.
For stand-alone shears, braking slides, etc. that require four quadrant operation, this is the only practical alternative. For these applications braking resistors are not a viable solution.
Common DC Bus AC Drives
An increasing popular option for AC drives is the common DC bus solution. Integrators can now package drive modules from most manufacturers such that a common rectifier feeds multiple inverter units. Any of the inverters that are braking regenerate to the common DC bus, this braking power is then used by other inverters that are motoring. This allows the use of a simple diode rectifier to be used and provide most of the benefits of the expensive IGBT inverter-rectifier. Normally a single chopper and resistor are incorporated, mainly used for e-stop conditions.
When an inverter regenerates power to the common DC bus, it charges up the capacitors in all the connected inverter modules. In a rolling mill all the inverter units need to be oversized to accommodate the 200% head end impact overloads, although only one drive is experiencing this impact at a time. The result in a large total capacitance compared to the used power. This is an ideal situation for braking. This large capacitor bank along with the motoring loads is able to absorb the immediate and frequent braking power of even start/stop shears, and then distribute this stored power to the motoring loads.
The more motors and inverters under the common DC bus, the more effective the use of the regenerative power. Also the total installed cost is reduced by using a few large drive systems, rather than multiple drive systems. By paralleling multiple rectifier modules common DC bus drive as large as total 4000kW can now be built. A single transformer is all that is needed to feed the system.
A common apprehension of common DC bus systems is that, if the rectifier fails, the complete line up fails. This is true. But in a rolling mill, normally any drive that fails will shut the mill down until it is repaired or a “work-around” is found. Normally larger common bus drive systems as used rolling mills require 2 or 3 such rectifier modules in parallel. Adding an additional rectifier module to provide redundant capacity is easily done, which alleviates this concern.
Common DC bus AC drives D4 and R8i modules
Today AC drives are very reliable, but in the past this was not always true. For that reason most drive component manufacturers have made great strides to improve the serviceability of their drive systems. Rather than attempting to replace IBGT’s or other components inside the drives; most drives are now constructed so that a rectifier or inverter module can be easily removed from the cabinet and replaced with a complete spare module. Some drive manufacturers supply the rectifier and inverter modules as complete three phase units on wheels with plug connections (similar to draw-out circuit breakers). A failed module can be replaced in about 10 minutes total.
The failed module can then be repaired on a bench, or sent back to the manufacturer for repair. Most manufacturers have a fixed exchange price, where they send out a complete module, while the customer sends the damaged module back. The manufacturer then repairs and tests the module and keeps it for use by the next customer.
Common DC bus systems combine the functions of a motor control center (MCC) with drives in a pre-packaged unit.
A single power feed to the rectifier, plus having all the components pre-assembled and tested in a cabinet saves considerable installation cost, and floor space.
Long product rolling mills have stringent requirements for the motor drives. The speed drop during head end impact presents the greatest challenge. This speed drop is dependent upon the total system inertia and the variable speed drive’s dynamic performance. With care taken regarding selection of components for inertia, AC drives have demonstrated superior performance to DC drive systems.
For the regenerative requirements needed for the frequent 200% braking power needed for start-stop shears, braking slides, aprons, etc.; DC drives have enjoyed a distinct cost advantage. However, systems with multiple AC motors and drives under a common DC bus system allow the braking energy of one motor (i.e. a shear) to be used by other motoring loads (i.e. stands). This system provides a cost effective approach, making AC drives systems competitive for long product rolling mill applications.
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