Drive selection of rolling mills

7/31/2019 Drive selection of rolling mills

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Drive selection or rolling millsLong product rolling mills have stringent requirements for motor drives. The speed drop duringhead end impact presents the greatest challenge and is dependent on total system inertia and

the drives dynamic performance. Modern AC drives have demonstrated superior performanceto DC drive systems. For the motor regenerative requirements, systems with multiple AC motors

and drives under a common DC bus system allow the braking energy of one motor to be used byothers, making AC drive systems very cost-competitive.

Long product rolling mills (see Figure 1) are one ofthe most demanding applications for motor drives.The modern rolling train consists of multiple rolling

stands arranged in an in-line conguration with each

rolling stand consisting of a top and bottom roll, driven

through a gearbox by an electric motor. Typically, 15-21

stands are used depending on the size of the feed billet

and the nished product. Finishing speeds of 10-15m/s

are common and typical motor sizes for modern mills are

600-1,200kW per stand using typically low voltage

(


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a

In January 2009, a rolling mill in the USA installed and

commissioned four stand motors and drives, consisting

of ABB ACS800 drive modules in a common DC bus

conguration, four 800kW Siemens low inertia AC motors,

and Prager gearboxes. This combination achieved better

than 0.16%.sec impact drop on all products.

Figure 3 shows an example from this plant with a

0.1%.sec impact drop.

DC vs AC DRIVESMost existing rolling mills have DC drives because,

until recently, AC drives did not produce the necessary

performance. Since all drives in the rolling train must havesimilar performance, early attempts at installing some

AC stands in a DC rolling train required the de-tuning

of existing DC drives. Today, the opposite is true. For

instance, the above-mentioned rolling mill has 17 stands,

with stands 1-13 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 efcient and require

less maintenance, the expense of changing mill stand

motors from AC to DC cannot always be justied. As well

as the cost of replacing power cables and of new motor

mounting modications, it is often necessary to replacethe gearboxes as rolling mill DC motors are most efcient

at low speed and in high torque applications, whereas AC

motors typically need to run above 800rpm to be cost-

effective. Also, the case for the higher efciency of AC

motors is many times negated by the wasteful braking

methods of some AC drive systems, as described below.

Having said that, an AC alternative should still be carefully

considered for motors below 200-300kW, or when new

products necessitate the changing of existing DC motors.

BRAKINGMany stand drives in the rolling train can be run with two

quadrant drives, but some braking is necessary for rapidcontrolled stopping for product changes or in emergency

stop situations. However, some edger stand and shear

drives, as well as many smaller drives in the nishing area,

require full four-quadrant operation.

In DC drive systems the braking power is typically

regenerated via a reverse armature bridge (four quadrant).

When controlled stopping is needed a controlled reversing

eld supply is all that is required. Regenerating DC drive

systems is inexpensive as modern DC drive modules

can go up to about 5,000 amps with only six thyristors.

The regenerative four-quadrant option requires, at most,

only six additional thyristors. Also, new Bi-DirectionalControlled Thyristors (BCTs) are now available, which

means four-quadrant DC drives up to 5,000A with only six

power BCTs are commonplace.

rFig 2 Defnition o a 0.25%.sec maximum

speed drop

Integrated speed drop: 0.25 percent*secondMeasured as the area under the curve in the abovediagram, typically as measured with the drivessoftware or chart recorder.

SA = Static accuracyISD = Integrated speed drop (%s)(area of speed drop triangle)ID = Impact drop percentt = Duration of speed drop in secondsI = 100% rated current

However, braking in an AC drive system is not as

straightforward. Low voltage AC drives consist of a rectier

(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 rectier section cannot

transfer this energy back to the incoming line. To handle

this, most drive manufacturers offer several types of

rectier sections.

RECTIfIER SECTION TypESStandard diode rectier This is the most common

and robust type. In most cases this rectier also includes

some thyristors or other switching devices to soft chargethe capacitors in the inverter sections. During braking

the DC bus voltage rises, then at a certain level, a

controlled chopper dumps the power into a large resistor.

This resistor, however, wastes the power, of ten negating

the efciency advantage of AC over DC motors.

Thristor rectier This is one of the older designs,

similar to a four-quadrant DC drive. During braking the

DC bus voltage rises, then at a certain level the forward

set of thyristor gate signals are removed and the reverse

set is gated. However, while a DC drive is feeding an

inductor (motor), the rectier in an AC drive is feeding acapacitor bank. In the event of an incoming line voltage

dip at the same time as the reverse thyristors are gated,

the forward set of thyristors may not turn off. This results


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in a shortcircuit across the DC bus, blowing all the fuses.

Although a little less expensive than a standard rectier,

this type of rectier is not recommended for most rolling

mill applications.

Inverter rectier Different drive manufacturers have

different names for this. Basically, it is an insulated gate

bipolar transistor (IGBT) inverter unit acting as a rectier.

It provides the true four-quadrant operation of a DC drive,with a near unity power factor and even a limited ability

to ride through some power dips. However, this is the most

expensive type, typically costing more than twice as much

as the standard diode rectier. The standard diode unit

provides a power factor better than 0.95, so the unity

feature is normally not of great benet. For standalone

shears, braking slides, etc, that require four-quadrant

operation, this is the only practical alternative. For these

applications braking resistor are not a viable solution.

COMMON DC BUS AC DRIVES

An increasingly popular option for AC drives is thecommon DC bus solution (see Figure 4). Integrators can

now package drive modules from most manufacturers

such that a common rectier feeds multiple inverter units.

rFig 3 Actual impact speed drop recording showing 0.1%.sec impact drop

Any of the inverters that are braking will regenerate to

the common DC bus. This braking power is then used by

other inverters that are motoring. This allows a simple

diode rectier to be used, providing most of the benets

of the expensive IGBT inverter-rectier. Normally a single

chopper and resistor are incorporated and used mainly for

emergency stop conditions.

When an inverter regenerates power to the common

DC bus, it charges up the capacitors in all the connectedinverter modules. In a rolling mill all the inverter units

need to be oversized to accommodate the up to 200%

head end impact overloads, although only one drive

at a time experiences the impact. This results in a large

total capacitance compared to the used power. It is ideal

for braking as the large capacitor bank, along with the

motoring loads, is able to absorb the immediate and

frequent braking power of even start/stop shears, then

distribute the stored power to the motoring loads.

The more motors and inverters there are under the

common DC bus the more effective the use of the

regenerative power. Also, the total installed cost isreduced by using a few large drive systems, rather than

multiple ones. By paralleling multiple rectier modules,

a common DC bus drive as large as 4,000kW can now


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A single power feed to the rectier, plus having all the

components pre-assembled and tested in a cabinet, saves

considerable installation cost and oor space.

CONCLUSIONSLong product rolling mills have stringent requirements

for motor drives. The speed drop during head end impact

presents the greatest challenge and depends on the

total system inertia and variable speed drives dynamic

performance. When care is taken in selecting components

for inertia, AC drives have demonstrated superior

performance to DC drive systems.

For the regenerative requirements needed for thefrequent 200% braking power 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 (eg, a shear) to be used by other

motoring loads (eg, stands). This provides a cost-effective

approach, making AC drives systems competitive for long

product rolling mill applications.MS

Eric Thorstenson is Director of Sales & Application

Engineering, North America, Russula Corporation,

Atlanta, Georgia, USA.

CONTACT: [email protected]

be built and a single transformer is all that is needed to

feed the system.

A common apprehension about DC bus systems is that if

the rectier fails, the complete line-up fails. Normally, any

drive that fails will shut the mill down until it is repaired

or a temporary solution is found. However, larger common

bus drive systems, as used in rolling mills, require twoor three such rectier modules in parallel, as adding an

additional rectier module to provide redundant capacity

is easily done, which alleviates this concern.

MAINTENANCE CONSIDERATIONSHistorically, AC drives were not very reliable, but most

drive component manufacturers have made great strides

to improve the serviceability of their drive systems. Rather

than attempting to replace IGBTs or other components

inside the drives, most are now constructed so that a

rectier or inverter module can easily be removed from the

cabinet and replaced with a complete spare module.

Some drive manufacturers supply the rectier andinverter modules as complete three-phase units on wheels

with plug connections (similar to draw-out circuit breakers),

and a failed module can be replaced in about 10 minutes.

The failed module can be repaired on a bench or at the

manufacturers. Most have a xed exchange price, sending

out a complete module if the customer sends back the

damaged one. The manufacturer then repairs and tests

the module for use by another customer. Figure 4 shows

common DC bus AC drives D4 and R8i modules.

INSTALLATION CONSIDERATIONS

Common DC bus systems combine the functions of amotor control centre (MCC) with drives in a pre-packaged

unit (see Figure 5).

rFig 5 Individual drives ed by an MCC (not shown)

rFig 4 Common DC bus drive system. Single

power eed, multiple motor outputs in a

pre-packaged and pre-tested cabinet

Documents

Drive selection of rolling mills