INDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: A CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE

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COLD ROLLING COLD ROLLING COLD ROLLING COLD ROLLING MILLMILLMILLMILL

BY

AKINYELE, DAMILOLA JOHNSON

(109043084)

BEING A TERM PAPER PRESENTED TO

THE DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING,

FACULTY OF ENGINEERING,

UNIVERSITY OF LAGOS,

LAGOS, NIGERIA.

IN PARTIAL FULFILMENT OF THE REQUIREMENT

FOR THE AWARD OF MASTER OF SCIENCE (M.Sc) DEGREE IN

ELECTRICAL AND ELECTRONICS ENGINEERING (CONTROL OPTION)

MAY, 2011.

1.0 ABSTRACT

This paper reviews the applications of electric drives in the industries, taking into consideration the

improving trend through the application of power electronics.

Application of a variable frequency drive (Chassis drive) employed in driving Rolling Mills Machinery at

Aluminum Rolling Mills (ARM), a subsidiary of Tower Aluminum Plc. Nigeria is briefly examined.

The examination includes analysis of the power circuit and control circuit with waveforms, Design of power

circuit, selection of devices employed in the drive system and problems associated with the use of the drive

in the Industry.

2.0 INTRODUCTION

Electric drive is an industrial system which performs the conversion of electrical energy to mechanical

energy or vice versa for running various processes. Drives are employed for systems that require motion

control – e.g. transportation system, fans, robots, pumps, machine tools, etc. Prime movers are required in

drive systems to provide the movement or motion and energy that is used to provide the motion can come

from various sources: diesel engines, petrol engines, hydraulic motors, electric motors etc. Drives that use

electric motors as the prime movers are known as electrical drives

Figure 1: Block Diagram of Electric Drive

There are several advantages of electrical drives:

a. Flexible control characteristic – This is particularly true when power electronic converters are

employed where the dynamic and steady state characteristics of the motor can be controlled by controlling

the applied voltage or current.

b. Available in wide range of speed, torque and power

c. High efficiency, lower noise, low maintenance requirements and cleaner operation

d. Electric energy is easy to be transported.

2.1 Components of Electrical Drives:

The main components of a modern electrical drive are the motors, power processor, control unit and

electrical source.

a) Motors:

Motors obtain power from electrical sources. They convert energy from electrical to mechanical therefore can

be regarded as energy converters. In braking mode, the flow of power is reversed. Depending upon the type

of power converters used, it is also possible for the power to be fed back to the sources rather than

dissipated as heat.

There are several types of motors used in electric drives – choice of type used depends on applications, cost,

environmental factors and also the type of sources available.

Broadly, they can be classified as either DC or AC motors:

DC motors (wound or permanent magnet)

AC motors:

Induction motors – squirrel cage, wound rotor

Synchronous motors – wound field, permanent magnet

Brushless DC motor – require power electronic converters

Stepper motors – require power electronic converters

Synchronous reluctance motors or switched reluctance motor – require power electronic converters

b) Power processor or power modulator:

Since the electrical sources are normally uncontrollable, it is therefore necessary to be able to control the

flow of power to the motor – this is achieved using power processor or power modulator. With controllable

sources, the motor can be reversed, brake or can be operated with variable speed. Conventional methods

used, for example, variable impedance or relays, to shape the voltage or current that is supplied to the

motor – these methods however are inflexible and inefficient. Modern electric drives normally used power

electronic converters to shape the desired voltage or current supplied to the motor. In other words, the

characteristic the characteristic of the motors can be changed at will. Power electronic converters have

several advantages over classical methods of power conversion such as:

1. More efficient – since ideally no losses occur in power electronic converters

2. Flexible – voltage and current can be shaped by simply controlling switching functions of the power

converter

3. Compact – smaller, compact and higher ratings solid–state power electronic devices are continuously

being developed – the prices are getting cheaper.

Converters are used to convert and possibly regulate (i.e. using closed-loop control) the available sources to

suit the load i.e. motors. These converters are efficient because the switches operate in either cut-off or

saturation modes

c) Control Unit:

The complexity of the control unit depends on the desired drive performance and the type of motors used. A

controller can be as simple as few op-amps and/or a few digital ICs, or it canbe as complex as the

combinations of several ASICs and digital signal processors (DSPs). The types of the main controllers can

be:

1. analog - which is noisy, inflexible. However analog circuit ideally has infinite bandwidth.

2. digital – immune to noise, configurable. The bandwidth is obviously smaller than the analog controller’s –

depends on sampling frequency

3. DSP/microprocessor – flexible, lower bandwidth compared to above. DSPs perform faster operation than

microprocessors (multiplication in single cycle).With DSP/microp, complex estimations and observers can be

easily implemented.

d) Source

Electrical sources or power supplies provide the energy to the electrical motors. For high efficiency

operation, the power obtained from the electrical sources need to be regulated using power electronic

converters.

Power sources can be of AC or DC in nature and normally are uncontrollable, i.e. their magnitudes or

frequencies are fixed or depend on the sources of energy such as solar or wind. AC source can be either

three-phase or single-phase; 3-phase sources are normally for high power applications .There can be several

factors that affect the selection of different configuration of electrical drive system such as:

a) Torque and speed profile - determine the ratings of converters and the quadrant of operation required.

b) Capital and running cost – Drive systems will vary in terms of start-up cost and running cost, e.g.

maintenance.

c) Space and weight restrictions

d) Environment and location

2.2 Comparison between DC and AC drives

Motors:

1. DC require maintenance, heavy, expensive, speed limited by mechanical construction

2. AC less maintenance, light, cheaper, robust, high speed (esp. squirrel–cage type)

Control unit:

1. DC drives: Simple control – decoupling torque and flux by mechanical commutator – the controller can be

implemented using simple analog circuit even for high performance torque control –cheaper.

2. AC drives, the types of controllers to be used depend on the required drive performance – obviously, cost

increases with performance. Scalar control drives technique does not require fast processor/DSP whereas in

FOC or DTC drives, DSPs or fast processors are normally employed.

Performance:

1. In DC motors, flux and torque components are always perpendicular to one another thanks to the

mechanical commutator and brushes. The torque is controlled via the armature current while maintaining

the field component constant. Fast torque and decouple control between flux and torque components

can be achieved easily.

2. In AC machines, in particular the induction machines, magnetic coupling between phases and between

stator and rotor windings makes the modeling and torque control difficult and complex. Control of the

steady state operating conditions is accomplished by controlling the magnitude and the frequency of the

applied voltage; which is known as the scalar control technique. This is satisfactory in some applications.

The transient states or the dynamics of the machine can only be controlled by applying the vector control

technique whereby the decoupling between the torque and flux components is achieved through frame

transformations. Implementation of this control technique is complex thus requires fast processors such

as Digital Signal Processors (DSPs).

3.0 HISTORICAL BACKGROUND

3.0.1 Electrical drives in industry

Prior to the availability of electronics, clever electromechanical solutions involving combinations of dc and ac

machines (e.g., Krämer and Scherbius systems) were developed early in the 20th century to control

the speed of electric machines in industrial processes. The emergence of mature triggered-arc power switch

technology (e.g., grid-controlled mercury-arc rectifiers, thyratrons, ignitrons)

in the 1920s and 1930s provided a major boost to dc commutator machines as preferred prime movers for

industrial drive application.

This situation persisted for several decades until solid-state thyristors finally provided the crucial power

switch breakthrough needed to build practical adjustable-frequency ac machine drives in the 1970s. Since

that time, new generations of gate-controlled power switches have successively improved the performance

and cost-effectiveness of ac drives in comparison to their dc drive counterparts.

Although most of today’s growth in the worldwide industrial drive market can be ascribed to ac drives,

modern generations of dc drives continue to hold a significant share of the total industrial drive market.

3.0.2 Induction motors

The AC squirrel-cage induction motors are the largest group of all electrical drives in the industry. It

has been estimated that they are used in 70-80% of all industrial drive applications, especially in fixed-speed

applications such as pump or fan drives. The benefits of induction motor are undisputable: simple

construction, low cost compared to other motors, simple maintenance, high efficiency and satisfactory

characteristics at the high speeds. With appropriate power electronics converters induction motors are used

in wide power range from kW to MW levels.

A common structure of induction motor is represented in Fig.1.4. Stator rotation field induces an

electromotive force in the short-circuited rotor winding. Due to the induced voltage and short-circuited

winding, current occurs in the rotor and electromagnetic torque is produced.

Induction motor was invented in the end of 19th century. Its theory is well-known and power

electronic converter technology provides appropriate variable-voltage/current, variable-frequency supply for

efficient and stable variable-speed control. Thus, it is possible to obtain a dynamic performance in all

respects better than which could be obtained with a phase-controlled DC drive combination. The significant

characteristic of the induction motor is a slip caused by the rotor lagging the rotating stator magnetic field.

Rotor copper losses are directly proportional to the slip. For example, the rotor copper losses in 4 kW

motor are approximately 4.7% of the nominal power if the efficiency of 85% is supposed. The slip has also

impact when a high dynamic performance is needed, since it derates the transient response of the motor.

The main disadvantage of the induction motor for the servo control systems is the nonlinear speed versus

torque and speed versus control voltage characteristics. Therefore such motors are inconvenient for the

implementation of the belt-drives.

Servo drives are applied for example in such areas of industry as:

• Paper industry

• Machine tools and metal working machinery

• Packaging machinery

• Woodworking

In spite of the wide spread occurrence of electrical motors, the precision requirements for servo drives

constantly increase.

3.0.3 Common motor types in motion control

With invention of the motor the important question arisen is: how it can be controlled?

The first speed-controlled drive was introduced by Harry Ward Leonard and it required

three machines for scheme implementation. After the invention of transistor and rapid

development of electronics, new possibilities for accurate control of DC motor, such as

PWM technology, appeared. Brushless DC motors with permanent magnets were

introduced in early 1960s, but their power was limited due to not enough powerful PM

materials. Brushless DC motors for higher power application became available only after

the invention of PM materials with high energy density in the beginning of 1980s. Later

on, when the field-oriented control was introduced, it was possible to apply AC motors for speed-controlled

applications. First speed-controlled AC drives were induction motors but in early 1990s the vector control

was also developed for PMSMs. Also, with rapid development of computer science became accessible control

of stepper motors directly from microcontrollers. All these motor types are available for wide range of motion

control applications.

3.1 CURRENT STATE OF THE DRIVE

This past decade has witnessed major advances in power electronics technology for both industrial and

traction drives. These advances have made it possible to significantly improve the electrical performance of

these systems while simultaneously reducing their size and weight and, perhaps most importantly, reducing

their cost. Improvements in all of these key metrics are expected to continue during coming

years, with a few trends meriting special attention:

1) Modularization and Automated Manufacturing:

Improved packaging of the power electronics components and subsystems is expected to be one of the

most active development areas because of the significant cost reductions that are achievable by

modularizing the power electronics and automating its manufacturing. Major progress toward these

objectives can already be observed in many of the new industrial drive products with ratings

of 10 kW or less. Wider acceptance of power electronics in automotive applications during coming years—

both for traction and accessory applications—will likely provide a substantial boost for this trend by focusing

the energies of the automotive industry on achieving these manufacturing economies.

2) Increasing Silicon Content:

As the cost of power switches and their driver circuitry continues to diminish, trends toward increasing the

proportion of active silicon- based power electronics in future drive products at the expense of passive

components (i.e., capacitors, inductors, transformers) is certain to continue. Particularly in industrial

drives, international trends toward tighter regulation of the input power quality of new products is expected

to motivate the wider introduction of active rectifier stages using additional power switches with less

dependence on special transformers and passive L-C filters. Direct ac/ac power converter topologies such as

the matrix converter are expected to appear in some new applications, although their

ability to broadly displace the dominant rectifier-inverter topology is very uncertain. Serious efforts are under

way to use new high-voltage power switches in innovative configurations to eliminate the need for heavy

and expensive transformers in future heavy-rail traction drives operating

directly from 25-kV ac catenary supplies.

3) Power Converter/Machine Integration:

The third notable technology trend, and perhaps the most controversial, is wider adoption of integrated

packaging of the power electronics and the electrical machine in the same mechanical

package. Several industrial drive manufacturers have introduced product offerings ( 10 kW) based on this

approach during recent years with limited initial market appeal, but anecdotal evidence indicates that

acceptance among skeptical customers is gradually improving. Automotive manufacturers are exploring this

approach as a long-term approach for modularizing the EV and HEV electric propulsion units while

simultaneously reducing EMI emissions by eliminating long interconnecting cables between the inverter

and machine. Significant technical hurdles associated with the ability of the power electronics to survive the

heat and vibration of the harsh machine environments are, gradually being overcome, making the “electronic

motor” concept increasingly attractive for many drive applications from both a technical and economic

standpoint.

3.2 APPLICATION OF POWER ELECTRONICS IN DRIVES

The power electronic converters are used to obtain an adjustable DC voltage applied to the armature of a DC

motor. There are basically two types of converters normally employed in DC drives: (i) controlled rectifier (ii)

switch –mode converter.

(i) Controlled rectifier:

Controlled rectifier can be operated from a single phase or three phase input.

Output voltage contain low frequency ripple which may require a large inductor inserted in armature circuit,

in order to reduce the armature current ripple. A large armature current ripple is undesirable since it may be

reflected in speed response if the inertia of the motor–load is not large enough. Controlled rectifier has a low

bandwidth. The average output voltage response to a control signal, which is the delay angle, is relatively

slow. Therefore cont rolled rectifier is not suitable for drives requiring a fast response. Eg. servo applications.

In terms of quadrant of operations, a single phase or a three phase rectifier is only capable of operating in

first and fourth quadrants – which is no suitable for drives requiring forward breaking mode. To be able to

operate in all four quadrants, configurations using back to back rectifiers or contactors shown below must be

employed.

Figure 2: Thyristor Configurations

The thyristor d.c. drive remains an important speed-controlled industrial drive, especially where the higher

maintenance cost associated with the d.c. motor brushes (c.f. induction motor) is tolerable. The controlled

(thyristor) rectifier provides a low-impedance adjustable 'd.c.' voltage for the motor armature, thereby

providing speed control.

3.2.1THYRISTOR D.C. DRIVES – GENERAL

For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-

phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used

to supply the field of the motor: the power is much less than the armature power, so the supply is often

single-phase, as shown in Figure 3(a). The arrangement shown in Figure 3(a) is typical of the majority of

d.c. drives and provides for closed-loop speed control.

Figure 3(a) D.C Drive

Figure 3(b): Basic three-phase voltage-source inverter bridge.

The main power circuit consists of a six-thyristor bridge circuit which rectifies the incoming a.c. supply to

produce a d.c. supply to the motor armature. The assembly of thyristors, mounted on a heatsink, is usually

referred to as the 'stack'. By altering the firing angle of the thyristors the mean value of the rectified voltage

can be varied, thereby allowing the motor speed to be controlled.

The controlled rectifier produces a crude form of d.c. with a pronounced ripple in the output voltage. This

ripple component gives rise to pulsating currents and fluxes in the motor, and in order to avoid excessive

eddy-current losses and commutation problems, the poles and frame should be of laminated construction.

It is accepted practice for motors supplied for use with thyristor drives to have laminated construction, but

older motors often have solid poles and/or frames, and these will not always work satisfactorily with a

rectifier supply. It is also the norm for drive motors to be supplied with an attached 'blower' motor as

standard. This provides continuous through ventilation and allows the motor to operate continuously at full

torque even down to the lowest speeds without overheating.

Low power control circuits are used to monitor the principal variables of interest (usually motor current and

speed), and to generate appropriate firing pulses so that the motor maintains constant speed despite

variations in the load. The 'speed reference' (Figure 1) is typically an analogue voltage varying from 0 to 10

V, and obtained from a manual speed-setting potentiometer or from elsewhere in the plant.

The combination of power, control, and protective circuits constitutes the converter. Standard modular

converters are available as off-the-shelf items in sizes from 0.5 kW up to several hundred kW, while larger

drives will be tailored to individual requirements. Individual converters may be mounted in enclosures with

isolators, fuses etc., or groups of converters may be mounted together to form a multi-motor drive.

4.0 APPLICATION OF DRIVES IN ALUMINIUM ROLLING MILLS(ARM).

Set up in 1983, Aluminium Rolling Mils was a major step in backward integration for Tower Aluminium.

Aluminium Rolling Mills ( ARM) is today one of the largest Rolling Mills in West Africa, with facilities for

converting its basic raw material, aluminium ingots, into high quality coils & discs. The present installed

capacity is 12,000 tons / annum.

PRODUCT RANGE

Plain Coils

• Thickness 0.43 to 1.2 mm – Used in roofing sheets.

• Thickness 1.2 mm upto 2 mm – Used for cash boxes, port cabins, pick up van bodies, metal

furniture.

Stucco Coils

• Thickness 0.43 to 1.2 mm – Used in channels, gutters, roofing.

Circles ( Discs )

• Used for aluminium kitchenware.

Special coils

• Used for high security registration plates for vehicles.

4.1 Roll Milling Machine

Steel rolling mill machinery is used for making or shaping the metal. This machinery is very helpful in

making or giving shape to the metals or steel. This steel rolling mill machinery varies on the basis of process.

This machinery is mostly used in steel and metal factories. This machinery is proving its worth in mechanical

industry. The unit which uses this steel rolling mill machinery is known as steel rolling mill plants. Steel is

rolled in steel rolling mill plants. Following type of items are rolled in steel rolling mill machinery plants:

Simple rounds, Steel rods, Wire rod in coils, Plates, Hot strips, H-beams, I-beams. Steel rolling mill

machinery is categorized in two ways that is hot rolling mill machinery and cold rolling mill machinery.

Hot rolling mill machinery- if the temperature of the metal is marked above its recrystallization temperature

then the metal is rolled into hot rolling mill machinery.

Cold rolling mil machinery- if the temperature of the metal is marked below the recrystallisation temperature

then the metal is rolled into cold rolling mill machinery. These procedures are carried out to harden the

metal according to the requirements of different companies.

The Chassis drive is one of the drives used in controlling the speed of the gears of the steel rolling mill

machines. This drive will be chosen for detailing in this paper.

4.2 Chassis drive

Squirrel-cage induction machines dominate ac industrial drive applications around the world for both new

and retrofit applications. Permanent magnet (PM) synchronous machines are increasingly popular choices for

high-performance servo applications because of their high power density. However, these represent only a

small fraction of the total number of industrial drive applications, the majority of which (i.e., pumps, fans)

can be satisfied with general-purpose induction motor drives using constant volts-per-hertz control.

Other types of brushless machines including switched reluctance account for only a small fraction of new

industrial drive applications. Three-phase voltage source inverters using the basic six-switch bridge topology

shown in Fig. 3(b) have become the overwhelming favorite for industrial drive applications less than 2 MW.

During the past decade, IGBTs packaged in compact plastic power modules have rapidly expanded their

voltage and current ratings so that they now dominate nearly all of the new industrial drive inverter designs.

Power modules are now available from several manufacturers with 1200-V IGBTs (appropriate for 460-V

machines) that are packaged either as single-switches (2400 A), dual-switch phase legs (800 A), or complete

six-switch bridges ( 400 A), depending on the required current ratings. Continuing improvements in IGBT

switching characteristics allow these devices to be used with PWM switching frequencies that range from 1

to 2 kHz in high-power drives ( 1 MW) to more than 20 kHz in lower power ratings ( 30 kW). IGBT power

modules are being offered with gate drives and protective functions built into the same package. Another

mark of progress is the growing availability of integrated power modules (IPMs) that provide all of the power

semiconductors for a complete industrial drive in a single package, including IGBTs and diodes for the

rectifier, inverter, and brake stages described in the following section.

These integrated modules are already being produced with ratings of 1200 V and 50 A, with higher current

ratings now in development.

5.0 POWER CIRCUIT AND WAVEFORM

Figure 4: Power Circuit of a Chassis drive

5.1 WAVEFORMS

Figure 5: (a) Stator current waveform (b) Stator voltage and current

The power waveform of the motor is as shown above using a computer based monitor.

6.0 DESIGN OF THE POWER CIRCUITS

The architecture for a typical panel-mounted chassis drive in the 50-kW power range is illustrated in Fig. 4.

The core of the power circuitry for this drive consists of an input three-phase diode rectifier, a precharge

circuit, dc link capacitor bank, and IGBT inverter stage. Other power components shown in this figure

including the input line ac reactor, input electromagnetic interference (EMI) filter, dc link reactor, dynamic

brake, and output filter are all optional depending on the application requirements.

1) Inverter Stage:

Modern IGBTs have matured to a high state of electrical ruggedness that makes it possible to design the

inverter stages with a minimum of additional snubber circuitry to limit either the rate of current change (

di/dt) or the rate of voltage change (dv/dt ) at the IGBT terminals. In fact, many new IGBT inverters are

designed without any snubbers at all. This approach has the advantage of saving cost, space, and losses in

the inverter—all positive effects. However, the resulting switching rates at the IGBT output terminals in

snubberless inverter designs can sometimes exceed 1000 A s and 10 000 V s, which is undesired and, in

some cases, harmful consequences. For example, it has been confirmed that the high switching rates can

interact with the inverter output cables and the machine windings to cause large transient voltages to appear

across the outermost turns of the stator windings. Field experience has demonstrated that this situation

sometimes leads to catastrophic failure of the stator winding insulation. Such conditions are most likely to

occur in installations with unusually long cables between the drive inverter and machine. Some machine

manufacturers have responded by strengthening the stator winding insulation, at least for the outermost

turns.

It was also found that the high switching rates can induce unbalanced charge build-ups on the machine rotor

by means of parasitic capacitances coupling the machine stator, the rotor, inverter switches, and ground.

This accumulated rotor charge eventually discharges through the bearings if no other galvanic current path

is provided. Over time, these discharges

can cause serious pitting of the bearing balls and races that eventually leads to bearing failures under some

circumstances that have been experienced in field installations. The introduction of insulated bearings or

grounded rotor shaft brushes is among the solutions adopted by machine manufacturers to prevent these

bearing failures.

A third problem caused by the high and switching rates is elevated levels of conducted EMI in

the drive input lines and ground that can easily exceed acceptable levels. Common-mode EMI can be

significantly reduced by adding common-mode inductors in both the input and output lines, together with

small capacitors between each dc link bus and earth ground to prevent these high-frequency currents from

reaching the utility grid or machine. Modifications of the PWM switching algorithm can also be introduced to

minimize the generation of common-mode EMI. Differential-mode EMI can be reduced by adding passive or

active filters on the inverter output lines. Another effective, albeit somewhat ironic solution to these EMI

problems is to use the IGBT gate drives to artificially slow their switching speeds, trading reduced EMI for

elevated switching losses.

2) Input Rectifier Stage:

By showing a diode rectifier stage in Fig. 4, the illustrated architecture is typical of the

large majority of today’s industrial drives ( 90%) that do not permit braking power to be returned to the

utility lines. Although the diode rectifier has the advantage of low cost, it also imposes several constraints

and disadvantages on the drive system that are attracting more attention every year.

Perhaps most importantly, the basic diode rectifier draws current from each input line that is significantly

distorted from its desired sinusoidal shape [Fig. 6(a)].

One of the purposes of including either a dc link reactor or three-phase input line reactors in the drive power

circuitry is to reduce the amplitude of the input current harmonics in addition to limiting the peak dc link

capacitor current and reducing the conducted EMI. However, Fig. 6(b) shows that, even when the ac line

reactors are introduced with typical ratings (3% of the drive base impedance, in this case), the input line

current harmonics are still very substantial, containing a fifth harmonic component greater than 40% of the

fundamental component. Another constraint imposed by diode rectifiers is the need for pre-charging of the

dc link capacitors to avoid dangerous surge current when the drive is initially connected to the ac line. A

variety of pre-charge circuits are used depending on the drive power rating, including the parallel

combination of resistor and bypass contactor illustrated in Fig. 4.

For industrial drives with ratings greater than 30 kW, the diodes in the input rectifier stage are often partially

or completely replaced by phase-controlled thyristors. Although more expensive than the diodes, the

thyristors serve multiple purposes by providing the pre-charge function and serving as a fast electronic

circuit breaker to remove power from the dc link in case of ground-fault or inverter failures.

Many industrial drive applications that demand only limited and infrequent machine braking capability (e.g.,

conveyors) can be satisfied by means of a dynamic brake assembly that dissipates the excess electrical

energy appearing on the dc link in external resistors (see Fig. 4). However, applications that involve frequent

and extended periods of

braking (e.g., cranes) can justify the introduction of a regenerative input converter bridge as shown in Fig. 4

to provide the drive with bidirectional power conversion capabilities.

The topology of the input converter bridge is identical to that of the inverter using six controlled switches

(i.e., IGBTs) and six diodes, as shown previously in Fig. 3. Three-phase input line reactors are essential with

the active input bridge configuration to provide the necessary input impedance characteristics to the bridge

terminals.

The addition of the regenerative active input bridge clearly represents an expensive solution that can only be

justified today in applications that truly need bidirectional power flow.

However, appropriate PWM control of the active input bridge results in significant system advantages by

eliminating all of the undesirable low-order current harmonics that are associated

with either diode or thyristor bridges, as well as providing independent control of the input power factor. Fig.

7 shows typical input line and dc link voltage/current waveforms for a regenerative drive system, illustrating

the major improvement in line current harmonics compared to the diode rectifier waveform shown earlier in

Fig. 6. This figure also illustrates the fast response that regenerative input stages can provide using vector

control principles handling a step change in drive operation from full-load motoring to full-load braking with

minimal perturbations of the dc link voltage.

3) Bus Capacitors: Electrolytic capacitors are the preferred choice for the dc link capacitors in almost all

industrial drives. Minimum dc link capacitance requirements are usually determined by the rms current-

carrying capacity of the electrolytic capacitors and the need for sufficient dc link energy storage to allow the

drive to “ride through” transient utility voltage sags. Typical electrolytic capacitor bank values for general-

purpose drives fall between 25 and 50 F per amp of inverter rms output current. Since the voltage ratings of

electrolytic capacitors are usually limited to 500 Vdc or less, series and parallel combinations are required in

many drive applications to support dc link voltages that exceed these ratings.

Newer film capacitors are available with higher rms current ratings per F of capacitance and better reliability

characteristics than their electrolytic counterparts. However, film capacitors are seldom used in place of

electrolytics today because they are more expensive and smaller bus capacitance provides less drive ride-

through protection during utility voltage sag events.

7.0 CONTROL CIRCUIT WITH WAVEFORMS

Fig. 6. Three-phase diode rectifier characteristics. (a) Input voltage and current

waveforms. (b) Input current frequency spectrum.

Figure 7: Block diagram of IGBT controlled drive of a 6Hi Cold Rolling Mill motors

8.0 DESIGN OF THE CONTROL CIRCUIT

The input voltage directly sets the oscillator and hence the inverter frequency. The input setting also

determines the link voltage, so maintaining the voltage/frequency relationship. In order to march the motor

speed with the increasing frequency, the frequency must start from zero and rise at a rate such that the

torque can accelerate the rotor to this rate.

The ramp unit delays the rise of voltage resulting from any step input, and allows the inverter

frequency to rise slowly, so maintaining stability during the accelerating period.

The accuracy of the drive depends on the oscillator frequency.The DC link voltage adjustment

controls the motor voltage magnitude and frequency within the inverter can be controlled.

The use of insulated gate bipolar transistors enables switching within the inverter to be faster and

more efficient than thyristor circuit.

Figure 8 : Diode-IGBT current switching characteristics for one inverter-leg, according to Martin (1996). T+

refers to the upper and T� to the lower transistor and+ is the freewheeling diode of the upper transistor.

9.0 APPLICATION IN THE LOCAL INDUSTRY WITH DIAGRAM

The Chassis drive is used for controlling the speed, torque, voltage and current of the 6Hi Cold Rolling Mill

motors.

The aim is to reduce the thickness of hot rolled coil.

Also, the drive is used for controlling the uncoiler motor, the edge trimmer, motor, and the recoiler motor as

shown below:

Figure 9: Block Diagram of Cold Rolling Mill Machine

Figure 10: 6Hi Cold Rolling Mill Machine

Figure 10(a) and (b) : Production Line pictures.

10.0 SELECTION OF DEVICES OF THE DESIGN SYSTEMS

The devices used are:

(a) Insulated Gate Bipolar Transistor based inverter

(b) Converter transformer

(c) Thyristor supply unit for conversion

(d) Motor

(e) Air Circuit Breakers (ACB)

(f) Moulded Case Circuit Breakers (MCCB)

(g) Contactor

(h) Relays

(i) Indicator Lamp

(j) Fuse

(k) Encoder

11.0 PROBLEMS ASSOCIATED WITH THE USE OF THE DRIVE IN THE INDUSTRY

1. The drive discussed above is a variable frequency drive, hence the introduction of harmonic

Pollution by non-linear elements in the make up of the drive is a challenge the industries

Face with the application of the drive.

2. The resulting switching rates at the IGBT output terminals in snubberless inverter designs

Employed in the drive can sometimes exceed 1000 A s and 10 000 V s, which is undesired and, in

some cases, harmful.

3. This high switching rates can induce unbalanced charge build-ups on the machine rotor by means of

parasitic capacitances coupling the machine stator, the rotor, inverter switches, and ground. This

accumulated rotor charge eventually discharges through the bearings if no other galvanic current path is

provided. Over time, these discharges can cause serious pitting of the bearing balls and races that

eventually leads to bearing failures under so circumstances that have been experienced in field installa

tions

4. Erratic power supply from the supply authority can cause huge damage during manufacturing

12.0 RECOMMENDATION

� The stator winding insulation should be strengthened, at least the outer most insulation.

� Necessary filter circuit should be introduced to reduced the effect of harmonics.

� Insulated bearings or grounded rotors should be Incorporated to reduce the effect of

bearing failure in the applications of the drive.

� Technology transfer method should be adopted to develop our local industry.

� Research and development of drives that suit the local industry should be vigorously pursued.

13.0 CONCLUSIONS

Drives are important in every manufacturing industry as they provide means of adjusting torque, speed,

voltage, current as regards to a given motor leading to reduction in the electrical energy demand of a

system and industry at large.

Local manufacture of drives will go a long way to developing our local technology as well as human

resources.

REFERENCES

[1] Ashfaq Amhed, “Power Electronics for Technology”, Prentice Hall, New Jersey, 1999.

[2] J.A. Odunsi, “Electrical Installation, Design and Drafting”, Yabatech Press, Lagos, 2000.

[3] Cyril W. Lander, “Power Electronics”, McGRAW-HILL Book Company, England, 1993.

[4] Katsuhiko Ogata, “Modern Control Engineering”, Prentice Hall, India, 2009.

[5] O.D. Osunde, “Bridgeless Asymmetrical Single Phase AC-DC Boost Converter for Power

Factor Correction”, Proceedings of the 3rd International Conference on Power Systems and

Telecommunications, UNILAG, July 22nd-24th, 2009, pp52-55.

[6] J.M.D. Murphy and F.G. Turnbull, “Power Electronic Control of AC Motors”, Pergamon Press,

New York, 1988.

[7] www.ABB.com(downloaded 25th April, 2010).

[8] T. M. Jahns and E. L. Owen, “AC adjustable-speed drives at the new millennium:

Howdid we get here?,” in Proc. 2000 IEEE Appl. Power Elec. Conf., New Orleans, LA,

Feb. 2000, pp. 18–26.

[9] D. P. J. Link, “Minimizing electric gearing currents in SSD systems,”IEEE Ind. Appl. Mag.,

pp. 55–66, July/Aug. 1999.

[10] S. Bhattacharya, L. Resta, D. M. Divan, and D.W. Novotny, “Experimental comparison

of motor bearing currents with PWM hard- and soft-switched voltage-source

inverters,” IEEE Trans. Power Electron., vol. 14, pp. 552–562, May 1999.

[11] M. Cacciato, A. Consoli, G. Scarcella, and A. Testa, “Reduction ofcommon-mode currents

in PWMinverter motor drives,” IEEE Trans. Ind. Applicat., vol. 35, pp. 469–477,

Mar./Apr. 1999.

[12] G. Oriti, A. Julian, and T. Lipo, “Elimination of common mode voltage in three

phase sinusoidal power converters,” in Proc. IEEE Power Elec. Spec. Conf., Baveno,

Italy, June 1996, pp. 1968–1972.

INDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: AINDUSTRIAL DRIVE: A CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE CASE STUDY OF CHASSIS DRIVE

EMPLOYED EMPLOYED EMPLOYED EMPLOYED IN 6Hi SINGLE STAND REVERSING IN 6Hi SINGLE STAND REVERSING IN 6Hi SINGLE STAND REVERSING IN 6Hi SINGLE STAND REVERSING

COLD ROLLING COLD ROLLING COLD ROLLING COLD ROLLING MILLMILLMILLMILL

BY

AKINYELE, DAMILOLA JOHNSON

(109043084)

BEING A TERM PAPER PRESENTED TO

THE DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING,

FACULTY OF ENGINEERING,

UNIVERSITY OF LAGOS,

LAGOS, NIGERIA.

IN PARTIAL FULFILMENT OF THE REQUIREMENT

FOR THE AWARD OF MASTER OF SCIENCE (M.Sc) DEGREE IN

ELECTRICAL AND ELECTRONICS ENGINEERING (CONTROL OPTION)

MAY, 2011.