Applications of Electric Drives | Electrical Engineering

In this article we will discuss about the applications of electric drives.

1. Machine Tool Drives:

Many machine tools require a simple more or less constant speed drive. For this squirrel- cage induction motors together with simple manual controls are adequate. For transmission of power to cutting tools or work pieces at different finite number of speeds, we use gear boxes and stepped pulleys. The use of gear mechanism gives rise to vibrations and noise, and thus affects the machining accuracy. Stepless speed control, if used, will provide better machine timings and surface finishes.

Electro-hydraulic and electromagnetic controls are used for providing desired smooth speed control. Though thyristor controlled drives have replaced these methods up to some extent. At present, in our country, the use of thyristorized drives is limited to special machine tools only, as the cost of such drives is sufficiently high.

In some cases pole changing is preferred over gear changing. If more than two speeds are required, two pole changing windings may be used in the same stator slots, e.g., 4/8 and 6/12 poles providing speeds in the ratio of 1 : 1.5 : 2 : 3. Such motor is convenient for small vertical drilling machines.

In some cases, speeds in excess of 3,000 rpm are required as for example in woodworking machinery. A direct drive is preferred over a geared drive, but with a 50 Hz supply this can be achieved only by commutator machines. If there are several such drives, it is worthwhile to install an induction type frequency-changer to provide high frequencies (100 Hz and/or 150 Hz). This permits the relatively cheap and robust squirrel cage motors to be employed for speeds approaching 6,000 and 9,000 rpm respectively.

Other drives need a variable speed. For a wider speed range the Ward-Leonard system is supreme but now it has been replaced by a cheaper alternative such as a rectifier-dc motor combination. For a smaller speed range, field-controlled dc shunt motor is adequate, and if dc supply is not available a free-firing rectifier may be employed. AC commutator motors with induction regulators or brush-shifting control of speed may also be employed for variable-speed drives.

For use such as internal grinding spindles for horological applications, drilling of printed circuit boards etc., high frequency, three-phase induction motors operating up to speeds of 180,000 rpm and providing fairly high output powers in small sizes are used. Typical ratings are 1.2 kW at 120,000 rpm for high-speed grinder drives and 0.5 kW at 150,000 rpm for high precision horological applications.

Some applications need braking torques. Many motors associated with machine-tool drives are of sufficiently small rating to allow capacitors to be employed for braking purposes without the size and cost being prohibitive. Where considerable inertia is involved, as in planning-machine drives, the regenerative braking of the Ward-Leonard set is a further advantage in addition to its others.

2. Cranes and Hoist Motor:

Essential requirement of a crane or hoist motor is that it should develop high starting torque and should withstand large number of switching operations. DC series or compound wound motors are preferred for cranes on account of their high starting torque and smooth speed control. The motors used for lifting, travelling and reversing, and for conveying and hoisting are dc compound wound, dc series wound and ac slip-ring induction motors (with rotor resistance control) respectively.

Hoist motors are provided with special electromechanical brakes, which by means of springs, are made to hold the load as soon as supply fails. On restoration of supply, solenoid connected across the terminals of motor is energized and brakes are released. Such motors are half-, or one-hour rated, developing starting torque twice of full-load torque, and robust to withstand severe strains to which these are exposed.

Drum-type controllers are used, operated by contactors, which are controlled by press buttons.

3. Lifts:

High smooth accelerating torque (twice full-load torque at start), high overload capacity and pull-out torque, high degree of silence and moderate speed are essential requirements. Motors are normally one-hour rated for duty cycle of 150 to 200 starts per hour. DC compound wound and ac slip-ring induction motors are mostly used. Induction repulsion and variable speed ac commutator motors are also used. In single phase installation a shunt type commutator motor is also used. In case of dc motors speed control is affected by shunt field variation or by variable voltage control (Ward Leonard). In case of polyphase slip-ring induction motor the speed is controlled by rotor resistance and involves copper loss (I²R loss).

4. Lathes, Milling and Grinding Machines:

Lathes are usually driven by constant speed squirrel cage induction motors. Multi-speed ac motors or adjustable speed dc motors are sometimes used.

Milling machines are usually driven by constant speed squirrel cage induction motors. Larger machines such as planer type milling machines have separate motors for each milling head and separate motors for each of feed motions. The general practice is to use adjustable speed motors for headstock drive and either constant or adjustable-speed motor for the wheel drive. Separate motors are often used to feed the wheel with respect to the work.

Grinding machinery varies considerably in the design of individual machines. The general practice is to employ adjustable speed dc motors for the headstock drive, constant speed squirrel cage or adjustable speed dc motors for wheel drive and constant speed squirrel cage motors for traverse drive.

Constant speed squirrel cage induction motors with the grinding wheels mounted directly on the motor shaft extension are used for bench, pedestal and centreless grinders.

5. Planers:

Planer consists of a bed having attached to it a platen which moves forwards and backwards on the bed. The job is clamped to the platen and is planned by a stationary clamped tool; at the end of the stroke the tool moves slightly in order to make a new cut. Cutting stroke is slow and return stroke is quick, therefore adjustable speed, reversing motors are required. In case of dc supply shunt or compound wound motor and in case of ac supply slip-ring induction motor is used.

6. Punches, Presses and Shears:

Special features are high peak loads and heavy starting torque. These are usually equipped with flywheels to supply momentarily demand of power. Motors used for these machines should have drooping speed-torque characteristics to allow the motor to slow down when heavy loads are encountered. High-slip squirrel cage induction motors, slip-ring induction motors or dc cumulative compound motors are used for such applications.

7. Frequency Converters:

Squirrel cage induction motors and synchronous motors are used for driving converters. Nowadays owing to the recent advances in the field of silicon controlled rectifiers, static frequency changers are being increasingly used.

8. Air Compressors:

Slip-ring induction motors and synchronous motors are usually employed in large size units. The latter type can be used for power factor correction in addition to driving air compressors if so designed. Squirrel cage induction motors are used for small compressors only.

9. Electric Traction:

DC series motors, which are simple and robust in construction, provide high starting torque and smooth speed control, are suitable for all types of services but more particularly for suburban services where high rate of acceleration is essential.

Single phase ac compensated series motors, which have speed-current and speed-torque characteristics similar to those of dc series motors, are extensively used for the main line work. Due to poor starting torque these motors are not suitable for suburban services where stops are frequent.

10. Pumps:

Drip proof or totally enclosed surface cooled polyphase induction motors are usually employed, often mounted on a common bed plate and direct coupled to the pump. Sometimes motor is flange mounted. In case the speed does not come within the range of the fixed motor speed, V- belt drive is employed. Squirrel cage motors with reduced voltage starters are used for centrifugal pumps, where starting torque required is about 40 to 50 per cent of full-load torque. Slip-ring induction motors are used for reciprocating pumps in which starting torque required may be 100 to 200% of full- load torque.

11. Refrigeration and Air Conditioning:

In vapour compression system of refrigeration, motor employed to drive the compressor is thermostat controlled. On re-starting the motor has to drive compressor against high head pressure. Motor used, therefore, should be capable of developing starting torque 2 to 2.5 times full-load torque.

Single phase, 230 V, capacitor-type induction motors with D-O-L starters are usually employed for small units. For large installations high torque squirrel cage induction motors or slip-ring induction motors are employed. For very large plants, synchronous motor, driving turbo- compressor, may suit especially if power factor correction is also required.

12. Belt Conveyors:

Belt conveyors are used for moving sand and gravel. Heavy loads are required to be accelerated. Normal starting current and high starting torque double cage induction motor with direct-on-line starter or wound rotor induction motors are used. These motors must be totally enclosed, surface cooled type as grit and dust remain present in the atmosphere.

13. Woodworking Machinery:

Screen-protected motors, except where there are chances of motor being buried in saw dust are usually used. In places where there are chances of motor being buried in sawdust totally enclosed surface cooled motor is used. Requirement of speed above 3,000 rpm, if required, can be met with induction motors in conjunction with induction type frequency converter or by using commutator motors.

14. Printing Machinery:

In printing machinery for constant speed work such as guillotines, driving platens or other small machines squirrel cage induction motors are used. Where line capacity and moderate starting torque permits single phase capacitor motors may be used. In case of rolling presses where a slow forward stroke and a quick return stroke is required, two squirrel cage motors-one for each stroke, are used.

For other purposes; where variable speed motors are required such as for rotary presses, dc compound or three- phase slip-ring induction motors with rotor resistance control or ac commutator motors may be used. For obtaining the steady crawling speed for inching the press, a pony or barring motor is used.

15. Petrochemical Industries:

In petroleum and chemical industries, fluid handling equipment is extensively used. Pumps have been driven by induction motors with flow control, when desired, accomplished by throttling valves. As per estimates 30% or more of electrical energy used by a typical refinery is wasted due to throttling effect of control valves. However, adjustable-speed pumping utilizing multi-speed induction motors or inverter-fed induction motors are quite economical because of reduced energy consumption and maintenance costs.

Other important tasks within the production processes of the petrochemical industry are gas liquefaction, compression, refrigeration and heat recovery. Compressor drive systems constitute important components in plants for such processes. The majority of the drives operate at a constant speed, employing a 4- or 6-pole motor with step-up gear.

For large ratings, synchronous motors with cylindrical rotors having both excitation and starting windings (damper bars) are preferred. Such motors are specially designed considering the oscillating torques developed during asynchronous acceleration owing to the magnetic and electrical anisotropy in the rotor and special cooling conditions.

16. Sugar Mills:

In sugar mills, a centrifuge is employed for separating out crystalized sugar from the syrup obtained from steam evaporator and for drying it out by the action of centrifugal force. The duty cycle of a centrifuge motor is illus­trated in Fig. 4.1. The functions involved are charging, inter­mediate spinning, spinning, regenerative and reverse current braking and plugging for ploughing. All these functions are to be performed at different speeds, variation of which may be as much as 1 : 30.

The motors used for this pur­pose are usually four-speed, pole-changing motors having two sets of stator windings that enable us to obtain syn­chronous speeds 1,500/750/214/107 rpm or 1,000/500/214/ 107 rpm. They not only are capable of providing the de­sired fixed speeds of operation, but also of returning a portion of energy back to the supply mains during regen­erative braking accomplished by switching over to a higher pole operation from a lower pole one.

In order to satisfy the duly cycle illustrated in Fig. 4.1, first 28 pole winding is switched on to provide a speed of approximately 200 rpm. During charging supply is cut off and then intermediate spin speed of about 450 rpm is obtained by energizing the 12 pole winding. The final spin speed of about 950 rpm is achieved by switching over to the 6 pole winding. After centrifuging operation is complete, supersynchronous regenerative braking is applied by connecting the motor for 12, 28 and 56 poles successively which brings the speed down to 500, 214 and 107 rpm respectively.

Ploughing speed of about 50 rpm is obtained by the application of reverse current braking. Automatic control of the entire duty cycle is achieved by feeding the control equipment from the output of a tachogenerator directly coupled to centrifuge motor.

Motors employed for driving centrifuges have their own special features in construction. They are vertically mounted so as to be coupled with the centrifuge shaft. The motors may be made with larger air gap so as to take care of the possible rotor oscillations about the vertical axis. Insulation used in motors must be humid proof so that it can operate successfully in a humid environment. For protection of motor against overheating, thermal elements are embedded in the windings.

These elements, which are also called sensotherms by trade name, operate few degrees below the maximum permissible temperature of the winding. Operation of thermal elements will either directly trip the motor or give visible and audible warning signals so that the particular duty cycle may be completed. Following this, next cycle cannot be commenced until temperature of the motor has come down to normal.

17. Cement Mills:

The driving motors used in cement works can be broadly classified as:

(i) Raw mill and cement mill drives

(ii) Kiln drives

(iii) Crusher drives

(iv) Waste gas fan drives and

(v) Compressor drives etc.

The starting torque for the mill motors for large cement plants is limited to 125% of the full-load torque and the pull- out torque is restricted to nearly 240% of the full-load torque. Slip-ring induction motors of 6.6 kV with liquid resistance starters are usually employed. Gearboxes are employed in order to get the desired mill speed of about 15 rpm and high voltage capacitors of adequate reliability and automatic capacitor control switchgear and circuit breakers are used for power factor correction.

Owing to large ratings (above 3,000 kW) required for the raw and cement mill drives and due to the limitations in the availability of large size gearboxes and motors, twin drives are used-the two motors employed for twin drives have to be more or less identical to each other and so also their liquid resistance starters.

The rotary kiln is an indispensable part of the cement plant. The rating of the motors employed for driving the kilns vary from 100-1,000 kW. The maximum speed of the kiln is about 1 rpm and speed range required is of the order of 1 : 10. The starting torque required may be between 200% and 250% of rated torque. In early years, variable speed ac commutator motors to be employed for kiln drives, which has been superseded by Ward-Leonard drives as ac commutator motors proved expensive and had maintenance problems. Nowadays use of dc motors with static supply is made. To cope up with increasing kiln capacities, the modern trend is to employ twin motor dc drives for kiln application.

Usually, the starting torque for crusher drives is limited to 160% of the full-load torque and pull-out torque is limited to 200-250% of the full-load torque. The slip-ring induction motors are usually used in crushers. Normally the motors are designed to withstand locked rotor current during running, without any external resistance inserted in the rotor circuit, for one minute-very important feature owing to frequent chances of getting crushers jammed.

The starting torque required for fan drives is about 120% of the full-load torque and the pull-out torque is from 200% to 250% of the full-load torque. The slip-ring induction motors with a speed variation are used for such drives. For starting and controlling the speed, the cast iron grid resistance controllers are normally used. The slip-rings and brush gears are totally enclosed and kept external to the motor enclosures for convenience in maintenance.

The air compressor motors are usually of smaller ratings, say 300-450 kW. Squirrel cage or wound rotor induction motors can be used as per power needs. The enclosures are usually of TEFC type and the speeds vary between 1,000 and 750 rpm.

18. Mining Work:

The electric motors employed in coal mines can be classified into two categories namely the auxiliary motors, used for driving auxiliaries such as compressors, fans, conveyors, pumps, hoists etc., and the mine face motors, used on continuous miners, drills, shuttle cars, cutting machines, loaders etc. The motors of the two categories differ considerably from each other. The auxiliary motors are normally modifications of general purpose industrial motors, while the face motors are specially designed motors as per specific needs. The duty of the auxiliary motors is, often, well defined and steady, while that of face motors consists of random loading and has a number of high-shock loads.

The motors employed for coal drills are usually short- time rated induction motors of rating 1 kW, 125 V, 2860 rpm, 50 Hz with a flame proof enclosure. Sometimes, high frequency motors (150 or 200 Hz) with the help of a frequency changer housed in the drill panel are also used.

Slip-ring induction motors with water rheostats are, usually employed for driving the colliery winder used for raising the coal, raising or lowering the persons and other loads. DC rheostatic braking is used initially and mechanical braking is employed only towards the fag end of the winding period. For higher capacities dc motors supplied from a Ward-Leonard generator are used for driving the mine winders as precise control of drum speed and position is necessary for accurate decking. The recent trends are to use thyristor conveners for supplying variable dc voltage supply.

The motors employed for haulage (mine-winches) work are subjected to frequent starting and stopping and also to braking and reversing. For this work the loads, often, require different speeds at different states of haulage. Hence slip-ring induction motors with drum resistance controllers are used. The drum controller and the rotor resistances are separately mounted in flame proof enclosures.

There are two types of ventilation fans used in mines namely axial flow type and radial flow type. Although squirrel cage induction motors having large number of poles can be used directly but high speed motors having 4 to 6 poles with a reduction gear are usually employed.

There are two types of colliery pumps-triple ram type driven by a high speed motor with a gear and the centrifugal type directly coupled to the driving motor. The ram type pump requires a starting torque of about 200% of rated torque, and, therefore wound rotor motors are used. For driving centrifugal pump, which requires only about 40% of rated torque squirrel cage induction motors are used.

Working surroundings is the chief factor which distinguishes electrical motors used in mining from those used in general industry. The motors employed in mining work must be flame proof, and all commutators and slip-rings must be in flame proof chambers with an inch metal to metal flange.

19. Textile Mills:

In early years the textile mills were usually equipped with diesel generators, steam turbines. The aspect of boiler, engine etc. made it essential to consider the electric drive. Then the ac power supply was preferred. This was on account of power cost to drive the mill. Thereafter, the group drive of machines were designed, but with the recent developments, the need for individual drive was recognised.

The standard motors in open type or in totally enclosed type cannot be used in a textile mills but specially designed motors are required on account of environmental (presence of fluff and dust fly, high humidity, restricted ventilation etc.), operating (wide voltage fluctuations, frequent starting and stopping, intermittent operation resulting in rapid change in torque and power requirement) and drive (restricted space, special mountings and special shaft extension requirement) conditions.

The special features of textile motors are:

i. Electrical:

Extra high or moderate torque with smooth acceleration characteristics.

ii. Thermal:

Normally high thermal withstand capacity for frequent starting, prolonged starting time or restricted ventilation or inching.

iii. Environmental:

Ability to withstand high humidity, temperature, dust and fluff.

iv. Mounting:

Foot, Cradle and Flat Base.

v. Construction:

Totally enclosed with axial or circumferential ribs with steam-lined shapes for smooth floor for fluff.

vi. Cooling:

Surface cooled or fan cooled.

Miscellaneous:

Terminal box or loose leads and special shapes for restricted overall dimensions, special shaft extensions.

Loom Motors:

In order to achieve a short pick-up time, the starting torque of the loom motor should be high (2-2.5 times of the rated torque). The loom machine operates on a reciprocating mechanism, which causes both torque and current pulsations. Also the loom motor is subjected to frequent starts and stops. This results in a high temperature rise and is taken care by having a liberal thermal dissipation capacity of the motor.

These motors are totally enclosed surface cooled. A high degree of fluff in the atmosphere calls for a smooth surface finish of the housing and shields so that the fluff do not get accumulated on the motor surface. The insulation of the motor must be able to withstand high moisture content. These motors have normally a higher slip or lower speed than that of a standard motor in order to give flywheel effect to smoothen the current pulsations.

The ratings of the motors employed for driving looms for light fabrics such as silk, rayon, cotton, nylon etc. are 0.37, 0.55, 0.75, 1.1 and 1.5 kW; while those of the motors used for making heavy fabrics (wool and canvas) are 2.2 and 3.7 kW. The motors are usually of 6 or 8 poles.

Carding Motors:

The inertia of the carding drum is high, as such the starting of such motors have to be designed to withstand prolonged starting time. Once the drum is started, the operation is continuous and uninterrupting. The commonly used motors are totally enclosed and totally enclosed fan cooled, 3-φ, high torque squirrel cage induction motors of rating 1.1 and 1.5 kW for light fabrics and 2.2, 3, 3.7 and 5.5 kW for heavy fabrics. Here again, the motors are usually of 6 or 8 poles.

Spinning Motors:

In spinning, the moderate starting torque and the smooth acceleration is essential. This calls for a low dip in the speed-torque curve of the motor. Totally enclosed fan cooled motors of ratings 5-30 kW are used for spinning frame operation. In general, three types of drives are employed for spinning frame operation namely single-speed motor, two-speed motor and two-motor drives. Usually a 4 pole or 6 pole squirrel cage induction motor is employed for single-speed drive. Two-speed motors (4/6 or 6/8 poles) are used in order to give maximum production with minimum breakage.

The increased production may compensate for the additional outlay owing to larger size and higher cost. In case of two-motor drive, two separate motors are employed to drive the common pulley of the ring frame.

Although this drive is costlier and requires more space, it has several advantages such as:

(i) Availability of any desired speed by adjustment of speed ratios,

(ii) Facility of adjustment of yarn tension independently,

(iii) Continued production even in case of failure of one of the motors.

The most common electrical method of controlled- torque starting involve the use of standard squirrel cage induction motors and different methods of applying reduced voltage to the motors during a selected starting period. One of the most effective methods of controlling the stator voltage of motors employed in textile mills is that of using series reactors. This method provides stepless, closed transition increase to approximately full speed. Variable iron-core inductors offer almost infinite choices of starting torques. Reactors with fixed tappings may be used to provide variety of starting torques. With the ever increasing use of solid state devices, nowadays ac regulators are being used to provide control of starting torque.

20. Woollen Mills:

Loom motors are totally enclosed type, capable of developing 3 times full-load torque at start if direct driven or alternatively started light and controlled through clutch. Motors including switchgear in dyeing section should be of totally enclosed type, and should be treated with acid proof paint as atmosphere is laden with fumes.

21. Paper Mills:

There are two main processes, that take place in a paper mill namely pulp making and paper making. The drives used are quite different for each one of these processes.

There are two methods of pulp making-purely by mechanical means and by both mechanical and chemical processing. The former method involves grinding logs of wood of about a metre length on large grind-stones. Grinders operate at almost constant speeds of 200-300 rpm and can be started under light load conditions. Hence constant speed synchronous motors and geared drives are most suitable. Usually, pulp making by purely mechanical means consumes more than half of the total power requirement of a paper mill, therefore, large size grinders driven by 3,000-4,000 kW motors are, usually, considered as economical.

Pulp can also be made by cutting wood logs into chips of several cms length and treating them with alkalies alongwith other raw materials such as grass, rags etc. During the chemical treatment the material is continuously beaten. Wood choppers have random load characteristics and their inertia is large, depending upon the size of the disc, on which the knives of the chopper are mounted. Beaters, usually are required to start with large load.

Depending upon the size of the mill, the ratings of motor used for chipping, beating, refining and storing range from several hundreds to thousand kilowatts. Except for beaters, synchronous motors are employed in these drives. Since beaters, very often, require speeds less than 200 rpm and large starting torque, wound rotor induction motors are, therefore, most suitable for such drives.

The paper making machine has to perform the job of forming sheets, removing water from sheets, drying of sheets, pressing of sheets and reeling up of sheets.

The basic requirements of paper making machine drive are:

(i) For the actual formation and production of sheets and from the point of view of economy, constancy of the speed of the drive is necessary.

(ii) For the paper machine to be multipurpose, its speed should be adjustable over a range as large as 10 : 1.

(iii) In the calendar section and reel section, variations in tension in sheet can occur even if the correct relative speeds are maintained due to uneven drying and other factors. It is, therefore, imperative to augment the speed control circuit by an overriding tension control.

(iv) While cleaning the wire, it has to be moved forward a few cms at a time for its proper cleaning and inspection. Hence the motor employed should be capable of running at inching speeds of 10-25 m/ minute as long as a particular button in pressed.

(v) Each section should be able to run at the crawling speed of 10-25 m/minute for running in felts, wire and heating up of dryer cylinders.

(vi) Smooth and quick starting of the sections, without causing excessive starting current, are required.

(vii) Control system employed should be flexible in nature.

There are two types of drives used for making paper from pulp, namely line shaft drive and sectional drive. In line shaft drive, the various sections of the paper machine are driven from a line shaft running the full length of paper machine. Cone pulleys and belt-combinations drive the various sections of the machine from line shaft through right angled gear reductions. Electric motors are usually employed for driving the transmission shaft. Both ac and dc drives can be employed for practically loss-less speed control.

In ac drive, only the ac commutator motor with shunt characteristic is suitable to provide an economic speed control system. However, the speed of an ac commutator motor depends on load and, therefore, its use as a paper making machine drive with stringent requirements of constant speed is not advisable.

Also, its speed range (usually of the order of 1 : 3) as well as the power required greatly affects the size of the motor. The open-loop speed control of the ac commutator motor is sluggish in comparison with a dc drive, as speed is varied by adjusting an induction regulator and shifting the brush rocker. In dc drives, the speed of the paper making machine is controlled by varying the armature voltage of a separately excited dc motor. The variable dc voltage is obtained from ac by means of either rotary converters or static converters.

In sectional drive each section of the paper making machine has its own electrical motor. The speed of the paper making machine can be controlled by varying the supply voltage. By adjusting the field excitation of any motor, it is possible to vary the speed of that particular motor with respect to other motors.

The requirements of a line shaft drive are not as stringent as those of the sectional drive. It has a number of major limitations as compared to the sectional drive system. The lower cost of the electrical equipment of a line shaft drive system is offset significantly by the additional cost of the mechanical equipment required and its maintenance.

22. Ship-Propulsion:

Except for small vessels, electric drive for ship-propulsion is universal. In electric ship-propulsion the prime mover (steam turbine or diesel engine) drives an ac or dc generator, which feeds power to ac or dc motor mounted on the propeller shafts. For large ships requiring power above 2,500 kW steam turbines with ac equipment and for smaller vessels diesel engines with dc equipment are generally employed.

In case of ac system 3-phase induction or synchronous motors are used. Speed is controlled partly by variation of applied voltage and partly by frequency variation. Speed of induction motors can also be changed by pole-changing method which gives two economical speeds. Reversal is achieved by changing the phase sequence of the supply. Power on the ship is generated by turbo-alternators at 2,200 or 6,600 volts.

In case of dc system, the speed control and reversing are carried out by varying field excitation of generator field in Ward-Leonard system. Power on the ship is generated by diesel- electric sets comparatively at low voltage of 650 volts.

Instead of constant voltage system, constant current system can be used to feed propeller motor armatures while their fields are supplied from constant voltage supply. Such a supply system is inherently proof against overloads, provides maximum safe torque without any risk of interruption of current. Motor is controlled through field control. This is quite suitable for small vessels requiring maximum maneuverability in restricted waters, such as tugs and ferries.

In case of very small vessels, propeller shafts are directly coupled to diesel engines through slip couplings. Speed of propeller shaft can be controlled by varying dc excitation, and hence the slip, of the coupling.

23. Rolling Mills:

Rolling of steel, the process during which the cross section of the metal gets reduced, while increasing its length proportionately, is the major function of steel mills. Rolling mills are usually classified according to the material produced by them.

The principal types of mills, classified according to their products rolled are:

Blooming mills (the mills used for producing blooms-any length of rolled metal having almost a square cross section), slabbing mills (the mills used to produce slab—any length of metal having a rectangular cross section), billet mills (the mills used for producing only a limited range of sizes which are further reduced in finishing mills), plate mills (the mills used to produce plates from slabs previously rolled by a blooming or slabbing mill), structural mills (the mills used for rolling beams, heavy angles, channels etc. from blooms or billets), rail mills (the mills used for rolling rails only from blooms or billets), merchant mills (the mills used for rolling small angles, channels, rounds, squares, etc. from billets), rod mills (the mills used for rolling small rounds, which are later drawn into wire), hot-strip mills (the mills used for rolling strips from slabs), cold-strip mills (the mills used for cold-rolling strips previously rolled on hot-strip mills to obtain sheets of high quality or to obtain gauges thinner than those produced on a hot mill), sheet mills (the mills used for rolling sheet bars or packs prepared from strips rolled on hot strip mills), skinpass or temper mills (the mills used to produce strips, previously rolled on a cold mill, of the desired temper, flatness, surface, and luster by using rolling pressure or a combination of pressure and tension and tube mills (the mills used for producing tubes, e.g., pipe and conduit, by either the seamless or the butt or lap- welded processes).

Direct drive is preferred whenever the speed of the mill permits. In general, it is not desirable to use direct drive (direct- connected motor) for speed less than 150 rpm and outputs less than 750 kW. In cases, the speed of the mills is not suitable for direct drive, gear drive is employed. It is common practice to use machine-cut, double helical gears for such work, although sometimes single helical gears are used, and provision is made to carry the thrust that is produced by such gears.

Spur gearing is used only for the lower speed drives where the pitch line speeds are 450 metres per minute or less. The use of gears introduces losses and offsets the increased efficiency of high­-speed motors, but in many cases the total cost of installation is less, and there are other advantages, e.g., higher power factor, with induction motors and sometimes a reduction of space required.

The flywheel where necessary, should be mounted between the gear and the mill, if the speed of the mill permits, so that gear does not have to transmit the peak loads which are carried by the flywheel. Where the mill shaft runs at a speed too low to permit economic flywheel design, the flywheel should be mounted on the pinion shaft of the gear unit in which case the peripheral speed of the wheel can be relatively high, and the weight correspondingly less, but the gears must be large enough to transmit the peak loads to the mill.

The dc motors, because of their inherent characteristics, are best suited for the rolling mills. Motors for reversing mills must have high starting torque, wide speed range, precise speed control, be capable to withstand overload and pull-out torque which may be as three times the rated value and have good commutation throughout. Acceleration from zero to base speed and then to top speed and subsequent reversal from top speed forward to top speed backward must be achieved in a couple of seconds.

The moment of inertia of the armature must be as small as possible. Requirement of higher constant torque at low speed needed for affecting heavy reduction in first few passes can be achieved by varying the applied voltage to the armature, keeping field constant. On the other hand low torque and higher speed required for rolling operation in final passes can be achieved by weakening the field. The motors employed for steel mill duty are, therefore, of totally enclosed, forced ventilated, higher class of insulation, lesser diameter and larger length.

In general, the use of dc motors increases the initial cost and introduces losses owing to the necessity of providing converting equipment, but the advantages obtainable by use of such motors greatly outweigh these additional losses and costs. The supply to dc motors for rolling mill drive can be made by means of grid controlled mercury-arc rectifier, silicon controlled rectifier or Ward-Leonard system.

The main advantages of grid-controlled mercury-arc rectifier are higher efficiency, higher overload capacity, stepless voltage control from 0-100% and quick response to very rapid changes in operating variables but suffers from the drawback of causing higher current peaks in the power system accompanied by wide voltage fluctuations.

The silicon controlled rectifier has the advantage of small size but suffers from the drawback that it does not have overload capacity. Hence grid controlled mercury-arc rectifiers or silicon controlled rectifiers are used for supplying the rolling mills where ac supply network is sufficiently strong. The main advantages of employing Ward-Leonard set are possibility of having speed variation within ± 100% of rated speed, possibility of electric braking by working the mill motor as a generator and of reducing the fluctuations in the power demand from the supply system. Ward-Leonard system, however, suffers from having low efficiency, longer response time and requires more maintenance.

Synchronous motors are used for driving constant speed mills and are advantageous where power factor correction is desirable or where slow-speed motors are necessary. These are particularly adaptable to continuous mills where the load is maintained for relatively long periods or for drives where the length of the material is such that the load exists for such long periods that a flywheel cannot be economically used and consequently the motor must carry the entire load. For slow speeds, synchronous motors can be built more economically than induction motors.

Slip-ring type induction motors are suitable for roughing and re-rolling mills where very precise speed control is not required. Their efficiency is low because of wastage of energy taking place in the rotor resistance. There is also abrupt rise in the motor speed when the material leaves the rolling stands. The disadvantage of the slip-ring induction motor with variable rotor resistance can be overcome by employing cascaded induction motor.

The low frequency rotor output is rectified by silicon rectifier and the rectified power is made to operate against an adjustable back emf provided by a separately excited dc motor mounted on the same shaft, which converts the rectified power into mechanical power. The slip-ring induction motor now has a shunt speed-torque characteristic and speed control is affected without power loss in the secondary resistances. The starting torque is about 4 times of the rated torque when the motor is started with full dc excitation on. The cascading arrangement becomes cheaper as compared to dc motors where speed variation desired is not more than 25%.