Electric drive

ELECTRIC DRIVESELECTION OF MOTOR FOR DIFFERENT TYPES OF LOADS OR SERVICE

BY

Prof. Sagar B. KudkelwarElectrical Engineering DepartmentBallarpur Institute of Technology,

Ballarpur.

Rolling Mills Metal rolling is one of the most important manufacturing processes in the

modern world. Metal rolling is often the first step in creating raw metal forms. The ingot or continuous casting is hot rolled into a bloom or a slab, There are two types of two types mills 1.Blooming Mill 2. Slabing Mill these are the basic structures for the creation of a wide range of

manufactured forms. Blooms typically have a square cross section of greater than 6x6 inches. Slabs are rectangular and are usually greater than 10 inches in width and

more than 1.5 inches in thickness .Rolling is most often, (particularly in the case of the conversion of an

ingot or continuous casting), performed hot.

rolling operation requires production of heavy torque at low r.p.m. to effect heavy reduction of material from ingots to blooms or slabs.

Blooming mill may have 2 rolls or 3 rolls.

two high blooming mill has arrangement of reversing the direction of rotation of the rolls in order to carry out rolling of material through successively reducing passes.

In case of three high mill direction of rotation need not to be changed.

By the use of tilt tables material can be pass between lower and middle rolls or between middle and top rolls.

Blooms and slabs as obtained from blooming mill are further subjected to rolling to produce plates, structural bars or billets.

Billets when further rolled produce rounds, square octagons ,flats angles etc.

Two Rolling Mill Three Rolling Mill

Depending upon type of mill selection of motor has to satisfy following conditions. 1) Motor should have robust construction to withstand severest duty

2) It should be capable of developing short time torque to the extent of 2 or 3 times of rated torque.

3) It should be capable of having wide speed variation.

4) It should be in a position to maintain preselected speed within tolerances from no load to full load. Therefore it should have quick response to change.

Various motors used for Rolling Mills Induction Motors:• we can change the speed of slip induction motor by introducing resistance

in the rotor circuit.

• This method is wasteful as the reduction in speed is proportional to copper losses taking place in rotor circuit.

• As such slip motors are used for small re-rolling mills where wide speed range is not required.

• besides wastage of energy taking place in rotor, introduction of resistance in rotor circuit converts the speed torque characteristic of induction motor from shunt to series.

• Therefore speed can not e maintained constant from no load to full load.

• This will therefore motor to fly off to no load as soon as stock passes out of the stand.

• this much reflects very much rolling on the quality stands of the rolling mill.

•As such slip ring motors are suitable for roughing stands of the rolling mill.

•Wastage of energy in the rotor of slip motor which is proportional to the reduction of speed can be avoided by employing cascaded induction motor

D. C. Motor• D.C. motor is the only type of motor which can satisfy the stringent

requirements of modern rolling mills.• By varying the applied voltage to the armature and keeping field constant it

is possible to change the speed of motor at constant torque up to rated speed.• In this way requirement of higher constant torque at low speed needed for

effecting heavy reduction in first passes is achieved.• on the other hand low torque and higher speed is required for rolling

operation in final passes when for the section of stock having become smaller, torque needed is low and elongated material requires higher speed to clear off in the shortest possible time.

• this achieved by weakening field. after rated speed is achieved by the increase of applied voltage to the armature.

• so in this way we get high speed at reduced torque i.e. H.P. remains same.• In case of reversible rolling mills, in order to ensure high rate of acceleration

,it becomes necessary to reduce the moment of inertia of motor to minimum .• this is achieved by increasing the length and diameter of armature.

• D.C. moors of rolling mills can be supplied from three phase a. c. system by any of the following methods: 1) Grid controlled mercury arc rectifier 2) Silicon controlled rectifier 3) ward Leonard set

• The main advantage of grid controlled rectifier are its overload capacity, steeples control of voltage from 0-100%, high efficiency and quick response to very rapid changes in operating variables.

• the only disadvantage of mercury arc rectifier is that load peaks cause by the operating of the mill are transferred to the feeding system.

• Due to this reason mercury arc rectifier or silicon controlled rectifier which works only as valve is recommended for supplying the rolling mill where a. c. supply net work is sufficient strong.

• the main advantage of silicon rectifier is its small size.

• the main advantage of employing ward Leonard set is that load peaks is not transferred o the supply.

SYNCHRONOUS MOTOR• Synchronous motors are used for driving continuous mill where load is

maintained for relatively long period.

• Flywheel can not be used and consequently the motor carries can the entire load.

• for slow speed, synchronous motors can be built more economically than induction motors and at the same time, get P.F. correction.

• Besides main mill drive , motors are required for auxiliary drive such as for driving the rolls of roller table, adjusting the gap between rolls, driving the manipulators, for chopping the tail ends, cutting the material to required length, winding the ends product into coils etc.

CRANESEssential Requirement of a hoist or crane motor o It should develop high starting torqueo It should withstand great number of switching operations.

D.C. MOTORS D.C. is usually employed on cranes in steel mills where safety and flexibility of

control are of first importance.

Series motor is by far the most commonly type of motor which is used on account of its inherent characteristic such that it adjust its speed to load.

This characteristics prevents excessive power demand on heavy loads.

During Dynamic breaking, series motor works as shunt generator..

A.C. Motors• on cranes using a. c. slip ring induction motor are used where speed control is obtained by rotor resistance.

• Speed range of slip ring motor is less than that of series motor .

• Dynamic breaking is applied by giving unbalanced supply to primary.

• This is done by giving single phase supply to the primary such that two phase windings are in parallel and this in turns is in series with thir winding.

• for outdoor service totally enclosed motors are best.

•Hoist motors have special electro-mechanical breaks which are applied by springs to hold the load as soon as supply to motor goes off.

PAPER MILL• Pulp stuff from storage tank after passing through strainers comes to

first section of paper making machine which consists of moving wire mesh.

• subsequently this sheet of paper goes in between series of heated press rolls, drying rolls, calendars, smoothing rolls and finally to reelers.

Following requirements for paper mill drive must be fulfilled :

1) To manufacture different types of paper we should be able to vary the speed of entire series of rolls.

2) Relative speeds of rolls should be constant otherwise tearing of paper may result.

3) We should be able to adjust the speed of any group of rolls relative to others in order to allow for the draw of the paper.

All above requirements are fulfilled by the use of

Schrage motor

Paper Mill Process

Marine Drive• Electric Drive propeller of ship is universal Propeller: A propeller is type of fan  that transmits power by conversion 

rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade.

It has following Advantages.1) Steam turbine or diesel engine has most economical speed of about 3000

r.p.m. and 400 to 800 r.p.m. respectively. Propeller speed on the other had is in between 100 to 200. r.p.m. We can run propeller and prime mover independently. This enables prime mover to run at its most economical speed

2) At the time of slow speeds we can run only few prime mover sets to supply all the propellers. Low speed cruising is now no longer uneconomical.

3) position of prime mover Speed control and reversing in case of electric propulsion are easy and can be carried out from any convenient position instead of signaling to engine room

• Position is not fixed by the propeller shaft as such there is greater flexibility in the layout.

• In Case of very big ships, electric drive will be by three phase induction or synchronous motors

• prime mover will be steam turbine, driving three phase alternator.

• Speed control is done by variation of applied voltage and partly by frequency variation.

• Reversal is obtained by changing the phase sequence of the supply.

• Speed of induction motor can also be change by pole changing method which give economical speed.

•Disadvantage of induction motor is that it’s P.F. at low speed is low between 0.7 to 0.75.

• Power at the ship is generated by turbo alternator at 2.2 to 6.6 kv

• Small ships can be driven by D.C. motors, which are controlled by means of ward Leonard method.

• Power on the ships is generated by diesel electric set, relatively at low voltage of 650 V

TEXTILE MILLS• Loom motors operate in atmosphere which is laden wool-lint and moisture.

• Further the motor should high starting torque of about 2to3 times full load torque.

• Loom motors have to start and stop many times in a day.

• this increases the motor heating during starting.

• Motors of low temperature rise under full load conditions are therefore selected do that under working conditions of frequent starting and stopping permissible temperature rise is not exceeded

• Keeping above requirements in view totally enclosed, fan cooled, high torque squirrel cage motors are selected

• Where only wool lint is present and no moist atmosphere ,screen protected motors are used.

• Splash proof motors are used in wet locations such as dye house.

• Brush shaft adjustable speed motor is used on printing machine

MINESMotor used inside mines should have flame proof enclosure.

For mine winders, it is essential to have both speed control as well as breaking capacity

Ward Leonard system is most effective drive.

Slip Induction motor with D.C. dynamic breaking is also used.

We may also used D.C. motor supplied from controlled rectifier.

PUNCHES PRESSES & SHEARS

• Since heavy sudden loads are applied , D.C. compound motor or slip ring motor with flywheel is used.

LATHE, GRINDING & MILLING MACHINE

• These machine tools require low starting torque and constant operating speed.

• Therefore D.C. shunt motors or squirrel cage motors are needed.

• In case of drills, requiring different operating speeds for different metals.

• motors having two sets of stator pole changing winding are used which give economical speed.

Lathe Machine

Grinding Machine

• Drip proof or totally enclosed surface cooled motors are most frequently used, often mounted on a common bed plate and direct coupled to the pump.

• Sometimes motor is flange mounted.

• Where pump speed does not come within the range of the fixed motor speed ,V belt drive is employed.

• starting torque necessary for centrifugal pumps is about 40 to 50% of full load torque.

PUMPS

• For reciprocating pumps starting torque may be 100 to 200 % of full load torque

• It is advisable to use slip ring motors for reciprocating pumps and squirrel cage motors for centrifugal pumps

Centrifugal Pump

REFRIGERATION & AIR CONDITIONING• In vapor compression system of refrigeration motor is used to drive the

compressor.

• This motor is controlled by thermostat .

• When motor is restarted , it has to drive compressor against high speed pressure.

• Motor should therefore be required developing starting torque of 200 to 250% full load torque.

• For small unit air conditioners, capacitor type single phase 230 V motor with D.O.L. starter is most commonly selected.

• High torque squirrel cage induction motor or slip ring induction motors are used for large installations

• For very large plant size, synchronous motor, driving turbo compressor may be found suitable specially when P.F. correction is also required.

Compressor

LIFTS• High smooth accelerating torque of 200 to 250% full load

torque at starting, high over load capacity and pull out torque an maximum degree of silence are essential requirements.

• Motor are normally one hour rated for duty cycle of 150 to 180 starts per hour.

• D.C. compound motors or slip ring motors suit the above requirements

BELT CONVEYERS

• Belt conveyors are used for moving sand, coal and others raw materials.

• Heavy loads have to accelerated.• High torque squirrel cage or preferably slip ring

motors should be made as per requirement.• Due to outside working conditions and presence

of grit and dust in atmosphere ,totally enclosed surface cooled motors are used.

DOMESTIC USE

• Universal series motor is used for house hold appliances such as Mixer, Washing Machine, Sewing Machine, Vacuum Cleaner

Mixer

Washing Machine

Washing Machine Motor

Sewing Machine

Vacuum Cleaner