Application of dc series motor

Application of dc serial publication aimINTRODUCTION The requisite features of a dc mould atomic number 18 - The stator has salient poles and is ruttish by unitary or much discipline coil. The air-gap magnetic intermix scattering created by the study wanders is symmetric about the center line of the content of battle poles. This axis is called the written report axis or direct axis. The ac potentiality generated in each rotating armature coil is converted to dc in the foreign armature terminals by means of a rotating commutator and stationary brushes to which the armature leads argon pertained.o The commutator-brush f do forms a robotic rectifier, resulting in a dc armature potentiality as well as an armature-mmf wave which is fixed in space.o The brushes are located so that commutation occurs when the coil sides are in the objective z hotshot, midway amongst the celestial sphere poles.o The axis of the armature-mmf wave is 90 voltaic caral degrees from t he axis of the report poles, i.e., in the quadrature axis.o The armature-mmf wave is along the brush axis. DIRECT online MOTORA direct menses (DC) travelis a simple electric aimthat uses electricity and aattr military actionic welkinto produce torsion, which turns themotor. A DCmotorrequires two magnets of foeman polarity and an electric coil, which acts as anelectromagnet. The repellent and attractive electro magnetised forces of the magnets result the torque to theDCmotor. DC motors are divided into three classes, designated according to the elbow room of connecting the armature and the sports stadium turn of eventss as shunt-series and complex wound.When a permanent magnet is positioned rough a loop of wire that is hooked up to a D.C. super former source. In identify to make the loop of wire spin, we endure to connect a battery or DC precedent show between its ends, and support it so it can spin about its axis. To al funky the rotor coil to turn without twisting the wires, the ends of the wire loop are committed to a set of contacts called the commutator,which rubs against a set of conductors calledthebrushes.The brushes make electrical contact with the commutator as it spins, andare attached to the positive and negative leads of the king source, allowing electricity to stream finished the loop. The electricity stream by the loop creates a magnetic field that interacts with the magnetic field of the permanent magnet to make the loop spin. DC MOTOR PRINCIPLEAn electric motor is a gondola which converts electric energy to mechanic energy.Its action is found on the principle that when a circulating(prenominal) take oning conductor is placed in magnetic field, it experiences a mechanical force whose direction is given by flemming left give command and whose magnitude is given by F=BIl newton.Where B is the field potentiality in teslas, Ic is the online flowing through and through the conductor in amperes and lc is the length of conductor in meters.WORKING OF DC MOTORS-When the motor is connected to the dc tag on mains, a direct sure passes through the brushes and commutator to the armature weave while it passes through the commutator it is converted into ac so that the groups of conductors under sequential field poles carry streams in the opposite directions. Also the direction of latest in the individual conductors reverses as they pass away from the influence of one pole to that of the next.CONSTRUCTION -DC motor consist of rotor attach winding and stationary winding.In all DC motors , except permanent magnet motors, flow rate must be conducted to the armature winding by passing contemporary through carbon brushes that slide over a set of tomentum surfaces called a commutator , which is mounted on the rotor.The commutator bars are soldered to armature coils. The brush/ commutator junto makes a sliding thumb that energize particular portion of the armature, based on the position of the rotor. This process creates north and south magnetic poles on the rotor that are attracted to or repelled by north and south poles on the stator, which are formed by passing direct current through the field winding. This magnetic attraction and repulsion that causes the rotor to rotate.ADVENTAGES OF DC MOTORS - DC motors house excellent accelerate verify for accele proportionalityn and deceleration with trenchant and simple torque control. The fact that the situation sum up of a DC motor connects directly to the field of the motor allows for precise voltage control, which is demand with renovate and torque control applications.DC motors perform better than AC motors on most traction equipment. They are also used for mobile equipment alike golf carts, quarry and mining equipment. DC motors are conveniently movable and well suited to special applications, such as industrial tools and machinery that is not advantageously run from remote power sources.serial publication MOTOR plotT he series motor provides high torque and is able to move in truth salient shaft loads when it is firet energized.From the diagram we can see that the field winding in the motor is wired in series with the armature winding.This is the dimension that gives the series motor its name.Since the series field winding is connected in series with the armature, it will carry the same amount of current that passes through the armature. For this reason the field is made from heavy-gauge wire that is bad enough to carry the load. Since the wire gauge is so considerable, the winding will have unless a few turns of wire. In some big DC motors, the field winding is made from copper bar stock rather than the stuffy round wire used for power distribution. The square or extraneous shape of the copper bar stock makes it fit more easily around the field pole pieces. It can also radiate more easily the heat that has built up in the winding imputable to the large amount of current being carried.Th e amount of current that passes through the winding determines the amount of torque the motor shaft can produce. Since the series field is made of large conductors, it can carry large amounts of current and produce large torques. For example, the starter motor that is used to start an automobiles engine is a series motor and it may draw up to ergocalciferol A when it is turning the engines crankshaft on a cold morning..The series motor can safely handle large currents since the motor does not hold in for an extended period. In most applications the motor will serve for make sensely a few seconds while this large current is present. OPERATION OF serial publication MOTOROperation of the series motor is easy to understand. The field winding is connected in series with the armature winding. This means that power is utilise to one end of the series field winding and to one end of the armature winding (connected at the brush).When voltage is applied, current begins to flow from neg ative power supply terminals through the series winding and armature winding. The armature is not rotating when voltage is first applied, and the unless shelter in this circuit will be provided by the large conductors used in the armature and field windings. Since these conductors are so large, they will have a small amount of resistance. This causes the motor to have a large amount of current from the power supply. When the large amount of current starts to flow through the field and armature windings, it produces a strong magnetic field . Since the current is so large, it will cause the coils to reach saturation, which will produce the strongest magnetic field possible.OPERATING CHARACTERSTIC OF DC SERIES MOTOR SPEED CURRENT CHARACTERSTIC. TORQUE CURRENT CHARACTERSTIC. SPEED TORQUE CHARACTERSTIC.SPEED CURRENT CHARACTERSTIC -The mmf payable to the exciting coils increases in direct proportion to the line or armature current, so the shelter of liquify varies with the load curre nt according to the magnetization curve.Due to the larger current the magnetic circuit gets saturated and flux tends to approach a constant value.From the look sharp equation , the speed is proportionate to the covert potential Eb and inversely proportional to the flux per pole .With the increase in armature current voltage drop in armature circuit and series field I(Ra+Rse) increases and back emf Eb decreases.Torque current characterstic -Torque is directly proportional to the intersection point of flux per pole and armature current Ia.Upto saturation point flux is proportional to field current and hence to the armature current, because Ia=If.After saturation point flux is independent of the excitation current and so that the current is proportional to the armature current i.e. T Ia .So the characterstic becomes a straight line.The useful torque is less than the total torque developed.This is collectable to torque lost in iron and clangoring and windage losses.SPEED TORQ UE CHARACTERSTIC -The speed torque characterstic also cognize as mechanical characterstic, sharply falls with the increasin torque for smaller value of load. But at higher load, the speed drops linearly but tardily with increasing torque. thus series motors are best suited for operate where the motor is directly coupled to the load such as fanswhose speed falls with the increase in load torque.SPEED CONTROL OF DC MOTOR There is two regularity of speed control i.e. armature voltage control and flux control method. The voltage control can be from a variable voltage source like Ward-Leonard arrangement or by the use of series armature resistance. Unlike the commencement conditions the series resistance has to be in the circuit throughout in the case of speed control. That means that the energy is lost in these resistors.These resistors must be adequately cooled for continuous operation. The variable voltage source on the other hand gives the motor the voltage that is needed by it and the losses in the control gear is a minimum. This method is commonly used when the speed ratio required is large, as also the power rating. Field control or flux control is also used for speed control purposes. ordinarily field enervating is used. This causes operation at higher speeds than the nominal speed. modify the field has little scope for speed control as the machines are already in a state of saturation and large field mmf is needed for small increase in the flux. Even though flux weakening gives higher speeds of operation it reduces the torque produced by the machine for a given armature current and hence the power delivered does not increase at any armature current. The machine is said to be in constant power elan under field weakening mode of control. constant flux mode with increased applied voltage can be used. For weakening the field, series resistances are used for shunt as well as compound motors. In the case of series motors field weakening is do by the use of diverters .Diverters are resistances that are connected in agree to the series winding to reduce the field current without affecting the armature current.BRAKING OF DC MOTORWhen a motor is switched off it coasts to rest under the action of frictional forces.Braking is employed when rapid stopping is required. The electric braking may be done for various reasons such as -1. To augment the stop power of the mechanical brakes.2. To save the life of the mechanical brakes.3. To regenerate the electrical power and remedy the energy capability.4. In the case of emergencies to step the machine instantly.5. To improve the through put in many production process by reducing the stopping time.In many cases electric braking makes more brake power to the braking process where mechanical brakes are applied. This reduces the wear and rub of the mechanical brakes and reduces the frequency of the replacement of these parts. By recovering the mechanical energy stored in the rotating parts and pumping it into the supply lines the overall energy efficiency is improved. This is called regeneration. Where the safety of the personnel or the equipment is at stake the machine may be required to stop instantly. Basically the electric braking involved is simple. The electric motor can be made to work as a generator by suitable terminal conditions and absorb mechanical energy.This converted mechanical power is dissipated/used on the electrical meshing suitably.Braking can be broadly classified into1. Dynamic2. Regenerative3. Reverse voltage braking or sightgingThese are now explained briefly with reference to shunt ,series and compound motors.DYNAMIC BRAKING SHUNT MACHINEIn energising braking the motor is split from the supply and connected to a dynamic braking resistance RDB. This is done by changing the switch from position 1 to 2 . The supply to the field should not be removed. Due to this rotation of the armature during motoring mode and due to the inertia, the armature c ontinues to rotate. An emf is induced due to the presence of the field and the rotation.This voltage drives a current through the braking resistance. The direction of this current is opposite to the one which was flowing . Thereforethe torque produced is reversed. The machine acts like a brake. The torque speed characteristics carve up by horny shunt of the machine under dynamic braking mode for a particular value of RDB. The positive torque corresponds to the motoring operation. The dynamic braking of a shunt excited motor and the corresponding torque-speed curve.The machine behaves as a self excited generator.Below a certain speed the self-excitation collapses and the braking action becomes Zero. SERIES MOTORIn the case of a series machine the excitation current becomes zero as soon as the armature is unlogical from the mains and hence the induced emf also vanishes. In order to arrive at dynamic braking the series field must be isolated and connected to a low voltage high curr ent source to provide the field.The motor is made to work like a one after another excited machine. When several machines are available at any spot, as in railway locomotives, dynamic braking is executable. Series connection of all the series field with parallel connection of all the armatures connected across a unmarried dynamic braking resistor is used in that case. COMPOUND GENERATORIn the case of compound machine, the situation is similar to shunt machine. A separately excited shunt field and the armature connected across the braking resistance are used.A cumulatively connected motor becomes differentially intensify generator and the braking torque generated comes down. If large braking torques are desired.there is needful to reverse the series fieldREGENERATIVE BRAKINGIn regenerative braking as the name suggests the energy recovered from the rotating masses is back into the d.c. power source. Thus this type of braking improves the energy efficiency of the machine. The arma ture current can be made to reverse for a constant voltage operation by increase in speed/excitation only. Increase in speed does not result in braking and the increase in excitation is feasible only over a small range, which may be of the order of 10 to 15%. Hence the best method for obtaining the regenerative braking is to operate the machine on a variable voltage supply. As the voltage is ceaselessly pulled below the value of the induced emf the speed steadily comes down. The field current is held constant by means of separate excitation. The variable d.c. supply voltage can be obtained by Ward-Leonard arrangement. Braking torque can be obtained right up to zero speed.PLUGGINGThe third method for braking is by plugging. method of connection for the plugging of a shunt motor. Initially the machine is connected to the supply with the switch S in position number 1. If now the switch is moved to position 2, thusly a reverse voltage is applied across the armature. The induced armatu re voltage E and supply voltage V aid each other and a large reverse current flows through the armature. This produces a large negative torque or braking torque. Hence plugging is also termed as reverse voltage braking. The machine instantly comes to rest. If the motor is not switched off at this instant the direction of rotation reverses and the motor starts rotating the reverse direction. This type of braking therefore has two modes i.e. 1) plug to reverse and 2) plug to stop. If we need the plugging only for bringing the speed to zero, then we have to exposed the switch S at zero speed. Plugging is a convenient mode for quick reversal of direction of rotation in reversible drives.During plugging it is necessary to limit the current and the torque, to reduce the stress on the mechanical corpse and the commutator. This is done by adding additional resistance in series with the armature during plugging. SERIES MOTORIn the case of series motors plugging cannot be employed as the fi eld current is reversed when reverse voltage is applied across the machine. This keeps the direction of the torque produced unchanged. This fact is used with advantage, in operating a d.c. series motor on d.c. or a.c. supply. Series motors thus change to be called as Universal motors. COMPOUND MOTORPlugging of compound motors increase on similar lines as the shunt motors. A cumulatively compounded motor becomes differentially compounded on plugging. The mmf due to the series field can over power the shunt field forcing the flux to low values or even reverse the net field. This decreases the braking torque, and increases the duration of the large braking current. To avoid this it may be advisable to deactivate the series field at the time of braking by short circuiting the same. In such cases the braking talk just as in a shunt motor. If plugging is done to operate the motor in the negative direction of rotation as well, then the series field has to be reversed and connected for g etting the prudish mmf. Unlike dynamic braking and regenerative braking where the motor is made to work as a generator during braking period, plugging makes the motor work on reverse motoring mode.REFERENCES http//www.solarbotics.net/starting/200111_dcmotor/200111_dcmotor2.html http//nptel.iitm.ac.in/courses/IIT-MADRAS/Electrical_Machines_I/pdfs/2_8.pdf http//www.engineersedge.com/motors/dc_series_wound_motor.htm http//www.tpub.com/content/neets/14177/css/14177_58.htm http//www.most.gov.mm/techuni/media/EP_02021_4.pdfBOOKS BASIC ELECTRICAL $ELECTRONIC ENGINEERING BY J.B.GUPTA. A TEXTBOOK OF ELECTRICAL TECHNOLOGY BY B.L.THERAJA A.K.THERAJA