Application of dc series motor

Application of dc serial publication motorINTRODUCTION The essential features of a dc cable car are - The stator has salient poles and is excited by one or more discipline coil. The air-gap liquify distribution created by the field tangleds is symmetric to the highest degree the center line of the field poles. This axis is called the field axis or look at axis. The ac potency generated in each rotating armature coil is converted to dc in the external armature terminals by mean of a rotating commutator and stationary thicketes to which the armature leads are affiliated.o The commutator-brush combination forms a mechanical rectifier, resulting in a dc armature voltage 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 zone, midway between the field poles.o The axis of the armature-mmf wave is 90 galvanical degrees from the axis of the field poles, i.e., in the quadrature axis .o The armature-mmf wave is along the brush axis. DIRECT received beat backA direct online (DC)motoris a simple electricmotorthat uses electricity and amagnetic fieldto produce crookedness, which turns themotor. A DCmotorrequires two magnets of opposite polarity and an electric coil, which acts as anelectromagnet. The repellent and attractive electromagnetic forces of the magnets provide the torsion to theDCmotor. DC motors are divided into three classes, designated according to the order of connecting the armature and the field meanders as ring road-serial publication and escalate wound.When a permanent magnet is conditioned around a circulate of wire that is hooked up to a D.C. post source. In order to make the loop of wire spin, we have to connect a battery or DC berth supply between its ends, and support it so it asshole spin rough its axis. To al hapless the rotor to turn without twisting the wires, the ends of the wire loop are connected to a set of assembles called the commutator,which rubs against a set of conductors calledthebrushes.The brushes make electrical contact with the commutator as it spins, andare connected to the positive and interdict leads of the power source, allowing electricity to flow through the loop. The electricity flowing through 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 tool which converts electric talent to mechanical energy.Its put through is based on the principle that when a underway carrying conductor is placed in magnetic field, it experiences a mechanical force whose management is given by flemming left pass away rule and whose magnitude is given by F=BIl newton.Where B is the field strength in teslas, Ic is the stream flowing 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 supply mains, a direct current passes through the brushes and commutator to the armature winding piece of music it passes through the commutator it is converted into ac so that the groups of conductors under successive field poles carry currents in the opposite directions. in any case the direction of current in the individual conductors lifts as they pass away from the influence of one pole to that of the next.CONSTRUCTION -DC motor consist of rotor mounted winding and stationary winding.In all DC motors , except permanent magnet motors, current must be conducted to the armature winding by passing current through carbon brushes that slide everywhere a set of copper surfaces called a commutator , which is mounted on the rotor.The commutator restraints are soldered to armature coils. The brush/ commutator combination makes a sliding switch 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 at tracted to or repelled by north and south poles on the stator, which are formed by passing direct current through the field winding. This magnetic attractive force and repulsion that causes the rotor to rotate.ADVENTAGES OF DC MOTORS - DC motors provide excellent quicken control for acceleration and deceleration with effective and simple torque control. The fact that the power supply of a DC motor connects directly to the field of the motor allows for precise voltage control, which is necessary with speed and torque control applications.DC motors perform give away than AC motors on most traction equipment. They are excessively used for mobile equipment alike(p) golf carts, quarry and mining equipment. DC motors are handily portable and well suited to special applications, such as industrial tools and machinery that is not easily run from remote power sources.SERIES MOTOR diagramThe series motor provides high torque and is able to move very banging shaft gists when it is fir et energized.From the diagram we mass see that the field winding in the motor is wired in series with the armature winding.This is the attribute that gives the series motor its name.Since the series field winding is connected in series with the armature, it bequeath carry the same amount of current that passes through the armature. For this reason the field is made from heavy-gauge wire that is large enough to carry the load. Since the wire gauge is so large, the winding will have only a few turns of wire. In some larger DC motors, the field winding is made from copper bar stock rather than the conventional round wire used for power distribution. The square or rectangular shape of the copper bar stock makes it encounter more easily around the field pole pieces. It can also radiate more easily the heat that has built up in the winding due to the large amount of current being carried.The amount of current that passes through the winding determines the amount of torque the motor sha ft 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 500 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 operate for an extended period. In most applications the motor will operate for only a few seconds while this large current is present. OPERATION OF SERIES MOTOROperation of the series motor is easy to understand. The field winding is connected in series with the armature winding. This substance that power is applied 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 negative power supply terminals through the series winding and armature winding. The armature is not rotating when voltage is firs t applied, and the only resistance 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 due to the exciting coils augments in direct proportion to the line or armature current, so the value of flux varies with the load current 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 speed equation , the speed is proportional to the back emf 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 product of flux per pole and armature current Ia.Upto saturation point flux is proportional to field current and whence 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 due to torque lost in iron and friction and wind exposure losses.SPEED TORQUE CHARACTERSTIC -The speed torque characterstic also k right awayn as mechanical characterstic, sharply falls with the increasin torque for smaller value of load. But at h igher load, the speed drops linearly but slowly with increasing torque.Hence series motors are best suited for services where the motor is directly join to the load such as fanswhose speed falls with the increase in load torque.SPEED CONTROL OF DC MOTOR There is two mode of speed control i.e. armature voltage control and flux control method. The voltage control can be from a unsettled voltage source like Ward-Leonard arrangement or by the use of series armature resistance. Unlike the starting 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 require is large, as also the power rating. Field control or flux control is als o used for speed control purposes. Normally field modify is used. This causes operation at higher speeds than the nominal speed. Strengthening 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 mode 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 parallel to the series winding to reduce the field current without affecting the armature current.BRAKING OF DC MOTORW hen 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 variant reasons such as -1. To augment the brake power of the mechanical brakes.2. To save the life of the mechanical brakes.3. To regenerate the electrical power and improve the energy efficiency.4. In the case of emergencies to step the machine instantly.5. To improve the through put in more 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 tear 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 machi ne may be required to stop instantly. Basically the electric braking involved is simple. The electric motor can be made to work as a germ by suitable terminal conditions and absorb mechanical energy.This converted mechanical power is dissipated/used on the electrical network suitably.Braking can be broadly classified into1. Dynamic2. Regenerative3. Reverse voltage braking or pluggingThese are now explained briefly with reference to shunt ,series and compound motors.DYNAMIC BRAKING ringway MACHINEIn dynamic braking the motor is disconnected 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 drive mode and due to the inertia, the armature continues to rotate. An emf is induced due to the front man of the field and the rotation.This voltage drives a current through the braking resistance. The direction of this current i s opposite to the one which was flowing . Thereforethe torque produced is change. The machine acts like a brake. The torque speed characteristics separate by excited 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 disconnected from the mains and hence the induced emf also vanishes. In order to achieve dynamic braking the series field must be isolated and connected to a low voltage high current source to provide the field.The motor is made to work like a separately excited machine. When several machines are getable at any spot, as in railway locomotives, dynamic braking is feasible. Series connection of all the series fields with parallel connection of all the armatures connected across a single 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 compounded generator and the braking torque generated comes down. If large braking torques are desired.there is necessary 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 fictitious character of braking improves the energy efficiency of the machine. The armature 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 excitati on 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 continuously 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, then a reverse voltage is applied across the armature. The induced armature 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 als o 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 open 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 render on the mechanical system 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 field 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) a d.c. series motor on d.c. or a.c. supply. Series motors thus qualify to be called as Universal motors. COMPOUND MOTORPlugging of compound motors offspring 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 proceeds 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 getting the proper 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 revers e 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