DC MACHINCES DC MACHINCES

DC MACHINCES 1 Introduction & Application ‰ ‰ Three electrical Machines (dc, induction & synchronous) are used extensively for electromechanical energy conversion. In these machines, conversion of energy results from the following two electromagnetic phenomena: 1. 2. When a conductor moves in a magnetic field voltage is induced in the conductor (generator action) When a current –carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force (Motor action) Mechanical Electrical System E, I Electrical Machines System T, n Motor ‰ ‰ Generator Note that the two systems in fig. 1, electrical and mechanical, are different in nature. 9 In electrical system the primary quantities involved are voltage & current 9 While in mechanical system, the analogous quantities are torque & speed. The coupling medium between these different systems is the magnetic field 2 ‰ The dc machines are versatile and extensively used in industry. ‰ A wide variety of volt-ampere or torque-speed characteristics can be obtained from various connections of the field winding. ‰ Dc machines can work as generators, motors & brakes. ¾ In the generator mode the machine is driven by a prime mover (such as a steam turbine or a diesel engine) with the mechanical power converted into electrical power. ¾ While in the motor mode, the machine drives a mechanical load with the electrical power supplied converted into mechanical power. ¾ In the brake mode, the machine decelerates on account of the power supplied or dissipated by it and, therefore, produces a mechanical braking action. 3 ‰ There is almost no modern use of dc machines as generators although in the earlier stages of electrical power generator and distribution, dc generators were the principle means of supplying electrical power to industrial and domestic consumers. ‰ Presently, all the land based electrical power networks are a.c systems of generation, transmission and distribution. ‰ The almost universal use of ac systems is on account of their ‰lower generation and transmission costs, ‰higher efficiency (large bulk of ac power can be transmitted and distributed over wide areas and long distance at much higher voltages that are impossible in dc system), ‰greater reliability on account of interconnection and control. ‰ No doubt, application like aircrafts, ships and road mounted vehicles which are isolated from land based ac networks employ dc sources including dc generators and secondary batteries for power supply but the modern trend is to use ac generators with the dc supply being obtained by rectification with the help of static power rectifiers. ‰ DC generators are still being used to produce power in small back up and stand-by generating plants driven by windmill and mountain 4 streams (minihydro-electric plants) to provide uninterrupted power supply. ‰As apart from dc generators, the dc motors are finding increasing applications, especially where large magnitude and precisely controlled torque is required. ‰Such motors are used in rolling mills, in overhead cranes and for traction purpose like in forklift trucks, electric vehicles, and electric trains. ‰They are also used in portable machine tools supplied from batteries, in automotive vehicles as starter motors, blower motors and in many control applications as actuators and as speed and position sensing device (tachogenerators for speed sensing and servomotors for positioning and tracing). 5 ‰ CONSTRUCTION The dc machines used for industrial applications have essentially three major parts: a) Field system (Stator) b) Armature (Rotor) and c) commutator 6 Cut-away view of DC Machines Fi el dwi M nd a Ar in ing -p m o at le co ur B m e m r us h ut at es or End-bearings Fr a m e End-shield Shaft Ventilator Bearings Basement 7 ‰ ‰ ‰ Field System The field system is located on the stationary part of the machine called stator. The field system is designated for producing magnetic flux and, therefore, provides the necessary excitation for operation of machine. The stator of dc machines comprises of 1. 2. 3. Main Poles Inter-poles Frame (Yoke) Yoke Main Pole Field Winding 8 STATOR ‰ The stator of a dc machines consists of a frame or yoke, and poles, which support the field windings. ¾ The frame or yoke in addition to being a part of a magnetic circuit serves as mechanical support for entire assembly. YOKE ‰ Earlier, cast iron was used for the construction of yoke but it has been replaced by cast steel. ‰ This is because cast iron has saturation density of 0.8 Wb/m2 while saturation occurs in cast steel at density of approximately 1.5 Wb/m2. ‰ Thus, the cross section of the cast steel frame or yoke is half that of iron cast and hence cast steel is used in case it is desired to reduce the weight of machine. ‰ Fabricated steel yokes are commonly used, as they are economical and have consistent magnetic & mechanical properties. ‰ For very small sized machines it may still be advantageous to use cast iron frames but for medium and large sizes rolled steel is used. 9 MAIN POLES ‰ Poles are made of sheet steel laminations of 1,0 to 1,2mm thickness (nowadays the thickness becomes 0.4-0.5mm) ‰ The pole shoes support the field coils placed on the pole body and also spread the total flux over a greater area, thereby reduce the air gap reluctance and giving the desired flux distribution to limit saturation in the teeth of the armature.(ℜ = l ) µA ‰ The poles are secured to the yoke by means of bolts. ‰ In small machines the pole are built of steel forgings, bolted directly to the yoke. ‰ In case of machines having compensating windings, the pole face is slotted to accommodate the windings. INTERLOPES ‰ In addition to the main poles, modern direct current machines are also provided with interlopes with windings on them in order to improve commutation under loaded conditions. ‰ They are arranged midway between the mains poles and are bolted to the yolk. ‰ Laminated interlopes are used in machine with sever commutation problems. 10 ‰ For small and medium size machines they could be solid. Armature ‰ The armature is the rotating part (rotor) of the dc machine where the process of electromechanical energy conversion takes pace. ‰ The armature is a cylindrical body, which rotates between the magnetic poles. ‰ The armature and the field system are separated from each other by an air gap. ‰ The armature consists of: ¾ Armature core with slots and ¾ Armature winding accommodated in slots 11 ‰ The armature of the dc machines is a cylindrical shape, consists of slots, teeth, winding and the core. ‰ The purpose of the armature is to rotate the conductors in the uniform magnetic field and to induce an alternating emf in its winding. ‰ The armature core is normally made from high permeability siliconsteel laminations of 0.4-0.5mm thickness, which are insulated from one another by varnish or ceramic insulation. ‰ The use of high grade steel is made: ¾ To keep hysteresis loss low, which is due to cyclic change of magnetization caused by rotation of the core in the magnetic field and ¾ To reduce the eddy current in the core which are induced by the rotation of the core in the magnetic field ‰ In order to dissipate the heat produced by hysteresis and eddy current losses etc, ventilating ducts are provided. ‰ By the fanning action of the armature, air is drawn in through these ducts, thus producing efficient ventilation. ¾ In the armature core of small diameters, circular holes are punched in the center of the laminations for the shaft. 12 The Commutator ‰ The commutator is mounted on the rotor of a dc machine and it performs with help of brushes a mechanical rectification of power from Commutator segment Riser V-groove Thread Bolt Insulator ¾ ac to dc in case of generators and ¾ dc to ac in case of motors. 13 ‰ The ends of armature coils are connected to the commutator, which together with the brushes rectifies the alternating emf induced in the armature coils and helps in the collection of current. ‰ It is cylindrically shaped and is placed at one end of the armature. ‰ The construction of the commutator is quite complicated because it involves the combination of copper, iron and insulating materials. ‰ The connection of armature conductors to the commutator is made with the help of risers. ‰ The commutator bars are built of a small wedge shaped segments of high conductivity hard drawn copper insulated from each other by mice or micanite of about 0.8mm thickness. ‰ V-shaped grove is provided at each end of the segments to prevent them from flying away under the action of centrifugal force. ‰ The commutator assembly is force and press fitted on the shaft. ‰ Satisfactory performance of dc machines is dependent under good mechanically stability of the commutator under all conditions of speed and temperature within the operating range. ‰ A mechanically unstable commutator manifests itself in a poor commutation performance and results in unsatisfactory bush life.14 BRUSHES AND BRUSH HOLDER ‰ Brushes are needed to collect the current from the rotating commutator or to lead the current to it. ‰ Normally brushes are made up of carbon and graphite, so that while in contact with the commutator, the commutator surface is not spoiled. ‰ The brush is accommodated in the brush holder where a spring presses it a gains the commutator with pressure of 1,5 to 2,0 Ncm2 ‰ A twisted flexible copper conductor called pigtail securely fixed in to the brush is used to make the connection between the brush and its brush holder. ‰ Normally brush holders used in dc machines are of box type. ‰ The numbers of brush holders usually equal to the number of main poles in dc machines. Pigtail brush holder Pressure adjusting lever Spring Brush 15 PRINCIPLE OPERATION OF DC GENERATOR 16 PRINCIPLE OPERATION OF DC GENERATOR ‰ An electrical generator is a machine, which converts mechanical energy into electrical energy. ‰ The energy conversion is based on principle of dynamically induced emf, whenever a conductor cuts magnetic flux, dynamically induced emf is produced in it (Faraday’s law). ‰ This emf cause a current to flow if the conductor is closed. ‰ The basic essential parts of an electrical generator are: ¾ A magnetic Field and ¾ A conductor or conductors, which can so move as to cut the flux. ‰ Fig. 1 shows the schematic diagram of a simple machine consists of a coil ABCD rotating in the magnetic field of a strong permanent or powerful electromagnet. ‰ The magnetic lines in the space between N and S poles are directed from the North Pole N to the South Pole S as shown in fig 1. ‰ The ends of the coil ABCD are connected to two copper rings R1 and R2, fixed on the shaft. ‰ Two brushes B1 and B2 connected to the external load circuit make contact with the copper rings R1 and R2 respectively. C B N S D A B1 R1 R2 B2 Load Fig.1 17 ‰ Let the coil be rotated in an ACW direction, with constant surface speed v in relation to the magnetic field. ‰ According to Faraday’s laws of electromagnetic induction, an emf will be induced in the rotating coil and is given by e = Blv volts ‰ As l and v are constant for particular case, e = constant × B volts ‰ Hence under the given conditions, the change in the magnitude of induced emf with time depends upon the magnetic flux density distribution under the poles. ‰ Though the flux density distribution under the poles is of a complex nature, it may be assumed that neglecting harmonics, it is a sine wave distribution. ‰ Thus the emf induced in the coil varies with time as a sine function. ‰ The direction of the induced emf in this case can be determined by Fleming’s right hand rule. 18 Right-hand Rule φ ν S N Motion e Flux EMF 19 ‰ Hence the conductor AB of the coil ABCD moves downward and CD moves upward, the direction of the induced emf in the coil is along DCBA as shown in Fig1. (a). ‰ The current in the external remains the same half a revolution of the coil starting from its vertical position. ‰ Similarly, in the next half of the revolution, the direction of the induced emf is reversed and hence the current flows from brush B2 to B1 as shown in Fig 1 (b). ‰ The magnitude of current in the external circuit also varies with time as per sine law; i.e. its magnitude is not constant with time. Fig.1 (a) In the 1st half revolution 20 Fig.1 (b) In the 2nd half revolution Current ‰ If the machine has P poles and the armature rotates at N revolutions per minute, then the frequency of the induced emf in the armature is, PN f = 120 ‰ The above discussion clearly indicates that the emf induced in the armature of a dc generator is of alternating nature, alternating with frequency of f hertz depending upon the number of poles in the machine and the speed of the armature. ‰ However the output voltage or the current of dc generator must be unidirectional and that too of a constant value. ‰ Thus to compel the above load circuit, the dc machine is furnished with a special device called the commutator θ 21 Fig.2 (a) In the 1st half revolution current ‰ Fig.2 show that the coil ABCD connected to a ring commutator split in two halves R1 and R2 well insulated from each other. ‰ The rings of the commutator are so arranged that during half the revolution of the coil, each half ring remain in contact with a particular brush. ‰ Fig 2 (a) while during the next half revolution, when the current is reversed, the same half ring is in contact with other brush as shown in fig.2 (b). ‰ As a result, current in the external load circuit remains in the same direction. 22 Fig.2 (b) In the 2nd half revolution time ‰ To overcome the above difficulty of the nature of a wave shape, consider two coils whose planes are inclined to each other at an angle of 900 and divided the commutator ring mounted on the same shaft into four parts. ‰ The leads of each coil are connected to the two diametrically opposite parts of the ring. In such case, the emf or current wave shape due to either coil will be of the same type but 900 out of phase, i.e. when the current in one reaches maximum value, the current in the other coil has zero value as shown in fig.4. ‰ The resultant current in the external circuit due to the rotation of the two coils simultaneously at the same speed can be obtained by superimposing the two current waves. ‰ Hence, the resultant current wave shape is less fluctuating. ‰ Similarly, if a large number of coils are provided on the rotating armature of the machine with double the number of commutator segments, the wave shape of the resultant current or the emf will practically be parallel to the time axis and hence constant with respect to time. Coil 2 current Coil 1 Resultant current wave 23 time Fig.4 TYPES OF DC MACHINES 24 TYPES OF DC MACHINES ‰ The field winding and the armature winding can be interconnected in various ways to provide a wide variety of performance characteristics. ‰ This can be taken as outstanding advantages of a dc machines. ‰ A dc machine can work as an electromechanical energy converter only when its field winding is excited with direct current, except for small dc machines employing permanent magnets. ‰ According to the method of their field excitation dc machines are classified into the following group: a) b) separately excited and self excited ‰ DC machines may have one or more field windings and their method of excitation, determines the performance characteristics of the dc machine. 25 IL a) Separately Excited ‰ ‰ Its field winding consists of several hundreds turns of fine wire and is connected to a separate or external dc source i.e. field winding are energized from an independent external sources of dc current. The voltage of the external dc source has no relation with the armature voltage, i.e. the field winding energized from a separate supply, can be designed for any convenient voltage. Ish Vdc Ia Eg VL Fig. separately excited dc machines i) I a = I L ii) E g = VL + I a Ra iii) Pdev = E g ⋅ I a iv) Pdel = VL ⋅ I L 26 ‰ ‰ ‰ ‰ b) Self Excitation When the field winding is excited by its own armature, the machines is said to be a self excited dc machine. In these machines, the field poles must have a residual magnetism, so that when the armature rotates, a residual voltage appears across the brushes. This residual voltage should establish a current in the field winding so as to reinforce the residual flux. According the connection of the field winding with the armature winding, a self-excited dc machine can be sub-divided as follows: i. Series Excitation ii. Shunt Excitation and iii. Compound Excitation 27 i. ‰ ‰ Series Excitation Ise The field winding consists of a few turns of thick wire and is connected in series with Eg Ia VL the armature. In other words, the series field current depends on the armature current and in view of this; a series field may be Fig.2 Series excited dc machine called a current operated field. i) I a = I se = I L ii) E g = VL + I a ( Rse + Ra ) iii) Pdev = E g ⋅ I a iv) Pdel = VL ⋅ I L 28 ii. ‰ ‰ ‰ ‰ Shunt Excitation The field winding consists of a large number of turns of fine wire and is connected in parallel (or in shunt) with the armature. Therefore the voltage across the armature terminals and the shunt field is the same and it is for this reason that a shunt field may be called voltage operated field. Remember that series field and shunt field windings are characterized by low and high resistance respectively. In some application, a shunt excited winding may be replaced by a separately excited winding. Ish Ia IL Eg VL Fig.3 Shunt excited dc machine VL i) I sh = Rsh ii) I a = I sh + I L iii) E g = VL + I a Ra iv) Pdev = E g ⋅ I a v) Pdel = VL ⋅ I L 29 iii. Compound Excitation ‰ ‰ ‰ A compound excitation involves both series-exited winding and the shuntexcited winding. From the view point of connections, a dc compound machine may have short-shunt connection or a long shunt connection. In short shunt connection of fig. 4(a) the shunt field or voltage excited winding is connected across the armature terminals. Ish IL i) I se = I L Ise Ia Eg VL ii) I sh = E g − I a Ra = iii) I a = I sh + I L Rsh VL + I se Rse Rsh iv) E g = VL + I a Ra + I L Rse v) Pdev = E g ⋅ I a Fig. 4(a) Short-Shunt Compound vi) Pdel = VL ⋅ I L 30 ‰ ‰ ‰ In long-shunt connection, the shunt field is connected across ¾ ¾ the series connection of the armature and series winding or the machine or line terminals as shown in fig 4(b). However there is no appreciable difference in the operating characteristics of short-shunt and long shunt. The choice between the two types, depends on mechanical considerations of connections or reversing switches Ish i) I a = I se E g − I a (Ra + Rse ) VL = ii) I sh = Rsh Rsh IL Ise Ia Eg VL iii) I a = I sh + I L iv) E g = VL + I a ( Ra + Rse ) v) Pdev = E g ⋅ I a vi) Pdel = VL ⋅ I L Fig. 4(b) Long-Shunt Compound 31 ‰ In a compound machine, the magnetic flux produced by the shunt field is stronger than the series field. ‰ When series field aids the shunt field, so that the resultant air gap flux per pole is increases, then the machine is said to be cumulatively compounded. ‰ In fig.5 (a) the direction of arrows corresponds to the direction magnetic flux produced by shunt and series field windings. ‰ As the two arrows are in the same direction in fig5 (a), this figure is for a cumulatively compounded dc machine. ‰ On the other hand if series field opposes the shunt field so that the resultant air gap flux per pole is decreased, the machine is called a dc differentially compounded machine as shown in fig. 5(b) Fig.5(a) Cumulative Compound Fig.5(b) differential Compound 32 ‰ ‰ ‰ ‰ ‰ ‰ ¾ ¾ In fig 4(a), each pole of compound machine is shown to possess shunt and series field windings. Fig. 6(a) illustrated how these windings are arranged on one pole of a dc machine. In this figure, shunt field coil is placed near yoke and series field coil near the pole shoe just for sake of clarity. Actually physical arrangement of these coils is shown in fig 6 (b). It is seen that first shunt field coil is wound around the pole body and over it is then wound the series field coil. The reasons for placing the series field coil outside are : (a) (b) Fig. 6 Series and shunt field windings on one pole of dc compound machine convenience in the construction and for its better cooling 33 EMF EQUATION OF DC GENERATOR Let φ = flux per pole in Weber Z = total number of armature conductors = Number of slots × Number of conductors per slot P = Number of poles a = number of parallel paths in armature N= armature rotation in revolutions per minute (rpm) E = emf induced in any parallel path in armature Generated emf, Eg= emf generated in one of the parallel path Average emf generated / conductor = dφ , volt Now, flux cut / conductor in one revolution, dφ = φ P , Wb dt N = , sec ond Number of revolution / second 60 Hence according to Faraday’s law of electromagnetic induction emf generated / conductor = dφ φ P N = ,volt dt 60 34 For wave winding Number of parallel path a = 2 Z Number of conductors (in series) in one path = ∴ emf generated / path = For lap winding φPN Z 60 ⋅ 2 = φPZ N 2 × 60 2 volt Number of parallel path a = P Number of conductors (in series) in one path= Z P φPN Z φZ N ∴ emf generated / path = ⋅ = volt 60 P 60 ∴ In general, the Generated emf Where, a =2 a=P E g = K aφ N Where, K a = ZP 60 ⋅ a Eg = φZ N ⎛P⎞ × ⎜ ⎟ volt 60 ⎝a⎠ for wave winding for lap winding is machine constant 35 1. A 6-pole d.c. machines has 300 conductors and each conductor is capable of carrying 80-A without excessive temperature rise. The flux per pole is 0.015-Wb and the machine is driven at 1800 r.p.m. Compute the total emf and power developed in the armature , if the armature conductors are: a) b) 2. lap connected and wave connected A 220-V compound generator is supplying a load of 100-A at 220-V. The resistance of its armature, shunt and series windings are 0.1Ω, 50Ω and 0.06Ω respectively. Find the induced emf and the armature current when the machine is connected a) b) Short-shunt Long-shunt 36 ARMATURE REACTION ‰ By armature reaction is meant the effect of magnetic field set up by armature current on the distribution of flux under main poles. ‰ In other words armature reaction is meant the effect of armature ampere-turns upon the value and the distribution of the magnetic flux entering and leaving the armature core. ‰ The armature magnetic field has two effects: 1. It demagnetizes or weakens the main flux & 2. It cross–magnetizes or distorts it 37 O Fm G.N.A. Polar-axis M.N.A. ‰ Let us illustrate (demonstrate) these two effects of armature reaction for 2-pole d.c generator. ‰ For better understanding let us see three cases. Case-I: ‰ Shows the distribution of magnetic flux when there is no current (Ia=0) in the armature conductors. For this case a) The distribution magnetic flux symmetrical with respect to the polar axis. b) The magnetic neutral axis or place (M.N.A.) coincides with geometrical neutral axis or plane (G.N.A) 9 M.N.A- may be defined as the axis along which noemf is produced in the armature conductors because they move parallel to the lines of flux. 9 OR M.N.A. is the axis which is perpendicular to the flux passing through the armature ∆ brushes are always placed along M.N.A ∆ The mmf (Fm) producing the main flux is directed perpendicular to M.N.A Fig.(a) Magnetic flux distribution due to the main field poles only 38 O Case-II: ‰ Shows the field (or flux) set up by the armature conductors alone, when current carrying the field coils being unexcited (If = 0). ‰ The direction of the armature current is the same as it would be when the generator is loaded & determine by Fleming’s Right-hand rule. G.N.A. M.N.A. ‰ The magnetic fields, which are set up by armature conductor are symmetrical to G.N.A. ‰ The mmf of the armature conductor (depending on the strength of Ia) is shown separately both in magnitude and direction by the Vector OF, which is parallel to G.N.A. FA 39 Fig.(b) Magnetic flux distribution due to the armature excitation only ‰ So far, we considered the main mmf and armature mmf separately, as shown in case I and case II respectively, as if they existed independently, which is not the case in practice under actual load conditions. ‰ The two cases exist simultaneously in generator as shown in case III. Fm O FA FR Trailing Pole-tip θ Case-III: Leading G.N.A. W M. N.A .. Pole-tip NE ‰ Shows the combination of case I & II. ‰ In this case the main flux through the armature is no longer uniform and symmetrical about the pole-axis, rather it has be distorted by flux which set up by armature conductors so that the resultant magnetic flux will be the combination of these two fluxes this will be shown in fig below. Fig.(c) combined magnetic flux distribution 40 due to armature and field ‰As shown in fig (c) the resultant mmf (OFR) is found by vectorally combining OFm and OFA. ‰The new position of M.N.A which is always perpendicular to the resultant mmf vector OFR is shown in fig (a). ‰Due to the shift of M.N.A, say through an angle θ, brushes are also shifted so as to lie along the new positions of M.N.A. ‰Due to this brush shift (or forward, leads), the armature conductors and hence the armature current is redistributed, i.e. some armature conductors, which were earlier under the influence of N-pole, come under the influence of S-pole and viceversa. ‰Let us see this condition with help of diagram 41 ‰ Now the armature mmf is represented by vector FA that is no vertical but is inclined by angle θ to the left (fig below) ‰ This vector can be resolved into two rectangular components, Fd parallel to polar axis and Fc perpendicular to this axis, we find that 1. Component Fc is at right angle to the vector OFm (case I) representing the main mmf it produces distortion in the main field and is hence called the crossmagnetizing or distorting component of the armature Reaction. 2. Component Fd is in direct opposition to OFm, which represents the main mmf. It exerts a demagnetizing influence on the main pole flux. Hence, it is called the demagnetizing or weakening component of the armature reaction. X X X X X X X Fd θ FA FC 42 Conclusion ‰ The flux across the air gap is no longer uniform, but weakens under the leading pole tips and strengthened under the trailing pole tips. (The pole tip which is first met during rotation by armature conductors is known as the leading pole tip and the other as trailing pole-tip). ‰ Due to this the resultant mmf given rise to decreases flux. ‰ So that emf in the armature under loaded conditions is somewhat less than that of under no-load conditions. ‰ The brushes should be shifted in the direction of rotation to avoid a heavy short-circuit current and sparking at brushes. ‰ The field distortion cause, an increase in the iron losses as compared its no-load value because of increases peak value of flux density in the tooth. 43 COMMUTATION ‰ Since armature conductors carry current in one direction when they are under N-pole and in opposite direction when they are under the influence of S-pole. ‰ So when the conductors come under the influence of the S-pole from the influence of N-pole, the direction of flow of current in them is reversed. ‰ This reversal of current in a coil will take place when the two commutator segments to which the coil is connected are being short circuited by brush. ‰ The process of reversal of current in a coil is termed as commutation. The period during which the coil remains shortcircuited is called commutation period, Tc. ‰ This commutation period is very small of the order of 0.001 to 0.003s. ‰ If the current reversal i.e. the changes from +I to ZERO and then to –I is completed by the end of short circuit or commutation period, the commutation is Ideal. ‰ If current reversal is not completed by that time, then sparking is produced between the brush and the commutator, which results 44 in progressive damage to both. ‰ To make our discussion simple and clear, let us consider that the machine has a ring winding, a part of which is shown and concentrate on coil B. Motion B A I I I b c 2I Fig (A) IN FIG. A Coil B carries current in clock wise direction but it is about to be short circuited, A C I I 2I a B A C C I I I a B b 2I c 2I Fig (B) IN FIG. B Coil B is shown during the period of short circuit and it is observed that current can reach the brush with out passing through coil B, so coil B has no current. a b 2I c Arc (spark) Fig (C) IN FIG.C Coil B is shown immediately after short circuit and carries current in a counterclockwise direction. 45 Ideal commutation +I (under commutation) Incomplete commutation (under commutation) -I Tc Current jumps and produce a spark Commutation in Coil B 46 ‰ During the period of short circuit, period of commutation, the current in the short-circuited coil should be reversed to full value. ‰ Rapid reversal of current in the short circuited coil does not attain its full value in the reverse direction by the end of short circuit. ‰ The failure of current in the short-circuited coil to reach the full value in reverse direction by the end of short circuit is the basic cause of sparking at the commutator ( as shown in fig. c current jump from commentator segment “ b” to brush in the form of an arc). ‰ The reason for sparking at brushes of dc machine is due to reactance voltage (self-inducted e.m.f.), which sets-up by rapid reversal of current in the armature coil and tend to delay the current reversal in the coil. ‰ Because coil B has some inductance L, the change of current ∆I in a time ∆t induce a voltage L(∆I ∆t ) in the coil. ‰ According Lenz’s law, the direction of this voltage is opposite to the change ∆I that is causing it. ‰ As a result, the current in the coil does not completely reverse by the time 47 the brushes move from segment b to a. ‰ Methods of improving commutation There have been adapted two practical ways of improving commutation i.e. of making current reversals in the short-circuited coil as sparkles as possible. The two methods are: ‰ resistance commutation and ‰ e.m.f. commutation. ‰ This method is achieved by a) By replacing low-resistance cu brushed by comparatively high resistance carbon brush. However , it should be clearly understood that the main causes of the sparking commutation is the self induced emf , so brushes alone do not give a sparkles commutation, though they do help in obtaining it. b) By the help of inter poles, neutralize the self- reactance voltage by producing reversing emf. In this method, arrangement is made to neutralize the reactance voltage by producing a reversing emf in the short-circuited coil under commutation. This reversing emf, as the name shows, is an emf in opposition to the reactance voltage and if its value is made up equal to the latter, it will completely wipe it off, there by producing quick reversal of current in short-circuited coil which will result in sparkles commutation. 48 ‰ ‰ Interpoles or Compoles These are small poles fixed to the yoke and spaced in between the main poles. They are wound with comparatively few heavy gauge Cu wire turns and are connected in series with the armature so that they carry full armature current. Their polarity, in the case of a generator, is the same as that of the main pole ahead in the direction of rotation. Main-Poles The function of interpole is two fold: i. ii. As their polarity is the same as that of the main pole ahead, the induced an emf in the coil (under commutation) which helps the reversal of current. The emf induced by the Compoles is known as commutating or reversing emf. The commutation emf neutralizes the reactance emf thereby making commutation sparkles. As Interpoles carry armature current, their commutating emf is proportional to the armature current. This ensures automatic neutralization of the reactance voltage, which is also due to armature current. Another function of the Interpoles is to neutralize the cross-magnetize effect of armature reaction. Hence, brushes are not to be shifted from the original position. Neutralization of cross- magnetization is automatic and for all loads because both are produced by the same armature current. Inter-Poles 49 Compensating winding ‰ The effect of cross-magnetization can be neutralized means of compensating winding. ‰ These are conductors embedded in pole faces, connected in series with the armature windings and carrying current in an opposite direction to that flowing in the armature conductors under the pole face. ‰ Once cross-magnetization has been neutralized, the M.N.A does not shift with the Load and remains coincident with the G.N.A. at all loads. 50 CHARACTERISTICS OF DC GENERATORS ‰ The behavior of various types of dc generators can be studies by their characteristic. ‰ The three most important characteristic curves of a dc generator are: 1. Magnetization characteristic or open-circuit characteristic (O.C.C.) 9 shows the relationship between the field current If and the generated emf Eg at no load and at constant given speed 2. External characteristic 9 shows the relationship between the terminal voltage Vt across the load and the current IL flowing in the external load circuit. 3. Internal characteristic 9 Shows the relationship between the emf generated Eg (after allowing for demagnetizing effect of armature reaction) at load and the armature current Ia. 51 ‰ 1. Magnetization characteristic (O.C.C.) The emf generated in the armature winding of a dc machine under no load condition is given by Eg = Eg C Pφ NZ 60a D B P, Z and a are constants for a particular generator, hence at constant given speed. ∴ ‰ Eg α φ The generated emf is directly proportional to the flux per pole (speed being constant), which in turns depends upon the field current If The characteristic curve plotted between generated emf Eg and the field current If at constant speed of rotation is called the magnetization curve or O.C.C. of the dc generator A O If AB- Unsaturated Region ( Straight Line) BC- Knee of the curve ( Operating Region) CD- Saturated Region 52 ‰ The magnetization characteristics of a separately excited generator or shunt generator can be obtained as explained below. ‰ Fig (1) shows the connections of the generator and the field for determination of O.O.C. ‰ A potentiometer arrangement has been made to supply the field winding so that the field current can be varied over a wide range by moving the contact K. ‰ Ammeter indicate the field current and voltmeter indicate the generated emf. ‰ The field current is increased in steps from zero to maximum and the corresponding value of If and Eg are noted down at each step. ‰ On plotting these results, a curve of the form shown in fig. 2 is obtained Eg C D B A O If Fig (2) Magnetization curve or O.C.C. OA- Residual EMF ( due to Residual Magnetism) AB- Unsaturated Region ( Straight Line) BC- Knee of the curve ( Operating Region) Fig (1) Circuit diagram for determination of magnetization characteristics CD- Saturated Region 53 ‰ On analyzing the above curve, it is observed that a small emf OA is generated by the generator, even when the field current is zero. ‰ The reason for this generated emf is the residual magnetism in the poles. ‰ This emf which is due to residual magnetism is normally 1 to 5% of the normal voltage of the generator. ‰ The magnetization curve of a shunt generator and a series generator can also be obtained in a similar manner. ‰ However, a shunt generator differs compared to separately excited one, in the manner that the field current in shunt generator is due to the generated emf only, where as the field current is independent of the generated emf in case of separately. ‰ This magnetization curve is of grate importance because it represents the saturation level in the magnetic system of the dc machine for various value of the excitation mmf (current). 54 VOLTAGE BUILD-UP PROCESS IN SHUNT (SELF-EXCITED) GENERATOR ‰ In the shunt or selfexcited generator the field is connected across the armature so that the armature voltage can supply the field current. ‰ Under certain conditions, to be discussed here, this generator will build up a desired terminal voltage. ‰ If the machine is to operate as a self-excited generator, some residual magnetism must exist in the magnetic circuit of the generator. ‰ Fig (3) shows the magnetization curve of the dc machine. ‰ Also shown in this figure is the field resistance line, which is a plot of Rf If Versus If. Field Resistance Line I f R f Vs I f Eag Ea4 Ea3 Ea2 Ea1 Ear If1 If2 If3 If4 If6 If 55 Fig (3) voltage build-up process in self excited dg generator ‰ Figure (5) shows the voltage buildup in the self-excited dc generator for various field circuit resistances. ‰ At some resistance value Rf3, the resistance line is almost coincident with the linear portion of the magnetization curve. ‰ This coincidence condition results in an unstable voltage situation. This resistance is known as the critical field circuit resistance. ‰ If the resistance is greater than this value, such as Rf4, buildup (Vt4) will be insignificant on the other hand, if the resistance is smaller than this value, such as Rf1 or Rf2, the generator will build up higher voltages (Vt1, Vt2). Eag Rf2 Rf4 Rf1 Rf3 Vt2 Vt1 If Fig (5) effect of field resistances on voltage build-up process 56 Summary ‰ To sum up, four conditions are to be satisfied for voltage buildup in a self-excited dc generator. 1. Residual magnetism must be present in the magnetic system 2. Field winding mmf should aid the residual magnetism 3. Field circuit resistance should be less than the critical field circuit resistance. 4. The speed at which the armature is rotating should be greater than the critical speed. 57 2. ‰ ‰ EXTERNAL CHARACTERISTICS The external characteristics of a dc generator express the relationship between the terminal voltage and the load current at a constant speed and with the field current keeping the same as under the no load condition. The shape of this curve depends upon: i. ii. The armature reaction Voltage drop in the armature winding, series , inter pole and compensating windings iii. Voltage drop at the brush contact (0.8-1.0V per brush ) and iv. The drop in terminal voltage due to (i) and (ii) results in a decreased field current which further reduces the induced emf. 58 SPEARATLY EXCITED GENERATOR ‰ In separately excited generators, the field current is independent of the load current. ‰ If there were no armature reaction and no voltage drop in various windings the terminal voltage will be equal to the generated emf and would be constant for various values of VL load current as indicated by curve I in fig 6. I ‰ However, the armature reaction will cause a decrease in the voltage, which depends upon the load current. II ‰ Considering the effect of armature action only, the curve of terminal voltage verses armature current will be slightly drooping as shown by curve II in fig (6). III ‰ Curve II of the generator, which takes into account the effect of armature reaction, gives to a different scale the emf induced in the armature and thus, it is normally called the internal characteristics of the generator. ‰ The curve of terminal voltage verses load current or ature m r armature current is obtained by subtracting the olmic a in p o r d drop in the armature winding with respect to the hmic O armature current is represented by the straight line passing through the origin as shown Fig (6). IL / Ia ‰ When the ordinates of straight line representing the voltage drop in the armature winding (IaRa) are deducted from those of curve II, a cure III is obtained, which given the external characteristic of the generator i.e. Fig (6) external characteristics Curve III = Curve II - Ia Ra ‰ External characteristics clearly indicate that the terminal voltage falls as load on the generator increase. of separately excited generator 59 SHUNT-WOUND GENERATOR ‰ In this type of generator, the field winding is connoted across the armature winding. The generator will therefore build up its own magnetism. ‰ The voltage across the shunt field winding is equal to the terminal voltage of the generator as discussed above, the terminal voltage of the generator will fall down due to the armature reaction and the ohmic drop in the armature winding, as the load on the generator increases. ‰ Thus the voltage across the field will not remain constant as the load on the shunt generator increases. ‰ The voltage across the field winding decreases with an increase in the load current, which causes a decrease in the exciting current. ‰ The terminal voltage further falls down incase of a shunt generator because of decreases in excitation current as explained earlier with increasing load current. ‰ Hence the total decreases in the voltage in case of shunt generators is mush greater than in separately excited generators. 60 ‰ ‰ ‰ ‰ ‰ ‰ VL Figure (7) shows the external characteristics, of a particular generator, when it is run as a A separately excited generator (curve IV) and when run as a shunt generator (Curve III). Comparing these two curves for the same generator, it is observed that with self-excitation the external characteristic is lower than that obtained with separate excitation. The basic reason for the difference in the two curves is that, in the former case the shunt field current decreases with decreasing terminal voltage, while in the case of separate excitation the field current remains constant. If the load on the shunt generator is gradually O increased by decreasing the resistance in the external circuit, its terminal voltage tends to fall by a process of exactly a reverse nature to that of building up. Up to the normal load current, steady conditions are obtained without a serious fall in the terminal voltage as shown by the thick line of curve III. When the load on the shunt generator increase beyond its full load value, the drop in terminal voltage becomes more appreciable as shown by the dotted line of curve III. I IV II III E D IC Fig (7) external characteristics of shunt wound generator 61 IL ‰ Up to the point D on curve III, the load current increases upon decreasing the external resistance in the load circuit, where the terminal voltage has fallen to an appreciably low value. The current corresponding to this condition is generally termed as critical current Ic. ‰ A further decrease in the external load resistance beyond the point D, does not increase the current in the load circuit, but on the other hand decreases it, because the load resistance shunts the field winding to such an extent the terminal voltage decreases more rapidly than the load resistance. ‰ Hence the external characteristic turns back and the terminal voltage is zero when the armature is actually short-circuited. The armature current at this instant is shown by a vale OE that is purely due to residual magnetism of the generator. ‰ To obtain the internal characteristics of the dc shunt generator, the sum of the voltage drop in the armature winding including the brush contact drop is added to the external characteristic, thus obtaining curve II representing this characteristic. ‰ Figure (7) also shows the no load voltage Eo of the generator represented by the dotted line I. The voltage drop between curve II and line I is due to reduction in flux caused by the combined action of armature reaction and the fall caused by the combined action of armature reaction and the fall in the shunt field current. Fig (7) external characteristics of shunt wound generator 62 SERIES WOUND GENERATOR ‰ In series- wound generators, the field winding is connected in series with the armature winding. ‰ Thus, the current in the field winding is the same as the current in the armature winding. ‰ If the generator is driven at the constant rated speed, and the armature current is varied by varying the external resistance in the load circuit, a curve III of Fig.8 is obtained by plotting the terminal voltage Vs load current or armature current. ‰ The internal or total characteristic of the same generator is represented by curve II in Fig.8 which can be obtained by adding the terminal voltage (curve III). ‰ Curve I, in Fig. 8, shows the magnetization characteristics of the same generator. The voltage drop between the curves I and II is caused by armature reaction Fig (8) external characteristics of series wound generator 63 COMPOUND GENERATION ‰ The shunt generator already discussed has a drooping external characteristic, i.e. the terminal voltage falls with load, whereas series generators have an external characteristic, in which the terminal voltage rises with the load. ‰ Hence, a series field winding in dc generators can compensate for the tendency of the shunt generator to lose voltage with load, thus maintaining practically a constant voltage at all loads. ‰ For this reason, the majority of dc generators in service have both shunt and series windings. ‰ Such a dc generator having both shunt and series windings is called a compound generator. 64 ‰ Curve I shows the external characteristic, in which the ‰ ‰ ‰ ‰ ‰ ‰ series excitation is such that the terminal voltage on full load is the same as on no load and the terminal voltage remains practically constant from no load to full load. A dc compound generator giving such an external characteristic is called level-compounded generator. The external characteristic shown by curve II indicates that the terminal voltage rises with the load. Such a compound generator with this external characteristic is said to be over compounded generator. The compound generator having an external characteristic of the nature represented by curve III is called under compounded generator. In all the above three types of compound generators, i.e. level-compounded , over-compounded and undercompounded, the series field aids the shunt field and thus these compound generators can also be called as cumulative compound generator. In case the series field opposes the shunt field, the external characteristic of the generator will be highly drooping with large demagnetizing armature reaction as shown by curve IV in fig 9. Such a compound generator said to be differentialcompound generator. II VL VNL I III IV IFL IL Figure (9) the external characteristics of dc compound generator I- Level (flat) compounded II- Over compounded III- Under compounded 65 IV- Differential compounded Application ‰Cumulative compound generator is most widely used in practice. ‰Their external characteristic can match to all classes of service. ‰These types of generators used for 9 electric railways, for supplying current of incandescent lamps, etc. ‰Differential compound generators find their field of application in 9arc welding where a large voltage drop is desirable, when the current increase. 66 DIRECT CURRENT MOTORS Working principle ‰ The principle upon which a dc motor works is very simple. If a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor, the direction of which is given by Fleming's left hand rule (also called motor rule) and hence the conductor moves in the direction of force. ‰ The magnitude of the mechanical force experienced n the conductor is given by F = BIc lc, Newtons Where B is the field strength in teslas (wb/m2), Ic is the current flowing through the conductor in amperes and lc is the length of conductor in meters. ‰ When the motor is connected to the dc Supply mains, a direct current passes through the brushes and commutator to the armature winding. While it passes through the commutator it is converted in to a.c. so that the group of conductors under successive field poles carries currents in the opposite directions, as shown in fig.1. ‰ Also the direction of current in the individual conductor reverses as they pass away from the influence of one pole to that of the next. 67 ‰ In fig.1, a 4-pole d.c motor is shown when the filed and armature circuits are connected across dc supply mains. ‰ Let the current in armature conductors be outwards under the N-poles (shown by dots) and inwards under Spoles (shown by crosses). ‰ By applying Fleming’s left hand rule, the direction of force on each conductor can be determined, which has been illustrated in fig.1. ‰ From fig.1 it is observed that each conductor experiences a force which tends to the motor armature in clock-wise direction. These forces collectively produce a driving torque. 68 COMPARISON OF MOTOR AND GENERATOR ACTION ‰ As mentioned above, dc motor and the dc generator are the same devices, at least theoretically. ‰ The machine operating as a generator is driven by some external driving force and dc out put is obtained from it where as the machine operating as a motor is supplied by electric current and mechanical rotation is produced. ‰ Let us first consider the generator operation. In fig.2 a dc machine driven, in a clock-wise direction, by its prime mover and supplying direct current to external load circuit is shown. ‰ The machine is working as a generator and the direction of the generated e.m.f. and current flowing through the armature conductors, as determined by Fleming's right hand rule, will be as shown in the fig.2.a. X X X X X X X X X X X X Rotation X X X X X 69 ‰ Since the armature is carrying current and rotating in a magnetic field, Electro-magnetic forces will be given by Fleming's left hand rule. These Electro magnetic forces acting on the armature conductors will collectively result in torque acting on the armature in a counter-clockwise direction. ‰ This Electro-magnetic torque, therefore, opposes the outside driving torque, which is causing the rotation of the machine and called the backward torque or magnetic drag on the conductors. ‰ The prime mover has to work against this magnetic drag and the work so done is converted in to electrical energy. ‰ The larger the output current, more will be the backward torque and, therefore, more mechanical energy will be required to be supplied to the generator. 70 ‰ In fig.2.b the same machine operating as a motor is shown, ‰ This operation takes place when the prime mover is uncoupled from the machine and the machine is connected to the dc supply mains. ‰ With the directions of field and armature current shown in the fig.2b, the torque developed by Electro-magnetic actions will rotate the machine in a clockwise direction (as determined by Fleming's left-hand rule). ‰ The friction of the machine and the mechanical load that the motor is driving will exert a torque in counterclockwise direction, opposing the rotation of the motor. ‰ Since the armature conductors are revolving in the magnetic field, e.m.f. is induced in the armature conductors. ‰ The direction of e.m.f. so induced, as determined by Fleming's right hand rule, is in direct opposition to the applied voltage. ‰ That is why the induced e.m.f. in motor often is called the counter e.m.f. or back e.m.f. Eb. X X X Rotation X X X X X X X X X X X X X X 71 ‰The applied voltage must be large enough to overcome this back e.m.f. and to send the current through the resistance of the armature. ‰The electric energy supplied to overcome this opposition is converted into mechanical energy development in the armature. ‰Thus we see that an e.m.f. is generated in both generator and motor, therefore, there is a generator action in both motor and generator operation. ‰However, in generator operation the generated e.m.f. produces the armature current, where as, in motor operation the generated e.m.f. opposes the current direction. ‰We also observe that Electro-magnetic torque is developed in generator as well as motor i.e. there is a motor action in both generator and motor, operation. ‰However, in motor operation the Electro-magnetic torque developed causes the armature rotation, where as in a generator operation the Electro-magnetic torque produced opposes the rotation. 72 TYPES OF DC MOTORS ‰ All dc motors must receive their excitation from an external source; therefore, they are separately excited. Their field and the armature windings are connected, however, in one of the three different ways employed for self-excited dc generators, and so according the field arrangement there are three types of dc motors namely; i. Series wound ii. shunt wound and iii. compound wound. 73 i. Series wound motor ‰ A series motor is one in which the field winding is connected in series with the armature so that the whole current drawn by the motor passes through the field winding as well as armature. Connection diagram is shown in Figure 4.42. Ise IL + Ia Series Winding Eb VL I a = I se = I L Eb = VL − I a (Ra + Rse ) Pdrawn = VL ⋅ I L Important relationships Pdev = Eb ⋅ I a 74 ii. ‰ Shunt wound motor A shunt wound motor is one in which the field winding is connected in parallel with armature as illustrated in Figure 4.43. The current supplied to the motor is divided into two paths, one through the shunt field winding and second through the armature. Important relationships I sh = VL Rsh I L = I sh + I a Eb = VL − I a Ra Pdrawn = VL ⋅ I L Pdev = Eb ⋅ I a 75 iii. Compound wound motor ‰ A compound wound motor has both series and shunt windings which can be connected as short-shunt or long shunt with armature winding as illustrated in figure 4.44. Ish IL Ia i. Shunt Winding Eb + Ise Series Winding (a) short-shunt compound motor VL I se = I L Important relationships E +I R V −I R I sh = b a a = L se se Rsh Rsh I L = I sh + I a E b = V L − I a Ra − I L R se Pdrawn = VL ⋅ I L Pdev = Eb ⋅ I a 76 Ish IL Ia + Ise Important relationships Shunt Winding Eb Series Winding VL _ (b) long -shunt compound motor I a = I se E b + I a (Ra + R se ) V L I sh = = R sh R sh I L = I sh + I L E b = V L − I a (Ra + R se ) Pdrel = VL ⋅ IL Pdev = E b ⋅ I a 77 DIRECTION OF ROTATIONS ‰It is clear that, from principle operation of dc motor, if the armature current were reversed by reversing the armature terminal leads, but leaving the field polarity the same, torque would be developed in a counter-clock wise direction. ‰Likewise, if the field polarity were reversed leaving the armature current as shown torque would be developed in a counter-clockwise direction. ‰ However if both the armature current direction and field polarity were reversed torque would be developed in a clock-wise direction as before. ‰Hence the direction of rotation of a motor can be reversed by reversing the current through either the armature winding or the field coils. If the current through both is reversed, the motor will continue to rotate in the same direction as before. 78 SIGNIFICANCE OF BACK E.M.F. ‰ As explained earlier, when the motor armature continues to rotate due to motor action, the armature conductors cut the magnetic flux and therefore e.m.fs are induced in them. ‰ The direction of this induced e.m.f. known as back e.m.f. is such that is opposes the applied voltage. ‰ Since the back e.m.f. is induced due to the generator action , the magnitude of it is, therefore , given by the same expression as that for the generated e.m.f. in a generator Eb = φZN 60 × P volts , a (*) All symbols have their usual significance. ‰ The armature circuit is equivalent to a source of e.m.f. Eb in series with a resistance, Ra put across a dc supply mains of V volts. Fig.3 Equivalent circuit of a motor Armature 79 ‰ It is evident from fig.3 that the applied voltage V must be large enough to balance both the voltage drop in armature resistance and the back e.m.f. at all times i.e. V = Eb + I a Ra (**) Where V is the applied voltage across the armature, Eb is the induced e.m.f. in the armature by generator action; Ia is the armature current and Ra is the armature resistance. ‰ The expression (**) may be rewritten as I a = V − Eb Ra to give armature current in terms of applied voltage V, induced e.m.f. Eb and armature resistance ‰ As obvious from expressions (*) and (**) the induced e.m.f. in the armature of a motor, Eb depends among other factors upon the armature speed and armature current depends upon the back emf Eb for a constant applied voltage and armature resistance. ‰ If the armature speed is high, back e.m.f. Eb will be large and therefore armature current becomes small. ‰ If the speed to the armature is low, then back e.m.f. Eb will be less and armature current Ia will be more resulting in development of large torque. ‰ Thus it is evident that back e.m.f. Eb acts like a governor i.e. it makes a motor 80 selfregulating so that it draws as much current as just required. TORQUE EQUATION ‰ The back emf of dc motor is given by Eb = V − I a Ra Eb I a = VI a − I a2 Ra Multiplying both sides this equation by Ia, (i) In eq. (i) VIa = Total electrical power supplied to the Armature of the dc motor (armature input) & ‰ The difference between the armature input and the armature copper loss is equal to the mechanical power developed by the armature of the motor. ‰ Hence, Ia2Ra = power wasted in the armature (armature copper lass) Mechanical power developed = E b I a ( ii ) ‰ If Ta is the torque in Newton meter developed by the armature of the motor, running at N revolutions per minute, then Mechanical power Developed = 2 π N Ta watts 60 (iii) 81 ‰ Equating eqs. (ii) & (iii) 2πN Ta Eb I a = 60 Torque , Ta = ( 60 ) Eb I a . ..................................( iv ) 2π N ‰ However back emf Eb = pφ NZ ..................( v ) 60.a ‰ Substituting Equation (v) into equation (iv) PΦI a Z 60 PΦNZIa = 0.159. [ N .m ] . Torque, Ta = a 2π 60 a N ‰ For a particular dc motor; P, Z & a are fixed. Hence, Ta ∝ ΦI a ∴ The torque developed by the armature of dc motor is proportional to the product of armature current and the flux 82 per pole. For dc shunt motor ‰The flux per pole is practically constant , hence the torque developed is directly proportion to the armature current , i.e. Taα Ia For dc series motor ‰The flux per pole is directly proportional to Ia hence the torque developed is directly proportion to the square of the armature current , i.e. Ta α I 2 a 83 SPEED EQUATION ‰ The back e.m.f. for dc motor is given by Pφ N .Z Eb = volts 60 a Also Eb = V − I a Ra Pφ N .Z = V − I a Ra 60 a 60.a 1 N = ( V − I a Ra ). . PZ φ Combing the above two equations, or For a given particular motor; P,Z and a are fixed. Hence N =K ( V − I a Ra φ =K φ Eb ‰ Thus the speed of dc motor is directly proportional to the voltage applied to the armature or 84 the back e.m.f. & inversely proportional to the flux per pole. For dc shunt motor, ‰the flux per pole is approximately constant and hence the speed of dc shunt motor is directly proportional to the back e.m.f. i.e. N α Eb For dc series motor ‰the flux per pole is directly proportional to the armature current and hence the load on the motor. Thus the speed of dc series motor is inversely proportional to the flux per pole or the armature current i.e. 1 N∝ Φ ∴The speed of the motor increases with the fall in flux. 85 DC MOTOR CHARACTERISTICS ‰ The 3 Important characteristic curves of dc motors are: 1. TORQUE-ARMATURE CURRENT CHARACTERISTIC This characteristic curve gives relation between mechanical torque T and armature current Ia . This is known as electrical characteristic. 2. SPEED-ARMATURE CURRENT CHARACTERISTIC ‰ This characteristic curve gives relation between speed N and armature current Ia 3. SPEED-TORQUE CHARACTERISTIC: This characteristic curve gives relation between speed N and mechanical torque T. This is also known as mechanical characteristic . This curve can be derived from the above two curves. 86 CHARACTERISTICS OF DC SERIES MOTORS ‰ From expression of mechanical torque T it is obvious. TORQUE-ARMATURE CURRENTCHARACTERISTICS Tα Φ Ia ‰ Up to saturation point flux is proportional to field current and hence to the armature current because Ia=If . ‰ Therefore on light load T α Ia2 and hence curve drawn between T and Ia up to saturation point is a parabola . ‰ After saturation point flux φ is almost independent of excitation current and so T α Ia . ‰ Hence the characteristics, becomes a straight line ‰ From the torque-armature current curve it is evident that series motor develops large starting torque to accelerate the heavy masses. ‰ Hence series motors are used where large starting torque is required such as in hoists electric railways, trolleys and electric vehicle T I I 87 SPEED-CURRENT CHARACTERISTIC: CHARACTERISTIC From expression for speed, it is obvious that ‰ ‰ ‰ ‰ Nα V − IaR a Φ If V remains constant, speed is inversely proportional to flux per pole, so if a curve is drawn between reciprocal of flux and current I, the speed current characteristic is obtained which is a rectangular hyperbola in shape. N.B Since on no load the speed is dangerously high as obvious from speed-current characteristic curve, which will result in heavy centrifugal force which in turn, will damage the motor That is why, series motors are never started on no load. These motor are suitable for gear drive, because gear provides some load on account of frictional resistance of the gear teeth in case of sudden release of load. 88 SPEED-TORQUE CHARACTERISTIC ‰ The speed- torque characteristic can be drawn with help of above two characteristics, which shows that as the torque increases, speed decreases (↑ T → ↓ N ) ‰ Hence series motors are best suited for the services where the motor is directly coupled to load such as fans whose speed falls with the increase in torque. ‰ Hence , a series motor should never started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damage due to heavy centrifugal force so produced. ‰ It should be noted that series motor is a variable speed motor. 89 CHARACTERISTICS OF DC SHUNT MOTORS ‰ If applied voltage V is kept constant, the field current will remain constant hence flux will have maximum value on no load but will decrease slightly due to A.R. as the load increase but for more purpose the flux is considered to be constant neglecting the effect of AR. ‰ From expression of speed, Nα Eb or (V - I R ) SPEED-CURRENT CHARACTERISTIC: Φ a a ‰ Since flux is considered to be constant as mentioned above, so with the increase in load current the speed slightly falls due to increase in voltage drop in armature IaRa. ‰ Since IaRa at full-load is very small as compared to applied voltage so drop in speed from no load to full load is very small and for all practical purposes the shunt motor is taken as a constant speed motor. ‰ Shunt motors being constant speed lathes, milling machines, conveyors, fans and motors are best suited for driving of line shafts, machine for all purposes where constant speed is required. 90 ‰ From the expression for the torque of a dc motor i.e. TORQUE-CURRENT CHARACTERISTIC Tα Φ Ia ‰ Torque is directly proportional to the product of flux and armature current. ‰ Since in case of dc shunt motors the flux is constant therefore torque increase with the increase in load current following linear law i.e. torque-armature current characteristics is a straight line passing through origin 91 SPEED-TORQUE CHARACTERISTIC This characteristic curve can be drawn from the above two characteristics. 92 CHARACTERISTICS OF COMPOUND WOULD MOTORS ‰ As the load is increased, the flux due to series field winding increase and causes the torque greater than it would have with shunt field winding alone for a given machine and for given current. ‰ The increase in flux due to series field winding on account of increase in load cause the speed to fall more rapidly N T than it would have done in shunt motor. ‰ The cumulative compound motor develops a high torque with increase of load. It also has a definite speed of no load, so does not run away when the load is removed Cumulative compound wound motors: ‰ Since the flux decrease with the increase in load, so the speed remains nearly constant as the load is increased and in some cases the speed will increase even. ‰ The decrease in flux with the increase in load causes the torque to be less than that of a shunt motor. The characteristics are similar to those of a shunt motor. ‰ Since the shunt motor develops a good torque and almost constant speed, therefore differential compound motor is seldom used. . Differential Compound wound motor 93 Speed-Torque characteristics Compound-wound Motor Application: ‰Cumulative compound wound motors are used in driving machines which subject to sudden applications of heavy loads , such as occur in rolling mills, shears or punches. ‰This type of motor is used also where a large starting torque is regard but series motor cannot be used conveniently such as in cranes and elevator 94 ‰ STARTING OF DC MOTOR If dc motor is directly connected to a dc power supply, the starting current will be dangerously high. From fig. a. Ia = ‰ ‰ ‰ 1. 2. V − Eb Ra The back emf Eb = ( K aφωm ) at start is zero. Therefore I a start = V Ra Since Ra is small , the starting current is very large. The starting current can be limited to a safe value by the following methods. Insert an external resistance , Rae (fig.b), at start Use a low dc terminal voltage (V) at start. This , of course , requires a variable-voltage supply 95 ‰ With an external resistance in the armature circuit, the armature current as the motor speeds up is Ia = Vi − Eb Ra + Rae ‰ The back e.m.f. Eb increases as the speed increases. ‰ Therefore, the external resistance Rae can be gradually taken out as the motor speeds up without the current exceeding a certain limit. ‰ This is done using a starter, shown in fig.c. At start, the handle is moved to position 1. ‰ All the resistances , R1, R2, R3 and R4 appear in series with the armature and thereby limit the starting current. ‰ As the motor speeds up the handle is moved to positions 2,3,4, and finally 5. ‰ At position 5 all the resistances in the starter are taken out of the armature circuit. ‰ The handle will be held in position 5 by the electromagnet , which is excited by the field current If. 96 SPEED CONTROL OF DC MOTOR Factors controlling Motor Speed ‰ It has been shown earlier the speed of a motor is given by the relation N= V − I a Ra ZΦ V − I a Ra ⎛ A⎞ ⋅⎜ ⎟ = K Φ ⎝P⎠ r . p .s . Where Ra =armature circuit resistance ‰ It is obvious that the speed can be controlled by varying a) Flux/pole i.e. Flux control b) Resistance Ra of the armature circuit i.e. Rheostatic Control and c) Applied voltage V i.e. Voltage control 97 Speed control of shunt motors a) Variation of Flux or Flux control Method ‰ ‰ ‰ ‰ ‰ ‰ ‰ ‰ It is seen from above equation that N∝ φ 1 . By decreasing the flux, the speed can be increase and vice versa. Hence, the name flux or field control method. The flux of dc motor can be changed by changing Ish with help of a shunt field rheostat. Since Ish is relatively small, shunt field rheostat has to carry only a small current, which means I2R loss is small. So this method is, therefore, very efficient. In non-interpolar machines, the speed can be increased by this method in the ratio of 2:1. Any further weakening of flux φ adversely affects the commutation and hence puts a limit to the max speed obtainable by this method. In machines fitted with interpoles, a ratio of maximum to minimum speeds of 6:1 is fairly common. 98 ‰ ‰ ‰ ‰ ‰ b) Armature or Rheostatic Control Method This method is used when speeds below the no-load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or resistance (called controller resistance) in series with the armature circuit as shown in fig below. As controller resistance is increased, Potential difference across the armature is decreased, thereby decreasing the armature speed. For a load of constant torque, speed is approximately proportional to the Potential difference across the armature. From the speed/armature current characteristics, it is seen that the greater the resistance in the armature circuit, greater is the fall in speed. 99 Rag c) ‰ ‰ ‰ ‰ ‰ ‰ ‰ Armature-terminal voltage control Utilizes the face that the change in the armature terminal voltage of a shunt motor is accompanied in the steady state by a substantially equal change in the speed voltage (Eb) and, with constant motor flux, a consequent proportional change in motor speed. One common scheme, called the Ward Leonard System, required an individual motor-generator set to supply power to the armature voltage of the motor whose speed is to be controlled. Frequently the control of generator voltage is combined with motor-field control, as indicated by the rheostat in the field of motor M in fig below, in order to achieve the widest possible speed range. With such dual control, base speed can be defined as the normal-armature voltage full field speed of the motor. Speeds above base speed are obtained by motor field control; speeds below base speed are obtained by armature-voltage control. As discussed in connection with field-current control, the range above base speed is that of constant power drive. The range below base speed is that of a constant torque drive because, as in armature-resistance control, the flux and the allowable armature current remain approximately constant. Ram + 3Φ - supply AC motor Vt Eag Eam G M N Lo ad _ Ifg Rfeg + Rfem Ifm - - + M-G Set DC drive motor P T T P Nbase N100 V control If control SPEED CONTROL OF DC SERIES MOTORS 1. Flux Control Method ‰ a) ‰ ‰ ‰ Variation in the flux of a Series motor can be brought about in any one of the following ways: Field Divertors The series winding are shunted by a variable resistance knows as field divertor (fig.1) Any desired amount of current can be passed through the divertor by adjusting its resistance. Hence the flux can be decreased, consequently, the speed of the motor increased. Fig.1 101 b) • • • • Armature Divertor A divertor across the armature can be used for giving speeds lower then the normal speed. For a given constant load torque, if I­a is reduced due to armature divertor, then φ must increase (∴ Ta α φ Ia). This results an increase in current taken from the supply (which increases the flux) and a fall in speed (N ∝ 1/φ). The variations in speed can be controlled by varying the divertor resistance Fig.2 102 c) Tapped Field Control ‰ This method is often used in electric traction (Show in fig 3) ‰ The number of series field turns in the circuit can be changed at will as shown ‰ With full field, the motor runs at its minimum speed, which can be raised in steps by cutting out some of the series turns. Fig.3 103 Variable Resistance in series with Motor ‰ By increasing the resistance in series with armature, the voltage applied across the armature terminals can be decreased. ‰ With reduced voltage across the armature, the speed is reduced. However, it will be noted that since full motor current passes through this resistance, there is a considerable loss of power in it. Fig.4 Speed 2. 104