DC Machines Chapter 1: Theory Of Operation & Construction Chapter 2: DC Machines On Load Chapter 3: DC Generators Chapter 4: DC Motors 1 References S.J. Chapman, "Electric Machinery McGraw Hill, 4th edition, 2005. Fundamentals", 2 Chapter 1 Theory Of Operation & Construction 1. 2. 3. 4. 5. 6. Introduction Electromechanical Energy Conversion DC Machines Theory of Operation Construction Classification of DC Machines Armature Voltage and Developed Torque 3 Electric Machine Mechanical Input Electrical Output Generator Electrical Input Motor Mechanical Output Electromechanical Energy Conversion Electrical system v + _ i w Ideal Electric Machine Motor Energy Flow Generator T Mechanical system v i=T w 4 Principle of Operation If a conductor of length l moves at a linear speed v in a magnetic field the induced voltage in the conductor is Faraday’s law or flux cutting rule e=Blv e N v v B S B Right hand rule 5 e Principle of Operation For the current carrying conductor of length l the force known as Lorentz force produced is i F i N F F=Bli B S B Left hand rule 6 Induced e.m.f e e N v v B e=Blv S B Right hand rule Electromagnetic Force F i i N F B F=Bli S B Lift hand rule 7 DC Machines theory of operation e N e w S e N N w w S S and for multiturn machine 8 Generated Voltage (EMF) For counter-clockwise motion the rotor terminal under the N pole is always positive with respect to that under the S pole. The brush terminal B1 is always positive and B2 is negative so e12 is unidirectional 9 DC Machines DC currents are required for both its field winding ( stator ) and armature winding (Rotor) DC machine mainly used as DC motors DC motors is characterized by superior torque and a wide range of speed DC machine is more costly than comparable AC machine and their maintenance cost is higher. 10 Construction 4 Pole DC Machine 11 4 Pole DC Machine 12 2 Pole DC Machine Shaft Armature Commutator Stator pole Field coil 13 DC Machine 14 Construction Of DC Machine Armature The armature (rotor) is made up of cylindrical steel structure. However, it cannot be fabricated from solid steel, since eddy and hysteresis losses may reach prohibitive values and damage the armature windings insulation. Reduction of eddy current losses can be achieved by fabricating armature core of insulated laminated silicon steel. Sheets of silicon steel of 0.35 mm thick are punched to form the shape of armature slots and teeth, then insulated on both sides and assembled to form armature core. Hysteresis loss can be reduced by adjusting the amount of silicon in the steel such that the hysteresis loop area of the alloy is kept as low as possible. 15 The Armature Slots Cooling ducts for air circulation Teeth 16 Eddy Currents 17 Eddy Currents (cont.) Dividing the iron core into thin insulated laminations minimizes eddy current loss. 18 Hysteresis Loop 19 Hysteresis Loss The amount of heat released per cycle expressed in J/m3 is equal to the area in Tesla.A/m of the hysteresis loop. To reduce hysteresis losses, we select magnetic materials that have a narrow hysteresis loop, such as the grainoriented silicon steel used in the cores of alternating-current transformers. 20 Construction Of DC Machine (Cont.) The Commutator The commutator is made up from a number of hard drawn copper segments forming a cylinder rotating with the shaft. For small machines this is achieved by mounting the segments on a Bakelite ring fitted on the shaft. The segments are insulated from each other and fixed on the Bakelite ring using adhesives. For machines with high ratings, the commutator is assembled from commutator segments shaped as shown. The segments are fitted into two guides as shown in figure and insulated from each other and from the guides using mica paper. 21 The Commutator Commutator: is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush. It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments. Commutator 22 Construction Of DC Machine (Cont.) The Brushes Brushes are fitted to collect or conduct current from or into the armature. They are made of graphite with high hardness to ensure long life time during service. Brushes are usually fitted in brush box (holder) are pressed on commutator segments by virtue of spring. The brush holder is fixed in the machine frame and insulated from it. Details of brush holder are shown in figure. 1. Brush holder box 2. Brush 3. Pressure spring 4. live pigtail 23 The Brushes The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit. It is made of carbon and rest on the commutator. Commutator and Brushes 24 Construction Of DC Machine (Cont.) Field System The magnetic flux in a DC machine is established by electromagnets shaped in the form of salient poles attached to a cylindrical yoke. The pole itself is made from two main parts, shank and shoe. The shank may be of circular or rectangular cross section while fabricated from cast steel, wrought iron or cast iron. There is no need for laminating it since flux flow is unidirectional. As for the shoe, it keeps field windings in place and distributes flux lines over larger area and thus avoids saturation of flux lines in armature teeth. Pole shoe is essentially laminated to keep iron loss in it to a minimum. Iron loss in pole shoe is produced by tooth pulsations. Field windings are made by concentric number of turns of insulated copper wire wound on a former. The former is made of Bakelite and is shaped as shown in figure. 25 Construction Of DC Machine (Cont.) Armature Windings Armature windings are made of the shape of pre-wound coils as shown in figure. The armature is wound in a double layer fashion to avoid irregular shape of end connections. Moreover, coil pitch should be as near as possible to pole pitch to ensure voltage summation around the coil. On the other hand, to obtain a reasonable value of collected voltage, the coils are connected in series maintaining that their voltages are in the same sense. This is achieved in two alternative methods of windings; namely, Lap and Wave types of windings. 26 Construction Of DC Machine (Cont.) The turn, coil, and the winding are shown schematically as: End connection Conductors Turn Coil Winding A turn consists of two conductors connected to one end by an end connector. A coil is formed by connecting several turns in series. A winding is formed by connecting several coils in series. 27 Construction Of DC Machine (Cont.) Lap Windings In this type of windings, the successive coils of the armature overlap each other as shown in figure. Successive coils are connected in series with their ends connected to successive commutator segments. The pitch of the coil as viewed from commutator end is termed as the front pitch while that measured from other end is called the back pitch. These pitches should not be equal to ensure winding progression. If an armature is wound with C coils and having 2p number of poles, then the back pitch should be equal to the number of coils occupying one pole pitch, i.e. Back pitch y b= C/2p coils and hence, Front pitch yf = yb ± 1 coils Positive sign is used for winding retrogression while the negative sign is for winding progression. Commutator pitch is then given by : yc = 1 Number of Commutator Segments = C 28 Construction Of DC Machine (Cont.) Example Design a suitable armature winding for an armature with 12 slots each containing two coil sides. The winding is double layer and number of poles is 4. Solution C = 12 x 2 / 2 = 12 coils yb = C/2p = 12/4 = 3 coils yf = 3 + 1 = 4 for retrogressive winding or yf = 3 - 1 = 2 for progressive winding yf is taken equal to 2 to ensure less end connection length. 29 Construction Of DC Machine (Cont.) 30 Construction Of DC Machine (Cont.) 31 Construction Of DC Machine (Cont.) In a lap winding, the number of parallel paths (2a) is always equal to the number of poles and also to the number of brushes. Bottom coil sides Top coil sides Commutator N S 1 2 3 Brush Elements of Lap Winding 32 Construction Of DC Machine (Cont.) 33 Construction Of DC Machine (Cont.) Wave Windings In this type of windings, the coils connected in series are either progressing or retrogressing all the way. The figure show schematic representation for a partially wound armature with wave windings. The distance between successive coils sides occupying nearly the same location under similar poles is termed the resultant pitch “ y “ and is equal to; y = yb + yf where yb and yf are the back and front pitches of the winding. The resultant pitch “ y “ occupies double pole pitch, therefore for a machine with “ 2p “ poles; y.p=C±1 On the other hand, commutator pitch in terms of commutator segments will be then given by; yc = y 34 Construction Of DC Machine (Cont.) Example Design a suitable armature winding for an armature with 14 slots each containing two coil sides. The winding is double layer and number of poles is 6. Solution C = 14 x 2 / 2 = 14 coils y = C ± 1 / p = 14 ± 1 / 3 = 5 coils progressive only yb = 3 & yf = 2 or yb = 2 & yf = 3 35 Construction Of DC Machine (Cont.) 36 Construction Of DC Machine (Cont.) 37 Construction Of DC Machine (Cont.) In a wave windings, the number of parallel paths (2a) is always two. Top coil sides N 1 S N Bottom coil sides S 2 Brush Elements of Wave Winding 38 Construction Of DC Machine (Cont.) From the previous example, it is clear that: a) There is only two parallel circuits whatever the number of poles is. b) Two brushes arms can only be used for voltage collection ( A & B ). However, extra brush arms are used to limit the current in each brush arm ( C, D, E & F ). Therefore number of brush arms = number of poles as in the case of Lap windings. c) Again brush location is decided relative to position of poles and best location is such that brushes should be in contact with coil sides in the neutral zone. D) Number of parallel circuits in this type of windings whatever the number of poles is always equal to TWO. i.e. For wave windings 2a = 2 39 Classifications of DC Machines Field Armature Separately excited Field Self excited 1- Shunt Field Armature Armature Self excited 2- Series 40 Self Excited 3- Compound ff F2 F1 ff fs A1 D1 D2 F1 D1 F2 i- Short-shunt Cumulative F1 A2 ii-Long-shunt Cumulative ff fs A1 F2 D2 A2 A2 ff fs A1 D1 iii- Short-shunt Differential D2 F1 A1 F2 fs D1 D2 A2 iv-Long-shunt Differential 41 Armature Voltage Let Z = total number of armature conductors N = total number of turns in the armature winding = Z/2 2p = number of poles 2a = number of parallel paths = 2 for wave winding 2 p for lap winding f = flux per pole Weber Nm= speed of the motor in the revolutions per minute, Time of 1 revolution = 60 N seconds m wm= speed of the motor in radians per second 2 p Flux / Re v . 2 pN m / 60 Emf generated in each conductor = Time / Re v . (60 / N m ) Total emf between brushes =emf/conductor * Number of conductor/path (2 pNm / 60)(Z / 2a) But, wm 2Nm / 60 Where K a , therefore Z .2 p N .2 p 2 .2a .2a Ea (2 pwm / 2 )(Z / 2a) Kawm (armature constant) 42 Developed (or Electromagnetic) Torque Consider the turn shown in the following Figure. Area per pole A = 2rl 2p 2p Flux density B A 2 r l Current / conductor is Ic The force on a conductor is Ia 2a fc B l Ia 2a I .2 p. I a a r The torque developed by a conductor is Tc f c r B l 2a 2 .2a The total torque developed is Te Z .2 p.I a E I K a I a a a 2 .2a wm 43 Example 1 Determine the induced voltage induced in the armature of a dc machine running at 1750 rpm and having four poles. The flux per pole is 25 mWb, and the armature is lap-wound with 728 conductors. 44 Solution N m 1750rpm 2p 4 f 25mWb 2a 2 p 4 Z 728 Ea K afwm pZ fwm 2 a pZf nm 60a 728 25 103 1750 60 530.83V 45 Example 2 A lap-wound armature has 576 conductors and carries an armature current of 123.5A. If the flux per-pole is 20 mWb, calculate the electromagnetic torque. 46 Solution 2a 2 p f 20mWb Z 576 I a 123.5 A Te K afI a 2 p.Z fI a 2 .2a 576 20 10 3 123.5 2 226.43Nm 47 Magnetization Curve of a DC Machine The magnetizing curve is obtained experimentally by rotating the dcmachine at a given speed and measuring the open-circuit armature terminal voltage as the current in the field winding is changed. Ea Saturation E a K a wm wm1> wm2 Field Linear Ea wm2 If Flux-mmf relation in a dc machine If Nf If p Magnetization curve The magnetization curve is of great importance because it represents the saturation level in the magnetic system of the d.c. machine. 48 Chapter 2 DC Machines on Load 1. 2. 3. 4. Armature reaction L di/dt Voltages Commutation in DC Machines Methods Of Improving Commutation 49 Armature Reaction 50 Armature Reaction (cont.) 51 Armature Reaction (cont.) 52 Armature Reaction (cont.) 53 Armature Reaction (cont.) 54 Armature Reaction (cont.) 55 Armature Reaction (cont.) 56 Armature Reaction (cont.) 57 Armature Reaction (cont.) 58 Armature Reaction (cont.) 59 Armature Reaction (cont.) 60 Armature Reaction (cont.) 61 Armature Reaction (cont.) 62 Armature Reaction (cont.) 63 Armature Reaction (cont.) 64 L di/dt Voltages 65 L di/dt Voltages (cont.) 66 L di/dt Voltages (cont.) 67 Commutation in DC Machines 68 Commutation in DC Machines (cont.) 69 Methods Of Improving Commutation 70 Methods Of Improving Commutation (cont.) 71 Methods Of Improving Commutation (cont.) 72 Methods Of Improving Commutation (cont.) 73 Methods Of Improving Commutation (cont.) 74 Methods Of Improving Commutation (cont.) 75 Methods Of Improving Commutation (cont.) 76 Methods Of Improving Commutation (cont.) 77 Methods Of Improving Commutation (cont.) 78 Methods Of Improving Commutation (cont.) 79 Methods Of Improving Commutation (cont.) 80 Methods Of Improving Commutation (cont.) 81 Methods Of Improving Commutation (cont.) 82 Methods Of Improving Commutation (cont.) 83 Armature Reaction It is the effect of armature ampere-turns upon the value and the distribution of the magnetic flux in the air gap. Fluxes added each other Fluxes oppose each other N S If N Fluxes added each other Near one tip of a pole, the net flux density increases while it decreases near the other pole tip, as a result the zero flux density shifts from the brush-axis. If the increased flux density causes magnetic saturation, the flux per pole decreases. This magnetizing effect of armature current . . ++ + + + + + S Fluxes oppose each other AT Load At No-Load . ... . Saturation effect increases as the armature current increases. B (resultant) Bf+ Ba Bf Ba 84 Compensating Winding The armature mmf distorts the flux density distribution and also produce demagnetizing effect as a result the zero flux density shifts from the brush-axis, and this causes poor commutation leading to sparking. Much of the armature mmf can be neutralized by using a compensating winding, which is fitted in slots cut on the main pole faces MMF produced by compensating winding opposes the armature mmf. The compensating winding is connected in series with the armature winding so that its mmf is proportional to armature mmf. Compensating windings Field Armature 85 Commutation The purpose of the commutator and brushes is to reverse the current in the conductor when it goes from one pole to the next. x N x y .. . .. . . ++ + + + ++ + S Brush Reactance voltage: the coil undergoing commutation is in the interpolar region. When this coil moves in this region, a voltage, called reactance voltage, is induced in the coil undergoing commutation. vr = L(di/dt) This high voltage naturally causes sparking at the brushes of the machine. Coil undergoing commutation y S N Commutator segments N Brush N S +Icoil t -Icoil 86 Interpoles or Commutator Poles To improve the commutation, a small poles, called interpoles or commutator poles, is created. Its winding carries the armature current in such a direction that its flux opposes the armature reaction flux and produces an emf that equal and opposite to the reactance voltage vr. Interpole Interpole winding i Ia N . . . .. .. ++ + + + ++ Ia S Armature winding a 87 Chapter 3 DC Generators 1. 2. 3. 4. 5. Basic Principles Classification of DC Generators Voltage Build Up Generator Characteristics Efficiency 88 Principle of Operation If a conductor of length l moves at a linear speed v in a magnetic field the induced voltage in the conductor is Faraday’s law or flux cutting rule e=Blv e N v v B S B Right hand rule 89 e Classification Of DC Generators 1- Separately Excited DC Generator Ia IL V f ( R fw R fc ) I f R f I f + ra + wm Rfw Rfc If Vt Ea + Vf RL Ea Vt I a ra E a K a wm Vt I L RL Ia IL 90 External Characteristic Curve It is a curve between terminal voltage and load current at constant field current and constant rotor speed Vt RaIa Ea Terminal characteristic with no armature reaction % rated voltage 100 DVAR 80 60 Terminal characteristic with armature reaction 40 20 0 20 40 60 80 100 % rated current It External characteristic of a separately excited dc generator 91 2- Self-Excited DC Generators 1- Shunt generator (Voltage build-up) If Rfc Rfw wm IL + Ia ra + Vt Ea Ea E a K a w m Vt I L RL Ia IL I f P RL V f R f I f Vt E a Vt I a ra Operating point (emf due to residual flux( Field resistance line IfRf versus If Ea2 Ea1 Ear If 0 If1 If2 Voltage build-up 92 Shunt generator (Cont.) Critical field circuit resistance Rf3 Ea Rf2 Rf1 Conditions for a successful voltage buildup Rf4 Residual magnetism must be present in the magnetic system Field winding mmf should aid the residual magnetism. Field circuit resistance should be less than critical field circuit resistance. Vt4 0 If Effect of field resistance 93 External Characteristic Curve (without Armature Reaction effect) The external characteristics of the self excited shunt generator can be obtained from the magnetization curve and the field resistance line, as illustrated in the following Figure. 94 2- Series Generator IL Ia Vt Ea I a (ra Rs ) + ra + Rs Vt Ea RL IL Ia I f Ea K a sw m Ea Vt Magnetization Curve Vt External Characteristic Slope =(Ra+Rs) Ia(Ra+Rs) Ia=If =IL IL 95 3- Compound DC Generator If If IL + Rfc Rs + Vt Ea Rfw Short Shunt Vt Ea I a Ra I L Rs If Ea I a Ra R fw R fc Ea K a ( sh s )w m Ea Rfw Ea K a sh s wm IL Ia I f Cumulative + Ia Ra + Ia Ra Rfc IL Differential Rs Vt Long Shunt Vt Ea I a Ra Rs IL Ia I f Vt If R fw R fc Ea K a ( sh s )wm 96 External Characteristic Curve Vt Over compound Vt(rated) Flat compound Under compound Differential (Useful as a welding generator) Ia Ia(rated) Feff Fsh Fs FA N f I f (eff ) N f I f N s I fs FA I f (eff ) I f Ns F I fs A Nf Nf 97 Graphical relationships for shunt generator without armature reaction 98 Graphical relationships for shunt generator with armature reaction 99 Graphical relationships for a series generator 100 Graphical relationships for a cumulative generator 101 Example: A 172-kW, 430-V, 400-A, 1800-rpm compounded dc generator shown in figure. The magnetization curve is given at 1800-rpm. The generator has compensating windings which eliminate armature reaction. The machine is being driven at 1800-rpm, and Radj is currently set to 55-Ω. (a) What is the no-load terminal voltage? (b) If this is connected cumulatively compounded, what is its terminal voltage at full-load (400-A)? What will its voltage regulation be? (c) If this generator did not have compensating windings and instead had an armature reaction of 500 A . turns at full-load, what would its terminal voltage be at full-load (400-A)? What would its voltage regulation be? The compounded dc generator 102 Solution (a): RF + Radj = 75-Ω, From the magnetization curve the no-load voltage is VT = 445-V. (b): If the load current IL = 400-A, then IA ~ 400-A too, and IA(RA + RS) = 24-V. And the effect of the series field mmf in terms of equivalent shunt field amperes is : (c): With armature reaction present in this case, the equivalent shunt field ampere is given by: The load triangle thus has a horizontal length of 0.7-A and a vertical length of 24-V. From figure, the output terminal voltage will be: VT = 425-V and the voltage regulation will be: The load triangle thus has a horizontal length of 1.2-A and a vertical length of 24-V. From figure, the output terminal voltage will be: VT = 440-V and the voltage regulation will be: 103 Solution 104 Example: If the generator in the previous example has compensating windings and is connected differentially compounded, what will its voltage be when it is supplying 200-A? Solution In this circumstances , the equivalent shunt current due to the series windings and without armature reaction is: The resistive voltage drop is IA(RA + RS) = 200 x 0.06 = 12-V. The output voltage of the generator can be found The load triangle thus has a horizontal length of 0.6-A and a vertical length of 12-V. From figure, the output terminal voltage will be: VT = 390-V. 105 Power Flow and Efficiency DC Generators IL If + Ia Rfc Ra + Rs Vt Ea Rfw Pinput= Pmech = Pshaft Ea I a Rotaional losses Va I a I a2 Ra Va I L Vt I L I 2f R f I L2 Rs Poutput= Pelectrical Poutput Vt I L Vt I L I 2 R RotationalLosses Vt I L Ea I a RotationalLosses Pinput Poutput Poutput Losses 106 Chapter 4 DC Motors 1. 2. 3. 4. 5. 6. Basic Principles Classification of DC Motors Motor Characteristics Efficiency Speed Control Starting 107 Principle of Operation For the current carrying conductor of length l the force known as Lorentz force produced is i F i N F F=Bli B S B Left hand rule 108 D.C. Motor Characteristics Torque-Speed Characteristics 1- Separately excited & Shunt motors Ia (f is independent of the load torque ) Vt Ea I a ra Ea K a w m Vt I a ra wm Ka T Ka I a Therefore , Vt ra wm T 2 K a ( K a ) wm Vt Ka ra Slope ( K ) 2 a T 109 Torque-Speed Characteristics 2- Series motors Ea Vt I a ( Ra Rs ) Ea K af wm Neglecting saturation f K1 I f K1 I a Ea K a K1 I awm K s I awm wm Vt R Rs a Ks Ia Ks But T K af I a K a K1 I a2 K s I a2 w m Ra Rs Vt Ks Ks T 110 Torque-Speed Characteristics 2- Compound motors Cumulative Compound ATt ATshunt ATseries Differential Compound ft fshunt fseries Shunt motor Vt ra wm T 2 K aft ( K aft ) 111 Example 3 A 250V shunt motor has an armature resistance of 0.25W and a field resistance of 125W. At no-load the motor takes a line current of 5.0A while running at 1200 rpm. If the line current at full-load is 52.0A, what is the full-load speed? 112 Solution If At no-load: It 5 A It Ia Ra Rfc n 1200 rpm w m m 1200 2 Rfw 125 .66 rad / sec 60 + wm + Vt Vt 250 If 2 A , I a _ NL I t _ NL I f 5 2 3 A R f 125 Ea _ NL Vt I a _ NL Ra 250 3 0.25 249.25V K af Ea _ NL wm _ NL 249.25 1.984 V.sec/rad 125.66 113 At full-load: I L 52 A I a _ FL It _ FL I f 52 2 50 A Ea FL Vt I a FL Ra 250 50 0.25 237.5V Ea _ FL K afw m _ FL Ea _ FL 237.5 wm _ FL 119.71 rad/sec K af 1.984 wm 60 nm _ FL 1142.4 rpm 2 114 Power Flow and Efficiency DC Motors IL If + Ia Rfc Ra Rs + Vt Ea Rfw Pinput = Vt I L Pelectrical Va I L I L2 Rs Va I a I 2f R f Ea I a I a2 Ra Rotational losses Poutput= Pmech= Pshaft Poutput Pinput Pinput Losses Pinput Vt I L I 2 R RotationalLosses Vt I L E a I a RotationalLosses Vt I L 115 Example 4 The field and armature resistance of a 220 V series motor are 0.2Ω and 0.1Ω, respectively. The motor takes 30 A of current while running at 700 rpm. If the rotational losses are 350W, determine the motor efficiency. 116 Solution Ia IL + Pin Vt I t 220 30 6600W Ra Pf I 2f R f 30 2 0.2 180W + Pa I a2 Ra 30 2 0.1 90W Vt Ea Rotational losses 350W Pout Pin Rs I 2 R Rotational losses Pout 6600 (180 90) 350 5980W Pout 5980 0.9061 Pin 6600 117 SPEED CONTROL Since Vt Ea I a ra Ea K a w m Vt I a ra wm K af So the speed of the d.c .motor can be controlled by controlling Vt , ra , or 1- Armature Voltage Control In this method ra and If (i.e.f) are kept constant, and Vt is varied to change the speed. The motor must be separately excited to use armature voltage control. Armature voltage control can control the speed of the motor for speeds below rated speed but not for speed above rated speed. This method is expensive because it requires a variable d.c. supply for the armature circuit. 118 SPEED CONTROL (Cont.) 2- Field current control In this method Vt and ra remain fixed and the speed is controlled by varying If . This is normally achieved by using a field rheostat as shown in the following Figure for shunt d.c. motor. Field control can control the speed of the motor for speeds above base speed but not for speeds below base speed. This method is simple to implement and less expensive, because the control is at the low power level of the field circuit. 119 SPEED CONTROL (Cont.) 3- Armature resistance control In this method, the armature terminal voltage Vt and the field current If are kept constant at their rated values. The speed is controlled by changing the resistance in the armature circuit. Armature resistance control is simple to implement. However, this method is less efficient because of losses in the resistance. This method can be used in all types of d.c. motors. Armature resistance control can control the speed of the motor for speeds below base speed. Shunt Motor. Series Motor. 120 SPEED CONTROL TECHNIQUES 1- Ward-Leonard System (Classical method) The system uses a motor-generator (M-G) set to control the speed of the DC drive motor. The motor of the M-G set runs at constant speed. By varying the generator field current, the generator voltage changes, which in turn changes the speed of the DC drive motor. The system is operated in two control modes. Vt Control: the speed is changed from zero to the base speed by keeping If constant at rated value and changing the terminal voltage. The torque can be maintained constant during operation in this range of speed. If Control: The field current control is used to obtain speed above the base speed. In this mode, Vt remains constant and If is decreased to obtain higher speeds. The armature current can be kept constant, thereby operating the motor in a constant horsepower mode. 121 SPEED CONTROL TECHNIQUES 2- Solid-State Control In Recent years, solid-state control have been used as a replacement of rotating (M-G) set. Both armature control and field control can be achieved using controlled rectifier or choppers. Controlled Rectifiers If the supply is ac, controlled rectifiers can be used to convert it to a variable voltage dc supply by changing the firing angle of the rectifier thyristors. 122 SPEED CONTROL TECHNIQUES Chopper A chopper converts a fixed-voltage dc supply into a variable voltage dc supply. The switch S can be a thyristor , a GTO or a power transistor. When the switching device S is on , Vt = V (supply voltage) and motor current increases. When S is off Vt = 0 and motor current decays through the diode. t on V V The average Vt is t T 123 Starting If a d.c. motor is directly connected to a d.c. power supply, the starting current will be dangerously high. V Ea at starting w 0 Ea 0 Ia t ra Ia Starting Vt ra Since ra is small, the starting current is very large. The starting current can be limited by the following methods: 1- Use a variable-voltage supply. 2- Insert an external resistance at start, as shown in the Figure. 124