CHAPTER 2 Basic DC Motor School of Computer

CHAPTER 2 Basic DC Motor. School of Computer and Communication Engineering, Uni. MAP Prepared By: Amir Razif b. Jamil Abdullah EMT 113: V-2008 1

2. 0 Basic Direct Current (DC) Motor. 2. 1 Introduction to DC Machines. 2. 2 Construction of DC Machines. 2. 3 DC Motor. 2. 4 DC Generator. 2

2. 1 Introduction to DC Machine. þ þ DC motors are dc machine used as motors. DC generator are dc machines used as generators. DC machine is most often used for a motor. The major advantages of dc machines are the (i) easy speed and (ii) torque regulation. þ However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. þ In the past, automobiles were equipped with dc dynamos to charge their batteries. 3

Cont’d… þ The same physical machine can operate as a motor or generator depends on the direction of power flow in it. þ DC machine are generators that convert mechanical energy to dc electric energy. þ DC machine are motors that convert dc electric energy to mechanical energy. þ DC motors are used in cars, trucks, and aircrafts because they use dc power system. þ DC motors are often compared by their speed regulation (SR) which is defined as; 4

Cont’d… Speed Regulation (SR) þ SR is the measure of the shape of the motor’s torque-speed characteristic. (a) Positive SR means that the motor’s speed drops with increasing load. (b) Negative SR means that the motor’s speed increasing with increasing load. (c) The magnitude of the SR tells approximately how steep the slope of the torque-speed curve is. 5

2. 2 Construction of DC Machine. þ The physical structure of the machine consists of two parts; (i) Stator or stationary part, (ii) Rotor or rotating part. Figure 2. 1: Stator (left) and Rotor (right) 6

Cont’d… þ Frame is the stationary part which provide physical support. þ Pole Piece which is projected inward and provide the magnetic flux in the machine. þ Pole shoes distributes flux evenly over the rotor surface. þ The expose surface of the pole shoes is the pole face. þ The distance between the pole face and the rotor is called air gap. Figure 2. 2: General Arrangement of a DC Machine 7

Cont’d… þ There are two principal windings on the dc machine; (i) armature windings, (ii) field windings. þ The armature windings are defined as windings in which a voltage is induced. þ The field windings are defined as the windings that produce the main magnetic flux in the machine. þ The armature winding is located on the rotor, and the field windings are located on the stator. þ The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. þ In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. þ The commutating poles are supplied by dc current. þ Compensating windings are mounted on the main poles. þ These short-circuited windings damp rotor oscillations. þ The poles are mounted on an iron core that provides a closed 8 magnetic circuit.

Cont’d… þ The rotor has a ring-shaped laminated iron core with slots. þ Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. þ The coils are connected in series through the commutator segments. Commutator þ The ends of each coil are connected to a commutator segment. þ The commutator consists of insulated copper segments mounted on an insulated tube. þ Two brushes are pressed to the commutator to permit current flow. þ The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce 9 arcing.

2. 2. 1 Commutator and Brushes Construction. Commutator. þ The commutator in a dc machine is typically made of copper bars insulated by a mica-type material. Figure 2. 3: Details of the Commutator of a DC Motor. 10

Cont’d… Brushes. þ The brushes of the machine are made of carbon, graphite, metal graphite or a mixture of carbon and graphite. þ The brushes have a high conductivity to reduce electrical losses and low coefficient of friction to reduce excessive wear. Operation. þ The commutator switches the current from one rotor coil to the adjacent coil, þ The switching requires the interruption of the coil current. þ The sudden interruption of an inductive current generates high voltages. þ The high voltage produces flashover and arcing between the commutator segment and the brush. 11

Cont’d… þ The commutator switches the current from one rotor coil to the adjacent coil, þ The switching requires the interruption of the coil current. þ The sudden interruption of an inductive current generates high voltages. þ The high voltage produces flashover and arcing between the commutator segment and the brush. Figure 2. 4: Details of the Commutator of a DC Motor. 12

2. 2. 2 Rotor Construction. þ The rotor or armature construction of a dc machine consists of a shaft machined from a steel bar with a core built up over it. þ The core is composed of many lamination stamped from a steel plate. þ The commutator is built onto the shaft of the rotor at one end of the core. þ The armature coils are laid into the slots on the core, and their ends are connected to the commutator segments. Figure 2. 5: Rotor of a DC Motor. 13

2. 3 DC Motor. Figure 2. 6: Cutaway View of a DC Motor. 14

2. 3. 1 DC Motor Operation. þ In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field. þ The rotor is supplied by dc current through the brushes, commutator and coils. þ The interaction of the magnetic field and rotor current generates a force that drives the motor. 15

Cont’d… þ The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S). þ The poles generate a magnetic field that is perpendicular to the current carrying conductors. þ The interaction between the field and the current produces a Lorentz force, þ The force is perpendicular to both the magnetic field and conductor DC Motor Operation Figure 2. 7: (a) Rotor current flow from segment 1 to 2 (slot a to b) Figure 2. 7: (b) Rotor current flow 16 from segment 2 to 1 (slot b to a)

Cont’d… þ The generated force turns the rotor until the coil reaches the neutral point between the poles. þ At this point, the magnetic field becomes practically zero together with the force. þ However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses. þ To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation. DC Motor Operation Figure 2. 8: (a) Rotor current flow from segment 1 to 2 (slot a to b) Figure 2. 8: (b) Rotor current flow 17 from segment 2 to 1 (slot b to a)

Cont’d… þ Before reaching the neutral zone, the current enters in segment 1 and exits from segment 2. þ Therefore, current enters the coil end at slot a and exits from slot b during this stage. þ After passing the neutral zone, the current enters segment 2 and exits from segment 1. þ This reverses the current direction through the rotor coil, when the coil passes the neutral zone. þ The result of this current reversal is the maintenance of the rotation. DC Motor Operation Figure 2. 9: (a) Rotor Current Flow From Segment 1 to 2 (slot a to b) Figure 2. 9: (b) Rotor Current Flow 18 From Segment 2 to 1 (slot b to a)

2. 3. 2 DC Motor Equivalent (1) Circuit. þ In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field. þ There are four major types of dc motor in general used; (1) Separately Excited DC Motor. (2) Shunt DC Motor. (3) Series DC Motor. (4) Compounded DC Motor. (2) (3) (4) 19

2. 3. 3 Separately Excited and Shunt DC Motors. þ Figure 2. 10 and Figure 2. 11 show the equivalent circuit of separately excited dc motor and shunt motor. þ The armature circuit is represented by the EA and RA. þ The field coil is represented by the LF and RF. (1) Seperately Excited DC Motor Figure 2. 10: The Separately Excited DC Motor Equivalent Circuit. þ From the above figure, 20

Cont’d… þ (2) Shunt DC Motor Figure 2. 11: The Equivalent Circuit of a Shunt DC Motor. þ From the above figure, 21

2. 3. 3. 1 Speed Torque Characteristics. þ Figure 2. 11 is the equivalent circuit of DC shunt motor. From Kirchhoff’s voltage law, þ The internal generated voltage, EA. þ Solving for the above equation yields; ω, þ The loss of field excitation results in over speeding for a shunt motor. Thus, care should be taken to prevent the field circuit from getting open. þ The armature current may be expressed as follows: þ The speed–torque equation of a DC shunt motor. 22

Cont’d… þ This equation is just a straight line with a negative slope. The resulting torque–speed characteristic of a DC shunt motor is shown in Figure 2. 12: Torque-Speed Characteristic of a Shunt or Separately Excited DC Motor with Compensating Windings to Eliminate Armature Reaction þ In an actual machine, however, as the load increases, the flux is reduced because of armature reaction. Since the denominator terms decrease, there is less reduction in speed and speed regulation is improved somewhat. Figure 2. 13: The Torque-Speed Characteristic of the Shunt Motor with Armature Reaction. 23

2. 3. 3. 2 Speed Control of Shunt DC Motor. þ Changing the field resistance - Assume that the field resistor increases and observe the response. - If the field resistance increases, then the field current decreases (IF = VT/RF increaces ), and as the field current decreases, the flux Φ decreases with it. - A decrease in flux causes an instantaneous decrease in the internal generated voltage, EA = KΦ ω, which causes large increase in machine’s armature currents, since - The induced torque in a motor is given by. Since the flux Φ in shunt DC motor decreases while the current IA increases. The increase in current predominates over the decrease in flux, and the induced torque rises: 24

Cont’d… þ Since , the motor speed up. þ However, as the motor speeds up, the internal generated voltage EA rises, causing IA to fall. As IA falls, the induced torque τind falls too, and finally τind again equals τload at a higher steady state speed than originally. Summary (1) Increasing RF causes IF ( ) to decrease. (2) Deceasing IF decreases Φ. (3) Decreasing Φ lowers EA ( ). (4) Decreasing EA increases IA ( ). (5) Increasing IA increases τind ( ), with the change in IA dominant over the change in flux. (6) Increasing τind makes , and the speed ω increases. (7) Increasing to increases EA = K Φ ω again. (8) Increasing EA decreases IA. (9) Decreasing IA decreases τind until at a higher 25 speed ω.

Cont’d… Figure 2. 14: The effect of field resistance speed control on a shunt motor’s torque speed characteristic: over the motor’s normal operating range. þ As the flux in the machine decreases, the no load speed of the motor increases, while the slope of the torque speed curve becomes steeper. þ Naturally, decreasing RF would reverse the whole process, and the speed of the would drop. 26

2. 3. 3. 4 Changing the Armature Voltage. Figure 2. 15: Armature voltage control of a shunt (or separately excited) dc motor. Summary (1) An increase in VA increases IA [= (VA – EA)/ RA]. (2) Increasing IA increases (3) Increasing τind makes increasing ω. (4) Increasing ω increases EA (=K ω ) (5) Increasing EA decreases IA [ = (VA – EA )/ RA] (6) Decreasing IA decreases τind until at a higher ω. 27

Cont’d… Figure 2. 16: The effect of armature voltage speed control on a shunt motor’s torque speed characteristic þ The effect of an increase in VA on the torque speed characteristic of separately excited is shown in Figure 2. 16. þ The no load speed of the motor is shifted by this method of speed control, but the slope of the curve remains constant. 28

2. 3. 4 Series DC Motor. Figure 2. 17: The Equivalent Circuit of DC Motor. þ A series dc motor is a dc motor whose field windings consists of a relatively few turns connected in series with the armature circuits, Figure 2. 17. þ The generated voltage EA across the armature has a polarity opposite to the applied voltage, VT. 29

2. 3. 4. 1 Induced Torque in Series DC Motor. þ The induced or developed torque is given by, þ The flux in this motor is directly proportional to its armature current. Therefore, the flux in the motor can be given by þ where c is a constant of proportionality. The induced torque in this machine is thus given by þ This equation shows that a series motor give more torque per ampere than any other dc motor, therefore it is used in applications requiring very high torque, example starter motors in cars, elevator motors, and tractor motors in locomotives. 30

2. 3. 4. 2 Terminal Characteristic of a Series DC Motor. þ The terminal characteristic of a series dc motor is based on the analysis on the assumption of a linear magnetization curve, and the effects of saturation of the graph. þ The assumption of a linear magnetization curve implies that the flux in the motor given by; þ The derivation of a series motor’s torque-speed characteristic starts with Kirchhoff’s voltage law: þ From the equation; the armature current can be expressed as: 31

Cont’d… þ Also, EA = K , substituting these expression in previous equation yields; þ We know, þ Substituting from the above equations and the induced torque equation can written as þ Therefore, the flux in the series motor can be written as : 32

Cont’d… þ From the previous equations yields, þ The resulting torque–speed relationship is þ One disadvantage of series motor can be seen immediately from this equation. When the torque on this motor goes to zero, its speed goes to infinity. þ In practice, the torque can never go entirely to zero, because of the mechanical, core and stray losses that must be overcome. 33

Cont’d… þ However, if no other load is connected to the motor, it can turn fast enough to seriously damage itself. þ Never completely unload a series motor, and never connect one to a load by a belt or other mechanism that could break. Figure 2. 18: The Ideal Torque-Speed Characteristic of a Series dc Motor 34

2. 3. 4. 3 Speed Control of the DC Motor. þ Controlling speed. - Change the terminal voltage of the motor. - If the terminal voltage is increased, the speed also increased, resulting in a higher speed for any given torque. - This is only one efficient way to change the speed of a series motor. þ By the insertion of a series resistor into the motor circuit, but this technique is very wasteful of power and is used only for intermittent period during the start-up of some motor. 35

Cont’d… þ The rotor conductors cut the field lines that generate voltage in the coils. þ The motor speed and flux equations are : 36

2. 3. 5 Compound DC Motor. shunt series Figure 2. 19: The Equivalent Compound DC Motor (a) Long-Shunt Connection (b) Short-Shunt Connection þ A compound DC motor is a motor with both a shunt and a series field þ Two field windings: One is connected in series with armature (series field) and the other is connected in parallel with the armature (shunt field). þ The shunt field is always stronger than the series field in a cumulative compound motor the mmf of the two fields add in a differential compound motor the series field is connected so the 37 mmf opposes the mmf of the shunt field

Cont’d… þ The Kirchhoff’s voltage law equation for a compound dc motor is: þ The currents in the compounded motor are related by : þ The net magnetomotive force given by F net = F F ± FSE - FAR Where, FF = magnetmotive force (shunt field) FSE = magnetomotive force (series field) FAR = magnetomotive force (armature reaction) 38

Cont’d… þ The effective shunt field current in the compounded DC motor given by: NSE = winding turn per pole on series winding NF = winding turn per pole on shunt winding þ The positive (+) sign is for cumulatively compound motor þ The negative (-) sign is for differentially compound motor 39

2. 3. 5. 1 Torque Speed Characteristic. þ The cumulatively compounded motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose entire flux is proportional to armature current). þ It combines the best features of both the shunt and the series motors. þ Like a series motor, it has extra torque for starting; like a shunt motor, it does not over speed at no load. þ At light loads, the series field has a very small effect, so the motor behaves approximately as a shunt dc motor. þ As the load gets very large, the series flux becomes quite important and the torque speed curve begins to look like a series motor’s characteristic. 40

Cont’d… Figure 2. 20: The Torque-Speed Characteristic of a Cumulatively Compounded dc Motor Compared to Series and Shunt Motors with the Same Full-Load Rating. Figure 2. 21 The Torque-Speed Characteristic of a Cumulatively Compounded dc Motor Compared to a Shunt Motor with the Same No-Load Speed. 41

Cont’d… þ In a differently compounded DC motor, the shunt magnetomotive force and series magnetomotive force subtract from each other. þ This means that as the load on the motor increase, IA increases and the flux in the motor decreased. þ As the flux decrease, the speed of the motor increases. The increase in speed causes another increase in load and IA, Further decreasing the flux, and increasing the speed again. þ The techniques available for control of speed in a cumulatively compounded dc motor are the same as those available for a shunt motor: Change the field resistance, RF Change the armature voltage, VA Change the armature resistance, RA þ The arguments describing the effects of changing RF or VA are very similar to the arguments given earlier for the shunt motor. 42

2. 3. 5. 2 DC Motor Efficiency. þ Prior to calculating the efficiency of a dc motor, determine the following losses. þ Type of losses; (1) Copper losses (I 2 R losses). (2) Brush drop losses. (3) Mechanical losses. (4) Core losses. (5) Stray losses. Pconv = Pdev = EAIA= indω Pin VTIL Pout I 2 R losses Mechanical Core loss Stray losses 43

Cont’d… (1)Electrical or Copper Losses: Copper losses are the losses that occur in the armature and field windings of the machine. The copper losses for the armature and field winding are given by : Armature Loss; PA = IA 2 RA Field Loss; P F = I F 2 R F The resistance used in these calculations is usually the winding resistance at normal operating temperature (2) Brush Losses: The brush drop loss is the power loss across the contact potential at the brushes of the machines. It is given by the equation: PBD = VBDIA (3) Magnetic or Core Losses: These are the hysteresis and eddy current losses occur in the metal of the motor. 44

Cont’d… (4) Mechanical losses: These are friction and windage losses. - Friction losses include the losses caused by bearing friction and the friction between the brushes and the commutator. - Windage losses are caused by the friction between rotating parts and air inside the DC machine’s casing. (5) Stray load losses: These are other losses that cannot be placed in one of the previous categories. Motor efficiency : 45

2. 3. 5. 3 Speed Regulation. þ The speed regulation is a measure of the change speed from noload to full load. The percent speed regulation is defined as, Speed Regulation (SR): (1) + Ve SR means that the motor speed will decrease when the load on its shaft is increased. (2) - Ve SR means that the motor speed increases with increasing load. 46

2. 4 DC Generators. þ DC generators are dc machines used as generator. þ There are five major types of dc generators, classified according to the manner in which their field flux is produced: (1) Separately Excited Generator: In separately excited generator, the field flux is derived from a separately power source independent of the generator itself. (2) Shunt Generator: In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generators. (3) Series Generator: in a series generator, the filed flux is produced by connecting the filed circuit in series with the armature of the generator. (4) Cumulatively Compounded Generator: In a cumulatively compounded generator, both a shunt and series field is present, and their effects are additive. (5) Differentially Compounded Generator: Generator In differentially compounded generator: In a differentially compounded generator, both 47 a shunt and a series field are present, but their effects are subtractive.

Cont’d… Figure 2. 22(a): The Equivalent Circuit of a DC Generator Figure 2. 22(b): A Simplified Equivalent Circuit of a DC Generator, with RF Combining the Resistances of the Field Coils and the Variable Control Resistor 48

2. 4. 1 DC Generator Operation. þ In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field. 49

Cont’d… DC Generator Operation þ The N-S poles produce a dc magnetic field and the rotor coil turns in this field. þ A turbine or other machine drives the rotor. þ The conductors in the slots cut the magnetic flux lines, which induce voltage in the rotor coils. þ The coil has two sides; one is placed in slot a, the other in slot b. Figure 2. 23: (a) Rotor Current Flow From Segment 1 to 2 (slot a to b) Figure 2. 23: (b) Rotor Current 50 Flow From Segment 2 to 1 (slot b to a)

Cont’d… þ In Figure 2. 24(a), the conductors in slot a are cutting the field lines entering into the rotor from the north pole, þ The conductors in slot b are cutting the field lines exiting from the rotor to the south pole. þ The cutting of the field lines generates voltage in the conductors. þ The voltages generated in the two sides of the coil are added. DC Generator Operation Figure 2. 24: (a) Rotor current flow from segment 1 to 2 (slot a to b) Figure 2. 24: (b) Rotor current flow 51 from segment 2 to 1 (slot b to a)

Cont’d… þ The induced voltage is connected to the generator terminals through the commutator and brushes. þ In Figure 2. 25(a), the induced voltage in b is positive, and in a is negative. þ The positive terminal is connected to commutator segment 2 and to the conductors in slot b. þ The negative terminal is connected to segment 1 and to the conductors in slot a. DC Generator Operation 2. 25: (a) Rotor current flow from segment 1 to 2 (slot a to b) 2. 25: (b) Rotor current flow from segment 2 to 1 (slot b to a) 52

Cont’d… þ When the coil passes the neutral zone: þ Conductors in slot a are then moving toward the south pole and cut flux lines exiting from the rotor. þ Conductors in slot b cut the flux lines entering the in slot b. DC Generator Operation Figure 2. 26: (a) Rotor Current Flow From Segment 1 to 2 (slot a to b) þ This changes the polarity of the induced voltage in the coil. þ The voltage induced in a is now positive, and in b is negative. Figure 2. 26: (b) Rotor Current Flow 53 From Segment 2 to 1 (slot b to a)

Cont’d… þ The simultaneously the commutator reverses its terminals, which assures that the output voltage (Vdc) polarity is unchanged. þ In Figure 2. 27(b) the positive terminal is connected to commutator segment 1 and to the conductors in slot a. þ The negative terminal is connected to segment 2 and to the conductors in slot b. DC Generator Operation Figure 2. 27: (a) Rotor current flow from segment 1 to 2 (slot a to b) Figure 2. 27: (b) Rotor current flow from segment 2 to 1 (slot b to 54 a)

2. 4. 2 DC Generator Equivalent Circuit. þ Figure 2. 28 is the DC equivalent circuit of a DC motor. Where (1) EA and a resistor RA is the armature which is represented by an ideal voltage source. (2) The brush voltage drop is represented by a small battery Vbrush opposing the direction of the current flow in the machine. (3) The field coils, which produce the magnetic flux in the generator, are represented by inductor LF and RF. (4) The separate resistor Radj represents an external variable resistor used to control the amount of current in the filed circuit. Figure 2. 28: Equivalent Circuit of a DC Motor. 55

Cont’d… þ The brush drop voltage is often only a very tiny fraction of the generated voltage in the motor and can be neglected. The approximate value is included in the value of RA. þ The internal resistance of the filed coils is sometimes lumped together with the variable resistor, and the total is called RF , Figure 2. 29 below. þ Figure 2. 29 is the simplified equivalent circuit of Figure 2. 28. Figure 2. 29: A Simplified Equivalent Circuit eliminating the Brush Voltage Drop and Combining Radj with the Field Resistance. 56

Cont’d… þ The internal generated voltage in this motor is, þ EA is directly proportional to the flux in the motor and speed of the motor. þ Refer to Figure 2. 30, the field current in dc motor produces a field magnetomotive force given by F = NF IF. Figure 2. 30: The magnetization curve of a ferromagnetic material (Φ versus F ) þ This magnetomotive force produces a flux in the motor in accordance with its magnetization curve. 57

Cont’d… þ The field current is directly proportional to the magnetomotive force and since EA is directly proportional to the flux, it is customary to present the magnetization curve as a plot EA versus field current for a given speed ω0 , Figure 2. 31: The Magnetization Curve of a dc Machine Expresses as a Plot of EA Versus IF, for a fixed speed ω0 þ The induced torque developed by the motor is 58

2. 4. 3 Separately Exited DC Generators. Figure 2. 32: Separately Excited DC Generator þ A separately excited DC generator is a generator whose filed current is supplied by a separately external DC voltage source where, VT = Actual voltage measured at the terminals of the generator. IL = current flowing in the lines connected to the terminals. EA = Internal generated voltage. IA = Armature current. 59

2. 4. 3. 1 Terminal Characteristic of a Separately Excited DC Generator. Figure 2. 33: The Terminal Characteristic of a Separately Excited DC Generator (a) with and (b) Without Compensating Windings (EA = K ). þ For DC generator, the output quantities are its terminal voltage and line current. The terminal voltage is VT = EA – IARA þ Since the internal generated voltage EA is independent of IA, the terminal characteristic of the separately excited generator is a straight line. 60

2. 4. 3. 2 Control of Terminal Voltage. þ The terminal voltage of a separately excited DC generator can be controlled by changing the internal generated voltage EA of the machine. V T = E A – IA R A þ If EA increases, VT will increase, and if EA decreases, VT will decreases. Since the internal generated voltage, EA = KΦω, there are two possible ways to control the voltage of this generator; (1) Change the speed of rotation. If ω increases, then EA = KΦω increases, so VT = EA IARA increases too. (2) Change the field current. If RF is decreased, then the field current increases (IF =VF/RF ). Therefore, the flux Φ in the machine increases. As the flux rises, EA= K ω must rise too, so VT = EA – IARA increases. 62

2. 4. 4 Shunt DC Generator. þ A shunt DC generator is a DC generator that supplies its own field current by having its field connected directly across the terminals of the machine. Figure 2. 34 : The Equivalent Circuit of a Shunt DC Generator. 63

2. 4. 4. 1 Voltage Built up in a Shunt Generator. þ Assume the DC generator has no load connected to it and that the prime mover starts to turn the shaft of the generator. þ The voltage buildup in a DC generator depends on the presence of a residual flux in the poles of the generator. þ This voltage is given by; þ This voltage, EA is essentially applied to the field circuit, and it causes a current IF to flow in the field coils. þ This field current produces a magnetomotive force in the poles, which increases the flux in them. þ EA , then VT higher causes and increase IF , which further increasing the flux and so on. þ The final operating voltage is determined by intersection of the field resistance line and saturation curve. This voltage buildup 64 process is depicted in the next slide

Cont’d… Figure 2. 35: Voltage Built up on Starting in a Shunt DC Generator. 65

Cont’d… þ Several causes for the voltage to fail to build up during starting which are; (1) Residual magnetism. If there is no residual flux in the poles, there is no Internal generated voltage, EA = 0 V and the voltage will never build up. (2) Critical resistance. Normally, the shunt generator builds up to a voltage determined by the intersection of the field resistance line and the saturation curve. - If the field resistance is greater than critical resistance, the generator fails to build up and the voltage remains at the residual level. - To solve this problem, the field resistance is reduced to a value less than critical resistance. Refer Figure 9 -51 page 605 (Chapman) 66

Cont’d… þ The direction of rotation of the generator may have been reversed, or the connections of the field may have been reversed. In either case, the residual flux produces an internal generated voltage EA. The voltage EA produce a field current which produces a flux opposing the residual flux, instead of adding to it. þ Under these conditions, the flux actually decreases below res and no voltage can ever build up. 67

2. 4. 4. 2 Terminal Characteristics of DC Shunt Generator. Figure 2. 36: The Terminal Characteristic of a Shunt DC Generator þ As the load on the generator is increased, IL increases and so IA = IF + IL also increase. þ An increase in IA increases the armature resistance voltage drop IARA, causing VT = EA -IA RA to decrease. þ However, when VT decreases, the field current IF in the machine decreases with it. This causes the flux in the machine to decrease; decreasing EA. Decreasing EA causes a further decrease in the 68 terminal voltage, VT = EA - IARA

2. 4. 4. 3 Voltage Control for Shunt DC Generator. þ There are two ways to control the voltage of a shunt generator: (1) Change the shaft speed, ωm of the generator. (2) Change the field resistor of the generator, thus changing the field current. þ Changing the field resistor is the principal method used to control terminal voltage in real shunt generators. If the field resistor RF is decreased, then the field current IF = VT/RF increases. þ When IF , the machine’s flux , causing the internal generated voltage EA. EA causes the terminal voltage of the generator to increase as well. 69

2. 4. 4. 4 The Series DC Generator. Figure 2. 37 : The Equivalent Circuit of a Series DC Generator þ A series DC generator is a generator whose field is connected in series with its armature. Because the field winding has to carry the rated load current, it usually have few turns of heavy wire. þ The shunt generator tends to maintain a constant terminal voltage. While the series generator has tendency to supply a constant load current. þ The Kirchhoff’s voltage law for this equation; 70

2. 4. 4. 5 Terminal Characteristics of a Series Generator. Figure 2. 38 : A Series Generator Terminal Characteristic with Large Armature Reaction Effects þ The magnetization curve of a series DC generator looks very much like the magnetization curve of any other generator. þ At no load, however, there is no field current, so VT is reduced to a very small level given by the residual flux in the machine. þ As the load increases, the field current rises, so EA rises rapidly. The IA (RA + RS) drop goes up too, but at the first the increase in EA goes up more rapidly than the IA(RA + RS) drop rises, so VT increases. þ After a while, the machine approaches saturation, and EA becomes almost constant. At that point, the resistive drop is the predominant 71 effect, and V starts to fall.

2. 4. 5 Cumulatively Compound DC Generator. Figure 2. 39 : The Equivalent Circuit of a Cumulatively Compounded DC Generator with a Long Shunt Connection þ A cumulatively compounded DC generator is a DC generator with both series and shunt fields, fields connected so that the magnetomotive forces from the two fields are additive. 72

Cont’d… þ The total magnetomotive force on this machine is given by Fnet = FF + FSE - FAR where FF = the shunt field magnetomotive force FSE = the series field magnetomotive force FAR = the armature reaction magnetomotive force þ NFI*F = NFIF + NSEIA - FAR 73

Cont’d… þ The other voltage and current relationships for this generator are; 74

The Cumulatively Compounded DC Cont’d… Generator þ Another way to hook up a cumulatively compounded generator. It is the “short-shunt” connection, where series field is outside the shunt field circuit and has current IL flowing through it instead of IA. Figure 2. 40 : The Equivalent Circuit of a Cumulatively DC Generator with a Short Shunt Connection 75

Cont’d… The Terminal Characteristic of a Cumulatively DC Generator þ When the load on the generator is increased, the load current IL also increases. þ Since IA = IF + IL , the armature current IA increases too. At this point two effects occur in the generator: (1) As IA increases, the IA (RA + RS) voltage drop increases as well. This tends to cause a decrease in the terminal voltage, VT = EA –IA (RA + RS). (2) As IA increases, the series field magnetomotive force FSE = NSEIA increases too. This increases the total magnetomotive force Ftot = NFIF + NSEIA which increases the flux in the generator. The increased flux in the generator increases EA, which in turn tends to make VT = EA – IA (RA + RS) rise. 76

Cont’d… Voltage Control of Cumulatively Compounded DC Generator þ The techniques available for controlling the terminal voltage of a cumulatively compounded DC generator are exactly the same as the technique for controlling the voltage of a shunt DC generator; (1) Change the speed of rotation. An increase in causes EA = K to increase, increasing the terminal voltage VT = EA – IA (RA + RS). (2) Change the field current. A decrease in RF causes IF = VT/RF to increase, which increase the total magnetomotive force in the generator. As Ftot increases, the flux in the machine increases, and EA = K increases. Finally, an increase in EA raises VT. 77

Cont’d… Analysis of Cumulatively Compounded DC Generators þ The equivalent shunt field current Ieq due to the effects of the series field and armature reaction is given by þ The total effective shunt field current is 78

Cont’d… Field Resistance IA (RA + RS) VT at no load condition will be the point at which the resistor line and magnetization curve intersect. As load is added to the field current Ieq and the resistive voltage drop [IA(RA + RF)]. The upper tip triangle represents the internal generated voltage EA. The lower line represents the terminal voltage VT 79

Cont’d… The Differentially Compounded DC Generator Figure 2. 41: The equivalent circuit of a differentially compounded DC generator A differentially compounded DC generator is a generator with both shunt and series fields, but this time their magnetomotive forces subtract from each other. 80

Cont’d… The net magnetomotive force is Fnet = FF – FSE – FAR Fnet = NFIF – NSEIA - FAR And the equivalent shunt field current due to the series field and armature reaction is given by : The total effective shunt field current in this machine is or 81

Cont’d… Voltage Control of Differentially Compounded DC Generator þ Two effects occur in the terminal characteristic of a differentially compounded DC generator are (1) As IA increases, the IA (RA + RS) voltage drop increases as well. This increase tends to cause the terminal voltage to decrease VT. (2) As IA increases, the series field magnetomotive FSE = NSEIA increases too. This increases in series field magnetomotive force reduces the net magnetomotive force on the generator, (Ftot = NFIF – NSEIA), which in turn reduces the net flux in the generator. A decrease in flux decreases EA, which in turn decreases VT. þ Since both effects tend to decrease VT, the voltage drop drastically as the load is increased on the generator as shown in next slide 82

Cont’d… þ The techniques available for adjusting terminal voltage are exactly the same as those for shunt and cumulatively compounded DC generator: (1) Change the speed of rotation, m. (2) Change the field current, IF. 83