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Transcript
EMT 113/4 ELECTRICAL ENGINEERING TECHNOLOGY Chapter 2 : DC Machines Chap 2: DC Machines 1 Contents Introduction DC Machines Construction DC motors : Principles of Operation, Equivalent circuit & Characteristics DC generators : Principles of Operation, Equivalent circuit & Characteristics Review Chap 2: DC Machines 2 Introduction: What is DC Machines? Are DC generators that convert mechanical energy to DC electric energy. Are DC motors that convert DC electric energy to mechanical energy. Chapman S.J., “Electric Machinery Fundamentals” Chap 2: DC Machines 3 Introduction DC machine can be used as a motor or as a generator. DC Machine is most often used for a motor. Cutaway view of a dc motor DC motors are found in many special industrial environments Motors drive many types of loads from fans and pumps to presses and conveyors The major advantages of dc machines are the easy speed and torque regulation. However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. In the past, automobiles were equipped with dc dynamos to charge their batteries. Chap 2: DC Machines 4 Types of DC motors DC motors are classified according to electrical connections of armature windings and field windings. Five major types of DC motors:• • • • • Separately excited DC motor Shunt DC motor Permanent Magnet DC motor Series DC motor Compounded DC motor Chap 2: DC Machines 5 DC Machines Construction DC motor stator with poles visible Rotor of a dc motor Chap 2: DC Machines 6 DC Machines Construction . DC machines, like other electromechanical energy conversion devices have two sets of electrical windings – field windings - on stator – amarture windings on the rotor. Chap 2: DC Machines 7 DC Machines Construction • The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. • In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current. • Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations. Chap 2: DC Machines 8 DC Machines Construction • The poles are mounted on an iron core that provides a closed magnetic circuit. • The motor housing supports the iron core, the brushes and the bearings. • The rotor has a ring-shaped laminated iron core with slots. • Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. Chap 2: DC Machines 9 DC Machines Construction • The coils are connected in series through the commutator segments. • The ends of each coil are connected to a commutator segment. • The commutator consists of insulated copper segments mounted on an insulated tube. • Two brushes are pressed to the commutator to permit current flow. • The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing. Chap 2: DC Machines 10 DC Machines Construction • The commutator switches the current from one rotor coil to the adjacent coil, • The switching requires the interruption of the coil current. • The sudden interruption of an inductive current generates high voltages . • The high voltage produces flashover and arcing between the commutator segment and the brush. Chap 2: DC Machines 11 Review of magnetism Lines of flux define the magnetic field and are in the form of concentric circles around the wire. The magnetic lines around a current carrying conductor leave from the N-pole and re-enter at the S-pole. "Left Hand Rule" states that if you point the thumb of your left hand in the direction of the current, your fingers will point in the direction of the magnetic field. The flow of electrical current in a conductor sets up concentric lines of magnetic flux around the conductor. Chap 2: DC Machines 12 Review of magnetism The poles of an electro-magnetic coil change when the direction of current flow changes. Chap 2: DC Machines 13 Review of magnetism • The motor has a definite relationship between the direction of the magnetic flux, the direction of motion of the conductor or force, and the direction of the applied voltage or current. • Fleming's left hand rule can be used. – The thumb will indicate the direction of motion – The forefinger will indicate the direction of the magnetic field – The middle finger will indicate the direction of current. • In either the motor or generator, if the directions of any two factors are known, the third can be easily determined. Chap 2: DC Machines 14 DC motor Operation Chap 2: DC Machines 15 Current in DC motor Chap 2: DC Machines 16 Magnetic field in DC motor Chap 2: DC Machines 17 Force in DC motor Chap 2: DC Machines 18 Basic principle of operation The generated voltage of a DC machines having (p) poles and (Z) conductors on the armature with (a) parallel path between brushes as below : pZ EA K 2a where K = pZ /(2πa) = machine constant The mechanical torque which also equal to electromagnetic torque, is found as follows: e m EAI A KI A In the case of a generator, m is the input mechanical torque, which is converted to electrical power. For the motor, e is developed electromagnetic torque, which used to drive the mechanical load. Chap 2: DC Machines 19 Basic Principles of Operation ARMATURE winding are defined as the winding which a voltage is induced. FIELD windings are defined as the windings that produce the main flux in the machines. The magnetic field of the field winding is approximately sinusoidal, thus AC voltage is induced in the armature winding as the rotor turns under the magnetic field of stator. The COMMUTATOR and BRUSH combination converts the AC generated voltages to DC. Chap 2: DC Machines 20 Basic Principles of Operation The induced or generated DC voltage (EA) appearing between the brushes is a function of the field current (IF) and the speed of rotation () of the machine. This generated voltage is : EA K ' I F Where K’ = voltage constant = rotation per min If the losses of the DC machine are neglected, the electrical power is equal to the mechanical power E A I A m Chap 2: DC Machines 21 Generation of Unidirectional Voltage As the rotor is rotated at an angular velocity (), the armature flux linkage () change and a voltage eaa’ is induced between terminal a and a’. The expression for the voltage induced is given by Faraday’s Law eaa ' d dt a) Flux linkage of coil aa’; b) induced voltage; c) rectified voltage Chap 2: DC Machines Two pole DC generator 22 Generation of Unidirectional Voltage The internal generated voltage in the DC machines defined as: EA K Where EA = armature voltage K = motor constant = fluks = rotation per min Chap 2: DC Machines 23 DC Motor Equivalent Circuit The brush voltage drop RA External variable resistor used to control the amount of current in the field circuit Armature circuit (entire rotor structure) Field Coils Note: Because a dc motor is the same physical machine as a dc generator, its equivalent circuit is exactly the same as generator except for the direction of current flow. Chap 2: DC Machines 24 Simplified Equivalent Circuit The brush drop voltage (Vbrush ) is often only a very tiny fraction of the generated voltage in the machine – Neglected or included in RA. Internal resistance of the field coils is sometimes lumped together with the variable resistor and called RF - Combining Radj with field resistance (RF). Chap 2: DC Machines 25 The Magnetization Curve of a DC machine The internal generated voltage in the motor E A K From the equation, EA is directly proportional to the flux () in the motor and speed of the motor (). The field current (IF) in dc machines produces a field magnetomotive force (mmf) This magnetomotive force (mmf) produces a flux () in the motor in accordance with its magnetization curve. IF mmf flux The magnetization curve of a ferromagnetic material ( vs F) Chap 2: DC Machines 26 The Magnetization Curve of a DC machine Since the field current (IF) is directly proportional to magnetomotive force (mmf) and……. EA EA is directly proportional to the flux, the magnetization curve is presented as a plot EA versus field current for a a given speed. The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0 Note: To get the maximum possible power, the motors and generators are designed to operate near the saturation point on the magnetization curve (at the knee of the curve). Chap 2: DC Machines 27 The Magnetization Curve The induced torque developed by the motor is given as EA ind KI A The magnetization curve of a dc machine expresses as a plot of EA versus IF, for a fixed speed ω0 Chap 2: DC Machines 28 The equivalent circuit of separately excited dc motor Separately excited motor is a motor whose field current is supplied from a separate constant-voltage power supply. IF VF RF IL IA VT E A I A RA Chap 2: DC Machines 29 The equivalent circuit of a shunt dc motor VT RF VT E A I A RA IF A shunt dc motor is a motor whose field circuit get its power directly across the armature terminals of the motor. IL I A IF Chap 2: DC Machines 30 Speed-Torque Characteristics Consider the DC shunt motor. From the Kirchoff’s Law VT E A I A RA Induced Voltage VT K I A RA EA K Substituting the expression for induced voltage between VT and EA. VT K I A RA Since, then current IA can be expressed as IA ind K VT K ind K RA Finally, solving for the motor's speed yield VT RA 2 ind K ( K) Chap 2: DC Machines 31 Torque-Speed Characteristic This equation is a straight line with a negative slope. The graph shows the torque-speed characteristics of a shunt dc motor. VT RA 2 ind K ( K) ind then , with constant VT, otherwise it affect the torque-speed curve Torque-speed characteristic of a shunt or separately excited dc motor Chap 2: DC Machines 32 Torque-Speed Characteristic Affect of Armature Reaction (AR) will reduce flux as the load increase (ind also increase), so it will increase motor speed (). If the motor has compensating winding, the flux () will be constant. VT RA 2 ind K ( K) Torque-speed characteristic of a motor with armature reaction present. Chap 2: DC Machines 33 Torque-Speed Characteristic In order for the motor speed to vary linearly with torque, the other term in this expression must be constant as the load changes. The terminal supplied by the dc power source is assumed to be constant – if not, then the voltage variations will effect the shape of the torque-speed curve. However, in actual machine, as the load increase, the flux is reduced because of the armature reaction. Since the denominator terms decrease, there is less reduction in speed and speed regulation is improved (as shown in previous slide). If a motor has compensating windings, of course there will be no fluxweakening problem in the machines, and the flux in the machine will be constant Chap 2: DC Machines 34 Speed Control of Shunt DC Motor Two common ways in which the speed () of a shunt dc machine can be controlled. • Adjusting the field resistance RF (and thus the field flux) • Adjusting the terminal voltage applied to the armature. The less common method of speed control is by • Inserting a resistor in series with armature circuit. Chap 2: DC Machines 35 1 : Changing The Field Resistance VT to decrease. 1. Increasing RF causes IF RF 2. Deceasing IF decreases . 3. Decreasing lowers EA K VT E A RA 4. Decreasing EA increases IA 5. Increasing IA increases ind ( K I A ) with the change in IA dominant over the change in flux (). 6. Increasing τind makes ind load and the speed ω increases. Chap 2: DC Machines 36 1: Changing The Field Resistance 7. Increasing speed to increases EA = K again. 8. Increasing EA decreases IA. 9. Decreasing IA decreases ind until ind load at a higher speed ω Decreasing RF would reverse the whole process, and the speed of the motor would drop. The effect of field resistance speed control on a shunt motor’s torque speed characteristic: over the motor’s normal operating range Chap 2: DC Machines 37 2: Changing The Armature Voltage Armature voltage control of a shunt (or separately excited) dc motor. 1. An increase in VA increases IA [= (VA – EA)/RA] 2. Increasing IA increases 3. Increasing τind makes ind ( KI A ) ind load increasing ω. 4. Increasing ω increases EA (=Kω ) 5. Increasing EA decreases IA [ = (VA – EA)/RA] 6. Decreasing IA decreases τind until ind load Chap 2: DC Machines at a higher ω. 38 2: Changing The Armature Voltage The speed control is shiftted by this method, but the slope of the curve remains constant The effect of armature voltage speed control on a shunt motor’s torque speed characteristic Chap 2: DC Machines 39 3 : Inserting Resistor in Series with Armature Add resistor in series with R Circuit A Equivalent circuit of DC shunt motor The effect of armature resistance speed control on a shunt motor’s torque – speed characteristic Additional resistor in series will drastically increase the slope of the motor’s characteristic, making it operate more slowly if loaded Chap 2: DC Machines 40 3 : Inserting Resistor in Series with Armature Add resistor in series with R Circuit A VT RA 2 ind K ( K) The above equation shows if RA increase, speed will decrease Equivalent circuit of DC shunt motor This method is very wasteful method of speed control, since the losses in the inserted resistor is very large. For this it is rarely used. Chap 2: DC Machines 41 The Series DC Motor Equivalent circuit of a series DC motor. The Kirchhoff’s voltage law equation for this motor VT E A I A ( RA RS ) Chap 2: DC Machines 42 Induced Torque in a Series DC Motor The induced or developed torque is given by ind KI A The flux in this motor is directly proportional to its armature current. Therefore, the flux in the motor can be given by cI A where c is a constant of proportionality. The induced torque in this machine is thus given by ind KI A KcI A 2 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. Chap 2: DC Machines 43 The Terminal Characteristic of a Series DC Motor. To determine the terminal characteristic of a series dc motor, an analysis will be based on the assumption of a linear magnetization curve, and the effects of saturation will be considered in a graphical analysis The assumption of a linear magnetization curve implies that the flux in the motor given by : cI A The derivation of a series motor’s torque-speed characteristic starts with Kirchhoff’s voltage law: VT E A I A ( RA RS ) From the equation; expressed as: ind KI A KcI A2 the armature current can be IA ind Kc Chap 2: DC Machines 44 The Terminal Characteristic of a Series DC Motor. Also, EA = K, substituting these expression yields: ind VT K We know I A c IA Kc ( RA RS ) ; c W Substituting the equations so the induced torque equation can written as e K 2 c ind k n o Therefore, the fluxw in the series motor can be written as : ; c ind K Chap 2: DC Machines 45 The Terminal Characteristic of a Series DC Motor. Substituting the previous equation for VT yields: ind c VT K ind ( RA RS ) K Kc The resulting torque – speed relationship is VT 1 Kc ind R A RS Kc 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. In practice, the torque can never go entirely to zero, because of the mechanical, core and stray losses that must be overcome. Chap 2: DC Machines 46 The Terminal Characteristic of a Series DC Motor. However, if no other load is connected to the motor, it can turn fast enough to seriously damage itself. NEVER completely unload a series motor, and never connect one to a load by a belt or other mechanism that could break. Fig : The ideal torque- speed characteristic of a series dc motor Chap 2: DC Machines 47 Speed Control of Series DC Motor. Method of controlling the speed in series motor. 1. 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. VT 1 Kc ind R A RS Kc 2. 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. Chap 2: DC Machines 48 The Compounded DC Motor. series shunt shunt series The equivalent compound DC motor (a) Long-shunt connection (cumulative compounding) (b) Short-shunt connection (differential compounding) A compound DC motor is a motor with both a shunt and a series field Two field windings : One is connected in series with armature (series field) and the other is connected in parallel with the armature (shunt field). Chap 2: DC Machines 49 The Compounded DC Motor. series shunt shunt series The equivalent compound DC motor (a) Long-shunt connection (b) Shortshunt connection If the magnetic fluxes produced by both series field and shunt field windings are in same direction, that is, additive, the dc motor is cumulative compound. If the magnetic fluxes are in opposite, the dc motor is differential compound. Chap 2: DC Machines 50 The Compounded DC Motor. series shunt shunt series The equivalent compound DC motor (a) Long-shunt connection (b) Shortshunt connection In long shunt compound dc motor, the series field is connected in series with armature and the combination is in parallel with the shunt field. In the short shunt field compound dc motor, the shunt field is in parallel with armature and the combination is connected in series with the series field. Chap 2: DC Machines 51 The Compounded DC Motor. The Kirchhoff’s voltage law equation for a compound dc motor is: VT E A I A ( R A RS ) The currents in the compounded motor are related by : IA IL IF IF VT RF The net magnetomotive force given by F net = F F ± FSE - FAR FF = magnetmotive force (shunt field) FSE = magnetomotive force (series field) FAR = magnetomotive force (armature reaction) Chap 2: DC Machines 52 The Compounded DC Motor. The effective shunt field current in the compounded DC motor given by: N SE FAR I IF IA NF NF * F NSE = winding turn per pole on series winding NF = winding turn per pole on shunt winding The positive (+) sign is for cumulatively compound motor The negative (-) sign is for differentially compound motor Chap 2: DC Machines 53 The Torque Speed Characteristic of a Cumulatively Compounded DC Motor 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). It combines the best features of both the shunt and the series motors. Like a series motor, it has extra torque for starting; like a shunt motor, it does not over speed at no load. At light loads, the series field has a very small effect, so the motor behaves approximately as a shunt dc motor. 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. A comparison of these torque speed characteristics of each types is shown in next slide. Chap 2: DC Machines 54 The Torque Speed Characteristic of a Cumulatively Compounded DC Motor Fig (a) The torque-speed characteristic of a cumulatively compounded dc motor compared to series and shunt motors with the same full-load rating. Fig. (b) The torque-speed characteristic of a cumulatively compounded dc motor compared to a shunt motor with the same no-load speed. Chap 2: DC Machines 55 The Torque Speed Characteristic of a Differently Compounded DC Motor In a differentially compounded DC motor, the shunt magnetomotive force and series magnetomotive force subtract from each other. This means that as the load on the motor increase, IA increase and the flux in the motor decreased, (IA) As the flux decrease, the speed of the motor increase, () This speed increase causes an-other increase in load, which further increase IA, Further decreasing the flux, and increasing the speed again. All the phenomena resulting the differentially compounded motor is unstable and tends to run away. This instability is much worse than that of a shunt motor with armature reaction, and make it unsuitable for any application. Chap 2: DC Machines 56 Speed Control in the Cumulatively Compounded DC Motor The techniques available for control of speed in a cumulatively compounded dc motor are the same as those available for a shunt motor: 1. Change the field resistance, RF 2. Change the armature voltage, VA 3. Change the armature resistance, RA The arguments describing the effects of changing RF or VA are very similar to the arguments given earlier for the shunt motor. Chap 2: DC Machines 57 DC Motor Starter In order for a dc motor to function properly on the job, it must have some special control and protection equipment associated with it. The purposes of this equipment are: 1. To protect the motor against damage due to short circuits in the equipment 2. To protect the motor against damage from long term overloads 3. To protect the motor against damage from excessive starting currents 4. To provide a convenient manner in which to control the operating speed of the motor Chap 2: DC Machines 58 DC Motor Problem on Starting DC motor must be protected from physical damage during the starting period. At starting conditions, the motor is not turning, and so EA = 0 V. Since the internal resistance of a normal dc motor is very low, a very high current flows, hence the starting current will be dangerously high, could severely damage the motor, even if they last for only a moment. Consider the dc shunt motor: VT E A VT IA RA RA When EA = 0 and RA is very small, then the current IA will be very high. Two methods of limiting the starting current : • Insert a starting resistor in series with armature to limit the current flow (until EA can build up to do the limiting). The resistor must be not permanently to avoid excessive losses and cause torque speed to drop excessively with increase of load. • Manual DC motor starter, totally human dependant Chap 2: DC Machines 59 Inserting a Starting Resistor in Series & Manual DC Motor Fig : A shunt motor with a starting resistor in series with an armature. Contacts 1A, 2A and 3A short circuit portions of the starting resistor when they close Fig : A Manual DC Motor Human dependant: • Too quickly, the resulting current flow would be too large. • Too slowly, the starting resistor could burnup Chap 2: DC Machines 60 DC Motor Efficiency Calculations To calculate the efficiency of a dc motor, the following losses must be determined : • • • • • Copper losses (I2R losses) Brush drop losses Mechanical losses Core losses Stray losses Pconv = Pdev = EAIA=indω Pout =out m Pin =VTIL I2R losses Mechanical Core loss Stray losses losses Chap 2: DC Machines 61 DC Motor Efficiency Calculations 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 = IA2RA Must consider RS for series Field Loss PF = IF2RF and compound DC Motors PA = Armature Losses PF = Field Circuit Losses The resistance used in these calculations is usually the winding resistance at normal operating temperature 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 Chap 2: DC Machines 62 DC Motor Efficiency Calculations Magnetic or core loss : These are the hysteresis and eddy current losses occuring in the metal of the motor. Mechanical loss : These are friction and windage losses. • Friction losses include the losses caused by bearing friction and the friction between the brushes andcommutator. • Windage losses are caused by the friction between rotating parts and air inside the DC machine’s casing. Stray losses (or Miscellaneous losses) : These are other losses that cannot be placed in one of the previous categories. (Is about 1% of full load-RULE OF THUMB) [[pg 318,Electric Machinery and Transformers, BHAG S. GURU] and [pg 525, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN] Chap 2: DC Machines 63 DC Motor Efficiency Calculations Rotational losses is when the mechanical losses, Core losses and Stray losses are lumped together. [pg. 193 Electromechanical Energy Devices and Power System, ZIA A. ZAMAYEE & JUAN L. BALA JR.] It also consider as combination between mechanical and core losses at no load and rated speed.[pg 317, Electric Machinery and Transformers, BHAG S. GURU] and [pg 593, Electric Machinery Fundamentals, STEPHEN J. CHAPMAN] Motor efficiency : Poutput Pinput X 100% Pinput Plosses Pinput X 100% Chap 2: DC Machines 64 Speed Regulation The speed regulation is a measure of the change speed from no-load to full load. The percent speed regulation is defined Speed Regulation (SR): nl fl X 100% fl or nl fl X 100% fl +Ve SR means that the motor speed will decrease when the load on its shaft is increased. -Ve SR means that the motor speed increases with increasing load. Chap 2: DC Machines 65