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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