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Dc motor
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- 2. MACHINES Electric Generator:Mechanical Energy Electrical energy. Electrical Motor: Electrical Energy Mechanical Energy
- 3. PRINCIPLE OF DCMOTOR ELECTIRCAL ENERGYMECHANICAL ENERGY According to Faraday’s law of electromagnetic induction, ‘When a current carrying conductor is placed in a magnetic field it experience a force’.
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- 5. MAJOR COMPONENTS 1.Frame and Yoke 2. Poles 3. Armature 4. Field winding 5. Commutator 6. Brush 7. Other mechanical parts
- 6. FRAME AND YOKE It is outer cover of dc motor also called as frame. It provides protection to the rotating and other part of the machine from moisture, dust etc. Yoke is an iron body which provides the path for the flux to complete the magnetic circuit. It provides the mechanical support for the poles. Material Used: low reluctance material such as cast iron, silicon steel, rolled steel, cast steel etc.
- 7. POLES AND POLECORE Poles are electromagnet, the field winding is wound over it. It produces the magnetic flux when the field winding is excited. The construction of pole is done using the lamination of particular shape to reduce the power loss due to eddy current.
- 8. POLE SHOE Poleshoe is an extended part of a pole. Due to its typical shape, it enlarges the area of the pole, so that more flux can pass through the air gap to armature. Material Used: low reluctance magnetic material such as cast steel or cast iron is used for construction of pole and pole shoe.
- 9. FIELD WINDING Fieldcoil wound on pole The coil wound on the pole core are called field coils. Field coils are connected in series to form field winding. Current is passed through the field winding in a specific direction, to magnetize the poles and pole shoes. Thus magnetic flux is produce in the air gap between the pole shoe and armature. Field winding is also called as Exciting winding. Material Used for copper conductor is copper. Due to the current flowing through the field winding alternate N and S poles are produced.
- 10. ARMATURE CORE Armaturecore is a cylindrical drum mounted on the shaft. It is provided with large number of slots all over its periphery and it is parallel to the shaft axis. Armature conductors are placed in these slots. Armature core provides low reluctance path to the flux produced by the field winding. Material used: high permeability, low reluctance cast steel or cast iron material is used. Laminated construction of iron core is used to minimize the eddy current losses.
- 11. ARMATURE WINDING Armatureconductor is placed in a armature slots present on the periphery of armature core. Armature conductor are interconnected to form the armature winding. When the armature winding is rotated using a prime mover, it cuts the magnetic flux lines and voltage gets induced in it. Armature winding is connected to the external circuit (load) through the commutator and brushes. Material Used: Armature winding is suppose to carry the entire load current hence it should be made up of conducting material such as copper.
- 12. COMMUTATOR It isa cylindrical drum mounted on the shaft along with the armature core. It is made up of large number of wedge shaped segments of hard-drawn copper. The segments are insulated from each other by thin layer of mica. Armature winding are tapped at various points and these tapping are successively connected to various segments of the commutator. Function of commutator: • It converts the ac emf generated internally into dc • It helps to produce unidirectional torque. Material Used: it is made up of copper and insulating material between the segments is mica.
- 13. BRUSHES Current areconducted from the armature to the external load by the carbon brushes which are held against the surface of the commutator by springs. Function of brushes: To collect the current from the commutator and apply it to the external load in generator, and vice versa in motor. Material Used: Brushes are made of carbon and they are rectangular in shape.
- 14. BACK EMF Whena motor rotates, the conductors housed in the armature also rotate and cut the magnetic lines of force. So an emf is induced in the armature conductors and this induced emf opposes the supply voltage as per Lenz’s law. This induced emf is call Back emf(or) counter emf.
- 15. TORQUE EQUATION Mechanicalpower required to rotate the shaft on mechanical side = 𝑇𝜔……………………………………………… T =Torque in Newton-meter 𝜔= angular velocity in radian /second Gross mechanical power produced by the motor on electrical side = 𝐸𝑏𝐼𝑎……………………………………………… Eb = back emf in volts Ia = armature current in ampere Equating equation and , , we get , 𝐸𝑏𝐼𝑎 = 𝑇𝜔……………………………………………….. 1 2 1 2 3
- 17. VOLTAGE AND POWEREQUATION
- 18. WORKING OF DCMOTOR The direction of the force is given by Fleming's left hand rule and it's magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field. Fleming's left hand rule: If we stretch the index(first) finger, middle(second) finger and thumb of our left hand to be perpendicular to each other and the direction of magnetic field is represented by the index(first) finger, direction of the current is represented by middle(second) finger then the thumb represents the direction of the force experienced by the current carrying conductor.
- 19. Above animationhelps in understanding the working principle of a DC motor. When armature windings are connected to a DC supply, current sets up in the winding. Magnetic field may be provided by field winding (electromagnetism) or by using permanent magnets. In this case, current carrying armature conductors experience force due to the magnetic field, according to the principle stated above. Commutator is made segmented to achieve unidirectional torque. Otherwise, the direction of force would have reversed every time when the direction of movement of conductor is reversed the magnetic field. This is how a DC motor works!
- 20. TYPES OF DCMOTOR
- 21. DC SHUNT MOTOR In dc shunt motor the armature and field winding are connected in parallel across the supply voltage The resistance if the shunt winding Rsh is always higher than the armature winding Ra Since V and Rsh both remains constant the Ish remains essentially constant, as field current is responsible for generation of flux. thus So shunt motor is also called as constant flux motor. ф α Ish
- 22. Eb= VL– Ia Ra Ia= IL + Ish VL = Eb + Ia Ra Ish = VL Rsh
- 23. TORQUE AND SPEED EQUATIONOF DC SHUNT
- 24. CHARACTERISTIS OF DC SHUNTMOTOR To study the performance of the DC shunt Motor various types of characteristics are to be studied. 1. Torque Vs Armature current characteristics. 2. Speed Vs Armature current characteristics. 3. Speed Vs Torque characteristics.
- 25. 1) TORQUE VsARMATURE CURRENT:
- 26. 2) SPEED VsARMATURE CURRENT:
- 27. 3) SPEED VsTORQUE:
- 28. APPLICATIONS OF DCSHUNT MOTOR
- 29. DC SERIES MOTOR In series wound motor the field winding is connected in series with the armature. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same mmf. Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.
- 30. Eb =VL – Ia Ra – Ise Rse Ia = IL = Ise VL = Eb + Ia Ra + Ise Rse
- 31. TORQUE AND SPEED EQUATIONDOF DC SERIES
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- 33. TORQUE Vs ARMATURECURRENT:
- 35. SPEED Vs ARMATURECURRENT:
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- 39. DC COMPOUND MOTOR Compound wound motor has two field windings; one connected in parallel with the armature and the other in series with it. There are two types of compound motor connections 1. Short-shunt connection 2. Long shunt connection
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- 41. Eb =VL – Ia Ra – ISE RSE Ia = IL – Ish IL = ISE VL = Eb + Ia Ra + ISE RSE Ish = VL Rsh
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- 43. Eb =VL – Ia Ra – ISE RSE IL = ISE = IL – Ish VL = Eb + Ia Ra + ISE RSE
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- 48. SPEED CONTROL Numerousapplications require control of speed, as in rolling mills, cranes, hoists, elevators, machine tools, and locomotive drives. DC motors are extensively used in many of these applications. Control of dc motors speed below and above the base (rated) speed can easily be achieved. The methods of control are simpler and less expensive than ac motors. Classis way used Ward-Leonard System, latest used solid-state converters.
- 49. SPEED CONTROL OFDC MOTOR The speed equation of dc motor is 𝑁 ∝ 𝐸𝑏 / ∅ ∝ (𝑉−𝐼𝑎𝑅 𝑎) / ∅ But the resistance of armature winding or series field winding in dc series motor are small. Therefore the voltage drop 𝐼𝑎𝑅𝑎 or 𝐼𝑎(𝑅𝑎 + 𝑅 𝑠) across them will be negligible as compare to the external supply voltage V in above equation. Therefore 𝑁 ∝ 𝑉 /∅ , since V>>>> 𝐼𝑎𝑅𝑎 Thus we can say 1. Speed is inversely proportional to flux ∅. 2. Speed is directly proportional to armature voltage. 3. Speed is directly proportional to applied voltage V. So by varying one of these parameters, it is possible to change the speed of a dc motor
- 50. SPEED CONTROL OFDC MOTOR Speed Control of Shunt Motors: Flux control method Armature and Rheostat control method Voltage control method 1) Multiple voltage control 2) Ward Leonard system Speed Control of Series Motors: Flux control method 1) Field diverter 2) Armature diverter 3) Trapped field control 4) Paralleling field coils Variable Resistance in series with motor Series -parallel control method
- 51. FLUX CONTROL METHOD In this flux control method, speed of the motor is inversely proportional to the flux. Thus, by decreasing flux and speed can be increased vice versa. To control the flux , he rheostat is added in series with the field winding will increase the speed (N), because of this flux will decrease. So, the field current is relatively small and hence I2R loss is decreased. This method is quite efficient. So in this method, the speed can be increased by reducing flux, it puts a method to reducing flux with this method, it puts a method to maximum speed as weakening of flux beyond the limits will adversely affect the commutator.
- 52. ARMATURE CONTROL METHOD In the armature control method, the speed of the DC motor is directly proportional to the back emf (Eb) and Eb = V- IaRa. When supply voltage (V) and armature resistance Ra are kept constant, the Speed is directly proportional to armature current (Ia). If we add resistance in series with the armature, the armature current (Ia) decreases and hence speed decreases. This armature control method is based on the fact that by varying the voltage across the required voltage. The motor back EMF (Eb) and Speed of the motor can be changed. This method is done by inserting the variable resistance (Rc) in series with the armature.
- 53. The basic equationof the armature control method, N is directly proportional to the V-Ia (Ra+Rc) where Rc is controller resistance and Ra is the armature resistance. Due to the voltage back in the controller resistance the back EMF is decreased. Since N is directly proportional to the Eb.
- 54. VOLTAGE CONTROL METHOD Multiple Voltage Control: In this method, the shunt field is connected to a fixed exciting voltage, and the armature is supplied with different voltages. So the Voltage across armature is changed with the help of a suitable switchgear devises. Armature speed is approximately proportional to the voltage across the armature. Ward-Leonard System: This Ward – leonard system is used where very sensitive speed control of the motor is required (e.g electric excavators, elevators, etc.). The arrangement of this system is as required in the figure shown below.
- 55. M2 isthe motor, it controls the speed of the generator. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The generator output voltage can be connected to the motor M2 and it can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.
- 56. FLUX CONTROL METHOD Field Diverter : A Rheostat is connected parallel to the series field as shown in fig(a). This variable resistor is also called as a diverter, as desired value of the current can be diverted through this resistor and hence current through field coil can be decreased. Hence flux can be decreased to desired amount and speed(N) can be increased. Armature Diverter : Rheostat (Divider) is connected across the armature of the coil as shown in fig (b). For a given constant load torque, if armature current is reduced, then flux must increase. As armature torque Ta α ØIa. This will result in an increase in current taken from the supply and hence flux Ø will increase and subsequently speed of the motor will decrease.
- 57. TAPPED FIELDCONTROL: This tapped field control method is shown in fig (c). In this method, field coil is tapped dividing the number of turns. Thus we can select different value of Ø by selecting a different number of turns. In this method flux is reduced and speed is increased by decreasing the number of the turns of the series field winding. The switch S can be short circuit any part of the field winding, thus decreasing the flux and raising the speed (N) with full turns of coil. PARALLELING FIELD COILS: This is used for fan motors several speed can be obtained by regrouping the field coils in series with the DC armature.
- 58. VARIABLE RESISTANCE IN SERIESWITH MOTOR In this method, an introducing resistance (R) is series with the armature of motor.The voltage across the armature can be reduced. So the speed reduces in proportion with it. It is seen that for a 4 pole motor, the speed of the motor can be obtained easily.
- 59. SERIES-PARALLEL CONTROL METHOD Thistype of the method can be widely used in electric traction, where two or more mechanisms coupled series motors are employed. If required low speed motors are joined in series, and for higher speed motors are joined in parallel. When motors are connected in series, the motors have the same current passing through them, although voltage across each motor is divided. When in parallel, the voltage across each motor is same, although current gets divided.
- 60. STEPPER MOTOR PRINCIPLE OFSTEPPER MOTOR: A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates indiscrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.
- 61. TYPES OF STEPPERMOTOR Variable – reluctance motor Permanent magnet motor Hybrid motor
- 62. WHEN TO USESTEPPER MOTOR? A stepper motor can be a good choice whenever controlled movement is required They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Because of inherent advantages listed previously, stepper motor have their place in many different applications.
- 63. CHARACTERISTICS OF STEPPER MOTOR Stepper motors are constant power devices. As motor speed increases, torque decreases. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. This vibration can become very bad at some speeds and can cause the motor to lose torque.
- 64. ADVANTAGES OF STEPPER MOTOR Excellent response to starting/stopping/ reversing. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. The motor has full torque at standstill (if the windings are energized)
- 65. DISADVANTAGES OF STEPPER MOTOR Resonances can occur if not properly controlled. Not easy to operate at extremely high speeds
- 66. APPLICATIONS OF STEPPER MOTOR Computer-controlled stepper motors are one of the most versatile forms of positioning systems. They are typically digitally controlled as part of an open loop system, and are simpler and more rugged than closed loop servo systems. Industrial applications are in high speed pick and place equipment and multi-axis machine CNC machines often directly driving lead screws or ball screws. In the field of lasers and optics they are frequently used in precision positioning equipment such as linear actuators, linear stages, rotation stages, goniometers, and mirror mounts. Other uses are in packaging machinery, and positioning of valve pilot stages for fluid control systems. Commercially, stepper motors are used in floppy disk drives, flatbed scanners, computer printers, plotters, slot machines, and many more devices. Some people looking for generators for homemade Wind Turbines found success in using stepper motors for generating power.
- 67. BRUSHLESS DC MOTOR Has no brushes and commutators. Rotation of the rotor depends on the accurate position with stator. Detected by Hall Sensor, mounted on rotor, shifted at 60º or 120º phase shift. Electronic commutation used to vary the PWM duty- cycle for speed control, using software.
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- 69. WORKING OF BLDCM As there is no commutator ,the current direction of the conductor on the stator controlled electronically. Rotor consists the permanent magnet where as stator consist a no. of windings. Current through these winding produces magnetic field and force. Hall sensor used to determine the position during commutation.
- 70. COMMUTATION OF BLDCM Brushless DC motor requires external commutation circuit to rotate the rotor. Rotor position is very important. HALL SENSOR senses the position of the coil accurately.
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- 72. APPLICATIONS PMBLDC motors areincreasingly being used in a wide spectrum of applications domestic equipments Automobiles nformation technology equipment Industries public life appliances Transportation aerospace, defence equipments, power tools, toys, vision and sound equipments medical and health care equipment ranging from microwatts to megawatts.
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