DC Machines POLL The primary winding of a C.T. has _________ a) a larger number of turns b) no turns at all c) intermediate number of turns d) a few turns DC MACHINES • DC MOTOR ❑ ❑ ❑ ❑ ❑ Working principles Classification Starting of DC Machines Speed control of DC Motor Applications of dc motors Fleming’s left hand rule Used to determine the direction of force acting on a current carrying conductor placed in a magnetic field . The middle finger , the fore finger and thumb of the left hand are kept at right angles to one another . The middle finger represent the direction of current The fore finger represent the direction of magnetic field The thumb will indicate the direction of force acting on the conductor . This rule is used in motors. Fleming’s left hand rule Fleming’s Right hand rule What are DC Machines? ➢ Are DC generators that convert mechanical energy to DC electric energy. ➢ Are DC motors that convert DC electric energy to mechanical energy. ❑ DC machine can be used as a motor or as a generator. ❑ DC Machine is most often used for a motor. Practice Slide Practice Slide Practice Slide POLL In dc motor the voltage is induced by: a) by moving conductor b) by changing magnetic flux c) intermediate number of turns d) None of the above DCMotorConstructionandWorking • Electric motors are the most important element of today's industrial world. • From a simple drill machine to a sophisticated robotic arm electric motors are used everywhere . Types There are two types of electric motors1.AC Motors 2.DC Motors •In these, because of some important characteristics like fast reaction, better speed control, simple construction, DC motors are very popular in automatic machines. AC motor DC motor Construction •The outer part of the motor is generally static, which is known as stator. •Stator consist of a frame, and it contains “Pole Shoes” which are projected inward. Construction •Core of these poles are made from silicon steel on which insulated copper wire is wound to make windings. •These poles are bolted and fixed inside the frame. Core Construction • When DC current is pass through the windings it creates a static magnetic field. • There are two types of pole in a DC machine. 1. Main/ Field Pole 2. Inter Poles Construction • The connections of these poles are brought out through connectors in the terminal box, so that we can give electric supply to them and even do the check for the fault. • Main poles create static magnetic field, when current is pass to its winding. Construction • In this magnetic field rotating part of motor lies, known as Rotor or Armature which is made by number of stampings of highly permeable material i.e. silicon steel, which allows magnetic field to pass through it easily. Construction • Slots are cut on the outer periphery of rotor or we can say “armature”, which receive coils/windings made up of copper conductor. • Each coil gets connected to an external DC source by a pair of commutator segments arranged in the form of a Ring. Construction •This complete assembly is housed over a cylindrical shaft made up of high quality steel. Working of Commutator Working of Commutator Working of Commutator Construction • Because of the bearings at both side of the shaft the rotor is capable of rotate between the field poles. Working • When electric current is forced to pass through the rotor conductors by a set of carbon brushes through commutator segments, it creates their own magnetic field which tries to distorts the magnetic field, created by the field pole. • Due to interaction between two magnetic fields i.e magagntic field of main poles and the rotor conductors. Electromagnetic Forces act on the rotor conductors and these forces act tangentially on the rotor surface. • Therefore a torque is produced at the rotor shaft and the rotor rotates. . • To understand the process more easily lets take a example of single loop having a set of its own commutator segment. Commutator segments • Due to the DC voltage applied by an external DC source current will flow in the coil in given direction from positive to negative potential and by Fleming's left hand rule , we can find that this side of coil receive force in upward direction and this side will receive a downward force and hence the loop rotates. Current • During the rotation of loop you can find that commutator segments comes in contact with brushes of opposite polarity one by one. • This way the conductor of left side always carry current in given direction, and of right side in opposite direction. • This helps to maintain the continuous rotation of coil. • We can also find that magnitude of force on the coil is maximum in this region after that, magnitude reduce reduces considerably. • This cause uneven torque and moment of rotor. • To make “maximum torque” always available at the shaft of rotor multiple number of coils with a pair of commutator segments are placed in the rotor. • In small DC motor permanent magnet can be use to create static magnetic field, but they can loose their magnetism due to heating, vibration, or aging, that’s why in higher rating motor electromagnets are used and their magnetic field density can be controlled to control output torque. POLL Which part will surely tell that given motor is DC motor and not an AC type? a) Winding b) Shaft c) Commutator d) Stator Types of DC motor 1. Separately Excited DC motor 2. Self Excited DC motor Practice Slide Practice Slide Starting of DC motors • A starter is a device to start and accelerate a motor. A controller is a device to start the motor, control and reverse the speed of the DC motor and stop the motor. While starting the DC motor, it draws the heavy current which damages the motor. • The starter reduces the heavy current and protects the system from damage. ➢ Need of Starters for DC Motors: The dc motor has no back emf. At the starting of the motor, the armature current is controlled by the resistance of the circuit. The resistance of the armature is low, and when the full voltage is applied at the standstill condition of the motor, the armature current becomes very high which damage the parts of the motor. Since at the time of starting the DC Motor, the starting current is very large. At the time of starting of all DC Motors, except for very small motors, an extra resistance must be connected in series with the armature. This extra resistance is added so that a safe value of the motor is maintained and to limit the starting current until the motor has attained its stable speed. POLL Which D.C. motor is generally preferred for cranes and hoists? (A) Series motor (B) Shunt motor (C) Cumulatively compounded motor (D) Differentially compounded motor Speed Control of DC motors According to the speed equation of a dc motor N ∞ Eb/φ ∞ V- Ia Ra/ φ Thus speed can be controlled by: Flux control method: By Changing the flux by controlling the current through the field winding. Armature control method: By Changing the armature resistance which in turn changes the voltage applied across the armature Flux Control Method Advantages: It provides relatively smooth and easy control Speed control above rated speed is possible As the field winding resistance is high the field current is small. Power loss in the external resistance is small . Hence this method is economical Disadvantages: Flux can be increased only upto its rated value High speed affects the commutation, motor operation becomes unstable Armature Voltage Control Method The speed is directly proportional to the voltage applied across the armature . Voltage across armature can be controlled by adding a variable resistance in series with the armature Potential Divider Control If the speed control from zero to the rated speed is required , by rheostatic method then the voltage across the armature can be varied by connecting rheostat in a potential divider arrangement .
