Back EMF(counter EMF) is an important parameter to be considered for a DC Motor. Back EMF enables Motor to be self regulating Motor, which is a unique feature of DC Motor among-st other Motors.
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What is a dc motor? Magnetic fields affect moving charges and a magnetic force in a charge moving with a velocity in a magnetic field B is given by the Lorentz force law Fmagnetic = q(v cross.product B). Isn't it wonderful that when we flip a switch which is wired to a ceiling fan, the fan rotates ! Take a moment to visualize the force which acts on the blades to keep them rotating while we are standing on the floor far below the fan. As if it is rotating by an unseen hand! It operates based on the principle that a current-carrying conductor placed in a magnetic field experiences a force and this is what was encapsulated in the Lorentz force law. A loop of wire with a current in it is a magnetic moment and when this loop is placed in a magnetic field, it rotates because there is a force generated by it. More the number of loops, more is the force created and faster the rotation. If we place the loops in an armature and attach a set of blades to the shaft of the armature, we have made an electric motor with a fan. The fan pushes air and the molecules of air when they leave the blades produce a reaction force, a torque as the blades spin. The fan motor should generate sufficient torque to overcome this opposing “load” torque and keep the blades spinning. The "back emf" is the trick nature uses to regulate the current to generate this required torque automatically! For a lecture by Prof Ruth Chabay on magnetic forces and a live demonstration (36.40 minute onwards) visit ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-D-F9-DqHzmY.html There is one subtlety involved in the generation of the force by the current carrying loop and that is that the force is due to Hall effect charges in the arms of the loop. For a more detailed discussion of motors including the role of surface charges in the loop wires of the motor, see the books referred below. Electrostatics and circuits belong to one science and not two, that of electricity and magnetism. To know how they are unified visit this link matterandinteractions.org/articles-talks/ and view the article 'A unified treatment of electrostatics and circuits. B. Sherwood and R. Chabay, unpublished. (1999)' pdf. For a live demonstration of surface charge and its effects in circuits visit ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-U7RLg-691eQ.html For a detailed discussion of surface charge, coulomb's law, electric fields, fields of dipoles and other charge configurations, and parallel plates, and a distinct approach using the surface charge concept in the study of advanced topics of capacitance, currents, conservation of charge, conservation of current, superposition of fields, superposition of potential, simple dc circuit, magnetic fields, magnetic fields of a current element, straight wire, current loop, solenoids, biot-savart law, voltage, voltage source, difference between e.m.f. and potential difference, ideal voltage sources, resistors, how current branches in a parallel circuit, capacitors, inductors, Faraday's law, inductance, ac circuits, transmission lines, Lorentz Force law, motors, generators, p-n junction diodes, electromagnetic waves, antennas and radiation, new electrodynamic theories on the nature of the electric field, see "Electric and Magnetic Interactions" by Chabay and Sherwood www.matterandinteractions.org or Fundamentals of electric theory and circuits by Sridhar Chitta www.wileyindia.com/fundamentals-of-electric-theory-and-circuits.html There is a "look inside" feature in the amazon.com webpage of the book "Fundamentals of electric theory and circuits" by Sridhar Chitta with a few pages of Chapter 1 which may be viewed and also which you may swipe left or press < icon to view the foreword, preface and Table of Contents. The contents of the above book by Sridhar Chitta, make a distinct unified approach to electrostatics and a few advanced circuits and topics like dc motor and load torques, motor current during starting, accelerating and steady-state, coupling signals to amplifiers, lending precision and clarity to the topics which is not found in most text books. The book comes alongwith a CD with animated power point presentations for all chapters and voltage regulator, RC phase shift oscillator, transformer-coupled audio amplifier and differential amplifier included additionally.
What is a dc motor? What is back emf? Magnetic fields affect moving charges and a magnetic force in a charge moving with a velocity in a magnetic field B is given by the Lorentz force law Fmagnetic = q(v cross.product B). Isn't it wonderful that when we flip a switch which is wired to a ceiling fan, the fan rotates ! Take a moment to visualize the force which acts on the blades to keep them rotating while we are standing on the floor far below the fan. As if it is rotating by an unseen hand! It operates based on the principle that a current-carrying conductor placed in a magnetic field experiences a force and this is what was encapsulated in the Lorentz force law. A loop of wire with a current in it is a magnetic moment and when this loop is placed in a magnetic field, it rotates because there is a force generated by it. More the number of loops, more is the force created and faster the rotation. If we place the loops in an armature and attach a set of blades to the shaft of the armature, we have made an electric motor with a fan. The fan pushes air and the molecules of air when they leave the blades produce a reaction force, a torque as the blades spin. The fan motor should generate sufficient torque to overcome this opposing “load” torque and keep the blades spinning. The "back emf" is the trick nature uses to regulate the current to generate this required torque automatically! For a lecture by Prof Ruth Chabay on magnetic forces and a live demonstration (36.40 minute onwards) visit ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-D-F9-DqHzmY.html There is one subtlety involved in the generation of the force by the current carrying loop and that is that the force is due to Hall effect charges in the arms of the loop. For a more detailed discussion of motors including the role of surface charges in the loop wires of the motor, see the books referred below. Electrostatics and circuits belong to one science and not two, that of electricity and magnetism. To know how they are unified visit this link matterandinteractions.org/articles-talks/ and view the article 'A unified treatment of electrostatics and circuits. B. Sherwood and R. Chabay, unpublished. (1999)' pdf. For a live demonstration of surface charge and its effects in circuits visit ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-U7RLg-691eQ.html For a detailed discussion of surface charge, coulomb's law, electric fields, fields of dipoles and other charge configurations, and parallel plates, and a distinct approach using the surface charge concept in the study of advanced topics of capacitance, currents, conservation of charge, conservation of current, superposition of fields, superposition of potential, simple dc circuit, magnetic fields, magnetic fields of a current element, straight wire, current loop, solenoids, biot-savart law, voltage, voltage source, difference between e.m.f. and potential difference, ideal voltage sources, resistors, how current branches in a parallel circuit, capacitors, inductors, Faraday's law, inductance, ac circuits, transmission lines, Lorentz Force law, motors, generators, p-n junction diodes, electromagnetic waves, antennas and radiation, new electrodynamic theories on the nature of the electric field, see "Electric and Magnetic Interactions" by Chabay and Sherwood www.matterandinteractions.org or Fundamentals of electric theory and circuits by Sridhar Chitta www.wileyindia.com/fundamentals-of-electric-theory-and-circuits.html There is a "look inside" feature in the amazon.com webpage of the book "Fundamentals of electric theory and circuits" by Sridhar Chitta with a few pages of Chapter 1 which may be viewed and also which you may swipe left or press < icon to view the foreword, preface and Table of Contents. The contents of the above book by Sridhar Chitta, make a distinct unified approach to electrostatics and a few advanced circuits and topics like dc motor and load torques, motor current during starting, accelerating and steady-state, coupling signals to amplifiers, lending precision and clarity to the topics which is not found in most text books. The book comes alongwith a CD with animated power point presentations for all chapters and voltage regulator, RC phase shift oscillator, transformer-coupled audio amplifier and differential amplifier included additionally.
