ELEC 3105 Basic EM and Power Engineering Rotating DC Motor PART 1 Mechanical Rotating DC Motor Linear motors are good for certain tasks, but industrial and transportation applications usually demand rotating motion. v Linear motor Assume external applied B field much greater than B field generated by current in bar. V bat R B I app B I app v Metal fixed rail Magnetic flux density into page External applied field Movable metal bar The DC rotating motor is commonly constructed with an armature on the rotor and a field generated by a permanent magnet instead of a stator winding. Stator: non-moving coil Rotor: rotating part motor, electric, machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintenance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor. Rotating DC Motor Terming this device a DC motor is not entirely clear, in fact, the current in the armature coil alternates in polarity, even though the supply is DC. WHY ? Armature coil Brushes Stator: non-moving coil Rotor: rotating part 97.315 Basic EM and Power Engineering Rotating DC Motor Theory Rotating DC Motor Consider a rectangular coil rotating in a uniform magnetic field B Axis 2r Magnetic field and current in loop interact in such a way as to generate a torque on the loop. B Figures extracted from Lecture 20 Slide 38 Rotating DC Motor Torque on a magnetic dipole r m B r Side view We will consider a dipole in a uniform magnetic field. We can use any shape we want for the dipole. 2r I out of page I I into page Wire loop Top view Rotating DC Motor Taken from Lecture 20 Slide 40 Torque on a magnetic dipole m F B r Side view Torque G = r F F Pivot point Torque attempts to align dipole moment m with B. Rotating DC Motor Taken from Lecture 20 Slide 41 Torque on a magnetic dipole F m r Side view G = 2Fr sin( ) F Pivot point F => Magnetic force on wire of length F = IB Through postulate 1 for magnetic fields Then G = 2rIB sin ( ) B Rotating DC Motor Axis If current always flows in the same direction then loop will only oscillate rather than rotate 2r B F Stable equilibrium G m Unstable equilibrium r 0 180 360 F ELEC 3105Basic EM and Power Engineering Rotating DC Motor Torque Taming Rotating DC Motor We need to reverse the direction of the current on each half cycle. Transform original torque versus angle curve from: G 0 180 360 TO G 0 180 360 Now torque always applied in same direction inducing loop to spin continuously in same direction. Rotating DC Motor This is accomplished by using a commutator (either mechanical or electronic cycle). G = 2rIB sin ( ) G 0 180 360 How it Works…. Metal ring attached to shaft split in two sections Rotating DC Motor The figure illustrates one method by which the commutation function might be accomplished. Rather than hard wiring the current source to the coil, the current is conducted through sliding contacts (brushes) connected to the current source. The brushes ride on the ends of the coil wires, thus conducting current through the coil. In this simplified motor, the brushes switch coil connections about once every 180o of rotation. Therefore, the direction of current flow remains fixed with respect to the magnetic field. Rotating DC Motor The torque produced by this design momentarily goes to zero every half cycle. Stall is possible, also start up may require a small push. In addition to this the torque versus rotation angle is not uniform bad better best Rotating DC Motor Link to related site Link to related site Rotating DC Motor How to get rid of undesirable torque fluctuations and momentary zero values? Answer: Redesign several features of the simple loop motor. ELEC 3105 Basic EM and Power Engineering Rotating DC Motor Field Taming Rotating DC Motor B B To obtain a more even torque, the magnetic field lines should look something like: F B B F B B And how do you get a magnetic field with that shape? Rotating DC Motor poles of magnet To obtain a more even torque, the magnetic field lines should look something like: B Rotor B field lines follow the path of least reluctance, so the curved poles create roughly a radial field pattern. Rotating DC Motor Redesign the permanent magnet poles. Insert soft iron rotor Rotating DC Motor Radius r Rotating DC Motor Consider a rectangular coil rotating in a uniform magnetic field B Motor dimensions: Axis 2r Magnetic field and current in loop interact in such a way as to generate a torque on the loop. B Figures extracted from Lecture 20 Slide 38 Depth Torque in radial field G = 2rBI Recall slide 9 of this lecture for parameters of the wire loop. B r Rotor ELEC 3105 Basic EM and Power Engineering Rotating DC Motor / Generator Motor / Generator Action As the motor turns, a back emf is produced: V = 2vB emf velocity of the outer edge of the rotor v = r V = 2rB emf There are two conductors of length in the loop. Motor / Generator Action bat G = 2rBI I R V emf v Expression of Vemf V = 2rB terminal emf V = IR V bat Loop Equivalent circuit V emf G V = R 2rB 2rB bat Slide extracted from linear motor and modified for loop motor. Motor / Generator Action Slide extracted from linear motor and modified for loop motor. G V = R 2rB 2rB Linear relation between speed and torque bat V = 2rB bat noload Current flows in a direction to charge the battery. Link Generator =0 2rBV G= R bat Stall torque Motor G Rotating DC Motor In a practical motor design, use many turns of wire on the rotor (rather than just one) to increase the torque. B r Rotor DC Motor Simulation ELEC 3105 Basic EM and Power Engineering Rotating DC Motor and Regenerative Braking Regenerative Braking Since the DC motor and a DC generator are virtually the same machine mechanically, it was immediately realized that a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy. The idea formed that if the power could be returned to the source, other trains could use it. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains. Various schemes were tried over many years with more or less success but it was not until the adoption of modern electronics that reliable schemes have been available. Regenerative braking Extra slides after this one Simple Motor PURPOSE: To illustrate possibly the world's simplest motor. DESCRIPTION: A small coil is mounted across the terminals of a battery as shown. The enamel is scraped off half of the coil wire where it contacts the battery terminals. The magnet is oriented such that when the coil is rotating it either pushes away or pulls toward the magnet in the appropriate part of its cycle. The other half-cycle the enamel prevents the coil from being activated; if it were it would counteract the torque which produces the desired rotation. SUGGESTIONS: REFERENCES: (PIRA unknown.) EQUIPMENT: Mounted battery and rotation coil with carefully polished lead wires. SETUP TIME: None. Simple Motor ST. LOUIS MOTOR PURPOSE: To demonstrate the structure and operation of a simple motor. DESCRIPTION: This is a two-pole DC motor with a split-ring commutator and permanent magnets. Operates with a 1.5 volt battery. SUGGESTIONS: REFERENCES: (PIRA 5K40.10) EQUIPMENT: St. Louis motor. SETUP TIME: None. ELEC 3105 Basic EM and Power Engineering Rotating DC Motor Dissection Inside the DC Motor Link by clinking on figure "How Electric Motors Work" describes how an electric motor works and explains the basic components found in any simple DC electric motor. In this article we will take apart an actual electric motor and see what's inside. The motor being dissected is a simple electric motor that you would typically find in a toy: Inside the DC Motor You can see that this is a small motor about as big around as a dime. From the outside you can see the steel can that forms the body of the motor, an axle, a nylon end cap and two battery leads. If you hook the battery leads of the motor up to a flashlight battery the axle will spin. If you reverse the leads it will spin in the opposite direction. Here are two other views of the same motor. Note the two slots in the side of the steel can in the second shot - their purpose will become more evident in a moment: Inside the DC Motor The nylon end cap is held in place by two tabs that are part of the steel can. By bending the tabs back you can free the end cap and remove it. Inside the end cap are the motor's brushes. These brushes transfer power from the battery to the commutator as the motor spins: Inside the DC Motor The axle holds the armature and the commutator. As described in "How Electric Motors Work", the armature is a set of electromagnets, in this case three. The armature in this motor is a set of thin metal plates stacked together, with thin copper wire coiled around each of the three poles of the armature. The two ends of each wire (one wire for each pole) are soldered onto a terminal, and then each of the three terminals is wired to one plate of the commutator. The figures below make it easy to see the armature, terminals and commutator: Inside the DC Motor The final piece of any DC electric motor is the field magnet. The field magnet in this motor is formed by the can itself plus two curved permanent magnets: One end of each magnet rests against a slot cut into the can, and then the retaining clip presses against the other ends of both magnets. ELEC 3105 Basic EM and Power Engineering Rotating DC Motor Building at Home Home Made DC Motor While this motor is very crude and inefficient, it cost me less than $5.00 to build from parts I mostly had around the house, and total construction time was under four hours. The hardest part was winding the field magnet and the armature coils. Note that you can click on many of the smaller images on this page to see larger versions. The wooden frame of the motor was constructed from various bits of scrap lumber I had laying around. If you build your own, look through all of these pictures and you can rig something up based on what YOU have laying around. Home Made DC Motor I could have used permanent magnets for the fields on this motor, but this is a section on electromagnetism - and I couldn't find any. I made my field coil by winding 75 feet of 26 gauge enameled magnet wire onto the "U" of a 2.5 in iron Muffler clamp. Note the fence staple on the left to provide a route for the wires. The coil is wound in several neat overlapping layers, with a layer of electrical tape between each. Wind a layer, then wrap with a single layer of electrical tape, and wind back over the coil you have already wound. Just make sure that you always wind in the same clockwise or counterclockwise direction in which you started. The arms of the "U" bolt are passed up through holes drilled in the bottom of the wooden frame. The whole assembly is held in place by gravity and by the nuts on the top of the frame assembly. Home Made DC Motor The bearings for the shaft are simply screw eyes screwed in to the sides of the wooden frame. The brushes, which will transfer current to the slip rings in the commutator assembly are made from 22 gauge solid copper wire with a couple of inches of the insulation stripped from each end. Note that one is mounted on the top of one wooden cross piece, while the other is mounted to the bottom of the other. This wire must be stiff enough to hold a shape, but not so stiff that it puts too much friction on the commutator assembly. Home Made DC Motor The armature is made from a section of iron nail which was cut to fit cleanly between the arms of the "U" bolt. Before winding the coil for the armature, wrap one turn of 12 gauge solid copper insulated house wiring around the very center of the nail. Bend the wire in such a way that it comes straight off the piece of nail, and that the nail is positioned in a ninety degree angle to the wire. Place the nail and wire on a chunk of waxed paper and place some two part epoxy on the union to bind them together. Wrap one layer of electrical tape around each half of the iron nail, then wind four layers of 26 gauge enameled magnet wire and tape onto the iron nail, making sure to always wind in the same direction. Simply cross over the 12 gauge wire shaft in the center and continue each layer on the other side, as if the shaft were not there. Home Made DC Motor The commutator in my motor is made from a section cut out of a broken shovel handle. Drill a hole in the center into which the wire shaft will fit fairly snugly, and cut a groove into each of it's sides. Slide this onto the short end of the shaft. The next step is to fashion the slip rings. I used a tuna fish can, and cut it into 2 strips the width of the commutator using tin snips. The ends of the strips should be folded down into the grooves in each side of the commutator. Use a small screwdriver to fold them neatly into the grooves in the wooden piece. It is important that the slip rings are as round as possible when the commutator is assembled, and that none of the metal extends past the edges of the wooden part or your motor will not function properly. Home Made DC Motor Cut a small notch in the folded part of the slip rings so that you have something to which you can attach the wires from the armature. Using a small butane lighter, burn the insulation from the ends of the armature wires and clean with a piece of steel wool. You can solder the wires in place if you wish, but I simply used some miniature alligator clips to hold it in place. Snap the slip rings onto the wooden block, and wrap half of them tightly with electrical tape to hold them in place. Make sure that they are as round as possible, and that they do not touch each other in the notches. Make sure that the 12 gauge wire shaft is straight and even, and that the slip rings on the commutator are as round as possible. The gap between the slip rings should be at about a 90 degree angle to the armature assembly. Try spinning the shaft in your fingers to be sure that the assembly is fairly well balanced. Home Made DC Motor Slide the armature assembly into the front bearing (a.k.a. a screw eye) from the center of the wooden frame until the armature is against the frame. If the other end of the wire shaft is too long to fit in the rear screw eye, trim it off a bit. Insert the back end of the shaft into the rear bearing, and slide the whole assembly back until the slip rings line up with the brushes. Home Made DC Motor You should have to bend the brushes slightly outward to get the commutator between them. If they don't touch the slip rings when you are done, slide the assembly forward enough to bend them in toward the shaft, then gently slide the commutator back between them. There is not much to the electrical wiring of the motor - I did not even use an on/off switch. The use of the terminal strip in the back of the motor is optional, but does make life a lot easier. Apply power to the motor by connecting a 12 volt lantern battery, and it should spin merrily away. If the armature wants to lock in position, then you have the wires to the commutator reversed, causing an opposite magnetic field. Even if you have the magnetic poles in the correct orientation, to get the motor to run properly you may have to disconnect the battery and adjust the position and tension of the brushes. You can also slightly adjust the speed of the motor by slightly rotating the commutator on the shaft so that you change the angle between the armature and the field coils. Home Made DC Motor If your motor still does not work properly, connect a couple of "D" cell batteries (3 volts dc instead of 12) and manually turn the shaft. You should be able to feel the magnetic fields as resistance or attraction at certain points in the rotation and you should then be able to figure out where the problem lies. Do not leave the motor connected to the batter for very long or the coils will get very hot, and the battery will get drained quickly, due to the extremely poor efficiency of this design. If you want your motor to work better than mine, and possibly at a lower voltage, figure out how to reduce the friction of the brushes, and use a more rigid shaft mounted in bearings for the armature. ELEC 3105 Basic EM and Power Engineering Rotating DC Motor Links Making DC Motors Click picture to link ELEC 3105 Basic EM and Power Engineering Rotating DC Motor Brushless Brushless DC Motors Brushless DC motors are referred to by many aliases: Brushless permanent magnet, permanent magnet ac motors, permanent magnet synchronous motors ect. The confusion arises because a Brushless dc motor does not directly operate off a dc voltage source. However, as we shall see, the basic principle of operation is similar to a dc motor. A Brushless dc motor has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics. The control electronics replace the function of the commutator and energize the proper winding. As shown in the animation the winding are energized in a pattern which rotates around the stator. The energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator. There are no sparks, which is one advantage of the bldc motor. The brushes of a dc motor have several limitations; brush life, brush residue, maximum speed, and electrical noise. BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable. However, BLDC motors require electronic control.
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