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Download 1E6 Electrical Engineering Electricity and Magnetism Lecture 21
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1E6 Electrical Engineering Electricity and Magnetism Lecture 21: DC Motors I 21.1 Introduction One of the main outcomes of the Industrial Revolution towards the latter part of the 18th century was the beginning of the changeover from labourintensive manufacturing to machine-assisted manufacturing. A century later, the dawn of the widespread use of electricity gradually saw the introduction of electrical machines into all manufacturing industries. Today, motors are commonplace items in all aspects of life. They range in size and power from the tiny stepper motors used in digital analogue-style watches to those used in domestic equipment like fans, dish-washers, washing machines and central heating pumps to the heavy-duty motors used in high-powered industrial machines like electric fork-lift trucks, production line conveyers, hoists, lifts assembly-line robots, etc. In medium and heavy duty applications, most motors are ac powered simply because they run off mains electricity which is generated and distributed on an ac system. However, there are areas such as the automotive field, where power is available in dc form as in the case of a car or lorry battery which can provide significant power. Here, a dc motor is used as the starter motor as well as for driving heater fans, air conditioning units, windscreen wipers and a wide variety of relays. 21.2 Current Carrying Conductors in Magnetic Fields Recall that when a conductor which is carrying an electric current, I, is placed in a magnetic field, the field generated around the current carrying conductor interacts with the magnetic field into which it is placed. The conductor shown in Fig. 1 is placed into the magnetic field created by two permanent magnets with opposite poles facing each other. The current flowing through the conductor generates a magnetic field around the conductor as shown. As can be seen in Fig. 1, the lines of flux in the magnetic field created by the permanent magnets in the region above the conductor run in the same direction as those of the field generated by the current flowing in the conductor. Consequently, in the region above the conductor the two fields tend to repel each other. If the permanent magnets are considered rigid but the conductor mobile, then this gives rise to a force tending to move the conductor downward. On the other hand, in the region underneath the conductor the lines of flux of the two fields run in opposite directions and therefore the two fields tend to attract each other. In this case there is a consequent attractive force on the conductor tending to pull it downward. The overall effect of the interaction of the two fields is that there is a net force acting in the downwards direction on the conductor, and at a right angle to it. 1 permanent magnet S N permanent magnet conductor carrying a current downward force experienced by the conductor Fig. 1 A Single Current Carrying Conductor in a Magnetic Field The resulting force acting on a current carrying conductor placed in a magnetic field is proportional to the intensity of the magnetic field, the magnitude of the current flowing in the conductor and the length of the conductor and is given by: Force = Magnetic Flux Density x Current x Length of Conductor F=BIl Newtons upward force experienced by the left conductor permanent magnet N S permanent magnet conductors carrying currents in opposite directions downward force experienced by the right conductor Fig. 2 Conductors Carrying Currents in Opposite Directions in a Magnetic Field 2 If the conductor in Fig. 1 is free to move it will eventually be forced out of the magnetic field. If two conductors are placed within the same magnetic field as shown in Fig. 2, with the current flowing in opposite directions, then the forces they experience will also be in opposite directions. In the example shown, the right hand conductor will experience a downward force and the left hand conductor will experience an upward force. If these two conductors are free to move they will also eventually be forced out of the magnetic field. 21.3 Current Carrying Loop in a Magnetic Field Consider the situation in Fig. 3 where the two conductors have been combined into one conducting loop, with each side of the loop carrying the current in opposite directions. This conducting loop is also anchored in the centre at each end so that the conductors must remain within the magnetic field. A suitable means is also found of feeding current into the loop. With the loop in the position shown the same forces act on the conductors as before. However, this time the conductors do not move out of the field as the loop is anchored in the centre at each end. In this case the loop is forced to rotate in a clockwise direction for the direction of current flow in the loop shown. Consequently, by fixing the centre of the loop, the previous linear motion due to the force experienced by the conductors is now converted into a rotational motion. This motion can be exploited by connecting the centre supporting structure to a shaft, which can in turn be connected to some mechanical actuator. There are, however, two important features of this arrangement. (i) The Angle of Motion As the conducting loop rotates within the field, the angle of movement of the conductor with respect to the direction of the magnetic field changes. This means that the force on the conductor which induces this motion also changes. When the loop is aligned horizontally as shown in Fig. 4(a), the direction of motion of a conducting side of the loop is at right angles to the direction of the magnetic field. In this case the force exerted on the conductor is a maximum. When the loop is aligned at an angle to the direction of the magnetic field as shown in Fig. 4(b), the force acting on the conductor is reduced and is proportional to the sine of the angle between the direction of the magnetic field and the direction of motion of the conductor. This angle changes as the conducting loop rotates. Note that when the conducting loop is aligned vertically as shown in Fig. 4(c), the force acting on it in the direction of motion is reduced to zero. The effective force acting on the conductor in one side of the loop in the direction of its motion is given as: Force = Magnetic Flux Density x Current x Length of Conductor x Sinθ F = B I l Sinθ 3 N upward force experienced by the left conductor permanent magnet N S permanent magnet Elevation conducting loop carrying current centre supporting structure downward force experienced by the right conductor centre supporting structure current flow into loop current flow out of loop permanent magnet N l S permanent magnet l length of conductor within magnetic field conducting loop carrying current Plan centre supporting structure Fig. 3 A Conducting Loop Placed in a Magnetic Field 4 direction of motion of conductor θ direction of magnetic field (a) θ = 90o direction of motion of conductor direction of motion of conductor θ direction of magnetic field direction of magnetic field (b) 0 < θ < 90o (c) θ = 0o Fig. 4 Changes in Direction of Motion of Loop Relative to Magnetic Field (ii) Direction of Current The second important feature for consideration is the direction of current flow in the conducting loop. It can be seen from Fig. 5 that if the direction of current fed into the loop is fixed, then if the loop rotates through 180O to the horizontal position again, the direction of the current in the left hand and right hand branches of the loop have effectively reversed. This means that the magnetic fields surrounding left hand and right hand branches of the loop have also reversed. Consequently, the direction of the forces acting on the right hand and left hand sides of the loop reverse so that there is now an upward force acting on the right hand side and a downward force acting on the left hand side. This means that the forces are now acting in directions which tend to cause rotation of the loop in the opposite anticlockwise direction. This would essentially cause the rotation of the loop in the previously clockwise direction to slow down, stop and then begin again in the anticlockwise direction. upward force experienced by the right conductor permanent magnet N S permanent magnet downward force experienced by the left conductor centre supporting structure conducting loop carrying current Elevation Fig. 5 Effect of Rotation of Conducting Loop in Magnetic Field 5 This effect can be seen in Fig. 6 where the change in the forces on each side of the loop as it rotates can be seen. It is clear from Fig. 6(a) that with the fixed direction of feed to the conducting loop the forces reverse direction once the loop passes vertical alignment. This would mean that the loop would oscillate backwards and forwards through an angle of 180o. Fig. 6(b), on the other hand, shows that if the direction of the current fed to the loop is itself reversed when the loop reaches vertical alignment, then the directions of the forces acting on each side of the loop are maintained. In the latter case, the direction of rotation will remain clockwise as shown. However, there is still a variation in the magnitude of the force experienced by each conductor due to the angle of motion changing with respect to the direction of the magnetic field. This gives rise to a continuous variation in speed when there is only one conducting loop as in the example shown, which means that the speed of rotation is not constant. F1max F1 = 0 F1 F2 max F2 F1 rev F1 F2 F2 = 0 F2 rev (a) Direction of Current Feed to Loop Fixed F1max F1 = 0 F1 F2 max F1 F2 F2max F2 F1max F2 = 0 (b) Direction of Current Feed to Loop Reversed at Vertical Position Fig. 6 Effect of Reversing Direction of Current Feed to Loop in Magnetic Field Conclusion: This discussion then identifies in particular the need for reversal of the direction of current in the loop as it rotates in order to maintain the force in the correct direction for rotation. This is essential if the force experienced is to be exploited as the foundation of the motor. The other issues of variation in the magnitude of the force experienced, the variation in speed etc., must also be addressed. 6 21.4 The DC Motor The familiar structure a dc motor in the form of a self-contained machine is shown in Fig. 7 below. Fig. 7 The Structure of a Typical DC Motor in Machine Form The housing is usually made of cast iron for heavy duty applications and is often bolted to a larger frame, but may be of steel or aluminium for light portable devices. The ends of the housing can be detached from the body and contain bearings to support the rotating parts at each end. The main housing acts as the Stator. This is the static part of the motor which is fixed and does not move. It contains the magnets which provide the magnetic field. These magnets can be of the permanent magnetised metal type, as is often the case for light motors used in portable applications like model toys or light tools. However the magnetic poles are more commonly electromagnets in heavy-duty motors. In this case, coils having several turns are used to generate a strong magnetic field in metal poles which provide a more consistent and efficient magnetic field. Very often, there are more than two magnetic poles to increase the strength and uniformity of the field and the coils used to provide these are interconnected and collectively are referred to as the Field Winding. Inserted into the housing is the Rotor, which is the essential moving part of the motor and includes a shaft which fits into the bearings in the ends of the housing. The shaft protrudes from one end (sometimes both ends) of the housing and this can then be connected to a mechanical gearing mechanism or a belt drive to transport the energy developed in the motor to some mechanical actuator to do physical work. The Rotor is itself divided into two essential parts as shown in Fig. 8. These consist of the Armature and the Commutator. The Armature usually contains several metallic segments which are insulated from each other but have slots cut in the top along their length. These slots contain the conductors which experience the force within the magnetic field when they are fed with a current from the power source. There are multiple loops formed by the coils mounted in the armature which are also interconnected and collectively these are referred to as the Armature Winding. The second element of the rotor is the Commutator which can also be seen in Fig. 8. This provides a means of connecting all of the individual coils in the armature winding to the power source but also provides a means of allowing the direction of current flowing in the conducting loops to be 7 Fig. 8 The Construction of the Rotor in a DC Motor reversed at the appropriate points in the cycle as they rotate within the magnetic field. The commutator has several segments to allow individual connection to separate groups of coils requiring opposite directions of current flow. Since the commutator rotates, power cannot be connected to it by soldered wires. Instead this is accomplished by using a set of Brushes. These are usually made of carbon and are spring-loaded to maintain contact with the commutator as it rotates. The power source is then connected to metal contacts on the back of the brushes. The brushes wear down with time and must be replaced periodically. Fig. 9 shows an end-on view through a dc motor with the rotor mounted inside the stator. This machine uses four magnetic poles to increase the strength of the magnetic field. It can also be seen that the faces of the poles are shaped so as to follow the circular curvature of the armature. This gives a magnetic field which is normal to the rotor right around the full circle of rotation. The poles are also positioned so that the magnetic field passes right through the rotor. The field winding is wound around the poles of the stator. The diagram also shows that the direction of current flow is different for particular groups of conductors in the armature winding at different points of the rotational cycle. It can be seen that the direction of rotation in this particular example is anticlockwise. 8 rotor / armature stator S field winding armature winding N N S magnetic field commutator shaft electromagnet Fig. 9 An End-on View of Rotor and Stator of a DC Motor The exploded view revealing the internal structure of a fairly heavy-duty dc motor is shown in Fig. 10. Some examples of dc motors ranging from miniature and light-duty to medium and heavy-duty are shown in Fig. 11. Light duty dc motors are very common in battery operated toys and such like. Heavier duty dc motors are found in tools like cordless drills, sanders and light portable appliances that operate off rechargeable batteries. Heavier dc motors can be found in automotive appliances like the windscreen wiper motor in cars, hoists in trucks and the drives in fork-lift trucks, etc. 9 Fig. 10 An Exploded View Showing the Internal Structure of a DC Motor Fig. 11 Examples of Miniature, Light, Medium and Heavy-Duty DC Motors 10