DC Motors I and Current Carrying Conductors in Magnetic Fields


One of the major outcomes of the Industrial Revolution towards the latter portion of the 18th century was the starting of the changeover from labour-intensive manufacturing to machine-assisted manufacturing. A century later, the dawn of widespread utilization of electricity gradually saw the introduction of electrical machines into all manufacturing business. Nowadays, motors are commonplace items in each and every aspects of life. They vary in size and power from the tiny stepper motors employed in digital analogue-style watches to such employed in domestic equipment such as fans, washing machines, dish-washers and central heating pumps to the heavy-duty motors employed in high-powered industrial machines such as electric fork-lift trucks, hoists, production line conveyers, lifts assembly-line robots and so on. In medium and heavy duty applications, most f the motors are ac powered simply since they run off mains electricity that is produced and distributed on an ac system. Though, there are areas like the automotive field, where power is accessible in dc form as in the case of a car or lorry battery that can give significant power. Here, a dc motor is employed as the starter motor and also for air conditioning units, driving heater fans, windscreen wipers and a broad variety of relays.
Current Carrying Conductors in Magnetic Fields:

Remember that whenever a conductor that is carrying an electric current, I, is positioned in a magnetic field, the field made around the current carrying conductor interacts with the magnetic field into which it is positioned. The conductor shown in figure below is positioned into the magnetic field made by two permanent magnets with opposite poles facing one other. The current flowing via the conductor produces a magnetic field around the conductor as shown.

As can be seen in figure below, the lines of flux in magnetic field made by the permanent magnets in the area above the conductor run in similar direction as those of the field made by the current flowing in the conductor. As a result, in the area above the conductor and two fields tend to repel each other. When the permanent magnets are considered rigid however the conductor mobile, then this gives mount to a force tending to move the conductor downward. On other hand, in the area beneath the conductor, the lines of flux of two fields run in opposite directions and thus the two fields tend to attract one other. In this case, there is a resultant attractive force on the conductor tending to pull it downward. The total effect of the interaction of two fields is that there is a total force acting in the downwards direction on conductor, and at a right angle to it.

Figure: A Single Current Carrying Conductor in a Magnetic Field

The resultant force acting on a current carrying conductor positioned in a magnetic field is proportional to the intensity of magnetic field, the magnitude of current flowing in the conductor and the length of conductor is given by:

Force = magnetic flux density x Current x Length of conductor

F = B I l Newtons

Figure: Conductors carrying currents in opposite directions in a Magnetic Field

When the conductor in figure above is free to move it will ultimately be forced out of the magnetic field.

When two conductors are positioned in the same magnetic field as shown in figure above, with the current flowing in the opposite directions, then the forces they experience will as well be in opposite directions. In the instance shown, the right hand conductor will experience a downward force and the left hand conductor will experience an upward force. When these two conductors are free to move they will as well ultimately be forced out of the magnetic field.

Current Carrying Loop in a Magnetic Field:

Consider the condition in figure shown below where the two conductors have been joined into one conducting loop, with each and every side of the loop carrying current in opposite directions. This conducting loop is as well anchored in the centre at each end and hence the conductors should remain in the magnetic field.

An appropriate means is as well found of feeding the current into loop. With loop in the position shown similar forces act on the conductors as before. Though, this time the conductors don’t move out of the field as loop is anchored in the centre at each end. In this condition, the loop is forced to rotate in a clockwise direction, for the direction of current flow in the loop as shown. As a result, by fixing the centre of loop, the prior linear motion due to the force experienced by the conductors is now transformed into a rotational motion. This motion can be exploited by joining the centre supporting structure to a shaft that can in turn be joined to some mechanical actuator. There are, though, two significant features of this arrangement.


Figure: Conducting Loop positioned in a magnetic field

a) The Angle of Motion:

Since the conducting loop rotates in the field, the angle of movement of conductor with respect to the direction of magnetic field changes. This signifies that the force on conductor that induces this motion as well changes. Whenever the loop is aligned horizontally as shown in figure (a), the direction of motion of conducting side of the loop is at right angles to the direction of magnetic field. In this condition the force exerted on the conductor is maximum. Whenever the loop is aligned at an angle to the direction of magnetic field as shown in figure (b), the force acting on the conductor is decreased and is proportional to the sine of angle between the direction of magnetic field and direction of motion of the conductor. This angle modifies as the conducting loop rotates. Note that whenever the conducting loop is aligned vertically as shown in figure (c), the force acting on it in the direction of motion is decreased to zero. The effective force acting on the conductor in one side of loop in the direction of its motion is as follows:
Force = Magnetic flux density x Current x Length of the conductor x sin θ

F = B I l sin θ

Figure: Changes in the direction of motion of Loop Relative to the magnetic field

b) Direction of Current:

The second significant feature for considering the direction of current flow in the conducting loop. This can be seen from figure below that when the direction of current fed into the loop is fixed, then if the loop rotates via 180O to the horizontal position again, then the direction of current in the left hand and right hand branches of loop have efficiently reversed. This signifies that the magnetic fields surrounding left hand and right hand branches of the loop have as well reversed. As a result, the direction of forces acting on right hand and left hand sides of the loop reverse and hence there is now an upward force acting on the right hand side and downward force acting on the left hand side. This signifies that the forces are now acting in the directions which tend to cause the rotation of loop in the opposite anti-clockwise direction. This would basically cause the rotation of loop in the earlier clockwise direction to slow down, stop and then start again in the anti-clockwise direction.

Figure: Effect of rotation of conducting loop in the magnetic field

This consequence can be seen in figure below where the change in the forces on each and every side of the loop as it rotates can be seen. This is clear from the figure shown below that with the fixed direction of feed to conducting loop, the forces reverse direction once the loop passes the vertical alignment. This would signify that the loop would oscillate forwards and backwards via an angle of 180o. Figure (b) on the other hand, exhibits that if the direction of current fed to the loop is itself reversed whenever the loop reaches the vertical alignment, then the directions of forces acting on each and every side of the loop are maintained. In latter case, the direction of rotation will remain clockwise as shown below. Though, there is still a variation in the magnitude of force experienced by each and every conductor due to the angle of motion changing with respect to the direction of magnetic field. This gives mount to a continuous variation in speed whenever there is only one conducting loop as in the example shown below, that means that the speed of rotation is not steady.

Figure: Effect of Reversing Direction of Current Feed to Loop in Magnetic Field


This discussion then recognizes in particular the requirement for reversal of the direction of current in the loop since it rotates in order to maintain the force in the right direction for rotation. This is necessary if the force experienced is to be exploited as the foundation of motor. The other issues of variation in the magnitude of force experienced, the variation in speed and so on should also be addressed.

The DC Motor:

The well-known structure of a dc motor in the form of a self-contained machine is as shown in figure below.

Figure: The Structure of a typical DC Motor in machine form

The housing is generally made up of cast iron for heavy duty applications and is frequently bolted to a bigger frame, however might be of aluminium or steel for light portable devices. The ends of housing can be separated from the body and have bearings to support the rotating portions at each end.

The major housing acts as the Stator. This is the static portion of the motor that is fixed and doesn’t move. It includes the magnets that offer the magnetic field. Such magnets can be of permanent magnetised metal type, as is frequently the case for light motors employed in portable applications similar to model toys or light tools. Though the magnetic poles are more generally electromagnets in heavy-duty motors. In this condition, coils containing several turns are employed to produce a strong magnetic field in metal poles that give a more consistent and proficient magnetic field. Very frequently, there are more than two magnetic poles to raise the strength and uniformity of the field and the coils utilized to give these are inter-connected and collectively are termed to as the Field Winding.

Inserted into housing is the Rotor that is the necessary moving portion of the motor and comprises a shaft that fits into the bearings in the ends of housing. The shaft protrudes from one end (that is, sometimes both ends) of the housing and this can then be joined to a mechanical gearing mechanism or a belt drive to transport the energy build up in the motor to certain mechanical actuator to do physical work.

The Rotor is itself classified into two necessary portions as shown in figure below. These comprises of Armature and Commutator. The Armature generally consists of some metallic segments that are insulated from one other however slots have cut in the top all along their length. Such slots include the conductors that experience the force in the magnetic field whenever they are fed with a current from the power source. There are multiple loops build up by the coils mounted in the armature that are as well interconnected and collectively such are termed to as the Armature Winding. The second element of the rotor is Commutator that can as well be seen in figure shown below. This gives a means of connecting all of the individual coils in an armature winding to the power source however as well gives a means of permitting the direction of current flowing in the conducting loops to be reversed at suitable points in the cycle as they rotate in the magnetic field. The commutator has some segments to permit individual connection to separate groups of coils needing opposite directions of the current flow.


Figure: The Construction of the Rotor in a DC Motor

As the commutator rotates, power can’t be connected to it through soldered wires. Rather this is accomplished by employing a set of Brushes. These are generally made up of carbon and are spring-loaded to sustain contact with the commutator as it rotates. The power source is then joined to metal contacts on the back of brushes. The brushes wear down with time and should be changed periodically.

Figure below exhibits an end-on view via a dc motor with the rotor mounted within the stator. This machine employs four magnetic poles to raise the strength of magnetic field. This can as well be seen that the faces of the poles are shaped therefore as to follow the circular curvature of the armature. This provides a magnetic field that is normal to the rotor right around the full circle of rotation. Poles are as well positioned and hence the magnetic field passes right via the rotor. The field winding is wound around the poles of stator. The diagram as well exhibits that the direction of current flow is dissimilar for particular groups of conductors in the armature winding at various points of the rotational cycle. This can be seen that the direction of rotation in this specific illustration is anti-clockwise.

Figure: An End-on View of Rotor and Stator of a DC Motor

The exploded view revealing the interior structure of a fairly heavy-duty dc motor is as shown in figure below. Various examples of dc motors ranging from miniature and light-duty to medium and heavy-duty are shown in figure below. Light duty dc motors are very ordinary in battery operated toys and such like. The heavier duty dc motors are found in tools such as cordless drills, sanders and light portable appliances which operate off rechargeable batteries. The heavier dc motors can be found in automotive appliances such as the windscreen wiper motor in cars, hoists in trucks and drives in the fork-lift trucks and so on.

536_Internal Structure of a DC Motor.jpg

Figure: An Exploded view exhibiting the Internal Structure of a DC Motor


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