Download 1E6 Electrical Engineering Electricity and Magnetism Lecture 21

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
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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
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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
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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
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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.
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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
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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.
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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.
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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
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