3. Page 3
Construction of DC Motor
• Armature is typically a soft iron drum
mounted on the motor shaft, with the
armature conductors set axially into
the surface of the drum. Also mounted
on the armature shaft are the
commutator segment, to which the
armature conductors are connected.
The armature shaft is mounted in ball
bearings in each end, the bearings
being held in the ends of the motor
casing.
• The field windings are attached to the
inside of the Yoke and form two poles
fitting closely around the armature with
a running clearance of about 2.5mm.
company presentation 2012
18/11/2021
4. Page 4
DC Motor
• A DC motor in simple words is a
device that converts direct
current(electrical energy) into
mechanical energy.
• The very basic construction of a dc
motor contains a current carrying
armature
• Armature is connected to the supply
end through commutator segments
and brushes
• It is placed within the north south
poles of a permanent or an electro-
magnet as shown in the diagram
company presentation 2012
18/11/2021
5. Page 5
Fleming Left Hand Rule
• Fleming’s left hand rule says that if we
extend the index finger, middle finger and
thumb of our left hand in such a way that
the electric current carrying conductor is
placed in a magnetic field (represented by
the index finger) is perpendicular to the
direction of current (represented by the
middle finger), then the conductor
experiences a force in the direction
(represented by the thumb) mutually
perpendicular to both the direction of field
and the current in the conductor.
company presentation 2012
18/11/2021
6. Page 6
Magnitude of Force
• When a current ( I ) carrying conductor of length ( L ) is placed in a
magnetic field ( B ), it experiences a force F which is given by
• F = B.I.L.Sin θ
• Where
• B is the Flux Density
• I is the magnitude of current
• L is the length of the conductor
• θ is the angle between Magnetic Field and Conductor
company presentation 2012
18/11/2021
7. Page 7
Torque
• Torque also called as Rotating force acts on the armature of diameter w
and is given by
• Torque = force, tangential to the direction of armature rotation X
distance
• Hence, τ = F cos α w =B.I.L.w.cos α
• Where
• w is the distance between
opposite two conductors
• α is the angle between the plane of
the armature turn and the plane of reference
company presentation 2012
18/11/2021
8. Page 8
Motor EMF
• Back EMF: The loop of an armature is moving through the stationary
field as the armature rotates and this inevitably induces an EMF in the
armature. This EMF produces a current flow that opposes the applied
current from the battery and therefore reduces the total armature current
flow. The induced voltage is known as back EMF
• Net EMF: The difference between the applied EMF and the back EMF is
known as the net EMF and it is this that determines the torque produced
in the armature shaft
• In order to ensure that the net EMF is sufficient the resistance of the
armature winding is kept as low as possible
company presentation 2012
18/11/2021
9. Page 9
Armature Current
• The initial current flow through the armature, before it begins to rotate, is
determined by the applied voltage and the armature resistance. If the
resistance is low the current flow will be very high. As the motor gains
speed the back EMF increases and reduces the current flow through the
armature.
• To avoid excess starting armature current, some DC motors have a
resistance built in to the armature windings, which automatically cuts out
as motor speed increases
company presentation 2012
18/11/2021
10. Page 10
Operation of DC Motor
• Initially considering
the armature is in its
starting point or
reference position
where the angle α =
0
• τ = BIL w cos0 =
BILw
company presentation 2012
18/11/2021
11. Page 11
Operation of DC Motor
• Once the armature is
set in motion, the
angle α between the
actual position of the
armature and its
reference initial
position goes on
increasing in the path
• τ = BIL w cos α
company presentation 2012
18/11/2021
12. Page 12
Operation of DC Motor
• Once the armature is
set in motion, the
angle α between the
actual position of the
armature and its
reference initial
position goes on
increasing in the path
• τ = BIL w cos 90 o = 0
company presentation 2012
18/11/2021
13. Page 13
Induced Magnetic
Field (Due to current)
Fixed Magnetic Field
Force
A Conductor in a Fixed Magnetic Field A Current Carrying Conductor in a Fixed
Magnetic Field
Motor Armature Rotation
14. Page 16
Types of Motors
DC Motor
Series Wound
Shunt Wound
Compound Wound
company presentation 2012
18/11/2021
15. Page 17
Motors – Series / Shunt / Compound
company presentation 2012
18/11/2021
16. Page 18
Series Wound
• The field coils are connected in series with the armature
• At starting, when the current flow is very high, consequently a
characteristics of the series wound motor is high starting torque. This is
useful in circumstances where the motor will be required to start against
a high load and where the running load is also high.
• Examples of instances where series wound motors are used are engine
starter motors, flap operating motors and landking gear operating
motors.
company presentation 2012
18/11/2021
17. Page 19
Shunt Wound
• The field coils are connected in parallel (shunt) with the armature
windings
• The resistance of the field coils is deliberately set to limit the field
current to that required for normal operation of the motor, and is much
higher than the armature resistance
• On start up the current flow through the armature is high, because of its
low resistance
• A Characteristics of shunt wound DC motor is low starting torque
• As the armature speed increases, increasing back EMF will cause the
armature current to decrease
• They are particularly useful where constant speed under varying load
conditions is requirement viz. Fuel Pumps and fans
company presentation 2012
18/11/2021
18. Page 20
Compound Wound
• It has two sets of field winding, one connected in series with the
armature and the other in parallel. The low resistance series winding
and higher resistance shunt windings.
• The compound wound motor is suited to applications where load may
vary from zero to maximum and where starting loads may be high
• In aircraft they are often used to drive hydraulic pumps and used as a
starter / generator.
company presentation 2012
18/11/2021
20. Page 22
Reversible DC Motor
• Reverse the rotating direction could be achieved by the means of
switching arrangement that reversed the polarity of the DC supply to
either the field or the armature (but not both). This would reverse the
magnetic attraction and repulsion and this reverse the direction of
rotation of the armature.
• Alternatively, split field windings may be utilized, where two sets of field
windings, either would in opposite directions on a common pole (or
core) or on alternate poles around the inside of the motor casing
• Used to operate flaps and landing gear
company presentation 2012
18/11/2021
21. Page 23
Speed Control
• Armature Control
• Field Control
company presentation 2012
18/11/2021
22. Page 24
Losses in D.C. Machines
• Copper Losses – takes place in armature winding and field winding
• Armature winding losses
• Field winding losses
• Iron Losses – takes place in armature and field core
• Hysteresis losses
• Eddy Current Losses
• Stray Losses – Iron losses + Friction Losses
• Friction losses
company presentation 2012
18/11/2021
Since α = 0, the term cos α = 1, or the maximum value, hence torque at this position is maximum given by τ = BILw. This high starting torque helps in overcoming the initial inertia of rest of the armature and sets it into rotation
Since α = 0, the term cos α = 1, or the maximum value, hence torque at this position is maximum given by τ = BILw. This high starting torque helps in overcoming the initial inertia of rest of the armature and sets it into rotation
virtually no rotating torque acts on the armature at this instance. But still the armature does not come to a standstill, this is because of the fact that the operation of dc motor has been engineered in such a way that the inertia of motion at this point is just enough to overcome this point of null torque. Once the rotor crosses over this position the angle between the actual position of the armature and the initial plane again decreases and torque starts acting on it again
The magnetic field surrounding a current carrying conductor interacts with an existing magnetic field.
Use the Left Hand Rule to Determine the Rotation Direction of the Armatures in A and B
Hint: You will have to turn your left hand upside down for example A
Notice that when the current through the armature is reversed, it moves (Rotates) in the opposite direction
Armature Control: Increase the armature current to increase the speed. Practically not possible, as armature current is very high and would need bulky variable resistor.
Field Control: Increase in field coil current, increases magnetic field, thus increasing Back EMF which reduces armature current resulting in reduction of speed
To compare the magnetic field to a viscous fluid: the thicker it gets, the harder it is for the armture to turn. The thinner it gets, the easier it is for the armature to turn.
