LECTURE NOTES
DC MOTORS NOTES AND SOLVED EXAMPLES
COURSE CONTENT
DC MOTOR
Explain Principle of working of a DC motor.
Explain concept of development of torque & back EMF in DC motor including simple problems.
Derive equation relating to back EMF, Current, Speed and Torque equation
Classify DC motors & explain characteristics, application.
State & explain three point & four point stator/static of DC motor by solid State converter.
Explain Speed of DC motor by field control and armature control method.
Explain power stages of DC motor & derive Efficiency of a DC motor.
, DC MOTOR
Principle of Operation
DC motor operates on the principle that when a current carrying conductor is placed in a magnetic field, it
experiences a mechanical force given by F = BIL newton. Where ‘B’ = flux density in wb, ‘I’ is the current
and ‘L’ is the length of the conductor. The direction of force can be found by Fleming’s left hand rule. From
the point of construction, there is no difference between a DC generator and DC motor. Figure 3.1 shows a
multipolar DC motor. Armature conductors are carrying current downwards under North Pole and upwards
under South Pole. When the field coils are excited, with current carrying armature conductors, a force is
experienced by each armature conductor whose direction can be found by Fleming’s left hand rule. This is
shown by arrows on top of the conductors. The collective force produces a driving torque which sets the
armature into rotation. The function of a commutator in DC motor is to provide a continuous and
unidirectional torque.
In DC generator the work done in overcoming the magnetic drag is converted into electrical energy.
Conversion of energy from electrical form to mechanical form by a DC motor takes place by the work done
in overcoming the opposition which is called the ‘back emf’.
Back EMF and its Significance:
What is back emf:
When the armature of a DC motor rotates under the influence of the driving torque, the armature conductors
move through the magnetic field and hence emf is induced in them as in a generator.
The induced emf acts in opposite direction to the applied voltage V (Lenz’s law) and is known as Back EMF
or Counter EMF (Eb).
The equation for back emf in a DC motor is given below,
The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small
when the motor is running under normal conditions.
How Back EMF Occur in DC Motor:
Consider a shunt wound DC motor
Therefore, driving torque acts on the armature which
begins to rotate. As the armature rotates, back emf Eb is
induced which opposes the applied voltage V. The applied
voltage V has to force current through the armature against the back emf Eb. The electric work done in
overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the
armature. It follows, therefore, that energy conversion in a dc motor is only possible due to the production of
back emf Eb.
, Net voltage across armature circuit = V – Eb
If Ra is the armature circuit resistance, then, Ia = (V – Eb)/Ra
Since V and Ra are usually fixed, the value of Eb will determine the current drawn by the motor.
If the speed of the motor is high, then back e.m.f. Eb (= PφZN/60 A) is large and hence the motor will draw
less armature current and vice-versa.
The significance of Back EMF:
The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as
much armature current as is just sufficient to develop the torque required by the load.
Armature current (Ia),
When the motor is running on no load, small torque is required to overcome the friction and windage losses.
Therefore, the armature current Ia is small and the back emf is nearly equal to the applied voltage.
If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the speed at
which the armature conductors move through the field is reduced and hence the back emf Eb falls.
The decreased back emf allows a larger current to flow through the armature and larger current means increased
driving torque.
Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the
armature current is just sufficient to produce the increased torque required by the load.
If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that
armature is accelerated.
As the armature speed increases, the back emf Eb also increases and causes the armature current Ia to decrease.
The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque
required by the load.
Therefore, the back emf in a DC motor regulates the flow of armature current i.e., it automatically changes the
armature current to meet the load requirement.
Torque Equation
When armature conductors of a DC motor carry current in the presence of stator field flux, a mechanical torque
is developed between the armature and the stator. Torque is given by the product of the force and the radius at
which this force acts.
Torque T = F × r (N-m) …where, F = force and r = radius of the armature Work
done by this force in once revolution = Force × distance = F × 2πr
(where, 2πr = circumference of the armature)
DC MOTORS NOTES AND SOLVED EXAMPLES
COURSE CONTENT
DC MOTOR
Explain Principle of working of a DC motor.
Explain concept of development of torque & back EMF in DC motor including simple problems.
Derive equation relating to back EMF, Current, Speed and Torque equation
Classify DC motors & explain characteristics, application.
State & explain three point & four point stator/static of DC motor by solid State converter.
Explain Speed of DC motor by field control and armature control method.
Explain power stages of DC motor & derive Efficiency of a DC motor.
, DC MOTOR
Principle of Operation
DC motor operates on the principle that when a current carrying conductor is placed in a magnetic field, it
experiences a mechanical force given by F = BIL newton. Where ‘B’ = flux density in wb, ‘I’ is the current
and ‘L’ is the length of the conductor. The direction of force can be found by Fleming’s left hand rule. From
the point of construction, there is no difference between a DC generator and DC motor. Figure 3.1 shows a
multipolar DC motor. Armature conductors are carrying current downwards under North Pole and upwards
under South Pole. When the field coils are excited, with current carrying armature conductors, a force is
experienced by each armature conductor whose direction can be found by Fleming’s left hand rule. This is
shown by arrows on top of the conductors. The collective force produces a driving torque which sets the
armature into rotation. The function of a commutator in DC motor is to provide a continuous and
unidirectional torque.
In DC generator the work done in overcoming the magnetic drag is converted into electrical energy.
Conversion of energy from electrical form to mechanical form by a DC motor takes place by the work done
in overcoming the opposition which is called the ‘back emf’.
Back EMF and its Significance:
What is back emf:
When the armature of a DC motor rotates under the influence of the driving torque, the armature conductors
move through the magnetic field and hence emf is induced in them as in a generator.
The induced emf acts in opposite direction to the applied voltage V (Lenz’s law) and is known as Back EMF
or Counter EMF (Eb).
The equation for back emf in a DC motor is given below,
The back emf Eb(= PΦZN/60 A) is always less than the applied voltage V, although this difference is small
when the motor is running under normal conditions.
How Back EMF Occur in DC Motor:
Consider a shunt wound DC motor
Therefore, driving torque acts on the armature which
begins to rotate. As the armature rotates, back emf Eb is
induced which opposes the applied voltage V. The applied
voltage V has to force current through the armature against the back emf Eb. The electric work done in
overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the
armature. It follows, therefore, that energy conversion in a dc motor is only possible due to the production of
back emf Eb.
, Net voltage across armature circuit = V – Eb
If Ra is the armature circuit resistance, then, Ia = (V – Eb)/Ra
Since V and Ra are usually fixed, the value of Eb will determine the current drawn by the motor.
If the speed of the motor is high, then back e.m.f. Eb (= PφZN/60 A) is large and hence the motor will draw
less armature current and vice-versa.
The significance of Back EMF:
The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as
much armature current as is just sufficient to develop the torque required by the load.
Armature current (Ia),
When the motor is running on no load, small torque is required to overcome the friction and windage losses.
Therefore, the armature current Ia is small and the back emf is nearly equal to the applied voltage.
If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the speed at
which the armature conductors move through the field is reduced and hence the back emf Eb falls.
The decreased back emf allows a larger current to flow through the armature and larger current means increased
driving torque.
Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the
armature current is just sufficient to produce the increased torque required by the load.
If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement so that
armature is accelerated.
As the armature speed increases, the back emf Eb also increases and causes the armature current Ia to decrease.
The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque
required by the load.
Therefore, the back emf in a DC motor regulates the flow of armature current i.e., it automatically changes the
armature current to meet the load requirement.
Torque Equation
When armature conductors of a DC motor carry current in the presence of stator field flux, a mechanical torque
is developed between the armature and the stator. Torque is given by the product of the force and the radius at
which this force acts.
Torque T = F × r (N-m) …where, F = force and r = radius of the armature Work
done by this force in once revolution = Force × distance = F × 2πr
(where, 2πr = circumference of the armature)
