• In this tutorial, we will logically understand the
operation and construction of a commercial DC motor.
• let’s first start with the simplest DC motor possible, the
stator provides a constant magnetic field on the
armature of rotor which is the rotating part, a simple
coil. The armature is connected to a DC power source
through a commutator rings.
• When the current flows to the coil, an electromagnetic
force is induced on it according to the Lorentz’s law.
• So, the coil will start to rotate.
• In physics, particularly electromagnetism, the Lorentz
force is the combination of electric and magnetic force
on a point charge due to electromagnetic fields. If a
particle of charge q moves with velocity v in the
presence of an electric field E and a magnetic field B,
then it will experience a force
• Variations on this basic formula describe the magnetic force on a
current-carrying wire (sometimes called Laplace force).
• The force F acting on a particle of electric charge q with
instantaneous velocity v, due to an external electric field E and
magnetic field B, is given by .
• Here, we see Lorentz force F on a charged particle (of charge q) in
motion (instantaneous velocity v). The E field and B field vary in
space and time.
• As the coil rotates, the commutator rings connect with
the power source of opposite polarity. As a result, on
the left side of the coil, the electricity will always flow
away, and on the right side, electricity will always flow
• This ensures that the torque action is always in the same
direction throughout the motion, so the coil will
continue rotating. But, if you observe the torque action
on the coil closely, you will notice that, when the coil is
nearly perpendicular to the magnetic flux, the torque
action nears 0.
• As a result, there will be an irregular motion of the rotor
if you run a such DC motor. Here is the trick to
overcoming this problem, add 1 more loop to the rotor
with a separate commutator for it.
• In this arrangement, when the 1st loop is on verticular
position, the 2nd loop will be connected to the power
source. So a magnetic force will be always presented in
• More over, the more such loops the smoother will be
the rotor rotation. In practical motor, the armature loops
are feeding inside slots of highly permeable steel layers.
This will enhance magnetic flux in reaction, brushes help
to maintain contact with the power source.
• A permanent magnet stator pole is used only for very
small DC motors, most often an electromagnet is used.
The field coil of the electromagnet is powered from the
same DC source, the field coils can be connected to the
rotor windings in 2 different ways: Parallel or Series.
• The results is 2 different kind of DC motor constructions,
a shunt and a series motor. The series one has good
starting torque but its speed slows down with the load.
• The shunt motor has a low starting torque but it’s able
to run almost in constant speed, in respective of the
load acting in the rotor.
• Unlike the other electrical machines, DC motors exibly
unique caracteristic: the production of back e.m.f.
• A rotating loop in magnetic field will produce an e.m.f
according to the principle of electromagnetic induction,
the case of rotating armature loop is always the same.
An internal e.m.f will be induced that opposes the
applied input voltage according to Lenz’s law.
• The back e.m.f reduces armature current by a large
amount, back e.m.f is proportional to the speed of the
rotor. At the starting of the motor, back e.m.f is too low
thus the armature current becomes too high. Leading to
the burn out of the rotor; so, a propre starting
mechanism that controls the applied input voltage is
necessary in large DC motors.
• One of the interesting varitions in a DC motors is the
universal motors, which is capable to run in both AC and
DC power sources.
• To know more about it, please check universal motor.