Construction and Operation of
DC Motor
I. Objectives:
At the
end of this lesson the student should be able to:
1- Describe the construction of DC motor.
2- Describe the principle of operation of DC motor.
II.
Introduction:
A DC (Direct Current) motor in simple words is a
device that converts direct current (electrical energy) into mechanical energy. It’s of vital
importance for the industry today, and is equally important for engineers to
look into the working principle of DC motor. In order to understand the operating principle of dc motor we need to first
look into its constructional feature. Then we can identify the types of DC
motors and finally describe the application of this motor in the industry.
1 Construction of DC motor:
The main parts of DC motor are:
1-
The armature.
2-
The commutator.
3-
The Field Poles.
4-
The Frame.
5-
The End Plates.
6-
The Brushes and Brush Holders.
Is the rotating part of the motor, It is made of
laminated iron, with slots for the coils of wire. The laminated core is pressed
onto a steel shaft that also holds the commutator as shown in figure(1).
It
makes electrical contact between the stationary brushes and the coils of wire
in the rotating armature. The commutator is located at one end of the shaft, and rotates with the armature. It is
made of copper segments. The segments are insulated with mica and each segment
is connected to one end of the armature winding, figure (2).
1.3
The Field Poles:
The
field poles could be either a permanent magnets or electro magnets. They Provide
the magnetic field required in order for the armature to generate a potential
difference between the ends of the coils. The field poles are mounted on the
side of the frame. They are secured to the frame with machine screws or bolts.
The field poles are made of laminated iron. The coils that generate the
magnetic field are wound around the iron poles as shown in figure (3).
1.4 The Frame:
It surrounds the working parts of the motor and gives them support and protection. It is made of cast iron or wrought iron as shown in figure(4).
1.5 The End Plates:
They support the weight of the armature, and keep the armature centered within the field poles. The end plates are bolted to the motor frame. They contain bearings to reduce friction on the rotating shaft. Figure(5).
1.6 The Brushes and Brush Holders:
The brushes are the points of contact for the motor to the load, they are made of carbon and held in place by brush holders, as shown in figure(6).
They ride on the commutator and are made to slide up and down in their holders. A flexible braided conductor connects each brush to the external circuit.
2
Principle of Operation of
DC motor:
The principle of
working of a DC motor is that "whenever a current carrying conductor
is placed in a magnetic field, it experiences a mechanical force". The
direction of this force is given by Fleming's left hand rule and it's magnitude is given by F = BIL. Where,
B = magnetic flux density, I = current and L = length of the conductor within
the magnetic field. Figure(7). 
2.1 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 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.
2.2 The Operation of DC motor
can be described in steps:
1.
The DC supply produces a current that flows in the armature coils.
2. A magnetic field will be produced in both sides of armature coils.
3.The field poles create a
field flux, with a north to south polarity.
4. The magnetic field in the
armature coils bends the field flux produced by
the field poles.
5. The bent flux create a net
"up" force on the left side of the armature and a net
"down" force on the right side. And this will cause the armature to
rotate, as shown in figure (8).
