DC Motor

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. 

1.1          The armature:

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).

         

      1.2          The commutator:

  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).