DC Motor


·    Principle of DC motors

  • Types of DC Motors
  • Construction of DC Machines
  • Working of DC Motor
  • Significance of back emf
  • Voltage, Current and Power equation
  • Advantages, Disadvantages and Application of DC motors
  • Jobs in DC motors

    Principle of DC Motor:


    • Faradays laws of Electro Magnetic Induction
    • “ When a current carrying conductor is placed in a magnetic field; it experiences a mechanical force”
    • The direction of this force is determined by Fleming’s left hand rule and its magnitude is given by the relation

      Types of DC Motor:


      • DC Shunt Motor : A DC shunt motor (also known as a shunt wound DC motor) is a type of self-excited DC motor where the field windings are shunted to or are connected in parallel to the armature winding of the motor. Since they are connected in parallel, the armature and field windings are exposed to the same supply voltage.

        This supply current in case of the shunt wound DC motor is split up into 2 parts. Ia, flowing through the armature winding of resistance Ra and Ish flowing through the field winding of resistance Rsh. The voltage across both windings remains the same.

        From there we can write

        Thus we put this value of armature current Ia to get general voltage equation of a DC shunt motor.

        Now in general practice, when the motor is in its running condition, and the supply voltage is constant and the shunt field current given by,

        But we know Ish proportional to Φ

        i.e. field flux Φ is proportional to filed current Ish

        Thus the field flux remains more or less constant, and for this reason, a shunt wound DC motor is called a constant flux motor.



        DC Series motor: A series wound DC motor like in the case of shunt wound DC motor or compound wound DC motor falls under the category of self-excited DC motors, and it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.

        If we are to take a close look into the wiring of the field and armature coils of this DC motor, its clearly distinguishable from the other members of this type.

        To understand that let us revert back into the above mentioned basic fact, that the this motor has field coil connected in series to the armature winding. For this reason relatively higher current flows through the field coils, and its designed accordingly as mentioned below.

        1. The field coils of DC series motor are wound with relatively fewer turns as the current through the field is its armature current and hence for required mmf less numbers of


        turns are required.


        1. The wire is heavier, as the diameter is considerable increased to provide minimum electrical resistance to the flow of full armature


        1. In spite of the above mentioned differences, about having fewer coil turns the running of this DC motor remains unaffected, as the current through the field is reasonably high to produce a field strong enough for generating the required amount of torque. To understand that better lets look into the voltage and current equation of DC series motor.



        Let the supply voltage and current given to the electrical port of the motor be given by E and Itotal respectively.

        Since the entire supply current flows through both the armature and field conductor.

        Where, Ise is the series current in the field coil and Ia is the armature current. Now form the basic voltage equation of the DC motor.

        Where, Eb is the back emf.

        Rse is the series coil resistance and Ra is the armature resistance. Since Ise = Ia, we can write,

        1. This is the basic voltage equation of a series wound DC

        Another interesting fact about the DC series motor worth noting is that, the field flux like in the case of any other DC motor is proportional to field current.


        But since here

        i.e. the field flux is proportional to the entire armature current or the total supply current. And for this reason, the flux produced in this motor is strong enough to produce sufficient torque, even with the bare minimum number of turns it has in the field coil.




        •      DC Compound motor

        • Cumulative
        • Differential

        Long Shunt: If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound DC motor or simply long shunt DC motor.


        The circuit diagram of a long shunt DC motor is shown in the diagram below.

        Short Shunt : If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt DC motor or more specifically short shunt type compound wound DC motor.

        • The circuit diagram of a short shunt DC motor is shown in the diagram

          Construction Diagram of DC Motor:

          Working of DC Motor:


          • Converts an Electrical Energy into mechanical energy
          • Field windings produces magnetic field
          • Armature conductors play a role of current carrying conductor and it experiences force
          • Conductors are placed in slots – experiences turning force called torque
          • Overall armature experiences a torque and starts rotating
          • The magnetic field is produced by a permanent magnet
          • In practical d.c motor, magnetic field is produced by field winding (Fig. a)
          • Any current carrying conductor produces its own magnetic flux around it
          • Conductor is excited by a separate supply – current in a particular direction
            • In a motoring action – there is a generator actionDirection of the flux – Right hand thumb rule

              • The flux produced by the permanent magnet called main flux
              • Flux produced by the current carrying conductor
              • One side of the conductor, both fluxes are in same direction – gathering of flux (high flux density)
              • Other side of the conductor, both fluxes are in opposite direction – weakening of flux (low flux density)
              • Flux distribution around the conductor acts like a stretched rubber band under tension

              𝐹 = 𝐵𝑖𝐿 𝑁𝑒𝑤𝑡𝑜𝑛


              • B = flux density due to the produced by the field
              • L = Active length of the
              • I = Magnitude of the current passing through the conductor
              • The direction of such force i.e. the direction of rotation of a motor can be determined by

              Fleming’s left hand rule.

              Significance of Back EMF:𝐸𝑏

            • Faradays laws of Electro magnetic induction
            • Lenz’s law – Direction of the induced emf is always so as to oppose the cause produced by it
            • Denoted by Eb

              Charecteristic Equations:


              𝑉 = 𝐸𝑏 + 𝐼𝑎𝑅𝑎 + 𝐵𝑟𝑢𝑠ℎ 𝐷𝑟𝑜𝑝

              𝑉 − 𝐸𝑏

              𝐼𝑎 =𝑅𝑎


              𝑉𝐼𝑎𝑏𝐼𝑎 + 𝐼𝑎2𝑅𝑎




              𝑉𝐼𝑎  = Net electrical power input to the armature measured in watts.

              𝐼𝑎2𝑅𝑎= Power loss due to the resistance of the armature called armature copper loss.

              𝑉𝐼𝑎 − 𝐼𝑎2𝑅𝑎 =

              𝐸𝑏𝐼𝑎 (𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑜𝑓 𝑔𝑟𝑜𝑠𝑠 𝑚𝑒ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑎𝑟𝑚𝑎𝑡𝑢𝑟𝑒) VIa − Ia2Ra = Pm

              Applications of DC Motors:

              Advantages and Disadvantages of DC Motor:

              Advantages Disadvantages
              DC motors are small in size DC motors have high initial cost
              DC motors are suitable for traction systems for driving heavy loads. Maintenance cost is high and increased operation due to the presence of brushes and commutator.
              Wide range of speed control Due to sparking at brush DC motors cannot operate in explosive and hazardous conditions.
              DC motors have quick starting, stopping, reversing, and fast acceleration As the speed increases, the shaft gets vibrated and the armature gets damaged.
              DC motors are free from harmonics We need converters to supply power to the motor

        Implement DC machine torque speed characteristics equations in MATLAB script file. It asks user to input data and plots speed (RPM) versus torque (Nm):

        1. Robotics: DC motors are widely used in robotics to power the movement of the robot’s wheels, arms, and other mechanical parts.
        2. Industrial machinery: DC motors are commonly used in industrial machinery such as conveyor belts, cranes, and hoists to control the movement of materials.
        3. Automotive industry: DC motors are used in a range of applications in the automotive industry, including power windows, windshield wipers, and power seats.
        4. Household appliances: DC motors are used in a range of household appliances, including vacuum cleaners, electric shavers, and hair dryers.
        5. Aerospace industry: DC motors are used in various applications in the aerospace industry, including aircraft actuators, landing gear systems, and missile guidance systems.
        6. Medical equipment: DC motors are used in various medical equipment, including blood pumps, ventilators, and dental drills.
        7. Renewable energy: DC motors are used in renewable energy systems such as wind turbines and solar panels to convert the energy from the wind or sun into electrical energy.


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