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Motor Types- DC Motor

Motor Types- DC Motor
  1. Shunt DC motor: The rotor and stator windings are connected in parallel.
  2. Separately Excited motor: The rotor and stator are each connected from a different power supply, this gives another degree of freedom for controlling the motor over the shunt.
  3. Series motor: the stator and rotor windings are connected in series. Thus the torque is proportional to I*I  so it gives the highest torque per current ratio over all other DC motors.It is therefore used in starter motors of cars and elevator motors.
For more related information please refer:
Motor Types-Introduction
Motor Types- DC Motor
Motor Types- AC Motor
Motor Types- Special Types of Motor
Servo Motors
Stepper Motor

  1. Permanent Magnet (PMDC) motors: The stator is a permanent magnet, so the motor is smaller in size.                            Disadvantage: only used for low torque applications
  2. Compounded motor: The stator is connected to the rotor through a compound of shunt and series windings, if the shunt and series windings add up together,the motor is called cumulatively compounded. If they subtract from each other, then a differentially compounded motor results, which is unsuitable for any application.


Principle

This DC or Direct Current Motor works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of electric current in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of dc motor established.

The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of electric current, then the thumb represents the direction in which force is experienced by the shaft of the dc motor.

Structurally and construction wise a Direct Current Motor is exactly similar to a D.C. Generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port. We can represent it by the block diagram shown below.



The input and output port variables of the Direct Current Motor are related by the parameter K.

   T = K.I & E = K.ω
So from the picture above we can well understand that motor is just the opposite phenomena of a D.C. Generator

Advantages of DC motors:
  1. It is easy to control their speed in a wide range; their torque-speed characteristic has, historically, been easier to tailor than that of all AC motor categories. This is why most traction and servo motors have been DC machines. For example, motors for driving rail vehicles were, until recently, exclusively DC machines. 
  2. Their reduced overall dimensions permit a considerable space saving which let the manufacturer of the machines or of plants not to be conditioned by the exaggerated dimensions of circular motors. 

Disadvantages of DC motors

1. Brush wear:Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases dramatically in low‐pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the brushes would need replacement after one hour of operation.

2. Sparks from the brushes may cause explosion if the environment contains explosive materials.

3. RF noise from the brushes may interfere with nearby t.v. sets, or electronic devices,..etc

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