Introduction to DC Motor
Principle of DC Motor
This DC or direct current motor works on the principle, 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 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 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 matually perpendicular to each other and if the index finger represents the direction of magnetic field , middle finger indicates the direction of 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 dc 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.
Here in a DC motor, the supply voltage E and current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed w from the mechanical port or output port.
The input and output variables of the direct current motor are related by the parameter K.
T=K I
Detailed Description of a DC Motor
The direct current motor is represented by the circle in the center, on which is mounted the brushes, where we connect the external terminals, from where supply voltage is given. On the mechanical terminal we have a shaft coming out of the motor and connected to the armature, and the armature shaft is coupled to the mechanical load . On the supply terminals we represents the armature resistance Ra in series.