Principles of DC motor and Operation of DC motor:
Principles of DC motor it is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand rule and whose magnitude is given by
Force, F = B I l newton
Where B is the magnetic field in weber/m2. I is the current in amperes and
l is the length of the coil in meter.
The force, current and the magnetic field are all in different directions.
If an Electric current flows through two copper wires that are between the poles of a magnet, an upward force will move one wire up and a downward force will move the other wire down.
BACK OR COUNTER EMF
When the armature of a dc motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence an e.m.f. is induced in them.
The induced e.m.f. acts in opposite direction to the applied voltage V (Lenz’s law) and is known as back or counter emf Eb.
SIGNIFICANCE OF BACK EMF
The presence of back emf makes the dc motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.
Back emf in a dc motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.
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