Basic Electrical Engineering (3110005)

BE | Semester-1   Winter-2019 | 11-01-2020

Q4) (a)

Explain working principle of D.C. Motor.

Working Principle of D.C. Motor
• An electric motor is a machine which converts electric energy into mechanical energy.
• It works on the principle of Lorentz Law, which states that ‘the current carrying conductor placed in a magnetic and electric field, experience a force’, whose direction is given by Fleming’s Left-Hand rule and whose magnitude is given by F = BIl Newton.
• Constructionally, there is no difference between a dc generator and dc motor.
• In figure above, a part of motor is shown.
• When its field magnets are excited and its armature conductors are supplied with current from the supply mains, they experience a force tending to rotate the armature.
• Armature conductors under N-pole are assumed to carry current downwards (crossed), and those under S-pole are assumed to carry current upwards (dots).
• By applying Fleming’s Left-Hand Rule, the direction of the force on each conductor can be found. It is shown by small arrows placed above each conductor.
• It will be seen that each conductor experiences a force F which tends to rotate the armature in anti-clockwise direction. These forces collectively produce a driving torque which sets the armature rotating.