Ex

  Physics (NTSE/Olympiad)  

2. Magnetic Effect of Current

Electric Motor (D.C. Motor)

Electric Motor (D.C. Motor)
(A) Principle : It works on the principle of motion of a current carrying conductor in a magnetic field, according to Fleming’s Left Hand Rule.
It is a device which converts electrical energy into mechanical energy of rotation.
(B) Diagram :


Figure : DC Motor
(C)Construction : It has four mains parts, named below.
A field magnet having concave magnetic poles N–S, which provides a strong magnetic field.
An armature, which is moving part of the motor. It has two parts :
(i) Laminated shaft X–Y.
(ii) Copper coil ABCD wrapped on end X of the shaft inside the field.
A pair of metallic split rings R B1 and R B2 (commutator). A pair of metallic carbon brushes B B1 and B B2.
(D) Working : A direct current (D.C.) source is connected between metallic brushes B1 and B B2. When current passes through the coil, it flows in arms CB and AD in a direction perpendicular to the magnetic field. Equal and opposite forces act on these arms (in a direction according to Fleming’s Left hand Rule) and they form a couple. The coil rotates in clockwise direction (Fig.). After half rotation, split parts of ring change brushes. Current becomes reverse in the arms but couple acts in same direction as before. The coil continues rotating the shaft on which it is wrapped. Thus, rotatory motion (motor action) becomes available.
The rotation is not smooth with single coil. It is made smooth by increasing number of coils (multiphase motor) or preferably by having a continuous winding.

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