Question

What is a D.C. electric motor? Explain its construction and working.

Answer 1

It is a machine which converts electrical energy to mechanical energy. It is based on the principle of magnetic effect of current.
Construction: It essentially consist of the following parts:
(a) Field magnet (NS): In common dynamo it is a horse shoe magnet but for strong field electromagnet is used.
(b) Armature (ABCD): It consist of a coil of insulated copper wire, wound on soft iron core. It is rotated between the pole pieces by some external device.
(c) Split ring commutator $(S_1,S_2)$: The commutator is just a copper ring split into two halves $S_1$ and $S_2$ insulated from each other and the shaft. The ends of the coil are connected to $S_1$ and $S_2$ respectively.
(d) Brushes $(B_1,B_2)$: These are carbon strips which slide against the split rings $S_1$ and $S_2$. The current induced in the armature flows through these brushes in the external circuit.
Working: The armature $ABCD$ is rotated in the clock wise direction in the field and at any instant, $S_1$ is in contact with $B_1$ and $S_2$ with $B_2$. Under such situation $AB$ will come up the plane of paper and $CD$ will go down. Then by fleming right hand rule, the direction of induced current will be along $ABCD{S}_1{B}_{1}R{B}_2{S}_{2}A$. So, the current in the external circuit flows from $B_1$ to $B_2$ through $R$.
After half rotation, split ring $S_2$ is in contact with $B_1$ and $S_1$ and the arms. $AB$ and $CD$ interchange their positions. Now again when the coil is rotated clockwise $CD$ comes up the plane of paper and $AB$ goes down. Then by fleming right hand rule the direction of induced current is $DCBA{S}_2{B}_{1}R{B}_2{S}_{1}D$. Hence, in the external circuit the current flows from $B_1$ to $B_2$ through $R$. So, the current in the external circuit flows in the same direction. Hence, direct current is obtained.