Question

What is d.c. motor? Explain its construction and working with the help of labelled diagram.

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 consists 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 consists 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 enternal 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. They 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 is 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 enternal circuit flows in the same direction. Hence, direct current is obtained.