When a DC motor is running sparks may generally be seen wher

When a DC motor is running, sparks may generally be seen where the carbon brushes contact the “commutator” segments. Explain why this sparking occurs, and also define the word “commutation” in its electrical usage.

What does this phenomenon indicate about the longevity of DC motors, and their suitability in certain environments?

Solution

The voltage generated in the armature, placed in a rotating magnetic field, of a DC generator is alternating in nature. The commutation in DC machine or more specifically commutation in DC generator is the process in which generated alternating current in the armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes.

Again in DC Motor, the input DC is to be converted in alternating form in armature and that is also done through commutation in DC motor. This transformation of current from the rotating armature of a dc machine to the stationary brushes needs to maintain continuously moving contact between the commutator segments and the brushes. When the armature starts to rotate, then the coils situated under one pole (let it be N pole) rotates between a positive brush and its consecutive negative brush and the current flows through this coil is in a direction inward to the commutator segments. Then the coil is short circuited with the help of a brush for a very short fraction of time(1500 sec). It is called commutation period. After this short-circuit time the armature coils rotates under S pole and rotates between a negative brush and its succeeding positive brush. Then the direction of become is reversed which is in the away from the commutator segments. This phenomena of the reversal of current is termed as commutation process. We get directcurrent from the brush terminal.

The commutation is called ideal if the commutation process or the reversal of current is completed by the end of the short circuit time or the commutation period. If the reversal ofcurrent is completed during the short circuit time then there is sparking occurs at the brush contacts and the commutator surface is damaged due to overheating and the machine is called poorly commutated.