Q. Explain Commutator Action in dc machine?

As a consequence of the arrangement of the commutator and brushes, the currents in all conductors under the north pole are in one direction and the currents in all conductors under the south pole are in the opposite direction. Thus, the magnetic field of the armature currents is stationary in space in spite of the rotation of the armature.

The process of reversal of currents in the coil is known as commutation. The current changes from +I to -I in time Δt . Ideally, the current in the coils being commutated should reverse linearly with time, as shown in Figure. Serious departure from linear commutation results in sparking at the brushes. Means for achieving sparkless commutation are touched upon later. As shown in Figure, with linear commutation, the waveform of the current in any coil as a function of time is trapezoidal.