Introduction:
In a DC machine, commutation refers to the process of transferring the armature coil current from one commutator segment to another, ensuring that the current flows in the same direction in the external circuit. Good commutation is essential for the smooth operation of the machine. However, under commutation can lead to sparking and other undesirable effects.

Explanation:
Under commutation in a DC machine occurs when the current in the armature coils is not completely transferred to the next commutator segment. This can happen due to various reasons such as excessive resistance in the circuit, high reactance, or improper brush positioning.

Sparking at the trailing edge of the brush:
Under commutation can cause sparking at the trailing edge of the brush. When the armature coil current is not completely transferred to the next commutator segment, a short circuit is created between the trailing edge of the brush and the commutator segment that it just left. This short circuit leads to arcing and sparking at the trailing edge of the brush.

Reasons for sparking:
1. Inductance: The inductance of the armature coils causes the current to lag behind the voltage. When the brushes shift from one commutator segment to the next, the current in the armature coils has not reached its maximum value. As a result, there is a residual magnetic field in the coil that tries to maintain the current flow in the same direction. This causes arcing and sparking at the trailing edge of the brush.

2. Reactance: The reactance of the armature coils also contributes to under commutation. The reactance opposes the change in current flow, causing a delay in the current transfer from one commutator segment to another. This delay causes sparking at the trailing edge of the brush.

Effects of sparking:
Sparking at the trailing edge of the brush is undesirable due to several reasons:
1. Brush and commutator wear: The repeated arcing and sparking cause erosion of the brush and commutator surfaces, leading to increased brush and commutator wear. This can result in poor brush contact, increased resistance, and decreased machine efficiency.

2. Electrical interference: The sparking generates electromagnetic interference, which can affect the operation of nearby electrical equipment and cause radio frequency interference.

3. Noise: The arcing and sparking produce audible noise, which can be a nuisance in certain applications.

Conclusion:
Under commutation in a DC machine can lead to sparking at the trailing edge of the brush. This sparking is caused by the residual magnetic field and reactance in the armature coils. The sparking can result in brush and commutator wear, electrical interference, and noise. Therefore, it is important to ensure proper commutation for the smooth and efficient operation of DC machines.