Understanding Inductance in DC Machine Armature: Current Flows in an Alternating Nature

In direct current (DC) machines, the current flowing through the armature winding appears to be alternating due to the commutation process. However, why don't we consider the inductance of the armature? This article explores the reasons behind this decision, delving into the concepts of inductance, commutation, and interpoles.

Introduction to Inductance and DC Machines

Inductance occurs when there is a rate of change of current. In DC machines, this rate of change is only observed during the commutation process, which is a very brief moment. This phenomenon is known as the inductive kick. The commutation process is managed by interpoles, so the effect of inductance is compensated, resulting in no significant inductance in DC machines.

The DC Machine Context

DC machines use a direct current (DC) supply, yet the current within the armature winding reverses direction based on the commutation process. This can create the impression of alternating current (AC), even though the fundamental supply is DC.

Current Magnitude in the Armature Winding

In a DC machine, the current direction changes every half revolution, forming a pattern like x - x. However, the magnitude of current within the rotor coils remains relatively constant as long as the load is fixed. This constant current means that there is no need for the rotor coils to induce any electromotive force (EMF) in other coils. Contrast this with an AC machine, where the current magnitude varies in a sinusoidal pattern and EMF is induced in the rotor coils due to the rotating magnetic field.

Commutation Process in DC Machines

The commutation process in a DC machine is key to understanding why inductance isn't a factor. During commutation, the change in direction of the current is due to the physical rotation of the commutator segments, which reverses the terminals of the specific coil. This change in direction provides the necessary alternating torque to keep the motor rotating.

Air Gap and Induced Flux in DC Machines

In a DC machine, the air gap under the poles is uniform, leading to a constant reluctance that results in a flat-topped flux waveform. Since the current is nearly constant in this area, there is no question of inductance arising. However, as the conductors reach the interpolar region or the brush axis, the current reverses. This reversal induces a reactance voltage due to the inductance of the coil undergoing commutation. Interpoles in this region help to generate a commutating flux that induces a voltage opposite to the reactance voltage, neutralizing the effect of inductance.

Conclusion

In summary, while the current in the armature winding of a DC machine reverses direction, the inductance is not a significant factor in the overall performance of the machine. The inductive kick is managed by interpoles, and the uniform air gap leads to a flat-topped flux waveform, negating the need to consider inductance in the analysis of DC machines.