Why are tapered interpoles used in large DC electric machines?

In large DC electric machines, tapered interpoles (also known as commutating poles) are employed for several important reasons, which are closely related to their role in improving commutation and the overall efficiency of the machine. The key reasons for using tapered interpoles include:

1. Improved Commutation: The primary purpose of interpoles is to neutralize the cross magnetomotive force (MMF) due to armature reaction under the main poles and to provide a short-circuit path for the current in the coils undergoing commutation. The tapering helps in making the magnetic field strength distributed more uniformly across the coil undergoing commutation, which is essential for spark-free commutation.

2. Efficiency in Space Utilization: Tapering the interpoles can help in optimizing the space within the machine. It allows for a more efficient use of the available space within the motor or generator, which can be particularly important in large machines where space and the arrangement of components are critical for performance, maintenance, and cooling efficiency.

3. Reduction of Magnetic Saturation: By tapering the interpoles, the concentration of magnetic flux at the tips of the poles can be reduced, helping to prevent the saturation of the pole tips. Saturation can lead to inefficient operation and increased losses, so by shaping the poles to distribute the magnetic flux more evenly, better performance of the machine can be ensured.

4. Control Over Flux Distribution: The shape of the interpo