# What is coil span or coil pitch

In the context of electrical engineering, “coil span” refers to the distance between the centers of two adjacent coils in an armature winding. It is an important parameter in the design and construction of electric motors and generators, as it affects the performance and efficiency of the machine.

The coil span is measured in millimeters (mm) or inches and is calculated by dividing the circumference of the rotor by the number of coils in the armature winding. The circumference of the rotor is the distance around the rotor measured in the same units as the coil span. The goal is to make the coil span as small as possible to minimize the cogging torque and to increase the power density of the machine. However, the coil span is also limited by the size of the stator core and the size of the coils.

The coil span can also affect the distribution of the current flowing through the coils. In a distributed winding, the current is evenly distributed among the coils and the coil span is equal to the full pitch (distance between the centers of the same pole). In a concentrated winding, the coil span is less than the full pitch, which allows a higher current density in the coils. However, this also increases the harmonic content of the current and reduces the efficiency of the machine.

Coil span is also used to calculate the winding factor, which is a measure of how uniformly the current is distributed among the coils. The winding factor is used to calculate the mutual inductance between the coils and the stator, and it affects the performance of the machine.