# Bipolar stepper motor tooth geometry

A normal 1.8 degree bipolar stepper motor has 50 rotor teeth evenly spaced around the rotor's circumference, but what is the normal geometry of the stator teeth?

In tutorials explaining stepper motors I've seen different geometries being presented. The stator teeth are always arranged in eight groups corresponding to the eight motor windings, but the number of teeth in each group seems to vary, five and six being the most common.

The angular displacement of the stator teeth within a group also seems to vary. I've seen examples where the angular displacement is 7.2 degrees meaning that the stator and rotor teeth can be perfectly aligned, but I've also seen examples where the displacement is less so that the teeth can never align perfectly.

What is the normal geometry in a real world bipolar stepper motor?

Figure 1. Teeth aligned
Image source: YouTube - Lesics "How does a Stepper Motor work?"

Figure 2. Teeth not aligned
Image source: YouTube - Nanotec Electronic "Stepper Motor - Operating Principle - Nanotec"

The first image shows the motor in full-step mode and locked in to the top pole. The lower image shows the motor in half-step mode so the rotor is locked between poles with what appears to be $$\ \frac 1 {\sqrt 2} \$$ times the torque on each - so probably $$\ \sqrt 2 \$$ times the torque of the full-step mode. In half-step mode the rotor will step between one-pole and two-pole mode so the torque is not even.