Torque ripple is produced by physics of how force is created by current flowing in a wire in the presence of a magnetic field. Ideally the angle between the current vector and magnetic field vector is 90 degrees, but many factors are involved that make the actual angle oscillate a small percentage about 90 degrees. The math behind this effect is captured in the ‘Comparison of Slotless and Slotted Motors’ technical paper.
Torque ripple is present in all motors but to varying degrees, mainly driven by the design of the motor (BLDC vs. BLAC) and drive used (Sine vs. Trap or 6 Step). The only motor drive combination that produces theoretically zero ripple is a BLAC with a Sine drive. In practice, zero ripple is never truly achieved due to drive offset voltages, commutation error, and mechanical tolerance of the stack and winding variations around each tooth.