Low-Cost Sub-Fractional HP BLDC Claw-Pole Motor Designs with Reduced Cogging Torque

The increasing performance requirements on automotive auxiliary drives have led to a paradigm shift in the design of sub-fractional hp BLDC motors. While minimum cost used to be the primary design criteria, thereby accepting sub-optimal motor behavior, their overall performance is gaining in importance. The large cogging torque and torque ripple of single-phase BLDC motors, which often lead to disturbing structure- and airborne noise, have become unacceptable in many applications. However, the implementation of cogging torque reduction measures typically involves additional fabrication steps, thus increasing the cost, which is a disqualifier for mass-produced low-cost applications.
This thesis presents single-phase BLDC motor designs in which the cogging torque and, in turn, the output torque ripple can be reduced at no increase to the manufacturing cost. On the basis of the claw-pole motor topology, consisting of a ring winding housed between two specially shaped deep-drawn steel sheet parts, various design improvements are proposed and analyzed in detail. Therefore, dierent analytic and numerical models are developed and used to study phenomena such as cogging torque, back-EMF, magnetic forces, and iron losses. Subsequently, the simulation
results are verified by experiments, notably by rheometer-based techniques presented for the first time. Implementing the claw-pole motor with an asymmetric air-gap can facilitate the starting and, simultaneously, provide maximum design freedom concerning the claw-pole shapes for the implementation of cogging torque reduction measures. In case of static eccentricity or a non-uniform magnetization pattern, the studied four-pole four-slot claw-pole motor shows pulsating unbalanced radial magnetic forces with a dominating fourth order harmonic component. Implementing a single-sided skew of 30 can successfully reduce the cogging torque by about 25%, while the back-EMF reduction is below 2 %. This cogging torque reduction results in an output torque ripple reduction of about 12%. The proposed auxiliary slots can effectively modulate the cogging torque of the claw-pole motor.
Realizing a single-sided skew of 45° in addition to auxiliary slots can successfully reduce the cogging torque by 70%. The cogging torque decline is accompanied by a reduction in the back-EMF of 12%, while the output torque ripple is reduced by 17%. The application of a single-sided skew generally induces axial magnetic forces which can reduce the bearing system’s lifetime as well as cause vibrations and noise. Owing to the proposed design with a V-skew, the axial magnetic forces are balanced while, concurrently, the cogging torque is successfully reduced by about 50% and
the back-EMF is marginally reduced by about 1%. Using a rheometer, the motor’s
cogging torque and hysteresis torque waveforms in the sub-milli-Newton meter range can be measured with excellent accuracy. Moreover, the rheometer is used to accurately determine the iron losses of the motors under investigation by studying the observed offset torque. It is advantageous that all manufacturing influences are included in the measured iron losses because, as opposed to the investigation of sheet material properties, the assembled motor is studied.
The single-phase BLDC claw-pole motor topology is suitable for the implementation of cogging torque reduction measures at no increase to the manufacturing cost. However, as motor performance parameters are typically interdependent, a beneficial change in one parameter is often linked to a detrimental change in another parameter. Cogging torque reduction always comes with a reduction in the average output torque or efficiency, but it can successfully reduce the output torque ripple. The developed design improvements are by no means limited to motors as part of automotive auxiliary drive applications; they can generally be used in any cooling system, especially those in electronics