Micro Multipole Magnets for Coreless Motors: 8 Poles from Ø6.3 mm

A slotless stator has no iron teeth, so there is no cogging torque, inductance is low, and the back-EMF waveform is set by the magnet and the winding geometry. In a slotted motor, tooth geometry, stator skew and pole face shaping all sit between the magnet and the waveform the controller measures, and an imperfect magnetization pattern is partly absorbed by the iron.

A slotless motor has none of those corrections available, so pole-to-pole variation in the ring appears directly in the measured back-EMF and in the torque ripple the control loop has to work with. The order of operations we set out for larger rotors in magnetic circuit design for PM motors applies here with less margin, because there is no iron structure to absorb a compromise made earlier in the design.

The word coreless covers two different machines. A brushed coreless motor uses a stationary 2-pole magnet core inside a rotating winding cup. A brushless slotless motor rotates a multipole magnet inside a stationary winding. The multipole capability described here applies to the brushless configuration.

Ring production splits at Ø20 mm outer diameter. Rings below Ø20 mm are made on a multipole-oriented ring process. At Ø20 mm and above, production moves to the radial-orientation process, which runs a standard envelope of OD 20 to 75 mm held to ±0.05 mm, with straight or skewed multipole magnetization.

Micro coreless rotors sit entirely below that split. Within the multipole-oriented process the working limits are 8 poles from Ø6.3 mm, and Ø4.4 mm as the smallest ring at 4 poles. Between those two points, pole count and diameter trade against each other, and the way to settle it is to send a drawing with both the pole count and the diameter marked and have the pair confirmed.

Skewed magnetization is available on multipole rings, which is of more interest where the same rotor platform also has to serve a slotted design, since a slotless stator has no cogging for the skew to reduce. Grade at these diameters is confirmed against the specific drawing rather than picked from a published window.

The NdFeB grades catalogue behind that runs N30 to N58, in three series: F-Series with no heavy rare earths and no MOFCOM export licence requirement, N-Series conventional, D-Series by grain boundary diffusion. Temperature classes go to AH at 230°C.

Pole count is a speed decision. An 8-pole rotor has four pole pairs, so at 30,000 rpm the electrical frequency is 2,000 Hz. Four poles at the same speed gives half of that. Frequency-dependent losses in the winding and in the drive electronics follow that number. High-speed micro drives, including the surgical and dental handpieces covered under medical devices, tend to sit at low pole counts for that reason.

Drives that run at moderate speed and care more about position control, such as finger and gripper joints in robotics applications or actuators in laboratory automation, can use the higher pole counts that the Ø6.3 mm capability now allows. Fix the speed range before fixing the pole count.

Changing pole count later means re-tooling the magnetization and re-designing the winding, and at these diameters both are slow changes to make.

One-piece sintered micro Halbach cylinders are also produced at this scale. Demonstrated parts: Ø3.3 × Ø1.0 × 8.0 mm at 2 poles; Ø5.3 × Ø1.5 × 15.7 mm at 4 poles. Both are single pieces with no bonded segments. At these wall thicknesses a bond line would take a real fraction of the section. Position accuracy is where magnetization tolerance shows up.

Sensor-grade magnets are held to a magnetization deviation angle under ±3°, with 100% post-magnetization inspection. In a small brushless drive that angle sits in the same error budget as the position sensor and the commutation timing, so put it on the drawing. If the magnet is supplied inside a rotor or sensor sub-assembly rather than loose, the magnet assemblies requirements come with it: concentricity to the shaft, retention at speed, magnetization fixturing, and measurement after assembly rather than before.

Documentation at these sizes runs the same as on larger programs. Quality systems are ISO 9001:2015 and IATF 16949:2016 at the certified plant in our production network, and PPAP Level 3 packages are available where the program needs them. Grade, pole count and diameter are confirmed together against the drawing, because at micro sizes they are not independent choices.

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