Flapjack Stator Thoughts

WARNING: THIS POST CONTAINS EXTREMELY SHADY ENGINEERING. HOLD NO ILLUSIONS ABOUT THE LEGITIMACY OF THE FIGURES AND METHODS. DO NOT TRY THIS AT HOME.

I’ve been think­ing about alter­na­tives in construct­ing the stator for Flap­jack, my latest 3 lb R/C combat bot, besides milling it out of 5 oz copper-clad circuit board mate­r­ial. I think I can wind coils that would be epox­ied into the cutouts on a stator retainer plate like this one:

Stator coil retainer plate

Physical fit

The retainer is cut out of 1/8 in thick poly­car­bon­ate, and my wind­ings would be four strands of 22 AWG magnet wire with double-thick­ness enamel. That would be 4 × 0.0276 in = 0.110 in tall within the cutout, save for the termi­nal of the coil which needs to come out some­where, which makes this a 5 × 0.0276 in = 0.138 in over­all height coil. If I file some away some indents in plas­tic between wind­ings, then they stick just 13 mil out of the retainer. Over­all, there’s a hair1 shy of 1/16 in of clear­ance on either side of the stator with respect to the magnets on the stator.

I’m concerned that a verti­cal strike to the weapon can deform the mild steel rotor plates more than that and cause the magnets and magnet retain­ers to rub against the stator assem­bly.

Weight

To save weight and to increase Kv, I would use seven turns in each coil, versus the eight on the P.S. WTFLOLs. This should have less effect on the motor than it initially appears, as each loop of the wire coils is signif­i­cantly larger (on aver­age) than the spiral­ing trace coils in the P.S. WTFLOL.

Gener­ously speak­ing, each turn of this coil is, on aver­age, 3.0 in of wire (the perime­ter of the cutout is about 3.4 in while the inner­most loop will be just 2.6 in in perime­ter).

\cfrac{(3.0 \mathrm{\ in} \times 7 \mathrm{\ turns} + 2 \mathrm{\ in}) \times 12 \mathrm{\ coils} \times 4 \mathrm{\ strands}}{501.5 \ \frac{\mathrm{ft}}{\mathrm{lb}}} = 2.9 \mathrm{\ oz}

Now, I’ve got about 3.5 oz of weight to spend on the stator exclud­ing fasten­ers, so with the poly­car­bon­ate retain­ers at 0.18 oz each, that leaves me with a quar­ter ounce of epoxy and tape to secure and insu­late the coils with. That seems shady2; I might drop down to six turns or three strands in order to make weight.

Resistance

Now for the whole point of moving to wire-wound coils: lower resis­tance.

(3.0 \mathrm{\ in} \times 7 \mathrm{\ turns} + 2 \mathrm{\ in}) \times 4 \mathrm{\ coils} \times 2 \mathrm{\ phases} \times \cfrac{16.14 \ \frac{\mathrm{m\Omega}}{\mathrm{ft}}}{4 \mathrm{\ strands}} = 62 \mathrm{\ m\Omega}

By the way, I’m adding 2 in to each coil for termi­na­tion, connec­tion to the next phase, etc. The over­all phase-to-phase resis­tance of 62 mΩ is a greater than 70% reduc­tion in wye-termi­nated phase-to-phase resis­tance compared to the 0.226 Ω of the P.S. WTFLOLs.

Summary

Using these coils would remove a big “ass” compo­nent of the perma­nent magnet core­less axial flux synchro­nous motor shell and printed spiral wound trace flux link­ing outer loops (PMCAFSMS, P.S. WTFLOL). Namely, I lose the P.S. WTFLOL part, but I am confi­dent a suit­able replace­ment acronym will come up. More­over, a bigger propor­tion of the air gap between rotor sets will be filled with torque-produc­ing copper and make for a much more power­ful, effi­cient motor.

Effi­ciency becomes a big deal when you consider how much power needs to be dissi­pated from such a compact robot, and the fact that the energy onboard is limited: the ~50 kJ stored in the batter­ies means I can draw less than 300 W aver­age during a 3 minute match. Consid­er­ing that prob­a­bly 10 kJ is spent on just oper­at­ing the drive­train, blow­ing (10. A)² × 226 mΩ = 23 W on copper heat­ing is defi­nitely some­thing I worry about.

These wind­ings I’m think­ing of make for just 6% of motor loss at 10 A × 11.1 V = 111 W input power, but that’s not includ­ing eddy current losses nor the high ripple currents slosh­ing around the motor due to PWM. That’s still a huge improve­ment over the trace wind­ings, where that loss is 20% at the same input power.

Eddy current losses would be much lower in compar­i­son too, as each turn of copper is verti­cally oriented and sepa­rated into four conduc­tors. However, unlike real litz wire, these conduc­tor sets are not twisted together. So, the slight differ­ences in flux density axially (see simu­la­tion results) mean that each strand produces a differ­ent EMF and that there will still some eddy currents between them.

To do

  • Build test jig for motor test­ing
  • Print a new chas­sis which can hold the exter­nal big-ass ass-induc­tors for the motor
  • Build wind­ing tool for coils
  • Wind, compact, pot, secure, and insu­late coils into stator retainer plate

Wah.

  1. Coarse East Asian hair ~170 µm thick []
  2. It’s about three post-1982 US copper-clad nickel pennies []