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 []