Two different coil shapes were tested on this small alternator to investigate why they did not seem to match expected power output and efficiency. The rotor and magnet spacing was kept the same. The stator held the coil faces approx. 0.080" from the magnets.
Rotor Plate: ¼" thick, mild steel ring (9.75" OD, 5.19" ID)
Magnets: Total of 24, 1" dia x ½ thick, N42, arranged in 12 poles in a figure-8 as shown in the pictures
68 turn coil: 14 AWG wound 0.5"-0.55" thick, leg width 0.8". Note the overlap on both sides of the round magnet when passing over the center.
http://www.fieldlines.com/images/scimages/11755/68t.jpg ">
67 turn coil: 16 AWG wound 0.4" thick, leg width 0.62". The inside wedge shape was opened up to avoid overlap.
http://www.fieldlines.com/images/scimages/11755/67t.jpg ">
The stator had an arm attached that pressed down on a digital scale, 9" from the center of rotation for the alternator. The single rotor rides on a spindle that passes through a bearing at each end of the test setup. The scale is zeroed out before each run to eliminate the off-center mass of the lever arm. The rotor is driven with 2 V-belts by a 36V electric golf cart motor, with several selectable speeds. The 3-phase alternator output is rectified and wired to a deep cycle battery. Alternator speed is checked with a photo tachometer and a reflective target on the alternator-side drive pulley.
Power is calculated according to the formula given at the following link:
http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
Where P (HP) = (Torque (lb-ft) * RPM)/5252
68T coil (Star configured):
21.7 VDC at 380 RPM which gives 0.0571 V/RPM, or 12V cut-in at 210 RPM.
No load test:
69 grams at 380 RPM (6W) and 220 grams at 890 RPM (46W)
68T coil (Delta configured):
7.61 VDC at 240 RPM which gives 0.0317 V/RPM
67T coil (Star configured):
13.48 VDC at 219 RPM which gives 0.0616 V/RPM, or 12 V cut-in at 195 RPM.
No load test:
80 grams at 219 RPM (4W) and 145 grams at 650 RPM (22W)
67T coil (Delta configured):
14.7 VDC at 412 RPM which gives 0.0357 V/RPM
No load test:
160 grams at 412 RPM (15W)
Here are the data tables used to determine the relative efficiency of each configuration:
And finally a chart of efficiency versus RPM for both sets.
Conclusions: These sets could use more data points, but it appears that the average of the efficiencies is a bit higher for the 67T coil versus the 68T coil (compare 68T Star to 67T Star, and 68T Delta to 67T Delta). The reasons are open to debate, but I'm guessing that the slightly thinner coil helps given the single rotor design.