The Firefly's wheel speed is limited by the voltage produced as the permanent magnets move across the motor coils (Faraday's law of induction). The faster it spins, the larger is the induced voltage, which eventually equals what the battery can produce. Then current and torque drop to zero, and the maximum speed has been reached. For this reason, a fully charged battery with higher voltage can go faster than a depleted one.
I have been experimenting with Field-Oriented Control which, among other advantages, can use the motor coils to generate fields that counteract the fields of the spinning permanent magnets. The technique is known as Field Weakening.
My tests with TI boards (LAUNCHXL-F28069M and BOOSTXL-8323RS) show a speed-up of up to 38 % compared to the original motor controller.
Original data where Kv0 = 10.46 rpm/V is the measured speed of the original controller:
Vsupply
Isupply
(A)
Speed
(rpm)
Kv
(rpm/V)
Kv/Kv0
Field
weakening
(A)
32.23
0.91
350
10.86
1.038
0
32.23
0.97
371.1
11.51
1.100
2.5
32.23
1.24
394.6
12.24
1.170
5
32.23
1.67
422.2
13.1
1.252
7.5
32.23
1.75
431.6
13.39
1.280
8
32.23
1.9
441
13.68
1.308
8.5
32.23
2.35
466
14.46
1.382
9.0
The data were taken on a stationary test stand, where the wheel just spins in air. It's not ready for vehicle testing yet.
