bspalteh said:
What happens if you increase the field weakening value beyond that 17%, say 30% or whatever?
We decided to see what would happen if you had a slow winding motor and then used field weakening to increase the top-end speed, since it's a scenario that we've seen quite often where people are mistakenly lead to purchase a slow motor wind under the generally false impression that it has extra torque, only to find that the performance at cruising speeds is quite underwhelming. The first thing we wanted to do was confirm the relationship between the field weakening current and the unloaded RPM.
RPM vs Field Weakening.jpg
In all these tests, we had the defined max motor current at 60A, so a 50% field weakening current is 30A and so forth. The unloaded motor starts off at about 310rpm with no field weakening. By the time we hit 20A of field weakening current, the RPM has increased to about 380 rpm, while the no load current increased from 1A to just under 3A. We don't get a 50% increase in the top speed until the field weakening current is at a full 40 amps, at which point the no load current just to spin the hub is about 7.5 Amps. To actually double the RPM of the motor in this setup required the full 60A of field weakening.
The resulting 17A of no-load current shows that it's pretty wasteful, and you could just watch the motor temperature start to skyrocket on the CA readout at this point.
So based on this, I'd say that the field weakening is viable to get upwards of 20-30% boosts to the unloaded speed on a slow DD hub if a higher voltage pack isn't an option, but more than this has rapidly diminishing returns.
As far as the efficiency goes, we then did a dyno test of the hub at both 0% and 50% (30A) field weakening currents, and in both cases with a 25A battery current limit:
Comparison with 50% Field Weakening.jpg
With no field weakening, peak efficiency was around 88%. In the case of the setup with automatic field weakening, it was at 80% efficiency once the weakening kicked in, and then dropped steadily to the 71-72% range. While the power roll-off from back-emf voltage started at about 250rpm with no field weakening, it was able to pull the full 25A right until 340 rpm with the 30A weakening current. So here you are getting a ~33% boost to the top speed, but at 70-80% efficiency levels compared to the 80-88% efficiencies you'd have gotten with a higher voltage pack or a faster wind hub.
Here is a repeat of the same experiment but using a 15A rather than a 25A battery current limit, representative of running part throttle. Comparison with 50% Field Weakening at 15A limit.jpg