Inexpensive moderate-inductance short-can BLDCs?

[fictivedesigns]

Hi all,

tl;dr: Are there any good sources for short can (<55mm from mounting plate to end of bell), moderate to high inductance (> 40uH phase to phase) BLDCs? Ideally the motor shaft is >= 6mm and the motor diameter is <= 50mm.

First, some background. I'm an undergrad developing a dual motor controller for an electric longboard as a personal project. It features sensored FOC (independent, on both motors) and 40KHz PWM frequency.
The controller is fast enough for cycle-by-cycle control, i.e., the controller uses fresh sensor angle feedback and current feedback to calculate new SVPWM periods every single PWM cycle.
The board itself is fully aluminum, so FET heatsinking is a non-issue. Due to the PWM frequency the board is very silent when riding, one of my key goals for this project.

Now, the issue. I currently have the sensor mounted to the pulley but the pulley moves around during acceleration and constantly screws up the feedback.
I would like to mount the sensor to the end of the motor, but there is no space. My current motors are Turnigy Aerodrive SK3 4250-350KV motors. By sheer luck their inductance is 50uH, compared to the 20uH of my NTM 5060.
Due to the nature of FOC, my controller does not work particularly well at higher voltages, or with low inductance motors (<25uH). This is because either situation causes current to ramp very quickly,
and I lose control over motor current.

http://imgur.com/a/WiN8o

From the picture you can see the lack of space between both motors. I am looking for a shorter motor (<55mm from front to back) with comparable inductance (~50uH) and power (>1KW).
If anyone has any suggestions or knows anything about the typical inductance of outrunners, I would greatly appreciate it. I was considering the Turnigy L5055A-400 but am hesitant
to buy due to its unknown inductance.

Thanks,
David

 

[liveforphysics]

Essentially core iron mass, and the Kv of the motor are your two big indicators. The higher the Kv, the lower the inductance. The higher the core mass, the higher the inductance.

 

[fictivedesigns]

Essentially core iron mass, and the Kv of the motor are your two big indicators. The higher the Kv, the lower the inductance. The higher the core mass, the higher the inductance.

Intuition tells me that is the case, but from the two samples I have the actual motor design appears to matter far more. In fact the NTM 5060 has both a lower KV and a higher core mass, yet its inductance is less than half of the smaller, higher KV SK3 4250.

David

 

[thepronghorn]

Just thinking out loud here, but I think core mass is playing a much larger role in the inductance of your motors here than the Kv.

The NTM 5060 motor has around 2x the volume of the 4250 motor but it is wound for a similar Kv. Interesting because the 5060's inductance is 40% of the 4250. Maybe winding double the core mass for the same Kv means the windings will have ~1/2 the inductance.

Or maybe I'm wrong.

Not sure if I can help you with your original question on other motors. I think user vanarian had some really low Kv motors made. Lower Kv means less power for the same voltage but you do get more inductance.

 

[fictivedesigns]

As an update: I bought the L5055-400, and it turns out to have an inductance of 47uH phase to phase. Perfect!

David