I apologize for taking so long to get back to you. Let me provide a brief review of the relevant relationships that will allow you to size an ESC for your application correctly. Furthermore, we are willing to ship you one ESC for evaluation purposes free of charge, so you could confirm my reasoning below empirically. Also, it would help us if you could just tell me what exact motors and propellers you are using; perhaps we have those on hand in our lab (we have a lot of various propulsion hardware here), in which case we can pre-tune the controller for you.
The amount of thrust created by the propeller is dependent on the mechanical power at the driving shaft. The mechanical power is a product of angular speed and torque. In an ideal electric motor (barring edge cases like magnetic saturation), the voltage across the windings (armature for DC) is linearly proportional to the angular speed, and the torque is proportional to the current through the windings (assuming the ideal case where the reactive power is zero. At any rate, any FOC drive will strive to eliminate the reactive power, excepting certain special operating modes like field weakening).
The electrical power is a product of current and voltage (although, in a three-phase system, an additional coefficient of 1.5 is introduced when said system is modeled as an equivalent two-phase alpha-beta or a DQ system, unless a power-invariant Clarke transformation is used (not in Telega); more on this in the quick start guide). Assuming zero losses (a typical total power conversion efficiency can be somewhere around 60%-90% for good sensorless drives, like Myxa), the electrical power (voltage times current) across the whole system remains constant.
Now we can apply the above information to your problem. Suppose you have the following setup (plug your numbers):
- Motor speed constant (KV): 350 RPM/V
- Nominal speed: 5000 RPM
- Mechanical power required at the nominal speed: 250 W
- Motor efficiency: 90%
- Driver efficiency: 90%
- DC link supply voltage: 6S (22 V)
Now, let’s work backward from the propeller to the DC link to estimate the electrical parameters of the drive. The back EMF of the motor at the nominal speed is 5000 [RPM] / 350 [RPM/V] = 14.3 V. The required motor phase current (assuming zero loss) is 250 [W] / 14.3 [V] = 17.5 A; loss current: 17.5 [A] * (100% - 90%) = 1.8 A. Total phase current: 19.3 A; total power delivered to the motor: 14.3 [V] * 19.3 [A] = 276 W (the difference of 276 - 250 = 26 W is due to energy losses in the motor). The required DC link power is 276 [W] + 276 [W] * (100% - 90%) = 303 W. Having the DC link voltage of 6S (22 V), the estimated DC link current is 303 [W] / 22 [V] = 14 A.
For Myxa, the maximum rated phase current is 40 A (may be increased in the future firmware revisions), and the maximum DC current is 30 A (unlikely to ever change).
One should beware that most sinewave drive controllers have certain limits on the magnitude of the output voltage they can modulate. For Myxa, the maximum attainable phase voltage is Vdc / sqrt(3) * 0.91. For example, with the DC link voltage of 22 V, the maximum attainable phase voltage would be 22.2 / 1.73 * 0.91 = 12 V. Which means that the above setup would be dysfunctional – the controller would be unable to reach the maximum power because the voltage modulation range will be exhausted before the desired power is reached. However, if the speed constant of the motor were higher, it would work out nicely:
5000 [RPM] / 430 [RPM/V] = 12 V
You are right to point out that the lack of any datasheet is frustrating. This is a very new technology, so we are still working out certain minor implementation issues. The current version of the firmware is only v0.1; v0.2 is just around the corner, and v1.0 (the first highly robust release) is due around January already. Despite this, we already have several large customers whom you have certainly heard of who are leveraging our controllers in production successfully; the only limiting factor for them is the lack of proper documentation.
As I said, we would be happy to help you integrate Myxa into your system.