Max current/watts power data sheet discrepancy

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1

The Zubax site says the myxa can handle 850 watts but the titan elite site has 600 watts.

My question is which is it and also i understand that the way these esc work is different than traditional hobby type and grade units but are they limited via current amounts? Meaning can I push 1200 or 1500 watts (60 amps or so at 25v) for a few seconds? Maybe 5-10 seconds? My biggest issue with these units and getting the survey company I work for to try this and hopefully transition all our units over is not understanding the data and what the true max of these units are or what we can do or expect to see that’s about double the capacity if we can’t push these to 12-1500 watts for 5-10 seconds.

Any help would be great.

2

The 600 watt rated power provided at the Titan Elite website is obsolete; it used to be the rated maximum in an early revision of the firmware until we’ve resolved certain stability issues at high power levels in a newer revision. I am going to ask them to fix that.

Regarding the power capability: the relationship between the DC current and the phase current circulating through the motor is rather complex; you can get a brief overview of the problem in my short post “Power balance explained”. To size the ESC properly, you need to rely on the required power output (dictated by the motor and the propeller), the required voltage at the rated speed (dictated by the motor KV parameter (RPM per volt)), and the required burst power capability. Having that data and relying on the relationships explained in the referenced post, you can come up with an estimate. However, if your propulsion system relies on low-cost poorly-characterized motors, you are probably better off just collecting empirical data by testing your setup on the bench. We can also assist you with tuning Myxa for your specific propulsion hardware to optimize its energy-efficiency and robustness; please let us know if that is relevant.

3

Noted on the 850 watts but the remainder is not exactly helpful or what I haven’t read.

Our company has about 35 fixed wing planes and we do surveying in northern Montana. We are going to double that amount in the next 2 months but also are looking at increasing flight times while not changing thebairframes of what we already use or our additional planes. I oversee the service of them to keep them all airworthy and testing of new hardware.

All are 2m flying wings and we cruise at about 250 watts but when we have our hyperspectral on board we are taking off (only takeoff) and pulling 1200-1500 watts 50-60 amps at 25v). Not really because it’s needed but because our power setups are capable of such and there is enough overhead from a power safety perspective. Probably last 10 - 15 seconds.

I’m interested in replacing everything with these Myxa speed controllers but I can’t even find a data sheet or know if our (300-400kv) motors will just overheat and kill the esc.

So some clarity would be great. Is current limited via the firmware? We’ve used other FOC esc before but none make the 11% claim of increased flight times (most are designed for skateboards except the T-motor ones but there isn’t any real control over the units) and on the initial foc units, I like how much granular control there is but I want to know what I’m dealing with or I can’t get approval to buy 1 let alone 70 plus spares.

Some help here would be great and some sort of data sheet. Is there an over current shutdown? If so what is it? Can we set it? What have you tested the max current at specific voltages to? How is the limiting put in place? Current is complex I understand and really heat dissipation plus voltage switching capability so unless you are blocking via watts, how does that play into things? I just want to get some numbers and know what ratings are so I can make a proposal and request for purchase but I need some manufacturer data to present.

Thanks

4

Hi Jason,

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.

5

Hi,
So the maximum modulation is 1/(sqrt(3) * 0.91)=63%?
This would mean that a 400kv motor on 50V(12s battery) would have a max rpm of 400 * 50 * 0.63=12600 rpm (neglecting losses) using zubax myxa?

6

Hi Magnus,

Yes, your calculations are correct. But don’t forget that this is rpm value of the unloaded motor. If you attach a propeller the maximum rpm value is lower.

7

Hi Dimitry,
This would mean that the max rpm is 6000 rpm from 24V (6s) battery for an unloaded 400kv motor. When I tested the Myxa with a KDE3520xf-400kv with a propeller mounted I’ve managed to run it up to 7240rpm (see attached screenshot). What is going on here? Is there field weakening in action or something doing this?

I’m also a bit confused about the max current for the Myxa. I’ve set it to 55A which seems to work fine for short bursts of peak power (1500W, although very low efficiency), but how reliable would is this? I tried 40A (since 40-50A is the max current according to some forum post) at first, but this didnt give enough peak thrust and power was limited to about 1000W of power when connected to 50V power source. I have tested the same motor and propeller with same battery on a VESC six with the same thrust at 950W as Myxa delivers at 1500W. At hover thrust they are pretty much the same in efficiency though ~470W. I would like to know what is going on here.

8

Hi Magnus,

I don’t see the screenshot. It’s really difficult to say what exactly happens without plots and logs. You can check logs, there should be messages “FIELD WEAKENING IS ACTIVATED”. In the field weakening mode the efficiency is poor indeed. To achieve the same thrust with higher efficiency you can try oversaturation (ctl.vm_oversatur: true).

9

Hi,

What is meant by oversaturation here? Will it run at higher modulation than 63%?

10

Hi Magnus,

Oversaturation means that the input signal (vector) amplitude, coming on SVPWM modulator exceeds 1. The motor phase voltage 1st harmonic amplitude becomes higher than \begin{equation} \frac{0.91\text{V}_\text{DC}}{\sqrt{3}} \end{equation} .
But at the same time, voltage and current distortions appear, which leads to instability and the appearance of vibrations as in conventional BLDC controllers.