Hello, I’m a student looking to use the T200 for a project I’m working on, and I’d like to ask for some clarification on how overvoltage conditions affect the power efficiency. Due to the constraints of the platform, we’ll have to power it with ~22-24V, and I see others in the forum have asked about this before, particularly this post:
I believe I understand the explanation about a given thrust requiring a fixed current, so using a higher voltage would necessarily draw more power. However, according to this chart on the T200’s page, the 16V and 12V plots use approximately the same amount of power for a given thrust.
Is there something I’m missing that explains how both of these can be true? Also, part of the concern of the platform is how hot the ESCs will get, and if applying overvoltage will run them hotter than usual. Is all of the wasted overvoltage energy dissipated as heat in the thrusters themselves, or is any significant portion dissipated in the ESC instead?
First, I believe this explanation of mine will clarify the expected behavior of the T200 in beyond supported voltages:
The previous discussion you linked is quite old and dated back to only a few months after the launch of the T200. Since then, there has been a lot more knowledge gained and testing carried out.
Everything you say is true- torque is proportional to current only, and torque dictates propeller RPM and thus thrust. Since Ohm’s law (V=I*R) states there is direct relationship between current, voltage, and resistance, current must go up as voltage goes up since resistance (stator windings) remains the same. Also, as you correctly suggest, the plot you linked does show the T200 uses appropriately the same amount of power for a given thrust at both 12 V and 16 V. This is the expected behavior.
However, there is one more critical factor that must be considered, that @nick-wernicke actually points out in the original thread you linked- the ESC and how throttle is modulated. Critically, the voltage at the stator windings is not just Vsupply, but Vsupply * Duty Cycle, or the average PWM supplied voltage, AKA Vapparent. The ESC reduces “throttle” by literally chopping down the average voltage by only powering a phase a certain percentage of the time. Thus even though the supply voltage may be 16 V, the apparent voltage at the thruster will only be ~16 V at full throttle 100% duty cycle. At lower throttle, say 50% duty cycle, the apparent average voltage will be 16*0.5= 8 V. This is how an ESC can control “throttle” in the first place- by reducing duty cycle, apparent voltage, and thus current/torque/RPM/thrust are reduced.
In the T200 12 V and 16 V thrust vs. power chart you posted, note the purple 16 V line is significantly longer than the brown 12 V line, corresponding to a greater range of thrust due to a greater range of voltage. When powered at 16 V, the ESC does not actually feed 16 V to the thruster until duty cycle/throttle exceeds 75%. At 100% throttle and 12 V, apparent voltage is the same as 75% throttle and 16 V (0.75*16=12). Therefore regardless of Vsupply being 12 V or 16 V, approximately the same amount of power will be required for a given thrust.
Thanks a ton for your answer! Your explanation helped clear up most of the misconceptions I had about the way ESCs worked. This will help us decide how to limit our throttle. Looking at the efficiency vs. PWM chart, it looks like we’ll need it to keep our batteries running for long enough. Thanks again!