


Discussion
Peak current at partial throttle
I have seen in several threads the concept that at partial throttle the peak current pulled by a motor is the same as the full throttle current regardless of the throttle setting. I have asked why this is so in other threads, but so far no one has been able to explain it other than saying the current is switched and referring me to the Castle Creations article on the subject. If this is in fact true, I am hoping that someone here can explain this phenomenon in a more concise manner.
First I will tell you what I know about the subject. I have read the Castle Creations article and IMHO it is seriously flawed. For those of you who have not seen it or want to refer to it, you can find it at http://www.castlecreations.com/media/castlescribe.html . Select Issue #1 and scroll down to “Myths and Mysteries Explained”. Basically the article sets up a scenario where a motor pulls 15 amps at full throttle. It also states that a meter will measure an average current of 10 amps at threefourths, or 75% throttle. Also 75% throttle will give a 75% duty cycle where the ESC is on for threefourths of the time and off for onefourth of the time. Now we are told that during the ESC ontime the motor will pull the same current as at full throttle, in this case 15 amps. So far so good. Although the article doesn’t state it, full battery voltage is also applied to the motor during the ESC ontime. Therefore, the article implies that with full voltage and full current the motor will be producing full power for 75% of the time and no power for 25% of the time. Thus the motor is averaging 75% power at 75% throttle or duty cycle. This is where their logic falls short. At 75% duty cycle a motor does not produce 75% power. It produces only the power needed to turn the load. And since our load is a propeller, it only requires 42.2% power at a 75% duty cycle. This is because the power required by a propeller changes with the cube of the change in RPM. Since the average voltage seen by the motor is 75% of full battery voltage, the RPM will be 75% of full throttle RPM. Therefore the power required by the propeller will be the cube of 0.75 or 0.422. While the propeller requires 42.2% average power at the reduced RPM, we are still working at a 75% duty cycle. Therefore the peak power required during the ESC ontime will be 0.422 / 0.75 = 0.563 or 56.3% of full throttle power. Therefore, the peak current at 75% throttle will be 56% of full throttle current or 8.44 amps, not the full 15 amps as stated in the CC article. That’s my story and I’m sticking to it . That is unless someone can point out any errors in my logic. I am willing to learn. Larry 




I had doubts about the Castle article too.
The current drawn by the motor will be effected by the load and the inertia of the prop will even out any power pulses from the ESC. Also the damage done by high current is from heating and we have the thermal mass of the batteries and ESC to consider. Thats why the continueous rating on batteries and other electroic components is less than this peak rating. Regards Terry Rigden 


Staffs, UK
Joined Nov 2003
11,561 Posts

Quote:
What law of physics is it that makes 75% duty cycle equate to 75% rpms ? You're assuming that rotational speed is proportional to duty cycle but I don't know of any reason why that would be true (and the simple measurements I've done don't show anything like that). BTW to my mind the CC article is fine except that it assumes a resistive load. I.e. that simple Ohms Law applies. This isn't really the case for (highly inductive) motors. But I'm not convinced that's enough to explain your theory of rpm proportional to duty cycle. Steve 




Certainly the motor will draw less current at low throttle. But the problem with your theory is the assumption that current flow is constant during the 'on' portion of the PWM cycle. It isn't.
Take a look at these two oscillograms, showing measurements I made of current going into an ESC (GWS ESC25A, GWS 220515T, EP7035, PSU set to 7V). At full throttle my wattmeter showed 8A. The 'scope showed current climbing a bit during each phase, from a low of about 7A to a high of about 9A. At 2/3rd throttle my wattmeter showed just 3.5A, but the instantaneous current spiked to almost 9A! In this case the peak current at 2/3rd throttle was higher than the average current at full throttle! 



Very good graphs Bruce.
The full throttle trace shows how the motors inductance limits inrush and the current is fairtly steady. Part throttle shows the final current is higher by far..this is what you expect, because the back EMF from the motor is well below supply. Now to calculate the actual power losses in both cases... 



Thanks Bruce. Hope that helps folks show how their ESC's really work.
Also shows why most ESC's are so nonlinear in throttle response. charlie 



But, I'm still sticking to the contention that these "tests" were performed on a "bench" with the motor statically loaded. In flight with a lightly loaded model in a cruse condition, amp draw will be less, because the power needed to sustain flight is lower. I'm not saying the motor never draws full amperage at part throttle, but it does not draw full amperage all the time at part throttle. This depends on the load.




Bruce,
Thank you for the O'scope shots. The "full current at partial throttle" theory, is most often refered to when someone wants to limit amps by using partial throttle, when say their ESC is rated below their WOT current as measured for the setup, simple answer NOOOO This seems to be a recipe for smoke. Now I have seen references to the peak current being a problem for lipos in particular, but I am clueless, as I don't have the engineering level training to have an opinion on that one, any thoughts from anyone? Also, would there be any value in adding filter capacitors to our Wattmeters to give a more accurate "average" reading. Pete 



Thanks to all for attempting to enlighten those of us who are not "electron gurus" out here!
My basic question about all of this is just how the ESCs and motors are rated for maximum allowable load. Are they rated for the maximum amperage spike that they can survive, at a certain voltage, or are they rated for an "average" amperage, (as read by a watt meter)? I suspect that it's the latter, and I've always wondered why they aren't rated for maximum allowable wattage, as that should paint a truer picture of thermal and mechanical stress on the components. If the components are rated for an "average" amperage, then part throttle operation should provide a margin of safety, since a watt meter will show a much lower amperage draw upon seeing even a minor reduction of throttle setting. AmpAce 



Quote:
They are rated to not blow up and thats 99.99% from overheating. The heat is proportional to the RMS current through them. The meter will probably measure the average current. Thats why they CAN get hotter at part throttle, than full. May do some maths later.. 




After all that, let me say that most of what everyone perceives allows them to work out good combinations for flight. The only real problem is getting an ESC that is correct for your application, ie, whatever your peak voltage and current draw are.
The ratings of a motor are based mainly on the wire size, which dictates the amount of current that can be run through them without shorting. Secondly, the heat generated by that current has to be able to escape, which is why there is a Watt rating as well. Peak ratings mean that the immediate heat will not melt anything, but, that it needs a while for all that energy to get out of the motor. So, full throttle for a few seconds, then at partial throttle it has time to cool down. The watt meter is sufficient to measure both of those. ESC's are limited by the FET's (Field Effect Transisitors, which are like high frequency switches) capacities. Their load rating is based on peak voltage as well as current (heat generation). Too high a voltage can break down the circuit inside the FET. They still have to get rid of heat (like a motor does) which is why you see large heat sinks on ESC's for cars and boats. Part throttle operation does provide a margin of safety. But, other things can suffer if you take it to the extreme. If you cruise at 1/4 throttle, then you have only 1/4 of the stick movement to setup for landing. Kinda coarse controls. The key is to make sure your peak voltage is within the limits of the components, then keep the current draw to reasonable levels. You can rely on throttle control for this, but, it is risky. I've burned up a motor using this logic. At least it was only a sp400 and not a big brushless. charlie 



Mike,
The motor is not seeing constant voltage. It is seeing full pack voltage, then none, full, then none. Not all of your "excess" 75 watts are staying in the battery. Some are being converted to heat due to the inefficiencies of the system. Take a look at some of the power systems being used in 3D foamies with their absurdly low wing loadings. They often don't need more than a few clicks to maintain level flight, but the power systems can run very hot, despite being out in the breeze and running almost the entire flight at partial throttle. Static on the bench it is only seeing max load if you give it full power, same as in the air. Partial power settings in the air, and on the bench, are just that; partial power. Thayer Thayer, I know the voltage is not "on" continusly, and is switched at the ecs rate, What I'm talking about is when factoring watts, the voltage is "Constant" Like: Amps x Volts = Watts If a motor draws 10 amps on a bench at 1/2 throttle, in a lightly loaded plane flying at 1/2 throttle you say your still drawing 10 amps? I say its less and dependent on the motor load. Static on the bench it is only seeing max load if you give it full power, same as in the air. Partial power settings in the air, and on the bench, are just that; partial power. Thayer I thought your claiming its seeing full power (amps) but switched at a lower rate by the esc? 



Static load on the bench and load in the air are different beasts. Most of the time the static load at any power setting is higher than the same throttle setting in the air.
One exception is when you have a propellor blade that is stalled when testing static. That conditions tells you almost nothing about flight performance. Other exceptions (not all of them) are hovering, high AOA maneuvers, takeoff roll, rapid application of throttle. I use static testing to tell me if I am exceeding limits when at full throttle. Very little more than that. Please don't mix current draw, power, and voltage. They are different. The ESC switches full voltage on and off. The current draw that responds to the depends on the motor and load conditions. charlie 



Quote:
In your simple measurements were you using actual duty cycle or throttle position? They are not the same in most, if not all, ESCs. Some ESCs are better than others, but none have a completely linear throttle. Larry 




Quote:
Yes, my model is quite simplified and, to paraphrase if I may, your graphs definitely show that reality is stranger than theory. I do have a question about your wave forms. The major cycles are occuring at 125usec intervals which gives a frequency of 8khz. Is that the PWM switching frequency or the comutation frequency? Larry 

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