HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Oct 16, 2006, 02:12 AM
Senile Member
Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
6,000 Posts
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/castle-scribe.html . Select Issue #1 and scroll down to ďMyths and Mysteries ExplainedĒ. 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 three-fourths, or 75% throttle. Also 75% throttle will give a 75% duty cycle where the ESC is on for three-fourths of the time and off for one-fourth of the time. Now we are told that during the ESC on-time the motor will pull the same current as at full throttle, in this case 15 amps.

So far so good. Although the article doesnít state it, full battery voltage is also applied to the motor during the ESC on-time. 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 on-time 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.

Thatís my story and Iím sticking to it . That is unless someone can point out any errors in my logic. I am willing to learn.

Larry
Lnagel is offline Find More Posts by Lnagel
Reply With Quote
Sign up now
to remove ads between posts
Old Oct 16, 2006, 02:22 AM
Registered User
Terry Rigden's Avatar
UK, Bedworth
Joined Apr 2004
2,577 Posts
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
Terry Rigden is offline Find More Posts by Terry Rigden
Reply With Quote
Old Oct 16, 2006, 03:19 AM
Registered User
Staffs, UK
Joined Nov 2003
10,430 Posts
Quote:
Originally Posted by Lnagel
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.
......
Thatís my story and Iím sticking to it . That is unless someone can point out any errors in my logic. I am willing to learn.
Your arithmetic is fine but I do have one (fairly major) question about your logic.

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
slipstick is online now Find More Posts by slipstick
Reply With Quote
Old Oct 16, 2006, 03:46 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
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 ESC-25A, GWS 2205-15T, 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!
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old Oct 16, 2006, 04:17 AM
Registered User
vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,703 Posts
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...
vintage1 is offline Find More Posts by vintage1
Reply With Quote
Old Oct 16, 2006, 08:44 AM
Neophyte hacker
portablevcb's Avatar
Albuquerque, NM USA
Joined Sep 2003
15,648 Posts
Thanks Bruce. Hope that helps folks show how their ESC's really work.

Also shows why most ESC's are so non-linear in throttle response.

charlie
portablevcb is offline Find More Posts by portablevcb
Reply With Quote
Old Oct 16, 2006, 09:56 AM
Same Day Delivery
mike3976's Avatar
Economy, Pa.
Joined Mar 2002
3,448 Posts
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.
mike3976 is online now Find More Posts by mike3976
Reply With Quote
Old Oct 16, 2006, 10:07 AM
Registered User
pilotpete2's Avatar
The Northeast Kingdom, Vermont
Joined Jun 2004
5,202 Posts
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
pilotpete2 is offline Find More Posts by pilotpete2
Reply With Quote
Old Oct 16, 2006, 12:47 PM
Registered User
AmpAce's Avatar
North by Northwest
Joined Nov 2003
2,422 Posts
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
AmpAce is offline Find More Posts by AmpAce
Reply With Quote
Old Oct 16, 2006, 02:11 PM
Registered User
vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,703 Posts
Quote:
Originally Posted by AmpAce
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)?
neither.

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..
vintage1 is offline Find More Posts by vintage1
Reply With Quote
Old Oct 16, 2006, 02:30 PM
Neophyte hacker
portablevcb's Avatar
Albuquerque, NM USA
Joined Sep 2003
15,648 Posts
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
portablevcb is offline Find More Posts by portablevcb
Reply With Quote
Old Oct 17, 2006, 06:42 PM
Same Day Delivery
mike3976's Avatar
Economy, Pa.
Joined Mar 2002
3,448 Posts
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?
mike3976 is online now Find More Posts by mike3976
Reply With Quote
Old Oct 17, 2006, 07:58 PM
Neophyte hacker
portablevcb's Avatar
Albuquerque, NM USA
Joined Sep 2003
15,648 Posts
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
portablevcb is offline Find More Posts by portablevcb
Reply With Quote
Old Oct 18, 2006, 12:48 AM
Senile Member
Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
6,000 Posts
Quote:
Originally Posted by slipstick
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).
I don't know whether or not it's a law of physics but that's the whole basis of how an ESC works. The battery voltage is switched on and off to vary the average voltage applied to the motor. That average voltage is equal to the battery voltage times the duty cycle. The motor will then turn at an RPM that is roughly equivalent to the motor Kv times the average voltage. I didn't find it a real challange to then conclude that since the average voltage is proportional to the duty cycle, and the RPM is proportional to the average voltage, that the RPM is also proportional to the duty cycle.

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
Lnagel is offline Find More Posts by Lnagel
Last edited by Lnagel; Oct 18, 2006 at 01:22 AM.
Reply With Quote
Old Oct 18, 2006, 01:20 AM
Senile Member
Lnagel's Avatar
Moab, Utah, USA
Joined Apr 2003
6,000 Posts
Quote:
Originally Posted by Bruce Abbott
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.
Nice pictures, Bruce. Thanks for posting them. That is just the kind of input I was hoping for.

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
Lnagel is offline Find More Posts by Lnagel
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Juggling efficiency and power at partial throttle (cruise). macr0t0r Power Systems 208 Feb 18, 2007 10:58 AM
Discussion less amps with more lipos for a given RPM at partial throttle? Nethole Batteries and Chargers 30 Oct 12, 2006 05:50 AM
Discussion Partial Throttle/Gov: motor runs cooler at full derway Castle Creations 11 Apr 28, 2006 08:59 PM
Are outrunners more efficient at partial throttle DWS Power Systems 3 Feb 18, 2005 08:57 AM
Is flying at partial throttle with a sensorless brushless motor bad? Mitch G Power Systems 6 Dec 12, 2001 02:15 PM