less amps with more lipos for a given RPM at partial throttle? - RC Groups
Oct 11, 2006, 08:03 PM
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# less amps with more lipos for a given RPM at partial throttle?

I was pondering the brushless motor equations, looking at the 'power required' to turn a prop at a given RPM. This was suggesting to me that for a higher voltage, less amps were needed.

However, for a given motor there's a certain Kv which dictates the speed it will turn with a given voltage potential.

So, I ask the question, how does 'partial throttle' work with ESC?

If I have a situation with a given motor and a 3S lipo and it requires 10amps to turn a given prop at 6421 rpm. and Now, if I replace the 3S lipo with a 4S lipo, but 'throttle' the beast so that I still get 6421 rpm. Will it still require 10 amps (plus or minus a minutia for the difference in resistance of the batteries)?

If that's the case, then does it suggest that increasing the number of lipos in series, and decreasing the throttle (to match original rpm) does not provide any advantages in terms of amp draw on the battery.

Conversely, does that suggest, if I drop to a 2S lipo, and can still make the required RPM from the voltage in the battery, that the same 10 amps (+-) will be all that is required?
 Oct 11, 2006, 08:16 PM Titanium Has My Back!! delete Last edited by 9LIVES1968; Oct 11, 2006 at 08:39 PM.
 Oct 11, 2006, 08:18 PM Titanium Has My Back!! Also, if you drop to 2s to make the same rpm, you are going to really have to reduce the prop size to turn the same rpms and you wont be pulling anywhere near the 10 amps. Probably around 4-5 amps
 Oct 11, 2006, 08:26 PM Registered Lurker No, you didn't answer the question I posed. I'm not talking about full throttle. I'm specifically talking about partial throttle WITH the same prop AND at the same RPM. Please address this question, do not create your own question. I worded it this way for a reason.
 Oct 11, 2006, 08:30 PM Dimension Engineering Yes, it will generally draw less amps to turn the same speed for a given controller. How much less depends greatly on the partial throttle efficiency of the controller. Partial throttle efficiency is governed by a whole host of things, and most ESC manufacturers don't consider it a major design consideration (its expensive to do well, and increases the weight of the controller without increasing the current rating of the controller)
Oct 11, 2006, 08:36 PM
Southern Pride
Quote:
 Now, if I replace the 3S lipo with a 4S lipo, but 'throttle' the beast so that I still get 6421 rpm. Will it still require 10 amps (plus or minus a minutia for the difference in resistance of the batteries)?

Volts times amps . equals watts. Watts is the unit or power. Watts are what turns props. It takes X number of watts to turn any give prop. at a given RPM.

Real life data.
IKARUS 2023011 motor. GWS DD 9050 prop at 5000r RPM.

2S 6.9 volt , 3.7 amps. 27 watts
3S 10.8 volts ,2.8 amps. 30 watts

Note these are number obtained two years ago with LiPolys that dropped volts a lot more than the ones of today. Also note that reading vary a little and these are averages.

And yes the ESC and battery needs to be able to handle the max. amps at WOT even if using at reduced throttle according to the ESC experts. I have never fully agreed with them but it is not worth the aggravation to argue the point.e
Last edited by everydayflyer; Oct 11, 2006 at 08:42 PM.
 Oct 11, 2006, 08:38 PM Titanium Has My Back!! Doing like you outlined in you question is going to cause excess heat in your esc. Even though you are "throttling back" to your given rpm, the esc is switching harder to maintain the throttle setting. It is still seeing full voltage from the battery and the setting of the throttle. If you are using a 10 amp esc and do like you outlines, you are going to overheat your esc. There are plenty examples of this in the power system forum which is where this needs to go.
Oct 11, 2006, 08:52 PM
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Quote:
 Originally Posted by everydayflyer Simple answer no. Volts times amps . equals watts. Watts is the unit or power. Watts are what turns props. It takes X number of watts to turn any give prop. at a given RPM. Real life data. IKARUS 2023011 motor. GWS DD 9050 prop at 5000r RPM. 2S 6.9 volt , 3.7 amps. 27 watts 3S 10.8 volts ,2.8 amps. 30 watts Note these are number obtained two years ago with LiPolys that droped volts a lot more than the ones of today. Also note that reading vary a little and these are averages. Charles
Ok, this is sounding more like a powersystem question now (I concur with the commonsense guy's comment). I have no objections if an admin moves this to powersystems instead.

Now, perhaps the previous individual answered the question of how (his answer was 'switching'), but I'm curious how 'less volts' are provided to the motor for 'less than full' throttle. Or is it the 'max voltage' for less time.

Meaning that for a given motor with a specified Kv, it's supposed to spin at a given rpm when that potential difference is applied. Does this suggest that the motor is in a more-or-less constant state of acceleration/deceleration from the 'full battery voltage' being applied and disconnected repeatedly? I know it's a three phase motor so there's more to it than that. But I'm still trying to grasp how the ESC funtions in 'throttling down' a particular battery/motor configuation.

Thanks, Charles, for providing reallife numbers. This points out that my understanding of the powersystem with 'partial throttle' settings is incomplete.
Oct 11, 2006, 08:52 PM
KC5UH transmits -------->
Quote:
 Originally Posted by everydayflyer And yes the ESC and battery needs to be able to handle the max. amps at WOT even if using at reduced throttle according to the ESC experts. I have never fully agreed with them but it is not worth the aggravation to argue the point.e
This means that you still do not understand how the esc controls motor speed .

David
 Oct 11, 2006, 09:03 PM Titanium Has My Back!! Think of the esc as being a gatekeeper. Inside the are 10 gates, with each gate representing 1 amp of power being handled.Obviously I am describing a 10 amp esc. When full throttle is applied, all 10 gates are open. Now say that you only want 1/4 throttle. 3 or 4 gates stay open, and the remaining gates switch open and shut very fast to limit the current and amperage being delivered to the motor. All 10 gates are still seeing 1 amp each, but now each of the switching gates are working harder to open and shut to maintain the lower output. 3/4 throttle, 6-7 gates are fully opened and the remaining ones are switching rapidily to lower their share of the amp load in order to lower the overall voltage to the motor. That is why an esc will run cooler at full throttle versus partial since less heat is generated from staying "open versus switching" Now realize this is a very simplistic explanation as there are other factors, but this is the easiest to understand. I hope this helps. Dave
Oct 11, 2006, 09:13 PM
Southern Pride
Quote:
 Originally Posted by ham2405 This means that you still do not understand how the esc controls motor speed . David
I fully understand how ESC work. It is a switch which is either on or off. Sometimes on more sometime off more. Thing is that It is more complicated than the simple partial throttle explanations given by most.
The worst throttle position is at approx. 65-75 % throttle. At less than 60% or more than 80% it is not nearly s bad as most would lead you to believe.

I have owed Bob Boucher's Electric Motor Handbook since it was first published and I have read it many times and understand most of it.
I have flown electrics since a two minute flight reaching 100 foot elevation was a feat.

A snip from a post of Astro bob's

Quote:
 on iron motors my measurements indicates that maximum heat in the speed control occurs at between 85 and 90 percent throttle. so the 10 amps control gets the hottest if the motor is propped for 10 amps and run at 9 amps. for basket motors like the hacker and chinese hacker clones the lower inductance makes for more current ripple so max temp may occur at 70 to 80 percent throttle. in all cases 50% throttle will be cooler than 100 percent.

so even the type of motor makes a difference.

Charles
Last edited by everydayflyer; Oct 11, 2006 at 09:18 PM.
 Oct 11, 2006, 09:16 PM Titanium Has My Back!! I was waiting for that reply with baited breath!lol. Dont mess with our resident battery guru! Dave
 Oct 11, 2006, 09:21 PM Titanium Has My Back!! Nethole, if you have a 3s setup with a given prop and esc at partial throttle and take its amp reading and then change to a 4s with everything else being the same(esc, prop, motor), at the same partial throttle setting, you will have a higher amp draw usually. It doesnt matter if it is at full thottle or partial throttle, amp draw will be higher with the added cell if you dont change the prop size. Conversely, if you drop down a cell, then your amp draw will be the lower with everything being constant. What I had written earlier still applies whether wot or partial.
Oct 11, 2006, 09:33 PM
Registered Lurker
Quote:
 Originally Posted by 9LIVES1968 Nethole, if you have a 3s setup with a given prop and esc at partial throttle and take its amp reading and then change to a 4s with everything else being the same(esc, prop, motor), at the same partial throttle setting, you will have a higher amp draw usually. It doesnt matter if it is at full thottle or partial throttle, amp draw will be higher with the added cell if you dont change the prop size. Conversely, if you drop down a cell, then your amp draw will be the lower with everything being constant. What I had written earlier still applies whether wot or partial.
You're going to suggest that with a 4S lipo to provide 6000 rpm will require more amps, than is required with a 3S lipo?

again, you're answering a different question than I asked, I didn't say for a 'same throttle' setting for each lipo case, I said for the SAME RPM for each lipo case.

From the FIRST SENTENCE IN THIS THREAD
I was pondering the brushless motor equations, looking at the 'power required' to turn a prop at a given RPM
 Oct 11, 2006, 09:42 PM Southern Pride Last edited by everydayflyer; Oct 11, 2006 at 09:48 PM.