New Products Flash Sale
Thread Tools
Old Jun 05, 2015, 12:34 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
Discussion
3s = 30amps, 2s = 5 amps? Same prop

I have a motor that should use a maximum of 18 amps. With the prop I want to use it was pulling 30amps. So, I figured what the heck? I'd just switch to a 2s battery and I should be fine.

At wot with the 2s battery the motor is only pulling 5amps. I tried 2 batteries with different C ratings and got the same thing. There is also very little pulling power.

Shouldn't this have cut the 3s amps by 30-50%? What I am seeing is that the amps were cut by about 85%.

Obviously I need to use a different motor for this prop/plane combo. But it would be nice to be able to figure out why this is happening. Thanks!
bbells is offline Find More Posts by bbells
Reply With Quote
Sign up now
to remove ads between posts
Old Jun 05, 2015, 12:48 PM
Martyn McKinney is offline
Find More Posts by Martyn McKinney
Registered User
Toronto Canada
Joined Dec 2002
5,605 Posts
Ohm's law doesn't work with motors and prop loads.

For a propeller, Power is proportional to the RPM cubed.

RPM is proportional to Voltage.

When you use 2 cells instead of 3, the RPM has decreased to 2/3 of its original value and the Power has decreased to (2/3)^3 = 0.67^3 = 0.3 the original Power

But the Voltage has also decreased to 2/3 the initial value.

For a perfect motor, the current on 2 cells would then be 0.3 / 0.67 = 0.45 its original value.

Since the original current was 30A the actual current should be 0.45 X 30 = 13.5A which doesn't agree with your measurement! I'm stumped.

Did you measure the actual applied voltage or RPM? Perhaps in each case, one of the 2S cells was bad.
Martyn McKinney is offline Find More Posts by Martyn McKinney
Last edited by Martyn McKinney; Jun 05, 2015 at 01:24 PM.
Reply With Quote
Old Jun 05, 2015, 12:58 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
Hey, that looks impressive and quite possibly makes sense.
Must be that science thingee. You know, science is only for those with enough self respect to not allow morons to manipulate them. So, I definitely won't try an manipulate your response.
Works for me! Thanks!
bbells is offline Find More Posts by bbells
Reply With Quote
Old Jun 05, 2015, 01:39 PM
Martyn McKinney is offline
Find More Posts by Martyn McKinney
Registered User
Toronto Canada
Joined Dec 2002
5,605 Posts
Oh, Oh!

I have edited my initial response a number of times. After looking at the time of your response, I realized that you might be responding to an incorrect reply which I have subsequently edited.

Your original thinking is reasonable.

Again, I would measure the applied voltage to determine if your 2S cells were OK.

Also, did you measure the current in the same manner in each instance? Is there any chance that an ammeter has introduced some voltage drop which is reducing the applied voltage?
Martyn McKinney is offline Find More Posts by Martyn McKinney
Last edited by Martyn McKinney; Jun 05, 2015 at 01:57 PM.
Reply With Quote
Old Jun 05, 2015, 02:30 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
Well, this is interesting and embarrassing. You got me to give this another try. Another of the same brand/model esc did the same thing. So, I soldered together a fresh other brand esc and I now get 18 amps. Exactly where I want it to be since I will be using a slightly smaller prop on the plane. Everything is the same except for the esc so I think my esc was rpm challnged. Thanks for the help!
bbells is offline Find More Posts by bbells
Reply With Quote
Old Jun 05, 2015, 02:35 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
Well, even more embarrassing. The first 2 esc's were setup to use in my quadcopter. I went through the setup using the turnigy esc setup card and I ended up with the same 18 amps. Obviously the setup for my quad was different than the setup for my plane. Dah.
bbells is offline Find More Posts by bbells
Reply With Quote
Old Jun 05, 2015, 02:53 PM
Ron van Sommeren is offline
Find More Posts by Ron van Sommeren
homo ludens modellisticus
Ron van Sommeren's Avatar
Netherlands, GE, Nijmegen
Joined Feb 2001
11,789 Posts
  • Simple instructive table giving the extra current when you add/remove one extra cell to/from a pack
    www.rcgroups.com/forums/showt...0#post27889754
    (a.k.a. "why did my motor and/or controller and/or battery go up in smoke ??? )
  • Motorcurrent is proportional to voltage squared, and proportional to Kv cubed.
    A 10% change in battery voltage will give a 20% change in current, a 10% change in Kv will give a 30% increase. Not quite, because battery voltage will sag more due to higher current, but you'll get the idea.
Prettig weekend Ron
Ron van Sommeren is offline Find More Posts by Ron van Sommeren
Reply With Quote
Old Jun 05, 2015, 03:04 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
Great info to have! Thanks!
bbells is offline Find More Posts by bbells
Reply With Quote
Old Jun 05, 2015, 03:04 PM
Martyn McKinney is offline
Find More Posts by Martyn McKinney
Registered User
Toronto Canada
Joined Dec 2002
5,605 Posts
Quote:
Originally Posted by bbells View Post
Obviously the setup for my quad was different than the setup for my plane.
Great that you got it sorted out, but I am curious how lowering the number of cells could change the ESC endpoint. Does this ESC on initialization sense the battery voltage and change the endpoint or were you using your transmitter with a different program when testing the 2S setup?
Martyn McKinney is offline Find More Posts by Martyn McKinney
Last edited by Martyn McKinney; Jun 05, 2015 at 03:37 PM.
Reply With Quote
Old Jun 05, 2015, 03:44 PM
bbells is offline
Find More Posts by bbells
Registered User
Joined Jan 2011
291 Posts
The setup card for the esc had several things that may have been the culprit.
It was set for 3 cells, not 2.
I moved the timing mode to middle.
Moved the cutoff voltage to middle.
bbells is offline Find More Posts by bbells
Reply With Quote
Old Jun 10, 2015, 07:25 PM
vollrathd is online now
Find More Posts by vollrathd
Registered User
vollrathd's Avatar
Joined Jan 2007
1,420 Posts
Good use of Wattmeter

Quote:
Originally Posted by bbells View Post
I have a motor that should use a maximum of 18 amps. With the prop I want to use it was pulling 30amps. So, I figured what the heck? I'd just switch to a 2s battery and I should be fine.

At wot with the 2s battery the motor is only pulling 5amps. I tried 2 batteries with different C ratings and got the same thing. There is also very little pulling power.

Shouldn't this have cut the 3s amps by 30-50%? What I am seeing is that the amps were cut by about 85%.

Obviously I need to use a different motor for this prop/plane combo. But it would be nice to be able to figure out why this is happening. Thanks!
Here is where it was wise on your part to actually measure the watts/current/volts input to your motor while going from a 3S to a 2S battery pack. As you've found out, making what appears to be a minor change on these electric power systems can make a very significant difference in the actual power outpu.

Had you gone from a 3S to a 4S pack, your wattmeter would have warned you that you're about to fry your motor/esc/battery pack
vollrathd is online now Find More Posts by vollrathd
Reply With Quote
Old Jun 11, 2015, 10:08 AM
Ron van Sommeren is offline
Find More Posts by Ron van Sommeren
homo ludens modellisticus
Ron van Sommeren's Avatar
Netherlands, GE, Nijmegen
Joined Feb 2001
11,789 Posts
Quote:
Originally Posted by vollrathd View Post
... Had you gone from a 3S to a 4S pack, your wattmeter would have warned you that you're about to fry your motor/esc/battery pack
Increase in current by factor (4/3) = 1.7!, ignoring voltage sag due to higher current.

Vriendelijke groeten Ron
Ron van Sommeren is offline Find More Posts by Ron van Sommeren
Reply With Quote


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Sold Traxxas Slash VXL-3s 2wd Fully RTR Including Tx & 2S Lipo drewbagel423 Cars - Cars and Parts (FS/W) 0 Apr 30, 2015 12:25 PM
Sold Traxxas Slash VXL-3s 2wd Fully RTR Including Tx & 2S Lipo drewbagel423 Cars - Trucks and Parts (FS/W) 0 Apr 29, 2015 05:41 PM
Sold NIP Fatshark 5.8 GHz 250mW Vtx with 2S/3S balance plug power & antenna $55 OBO The RevKev FPV Equipment (FS/W) 4 Nov 12, 2013 04:21 PM
Sold Affordable Dynam & E-Sky 2S & 3S LiPo Chargers - $5 to $8 Shipped write2dgray Aircraft - Electric - Batteries & Chargers (FS/W) 1 Mar 07, 2009 10:32 AM
Sold Lipo Clearance - Lots of Low Cycle & New 2S & 3S Lipos + a Few NiMH - $5-12 a piece write2dgray Aircraft - Electric - Batteries & Chargers (FS/W) 13 Mar 03, 2009 12:40 PM