Jun 05, 2015, 12:34 PM 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!
 Jun 05, 2015, 12:48 PM 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. Last edited by Martyn McKinney; Jun 05, 2015 at 01:24 PM.
 Jun 05, 2015, 12:58 PM 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!
 Jun 05, 2015, 01:39 PM 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? Last edited by Martyn McKinney; Jun 05, 2015 at 01:57 PM.
 Jun 05, 2015, 02:30 PM 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!
 Jun 05, 2015, 02:35 PM 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.
 Jun 05, 2015, 02:53 PM homo ludens modellisticus 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
 Jun 05, 2015, 03:04 PM Registered User Joined Jan 2011 291 Posts Great info to have! Thanks!
Jun 05, 2015, 03:04 PM
Registered User
Joined Dec 2002
5,605 Posts
Quote:
 Originally Posted by bbells 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?
Last edited by Martyn McKinney; Jun 05, 2015 at 03:37 PM.
 Jun 05, 2015, 03:44 PM 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.
Jun 10, 2015, 07:25 PM
Registered User
Joined Jan 2007
1,420 Posts
Good use of Wattmeter

Quote:
 Originally Posted by bbells 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
Jun 11, 2015, 10:08 AM
homo ludens modellisticus
Netherlands, GE, Nijmegen
Joined Feb 2001
11,789 Posts
Quote:
 Originally Posted by vollrathd ... 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)² = 1.7!, ignoring voltage sag due to higher current.

Vriendelijke groeten Ron