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        Discussion Why isn't my motor drawing the voltage my battery supplies?

#1 JimmyKudo Jan 31, 2013 03:45 PM

Why isn't my motor drawing the voltage my battery supplies?
 
Here's the AdamOne caclulation of my setup, I plan to have two of these motors total:

http://i.imgur.com/s2VY8zo.png?1



What is the reason for the low voltage draw? When I reduce the pack size to 8 cells, which is still above 9.24V it's "currently" drawing, the voltage goes down to 8.6V and the current goes down 11.8 A give or take.

I want to utilize the full amps/voltage to get the most thrust out of thse two motors. I am open to another motor, nothing that will draw more than 16 A.

While I'm at it, does anyone know if I can reduce the current draw from a motor by putting a lower current rated ESC? Like a motor with a max current draw of 30A with a 25A ESC limiting the system to 25A?

Sorry for the dumb questions, I'm clearly a new guy. Thanks for your help!

#2 Dr Kiwi Jan 31, 2013 04:31 PM

Amp draw is determined by prop size.. in your computation, increase your prop size 'til you get to the amp draw you need (but stay within the motor's limit).

ESC's don't limit amps.. they let through what they are capable of letting through... exceed that and they fry.

The low current to the motor indicates that the pack cannot deliver what that calculator suggests is what the motor/prop would demand, without dropping voltage.

NiMh cells (1.2v-1.4v HOC) typically can maintain only about 1v under load... in the same way Lipo cells (4.2v HOC) will be at 3.5v-3.8v under load.

#3 Lnagel Jan 31, 2013 06:12 PM

Quote:

Originally Posted by JimmyKudo (Post 23993380)
What is the reason for the low voltage draw?

The calculator shows the following data:
Battery pack voltage = 12.5 volts
Cell Resistance = 0.022 ohms -- ten cells in parllel would be 0.22 ohms
ESC resistance = 0.0065 ohms
Motor current = 14.4 amps

The battery resistance and the ESC resistance are in series with the battery and the motor so the current through them is the same as the motor current and the voltage drop across them will be subtracted from the battery voltage. The voltage drop across the battery resistance and the ESC is calculated by multiplying their combined resistance by the current. So their combined voltage drop would be 0.2265 ohms x 14.4 amps = 3.26volts. Subtracting that from the 12.5 volts battery voltage leaves 9.24 volts to the motor.

Quote:

Originally Posted by JimmyKudo (Post 23993380)
When I reduce the pack size to 8 cells, which is still above 9.24V it's "currently" drawing, the voltage goes down to 8.6V and the current goes down 11.8 A give or take.

The same applies. with 8 cells battery voltage would be around 11 volts. 0.1825 ohms times 11.8 amps = 2.15 volts leaving 8.6 volts or so to the motor.

As long as the motor is drawing current the voltage being applied to it will always be lower than the battery full voltage because of these series resistances. The higher the current, the greater the difference and the higher the battery and ESC resistances, the greater the difference.

Larry

#4 JimmyKudo Feb 02, 2013 03:49 PM

Quote:

Originally Posted by Dr Kiwi (Post 23993845)
Amp draw is determined by prop size.. in your computation, increase your prop size 'til you get to the amp draw you need (but stay within the motor's limit).

ESC's don't limit amps.. they let through what they are capable of letting through... exceed that and they fry.

The low current to the motor indicates that the pack cannot deliver what that calculator suggests is what the motor/prop would demand, without dropping voltage.

NiMh cells (1.2v-1.4v HOC) typically can maintain only about 1v under load... in the same way Lipo cells (4.2v HOC) will be at 3.5v-3.8v under load.

Thanks for your help!

Quote:

Originally Posted by Lnagel (Post 23994751)
The calculator shows the following data:
Battery pack voltage = 12.5 volts
Cell Resistance = 0.022 ohms -- ten cells in parllel would be 0.22 ohms
ESC resistance = 0.0065 ohms
Motor current = 14.4 amps

The battery resistance and the ESC resistance are in series with the battery and the motor so the current through them is the same as the motor current and the voltage drop across them will be subtracted from the battery voltage. The voltage drop across the battery resistance and the ESC is calculated by multiplying their combined resistance by the current. So their combined voltage drop would be 0.2265 ohms x 14.4 amps = 3.26volts. Subtracting that from the 12.5 volts battery voltage leaves 9.24 volts to the motor.



The same applies. with 8 cells battery voltage would be around 11 volts. 0.1825 ohms times 11.8 amps = 2.15 volts leaving 8.6 volts or so to the motor.

As long as the motor is drawing current the voltage being applied to it will always be lower than the battery full voltage because of these series resistances. The higher the current, the greater the difference and the higher the battery and ESC resistances, the greater the difference.

Larry

Thank you for the clarification Larry; I'll keep resistances in mind when shopping for ESC's and batteries, if that even applies. Much appreciated!

#5 Martyn McKinney Feb 02, 2013 05:12 PM

Quote:

Originally Posted by Lnagel (Post 23994751)
Cell Resistance = 0.022 ohms -- ten cells in parllel would be 0.22 ohms
Larry

Ten cells in parallel would actually be 0.0022 Ohms.

#6 Lnagel Feb 02, 2013 06:16 PM

Quote:

Originally Posted by Martyn McKinney (Post 24014052)
Ten cells in parallel would actually be 0.0022 Ohms.

If they were lipo cells they would be hooked up in some sort of series/parallel configuration to get 12.5 volts. As such the resistance would be somewhere between 0.0002 ohms and 0.22 ohms. However, they are 1800 mah NiMH. The only way to get 12.5 volts from those would be to hook them up in series giving a total of 0.22 ohms.

Larry

#7 Martyn McKinney Feb 02, 2013 06:46 PM

My intent in posting was to correct the information shown in the quote below which was given to the original poster and is clearly incorrect.
Quote:

Originally Posted by Lnagel (Post 23994751)
ten cells in parllel would be 0.22 ohms
Larry

A corrected statement would read "ten cells in SERIES would be 0.22 ohms"

#8 Lnagel Feb 02, 2013 07:55 PM

Ah, I missed that. Thanks. But it might have been better to point out in the first place that I had typed parallel rather than series.

Larry

#9 Martyn McKinney Feb 02, 2013 08:12 PM

Quote:

Originally Posted by Lnagel (Post 24015455)
But it might have been better to point out in the first place that I had typed parallel rather than series.

Larry

I had a 50-50 chance in trying to determine whether your math was incorrect or your spelling and definition of parallel and series was incorrect.

It would appear from your response that I initially chose the wrong one.
Quote:

Originally Posted by Lnagel (Post 23994751)
ten cells in parllel would be 0.22 ohms
Larry


#10 Lnagel Feb 02, 2013 08:16 PM

Not only did I use the wrong word, but it appears that I misspelled parallel. :eek:

Larry

#11 Mark Wood Feb 02, 2013 10:25 PM

Jeez. What are we gonna DO with you guys?

mw

#12 cryhavoc38 Feb 04, 2013 02:34 PM

Carefull you guys don't go way over the OP's head here with all the talk about resistance, etc
:)

#13 Lnagel Feb 04, 2013 06:38 PM

It's hard to tell whether or not we are over his head when we don't know how tall he is. :)

Larry

#14 desertstalker Feb 04, 2013 10:46 PM

Quote:

Originally Posted by Dr Kiwi (Post 23993845)
ESC's don't limit amps.. they let through what they are capable of letting through... exceed that and they fry.

Plenty of ESCs have a soft cut (power reduction) on current overload. Usually combined with a hard cut (power off) if a large overload is detected.

Even the cheap hobbyking ones are supposed to have that, never actually overloaded one to check though.

#15 Lnagel Feb 04, 2013 11:18 PM

Quote:

Originally Posted by desertstalker (Post 24037438)
Plenty of ESCs have a soft cut (power reduction) on current overload. Usually combined with a hard cut (power off) if a large overload is detected.

Even the cheap hobbyking ones are supposed to have that, never actually overloaded one to check though.

That's fine if the ESC's maximum current is less than what the motor is trying to draw. But it isn't going to help a 30 amp motor propped to draw 40 amps being controlled by a 50 amp ESC.

Larry


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