Brushless motors & Li-Poly

Hi
I am just "getting into" Brushless motors and Li-Poly cells. However, something is puzzling me.
Looking at spec sheets from hobby stores, it appears that:-
1) The recommended prop size quoted for a 2-cell LI-Poly pack is sometimes HIGHER than for a 3-cell pack. For example, one store quotes the following for an "E-Pro 2828/26 13T Brushless Outrunner Motor" Propeller = 10 x 4.7 on 2 cell LiPO & 9 x 4.7 on 3 cell LiPO
2) Motors designed for a 2-cell pack seem to have higher quoted revs than motors designed for 3-cell packs.
These are the opposite results to what I would have expected. What gives??
Regards
KGB
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KGB (KGB) wrote:

For the same motor, giving it higher voltage will make it want to turn faster.
For the same prop, turning it faster will require more torque.
For the same motor, more torque means more current.
So to keep the motor current within reason you need a smaller prop with higher voltage, to maintain the same torque & therefore the same current. The prop _will_ see more power (same torque @ higher speed higher power), so you'll get more thrust & all that good stuff.

You're probably seeing the RPM/V rating -- for the same voltage a motor with a higher RPM/V rating will spin faster, and require more current to develop the same torque. A basic motor design is pretty much capable of a certain RPM and torque; you can change the voltage at which this RPM and torque happens by changing the number of winds and adjusting the wire size so everything gets filled to the same extent.
So if you're designing a motor for 7.4V you'd want 66% the turns of that same basic motor designed for 11.1V, with wire that has 150% larger cross-section area.
The end result of changing the turns count and wire size is that the voltage goes up and the current goes down while the RPM and torque stay the same. So the power in and power out stay pretty much the same but you can use different batteries.
There, I probably mangled that badly enough that you'll never understand, even if you get a good explanation.

--

Tim Wescott
Wescott Design Services
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KGB wrote:

The Amps drawn through a motor are determined by the Volts coming in, and the size of the prop on the motor:
1. more Volts = more Amps 2. bigger prop = more Amps
When you increase the Volts, you need to reduce the propeller size to keep the Amps within the limit of the motor/battery/ESC. Otherwise, you risk burning something up. You still have more power overall due to the increased Volts, spinning a slightly smaller prop at a signifigantly higher RPM.

As the recommended Voltage goes down, it's Kv rating, that is, its RPM/Volt rating tends to go up. Part of that design decision is so the motor continues to spin a propeller at a useful RPM on the reduced voltage.
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KGB wrote:

The Amps drawn through a motor are determined by the Volts coming in, and the size of the prop on the motor:
1. more Volts = more Amps 2. bigger prop = more Amps
When you increase the Volts, you need to reduce the propeller size to keep the Amps within the limit of the motor/battery/ESC. Otherwise, you risk burning something up. You still have more power overall due to the increased Volts, spinning a slightly smaller prop at a signifigantly higher RPM.

As the recommended Voltage goes down, it's Kv rating, that is, its RPM/Volt rating tends to go up. Part of that design decision is so the motor continues to spin a propeller at a useful RPM on the reduced voltage.
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KGB (KGB) wrote:

Correct. How could it be otherwise?

Correct, how could it be otherwise?

Your understanding appears at odds with the physical reality

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wrote:

With glow motors, the case generally is that the more powerful the motor, the larger the prop. As it is the battery which provides the power then logically, a bigger battery should turn a bigger prop.

When the voltage to a motor starts to drop, the motor runs slower; Logically therefore more volts should turn a motor faster.

It depends what you perceive as the physical reality. Funny things start to happen to reality at the quantum level. My cat for example hates going to the veterinary clinic in a cat basket. It once heard about the Schrodingers cat paradox and the poor thing doesn't know whether it is going to be alive or dead when we get there.
Quantum theory also explains why models crash. According to quantum theory, you can know the speed of something or its location, but NOT both; so as soon as you say, "Strewth, that model must be doing 90MPH", you know its speed and therefore cannot possibly know where it is in the sky - i.e. you have lost control. 8^)
Regards
KGB
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KGB (KGB) wrote:

It can. If coupled to a bigger motor :D-)
Matthew Orme some years ago likened the pack to the inlet and exhaust and combustion part of an IC engine, and the motor to the crankshaft..with a tiny crankshaft the only way to get more power is at higher RPM. Motors tend to be torque limited..therefore more volts nets you more RPM and more power at higher RPM.

Not if you are comparing a different motor..
To get the same power and RPM out of two motors of the same size and same magnetic assembly, but running on different voltages, you wind the low voltage one with less turns, of fatter wire, to have less resistance, more current and spin faster for a given voltage..the end result is exactly the same power RPM and efficiency, juts achieved at a lower voltage.

You don't need to get that sophisticated. The equations for electric motors are very old and very well understood.

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wrote:

Also less torque with more RPM's in the same motors. As the speed constant goes up , (kv rating) , the torque goes down.
Ken

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