[NSRCA-discussion] Use Two ESC's?

[Peter Vogel]

Hi Ron,

I don't think you can Y the motor connections that way, the ESC's are
putting out pulses very specifically timed to move the motor through to the
next set of poles and rotate it in the correct direction while one of the
three leads (and it changes for each phase of the rotation) is used to sense
the position of the motor to time the next magnetic pulse.  Unless the ESC's
are in *perfect* sync they will wind up fighting each other and could even
misinterpret the pulses from the other ESC as a timing pulse from the motor.


There are no shortcuts to a proper electric power system.  Prop, Motor, ESC
+ Batteries should be chosen to deliver the right amount of power (Watts) +
thrust to fly the airplane for the mission intended (70 watts/lb of all up
weight for a trainer, 200 watts/lb for aerobatic sport flying, 300 watts/lb
for 3D -- as a general rule of thumb).  Most motor manufacturers will
publish prop charts showing the amp draw on the intended voltage for the
motor, you should look for a prop that will not overtax the motor/ESC combo
on the voltage you are planning to use.

Remember that an electric power system is a *constant speed* system, as
opposed to an internal combustion system which is constant power.  What that
means is that the motor has a rated kV, the rotations per minute per volt
and whatever load you put on the motor (prop) the motor will try to spin at
that speed and pull as many amps as it needs in order to do that.  Also
remember that if you are measuring watts on the ground holding the plane,
that's a static measurement and the load should drop somewhat when the prop
is allowed to pull the plane through the air, reducing the current.  Adding
volts to the same motor (i.e. going to 6s batteries) doesn't necessarily
help because that motor's kV is the same, it may, in fact, make things
worse.  If you want to go to 6s, you need to get a motor with a different
wind to reduce the kV so that you are getting the same RPM as you did at 5s
with the other motor.  THEN you will reduce amp draw by about 20% because
you increased voltage by about 20% and you are still delivering the same
watts.

If he's running 71 watts in a static test at wide open throttle, I'd
consider reducing the diameter of the prop by 1 inch or reducing the pitch
of the prop by 1 inch, that should get him into a reasonable range for a 60
amp ESC (though I prefer about a 20% headroom -- check the amps at WOT after
propping down and if it's at or near 60 the unload in real flight should
give you about 10% headroom).

Peter+

On Mon, Oct 3, 2011 at 7:19 AM, Ronald Van Putte <vanputte at cox.net> wrote:

> I have an off-the-wall question that I don't have the answer to.  Somebody
> please tell me the answer.
>
> A young local modeler has an electric-powered airplane whose motor puts out
> 1200 watts.  He has 5S lipo batteries that will handle the load.  In a
> ground test, the maximum current was 71 amps,  He had tried a 45 amp and a
> 60 amp ESC and they failed.  Hence the reason for the ground test and
> confirmation that he needed an ESC with a higher amp capacity.
>
> I went looking at BP Hobbies for one of their inexpensive BP ESCs with an
> 80 amp capacity and they no longer sell them.  Other high amp ESCs are $100+
>
> Here's the question:  Can he put two 40 watt ESCs in parallel?  Both would
> go to the throttle channel via a Y-adapter and the motor connections would
> be similarly Yed together.
>
> I was also thinking of having him use a 6S battery pack and limit the
> throttle to 60 amps, but I'm not sure he has the room for the battery pack.
>
> Ron Van Putte
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>



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