[NSRCA-discussion] Use Two ESC's?

[Ronald Van Putte]

I was afraid that would be the case.  I am well aware of the airplane  
requirements regarding power required for various "missions".  It's  
just that I was not sure that the two ESCs would work together, due  
to the feedback from the motor to the ESC.

We'd have tried an E prop with less pitch or smaller diameter or  
both, but didn't have the right props with us.

Thanks for the input.

Ron



On Oct 3, 2011, at 9:38 AM, Peter Vogel wrote:

> 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
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion
>
>
>
> -- 
> Did you know? Arthritis affects people in all age groups including  
> nearly 300,000 children.
> Please help me ride 525 miles down the California coast to support  
> Arthritis Research
> http://2011cccnca.kintera.org/pvogel
>
> _______________________________________________
> NSRCA-discussion mailing list
> NSRCA-discussion at lists.nsrca.org
> http://lists.nsrca.org/mailman/listinfo/nsrca-discussion

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.nsrca.org/pipermail/nsrca-discussion/attachments/20111003/3efde2e5/attachment.html>