[NSRCA-discussion] Use Two ESC's?

[Bill Henderson]

Bill Henderson
On Oct 3, 2011 12:56 PM, "Ronald Van Putte" <vanputte at cox.net> wrote:
> 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
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>>
>>
>>
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