View Full Version : Oversizing ESC to Motor

Jessum Dumguy
10-10-2005, 05:11 PM
Hello All, I posted this question on another form and
Walt Thyng sugested I try here, So here goes....

I'm Hoping the Electrical GURU's can answer a
rather odd question about Brush-less ESC's.
Please forgive me if this is answered somewhere already.
I've been reading several hundred threads that came close but
none have answered my SPECIFIC question.

So Okay Here goes, Is there any preformance HIT
( Discounting Size and Weight )
in using an Oversized ESC? In particular the Brush-less breed.
(ie) Running a itty bitty 10 amp outrunner on 3 cell Lipoly (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=2321) with say a
90 amp ESC like say Hacker Master series 99-3p opto BL ESC?
( I told you it was an odd question ) http://image.rcuniverse.com/forum/image/s2.gif

Clearly the example above is a bit extreme but the question is
one of capability not logic or common sense. What I was wondering is,
Will the low amp draw effect the performance of the ESC's abillity to
effectively control the motor? In sizing Controlers to motors the
driving factor is ussually to insure the ESC can handle the
motors amp draw then adding a TAD extra to be safe.
So how much EXTRA is too much?
(Again discounting Size and weight.)

Anyway thats my silly question, My need for understanding.
Jessum Dumguy

10-10-2005, 05:17 PM
I always use larger, usually 20% or so. So If I draw 8 amps I go with 10amp.

I have a 11amp application with a castle 25 amp board, no biggie.

Just size and weight are my limiting factor.

The larger the board the better it will dissiminate heat as well, so that it an added benefit.

Cost is another issue, but usually the larger ones are not too much more.


10-10-2005, 05:43 PM
A litter over sizing is usually a good idea 10-20% is fine.

Larger ESCs have lower resistance because they have to carry very high currents without dissipating too much heat but generally also absorb more current because of the larger components.

10-10-2005, 05:46 PM
No penalties other than size and weight.


Jessum Dumguy
10-10-2005, 05:52 PM
Yup all true, And Cost is one of my issues for asking this question.
For those of us that are... Uhmmm, Finacialy challanged,
It makes " Dollars and Cents " To move ESC's from one bird to
the next when and where possible. But If I had say a
40 Amp BL ESC running a 30 amp motor in my last plane,
Then my next plane calls for a 40 Amp motor then I'm sitting
with either a substancial HIT in the wallet or NUETERD Plane.

So, IF it's not going to cause a performance hit ( Electrically ),
I'd just go with the 60, 80, 90 AMP now for my current 30 AMP motor.
The extra weight and size I can work around to some degree.
I'd rather spend money on a single GREAT ESC than
spend possibly more for a hand full of So-So cheap ones.
Does my method seem sound? I mean, If the plane and
motor can handle the weight and I can fit it in,
Wouldn't it be more cost effective to go with a
larger ONE that latter I can reuse in a bigger plane?
I got Six planes already, All Brushed Systems currently,
Two of them I'm looking at switching to Brushless but
I'll only ever fly one of them at a time. ( Any Givin Weekend )
I also got my eye on another plane ( Warbird ) that will
require possibly double the current draw of either of the
two ( 2M E-Glider warnliner ) I'm looking to switch over.

BTW, Thanks for the rapid reply.
Jessum Dumguy

10-10-2005, 06:52 PM
As long as you can live with the weight penalty and also can live with only being able to fly one plane at a time you should be fine.

Walt Thyng
10-11-2005, 03:39 PM
Thanks for helping Jessum out so quickly. Your answers all square with my experience.

Jessum Dumguy
10-11-2005, 07:49 PM
Yes Thanks all, You've pretty much confirmed my thinking on the matter.
I never really understood why most would try so hard to stay
within the 10 to 20 percent above the motors max amps if it
was only a matter of size and weight. I mean there really isn't
much of a size or weight difference between a 60 amp and
90 amp esc. Well not in my world anyway. :-)
But 160 bucks is easier to swallow than 100 now plus 160 later.
Again Thanks all. And Thanks Walt for the finger to the right place.

Jessum Dumguy

10-12-2005, 12:18 AM
I see you got your answer, but here's one more thing to consider. The larger rated ESC typically have a greater capacity on their integrated Battery Eliminator Circuits. As an example, the Castle Creations 25 amp ESC is rated for only 3 servos, while their 35 amp ESC is rated for 4 servos. There used to be a lot of 'free advice' that the 25 amp would handle 4 micro servos without a problem. I totalled a plane when my 25 amp ESC 'locked' up. I'll buy 35 amp ESC's now even though most of my motor/battery set-ups use less than 20 amps. Consider the servo load as well as the motor load when selecting an ESC. A little bigger is a lot better.

One more point, when I first started building electrics, I too thought that I would move the ESC and receiver from plane to plane. That didn't last long at all. It's so much easier and really not that expensive to fully set-up each plane. You would be surprised how often I find a good price on used CC ESC's and used Hitec 555 receivers.

Jessum Dumguy
10-12-2005, 01:30 AM
Darrell F
Ya I kind of agree with the idea of having an
ESC/Motor/Receiver/ Ect for each plane, And with
Brushed Motor systems I did pretty much.
For receivers I do kind of keep that with a
plane aswell with the servo's mounted and attached.
5 Planes shareing Three receivers, 2 Planes semi retired.
Just the way it worked out. Ones got so much CA and
Epoxy I dare say It may very well clasify as A COMPOSITE.:D
But I'm getting ready to move into Brushless and
eventually up into larger planes. To do what I did with
Brushed motor setups with brushless would pretty
much bankrupt me.:eek:
Thats pretty much whats kept me out of a
brushless system till now is the sticker shock.
But if I can move about 200 bucks from
plane to plane like I already do with my 200 bucks of Batterys,
That solves alot of my problem.
Since moving the 90 to 150 dollar motor compared to the
20 to 30 dollar can motors would be a pain, Moving the
100 dollars of Receiver and servo's a major headache.....
You see where my SPECIAL logic took me.
The ESC is easy enough to access and move as long as my
connections stay consistant from plane to plane.
And if I'm gonna SCRIMP, Sacrifice and SAVE my lolli pop money
for something, It's gonna be a Special something, Not just Adiquate.
Thats what got me thinking all weird and stuff..... I've already
SAVED in a CHEAP BL Motor and it's given me a headache already.
Nowhere in the paperwork does it even mention sustained or Max Amps.
Gotta Love Ebay.....

As far as Receiver and servo power goes I've about been sold on
the idea of a UBEC and going with an Opto ESC. Just provideds
alot more options. And ya, I'll probably be floating the UBEC from
plane to plane aswell. I've already been running into problems with my brushed systems with 4 servos on 2 cell Lipoly. Hit the throttle full and
the control surfaces get real slow....:eek:
I'll Series them into 3 or 4 cell in my next bird.
Depends on what I find out about my cheap motor, Hehe.

Anyway thanks for weighing in and the advice.

Jessum Dumguy

10-12-2005, 02:03 AM
I also am thinking about something bigger. I'm still at the point where I have nothing bigger than AXI 2808's, but I sure would like to convert a 40 size to electric. I can handle the cost of the motor and ESC, but everytime I look at the cost of a couple of 4s2p Li-polys I cringe. I'll continue to look for the bargins as I collect the things I need.

Walt Thyng
10-12-2005, 03:55 AM
geeze I don't often get thanks for giving a guy a finger (-:

10-13-2005, 06:17 PM
Resistance is another thing to consider when changing the gauge of wire you use.

My $0.02.:)