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Old Aug 18, 2004, 08:34 AM
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Higher volts vs. higher Amps - which is better?

It's my understanding that it all comes down to the Watts available, but is one better than the other when it comes to r/c planes? What would the differences be in performances...

11.1 V, 30A or
48 V, 6.9A
(both equal about 330 Watts)

Let's say they are the same weight also.
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Old Aug 18, 2004, 11:50 AM
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Lots to consider here.
Higher voltage looks better because current is lower so less heating (I^2 R).
BUT magnetic losses go up with more RPM (voltage).
Also high Volt systems usually mean gearbox which reduces efficiency.
Must make sure your motor can stand the high RPM.
Motocalc can help to work this out.
All that being said, I have a 22/20/2 on 4S LiPo geared 5:1 swinging an APCE 16X10 and drawing ~900W static. It flys a 1/5 scale GeeBee well for about 8min but does come back a little warm. The AXI's at this level do not require a gear box but weigh more.
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Old Aug 18, 2004, 11:55 AM
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Not too many BL motors wound for 48V, cobalt motors too heavy
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Old Aug 18, 2004, 03:21 PM
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Quote:
Originally Posted by feathermerchant
All that being said, I have a 22/20/2 on 4S LiPo geared 5:1 swinging an APCE 16X10 and drawing ~900W static. It flys a 1/5 scale GeeBee well for about 8min but does come back a little warm. The AXI's at this level do not require a gear box but weigh more.
Wow! If you are drawing 900W with a 4S Lipo (3.7V x 4 = 14.8V), doesn't that mean you are drawing about 60.8 Amps?? What Lipo's did you find that can handle that?
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Old Aug 18, 2004, 03:28 PM
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Quote:
Originally Posted by hall woo
Not too many BL motors wound for 48V, cobalt motors too heavy
You could also use...
11.1V @ 30A vs. 22V @15A

They're still both around 330 Watts.

I'd bet there are many BL motors that can handle 22V and if only 15A is needed, it may open the door to many more (less expensive and lighter) battery choices.
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Old Aug 18, 2004, 04:46 PM
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BRET - Tanic 3P 2200 - 66A OK
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Old Aug 18, 2004, 05:54 PM
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Bret
Having diddled with this issue since my Uncle Ed Lyerly dragged me kicking and screaming into the "40A Club", I'd go for a little more volts over a little more amps any day.

All things in moderation of course - you really don't want to strain your motor, regardless of size, right up to its limits on either. But, mostly because amps is the rate at which flight time is sucked out of whatever battery you have, "a few more volts and a few less amps" has always been good for me.

That's in good old knockaround sports aerobatics with pretty lightly loaded models - but if you're doing something really specialised, you should know what you're up to anyway.

Regards

Dereck
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Old Aug 18, 2004, 06:43 PM
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It depends on the motor/battery/prop/gearbox/controller you choose. Some motors have a realtively low Kv and really like running at hi voltages. Some have a very hi Kv and will only run safely at lower voltages.

Two things (other than crashes) will kill a brushless motor - over reving and heat.

If your goal is to get as much power out of a motor as possible without killing it, then you need to balance these two things - heat and rpm.

If your goal is to get max effeciency (it iS a different goal) then you do it a bit differently.

Heat loss in motors comes from (mostly) 4 areas:
1) Resistive losses = I*I*Rm
2) Io * V
3) Magnetic or histertesis (sp?) losses
4) 'Voodoo mystery' losses

I = motor current
Io = no load current
V= voltage
Rm = motor resistance

#1 above shows that motor heat goes up as the square of the amps
#2 goes up linearly with applied voltage
3 and 4 are mabey almost linear and mabey not (the math involved is way over my head so I just ignor it). For the most part tho - they go up with rpm but are highly variable depending on motor construction more than anything else - mabey

At low voltage/hi amp levels #1 above dominates.
At hi voltage/lo amps levels #2 starts to have more and more of an effect and becomes a bigger factor.
The higher the rpms the more 3 and 4 come into play.

As an example, lets take one of my motors - a Mega 22/45/3. We will ignore the tricky #3 and 4 losses because we have no choice These are actual numbers from my planes.

Kv = 1000
max rpm is 50000 or so
Io = .95
Rm = .044

Setup #1 12 subC cells, scratch bipe
V=12
I= 35

I*I*R = 53.9 watts
V*Io = 11.4 watts

65.3 watts lost for 420 watts input
Motor temps over 160 degrees


Setup#2 6S1P 2200 lipos Freedom 3D
V= 20
I= 22

I*I*R = 21.3 watts
V*Io = 19 watts

Total = 40.3 watts lost for 440 watts input
Motor temps normaly in the 120 range.

Setup #3 Logo 10 heli 9S2P 3100 lipos
V = 31
I= 15 (averaged for a flight)

I*I*R = 9.9 watts
V*Io = 29.5 watt

Total of 39.4 watts lost for 465 watts.
Motor temps in the 150 range

This makes the hi voltage set up look very good, but the motor actually runs hotter at the highest voltage setup on 9S. This is probably due to the fact that the heli loads it more consistantly and the other "voodoo" losses comming into play and also the Logo has much less air flow - so the temp differences are a little suspect.

So for THIS particular motor, hi voltage is definately better. Each motor needs to be evaluated on its own merrits and its own Io, Rm, Kv etc. You also need controllers that can handle the hi voltage.

Hope this helps.

Larry
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Old Aug 18, 2004, 06:48 PM
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I didnt get into other factors like how much money you want to spend, how much weight the total power system weighs, room in the fuse etc, etc. There are a great many factors and they all play a part.

In general tho - I go for the highest voltage I can get away with.

It works well on all my set-ups, but I have designed them with that in mind.

Larry
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