01-22-2012, 01:26 AM
Join Date: Apr 2008
Location: Wisconsin, USA
Thanked 558 Times in 544 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
[QUOTE=RONALDO_UK;853906]Hi – a thank you to everyone for their help, advice and information (and patience!). I understand design fundamentals but readily concede I’m just not grasping the electrical basics.
I realise that power required is related to plane weight/ performance required. And a motor/ battery combo is then chosen to deliver that power. And the chosen propeller causes the motor to draw that power.
A 2200kv motor with a 10V battery would turn at 22,000 rpm with no load (ignoring losses) and minimum current.
When the motor is hooked up to a propeller and an ESC, am I correct in thinking the ESC controls the current/amps delivered to the motor? And that at design maximum current/amps with a well chosen motor/ battery combo, the motor will approach the theoretical maximum speed (22,000 rpm) and deliver its maximum power?
Returning to the EZ upgrade, I’ve seen articles that show a 28mm diameter inrunner (ie same diameter as stock motor) fitted into the nacelle – no mention is made of any additional cooling requirement so I have assumed it’s not an issue. With outrunners on an EZ, I’ve assumed exposure to natural air cooling in flight is sufficient.
You're making progress The maximum RPM any brushless motor can reach is the no load condition with no prop (Or a terminal dive straight down!) Since the motor is not turning a propeller, the actual power output on the motor shaft is zero.
Or, if you take that motor and really load it down with far to large of a propeller, and have perhaps 500 RPM, that motor is going to pull a LOT of amps, and get hot, real quick, like 5 or 10 seconds before the magic smoke gets out. As for power output, the motor is going to have a lot of torque, but because of the low RPM, again the power output on the prop will be pretty low.
So, somewhere in the middle, between no load, and smoke from the motor windings is the best power output of the motor. Only it's not quite that simple. The highest power output is something like where 1/2 of the watts into the motor is turning the shaft, and 1/2 of the watts into the motor is turning into heat. Problem is, that motor running under this condition is likely not going to last very long.
So, the best "Loading Point" for that motor will be something where the motor shaft efficiency is somewhere around 80-90%. Some of the cheap motors might never reach 80% efficiency under any propeller load. And the $$$$ motors typically reach the high 80% area with the proper prop selection.
Now, you've got to balance that motor/ESC/battery/prop selection against the type of model you are going to fly. One extreme is the "Piper Cub" slow flyer type of model. That would not be a good design to put a motor turning a tiny prop at 20,000 RPM on this slow flying Cub. Might work, but over all efficiency would be awful. So, a big diameter prop with a slow turning motor would be a good choice for the Piper Cub.
On the other hand, if you're going for a real FAST 100 MPH plus "Foamie" model, you would not want that slow turning motor on the Foamie. You'd want a motor that runs 20,000 RPM turning a little toothpick prop.
That is the big difference between glow power and electric power. For the most part, the glow engines are restricted to a certain RPM range, say between 8000 and 12,000 RPM for a mid sized model.
On the same mid sized model, you can select electric motors that can turn over anywhere from say 4000 RPM to 30,000 RPM, just by varying the brand of the motor, prop diameter, and cell count with the KV of the motor.
All of this is confusing to say the least. That is where software programs such as www.motocalc.com have been designed by people with a lot more knowledge in this stuff than 99% of us (Including me) can save the day.
Motocalc will let you know if you are trying to put a 20K RPM motor on a Piper Cub, or some other combination that is not optimum.
Retired and the days are just too short, busier than ever!