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This is part of a series of articles put here, written by me to try and
explain electic flight to a newcomer. These articles are taken from the
spad combat
forum
in the electric section.
Motors
First off, there are two major types of motor - brushed and brushless.
Brushless motors are more efficient, much more flexible (you can get
very high torque/low rpm from them) and generally better.
Back to the basics of motor physics then. A motor works by having an
electromagnet on a rod, and two stationary magnets around it. The
electromagnet energises, one end becomes north and one end becomes
south, and both are attracted to their respective outside magnets, and
this attraction pulls the electromagnet round so that the north bit of
the electromagnet is next to the south bit of the stationary magnets,
and vice versa with the other pole.
Now if this was all that happened, you wouldn't have much of a motor.
You'd just have a rod stuck in position. But through a mechanical (in
brushed motors) or electronic (in brushless motors) process, the
current through the electromagnet is reversed and it carries on
spinning and turns to face the other way. This constant flipping of
current direction causes the motor to spin. This is a gross
oversimplification of what actually happens, but it's close enough for
everyone to understand. If you want to find more info on this, just
google for it.
In a brushed motor, brushes rub on the shaft in such a way that when
the shaft rotates 180 degrees, the polarity is reversed. This is
inefficient, and this is why brushed motors are not as good, or as
powerful as comparable brushless motors for a given current. In a
brushless motor, special circuitry in the ESC does the switching, so
there doesn't need to be any switching inside the motor. This makes it
far more efficient, and enables much higher rpms and currents to be
delivered. Basically, you can up the voltage to a brushless motor and
hence increase the current, and power, until the ESC explodes or motor
flies apart. Most brushless motors are rated quite close to their
tolerance limit.
Brushed motors
I don't know much about brushed motors, but they are the ones that come
with speed ratings (eg speed 400). Here's a link with some info on
them: http://www.ezonemag.com/pages/faq/a414.shtml. It basically says
that the speed rating has no relation whatsoever to actual power, but
it might be worth a read anyway. There's lots of info available on the
net about brushed motors, however, I think the prevailing opinion in
the leccy world is that they have had their day and brushless motors
are so cheap now there's no point getting brushed.
Brushless motors
Now we're talking! There is a HUGE variety of brushless motor
designs available, and it seems quite daunting at first. How do you
know which one out of all the thousands of available motors you need
for a certain plane? There are programs like motocalc (or motoGUESS as
it is more commonly known) which claim to find the right motor for you
depending on plane stats and performance needed, but more often than
not they give dodgy answers. They can help, but they aren't the be all
and end all. The only way to come up with a helpful answer is through a
combination of rough maths, trial and error and experience.
RPM VS Torque
Basically, there are two types of brushless motor, inrunners and
outrunners.
These aren't so different as people imagine. Electric motors are all
about balancing rpm and torque. Imagine at one end of a scale for a set
power level you have all torque and no rpm, and the other end you have
infinite rpm but no torque.
Obviously these extremes aren't possible in practical terms, but you
can slide as far as you like either way by changing the internal
configuration of the motor. It's like gearing - a motor wound for a lot
of torque and less rpm is the same as a faster rpm motor geared down.
Outrunners are, as a rule, high torque low rpm. Some outrunners slice
this differently, but all will focus on high torques with low rpms. For
example, you can get three different outrunners from just one brand
that all draw around 12-13 amps on 3s and swing 8x4, 9x6 and 11x6 props
respectively. The larger props will just go round at less rpms.
However, all will swing a decent sized prop in direct drive, without
need for a gearbox.
An inrunner works differently, they normally spin at very high rpms
with very little torque. this is great for extremely high speed light
planes, where they will spin something like a 5x5 at 20,000rpm. Of
course, for lower rpms and more torque you can use a gearbox, but what
is the point? Gearboxes waste a lot of power as friction and heat, and
why use one when you can just get an outrunner that will swing a prop
in direct drive.
Inrunners generally have two poles, and the ESC switches polarities
very fast to cause the shaft to rotate extremely fast. Outrunners are
slightly more complex, a picture is worth a thousand words in this
case. Go here and look at the picture of the outrunner: http://www.hackerbrushless.com/motors_a20.shtml
The outside case rotates, and the picture illustrates well why
outrunners have less rpm but more torque than inrunners.
So inrunners are at one end of the torque-rpm "spectrum" and outrunners
are at the other end. You choose which motor you want depending on the
application. For a 3d plane where a large diameter prop is needed, an
outrunner is definately a requirement, or at least a geared inrunner.
For a tiny foamy jet, an inrunner is perfect in direct drive with a
tiny prop for ridiculous top speeds.
Voltage and
current
Of course, on top of this, you also have hugely varying power levels.
Motors range from tiny 6 amp (on 7.4v) indoor flyer outrunners, through
30 amp (on 11.1v) .25 size outrunners and fast inrunners, to massive 70
amp (on 19.6v) outrunners to replace 50cc engines.
Motors will draw as much current as possible at any given speed/prop
setup. How much current they can draw is limited by voltage (more
voltage pushes more current over a certain resisitance - dictated by
V=IR) and the resistance of the motor windings.
It is true you can put a huge prop on a tiny motor and pump a tonne of
voltage through it to get as much power you want until the motor melts.
However, because smaller motors are limited by the diameter of the
wires in them, and heat dissipation along with part quality, more and
more of this energy is lost as heat and friction as the voltage is
pushed up.
For a given voltage, more current (and hence more power) can be drawn
by reducing the resistance of the wires in the motor. This also reduces
power lost as waste heat. More effiency means longer flight times and
less wear and tear on the batteries. It therefore makes a lot of sense
to get the size of motor that best suits your application.
A few rules:
-More voltage ALWAYS pushes more current and results in more power.
-A bigger prop will draw more current until you reach a point at which
the resistance of the wires in the motor becomes too great, and not
enough current can be drawn to turn the prop.
-Motors can be pushed beyond their recommended limits (and they really
are just recommendations - nothing magical or set about them) but the
more they are pushed beyond their maximum efficiency band, the more
energy is lost as waste heat, and eventually, the motor will just fall
apart or melt, possibly causing damage to the ESc or battery pack.
Disclaimer
I don't claim to be an expert. All of this information is
offered on an "as is" basis. All text here is hereby released under the
GNU FREE DOCUMENTATION LICENSE for anybody to use, copy or alter,
commercially or otherwise, as long as, AND ONLY AS LONG AS THIS TEXT IS
PRESERVED.
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Email: rcpoweruk@gmail.com
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