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For
years now, I have heard concerns about motor cost and complexity
from many R/Cers wishing to try electric-powered flight. The market
has responded with lower prices through competition, reduced complexity
through outrunners, and even "combo" packages of matched
components that work together. This has attracted more enthusiasts
than ever before to try their first electric-powered ARF or glow-to-electric
conversion. However, the recent increase in new motor manufacturers
and inventive marketing techniques has resulted in a confused customer
rather than a confident buyer. Some folks look for a complete document
or database that contains all the information they seek about selecting
electric power systems but become disappointed and frustrated when
they cannot find it. The various numbering schemes of electric motors
all seem foreign to someone used to selecting a .40-size or .60-size
glow engine.
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R/C
hobbyists come from all different backgrounds; some are more
technical and love to experiment while others may have limited
time, patience, or means.
Everyone
wants to get it right the first time so their effort and
money is not wasted. This month's issue will address the main
confusion I see now from many people wanting to try electric
power. That confusion is...
What
motor do I use?
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Choosing An Electric Power System
Many
R/Cers wishing to try electric-powered flight are concerned with
the cost of the motor and the added speed controller. The cost of
paying for fuel up front, like batteries and especially expensive
Lithium cells, was a foreign concept to most glow-powered R/C enthusiasts
wishing to try an electric conversion. They were comfortable with
paying for fuel as it was needed. Added complexity came from gearboxes,
gear ratios, new mounting patterns, selecting motor and prop sizes,
cell count, and series or paralleled cells. The wide dynamic range
of brushless motor operation mystifies most of the glow-oriented
R/Cers since a given size engine would produce a specific level
of power. Each vendor proclaimed that their product was the best
but shrouded specifications in various numbering schemes that were
foreign to someone used to selecting a .40-size or .60-size glow
engine.
Many
of these areas have changed for the better. Outrunners have eliminated
the need for a gearbox, reduced the prop size range to select from,
and, also reduced the range of cells that could be used for a particular
motor. Electronic Speed Controls (or ESCs) like the Jeti
Advance PLUS line greatly simplifies programming the setup by
using a tiny card that can reside in your field box. Castle Creations
Phoenix line of speed
controllers require no programming before flying since the standard
settings will work for most applications. New Lithium cells have
reduced the need to parallel cells and have increased safety by
being non-combustible at high charging voltages. Lithium safety
and pack longevity were further enhanced by adding taps to multiple
cell packs. New chargers and devices allow for balancing during
the charge cycle and the cell count was hardwired through the node
connector which eliminated false guessing techniques. Automatic
peak chargers for all cell types are now available. The resultant
added safety and lifespan of the flight pack helped balance the
cost.
What
does this leave us with? The selection of the electric power system
is now the biggest unknown. The question I hear from most people
now is, "What motor do I use"?
Today,
you can fly a 20lb plane on an electric power system for less than
$400. New ESCs allow for 12s (12-cell) Lithium packs to provide
up to 7 hp! One of the biggest confusions for most people selecting
an electric motor is, "What is a watt"? The glow guys
are used to horsepower and electric power systems are measured in
watts. (1 hp = 746 watts or about 750 watts)
Power
Level in Watts equals Voltage x Current where the voltage is affected
by cell count and the current is affected by prop size and throttle
setting, but, these interactions are not exclusive to each other
as one can affect the other. Instead of tuning a carburetor on a
fueled engine by listening to the sound by ear, the wattmeter now
becomes the primary tool for measuring the power of an electric
motor system. Watt meters measure power input (not watts to the
prop). Since brushless motors are around 80% - 90% efficient, most
of the power gets to the prop.
What's
so easy about selecting an electric motor?
A general
rule of thumb for electric powered flight was originated by Dr.
Keith Shaw several decades ago based upon the older Astro Flight
Cobalt brushed motors. The rule read something like this:
- 50-75w/lb
for Cub-like planes or Trainers
- 100w/lb
for Sport/Aerobatic/Pattern planes
- 150-200w/lb
for 3D, EDF, or other high performance planes
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Since
brushless motors are more efficient than brushed motors,
only 65w/lb is all that is needed for a take-off from grass,
to climb well, and, to perform mild aerobatics on a .40-size
trainer plane.
Swap
your NiMH or NiCd battery packs for newer Lithium technology
and you're flying duration doubles or quadruples, respectively.
However, this increase in performance through advanced battery
technology does come with an added cost.
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The
sequence for choosing the right electric motor is simplified by
a combination of Keith Shaw's findings and today's abundance
of information both on-line and in catalogs. Here is the sequence
that I use to help people select the right motor class.
Choosing
An Electric Power System
1)
Determine the flying weight (in pounds) of the plane and multiply flying weight
by 50, 100, or 150 depending upon plane type and expected performance
(50 for Cub-like or Trainer, 100 for Sport Aerobatics, or 150 to
200 for 3D, EDF, and high performance) The result is power in watts
needed from the motor at full throttle. This result determines the
power class of the motor needed for your application which is a
value typically displayed in the manufacturer's motor specifications.
When the power level is not given, you can simply multiply the maximum
current given by the number of Lithium cells with a voltage under
load of 3.6v. As a comfort check, recall that 1h.p. is equal to
about 750 watts so you can compare your result with the corresponding
glow or gas engine recommended for the model.
2)
Use Vendor Web site recommendations (most vendor sites post charts
of motor power levels as well as complete recommended setups for
a particular model)
3)
Copy a review setup. There are many existing reviews that you can
benefit from reading for your own application
4)
Copy a similar size plane with similar design and wing area. If a
review uses a certain power system on a .40-size high wing trainer,
it will likely work fine on your similar application.
5)
Use a computer program to assist you in the motor selection process
(Sid Kaufman's ElectriCalc
or Stefan VorKoetter's MotoCalc).
Both of these power system selection programs can give you a reasonable
start or verify the merit of your existing component choices.
A scan
of the top distributors of electric flight products reveals different
techniques used to help customers select the right motor or power
system for their application.
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Great
Planes, ElectriFly - Offers extensive conversion charts, reviews,
and step-by-step selection tools. An impressive set of tools.
Hobby Lobby - Extensive suggestions for plane weight and flying
type. Uses combo packages of match components and many reviews.
Horizon Hobby - Motor scheme matches glow engine equivalent
or old brushed motor number. Power levels are also specified.
Very useful for glow-oriented customers. Power levels are
good for watts per pound rule-of-thumb.
Hobby People - Combo packages and suggested replacements for
older brushed motors. Some motors specify peak watts output
which is good for watts per pound rule-of-thumb. Short customer
reviews.
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Brushed vs. Brushless
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Great
Planes has a brochure on Brushless Power
Systems (GPMZ0006)
that is an extensive publication on their products for electric
flight. If offers easy-to-use tools that help you find what
you need and charts on selecting the proper motor for many
of the popular top-selling glow airplanes or for converting
your own plane.
ElectriFly
also offers an Easy
Conversion Methods page which provides links to selecting
the right components for your model.
One
difference between brushed and brushless motors is what
rotates. In brushed motors, it's the windings that
rotate. In brushless motors, the windings stay put and the
magnets move. But for the most part, the names say it all.
Brushed motors have brushes to carry current and spin the
rotor. Brushless motors don't. A brushless electronic
speed control (ESC) energizes the electro-magnetic field,
which causes the motor to turn. And because of this, there's:
1.)
No brush-to-commutator contact - the #1 source of friction,
waste heat, inefficiency, wear and maintenance in brushed
motors.
2.)
No voltage drop due to arcing caused by the brush-to-commutator
contact - which minimizes power losses and prevents
radio noise and glitching.
3.)
And virtually no limits on top-end speed or motor life.
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When
the power level is not given, you can simply multiply the
maximum current given by the number of Lithium cells with
a voltage under load of 3.6v. As an example, the RimFire
35-36-1500 (GPMG4625) can handle 47amps on a 3-cell LiPo
pack. Multiply 47 * 3cells * 3.6v per cell to get 507 watts.
Then use the watts per pound rule of thumb.
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Inrunner vs. Outrunner
The
ElectriFly AMMO motor on the left is an inrunner. The outside
can is stationary and it only spins the shaft on the outside of
the motor. Typically, but not always, this type of motor requires
a gearbox which allows them to swing larger props efficiently.
Some inrunners, especially the brushless kind, can spin a larger
prop directly without the need for a gearbox. On many inrunners,
when a gearbox is not used, only a small prop can be used that
spins very fast.
The
RimFire motor on the right is an outrunner. An outrunner motor
spins both the shaft and the outer case. These motors generate
much more torque than a normal inrunner motor and can typically
spin a larger prop without the need for a gearbox. This is sometimes
referred to as direct drive since it drives a prop directly. Direct
drive reduces the cost, complexity, and, added weight of using
a gearbox. Outrunners also reduce the prop size range to select
from and reduce the range of cells that could be used for a particular
motor.
ElectriFly.com
offers an on-line Electric
Motor Configuration service to help you select the proper
motor for your application. You can also view the Glow
To Electric Conversion Guidelines PDF for hints on using larger
geared inrunner motors.
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Outrunners
like this black Atlas 5030
series come with two prop adapters. One is for mounting
the motor behind a firewall and the other is for mounting
it to the front of a firewall.
Outrunner
motors have come to dominate many consumer devices such
as computer hard drives, CD/DVD players, and PC cooling
fans. These low speed brushless DC motors are also used
in direct-drive turntables, electric vehicles, and some
industrial machinery. The recent increase of popularity
in electric-power model aircraft has spurred demand for
these outrunner motors.
Brushless DC electric motors from Wikipedia
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Understanding an Outrunner's Numbers
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Most
outrunners use a 6-digit numbering system like the AXI
5330/18. The first two numbers (53) represent the diameter
of the stator in millimeters. The stator is the fixed
part in the middle of the motor. The second two numbers
(30) represent the length of magnets in millimeters. These
long rectangular magnets are attached to the inside of
the rotating case. The third set of two numbers (18) represents
the number of wire winds, also called turns.
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Other
manufacturers use variations on this numbering scheme. Sometimes
the second set of numbers can represent the length of the
rotor or motor can instead of the magnet, or they have a
letter designation to represent a size like S for Short
and L for Long. In any case, I find it easiest to simply
remember that bigger and longer means more power!
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Bigger and longer
means more power!
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When
comparing motors of similar size, there are several characteristics
that you will notice. Motors with higher winds spin slower for
a given voltage. Higher winds means they have lower Revolutions
Per Minute (RPMs). This is also referred to as Kv in motor gain
terminology. These motors spin a larger propeller at slower speeds
than lower turn motors. Motors with lower winds spin faster for
every volt of electricity applied. They have a higher Kv (or RPM/V).
They can spin a smaller propeller at higher speeds than higher
turn motors.
Sometimes
I take advantage of the fact that lower Kv motors use thicker
guage wires compared to the same size higher Kv motor. This means
that you can draw more current through the motor and gain higher
burst power levels for 10-20 seconds without damage to the motor.
The result is an impressive fly-by or vertical climb when needed
during the flight.
Example:
A motor with a Kv of 1200 will turn 1200 revolutions per minute
per volt applied. On a 12v supply, it will turn 12 * 1200 = 14,400
RPMs.
Glow Conversion Example
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Let's
try an example using the Graupner Taxi Cup II which used
to be sold in the U.S. by Hobby Lobby. Although this plane
is a high wing aileron trainer, the clean design (low drag)
makes it unusually fast so a pilot needs intermediate skills
to fly it successfully. The manufacturer says that the plane
weighs 96oz (or 6lbs) when equipped with a .40-size glow
engine.
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Using
the middle of our rule of thumb power level for trainer-type planes,
we multiply 65watts times 6lbs to get 390 watts which is about ?
hp The result of about 400 watts is the motor class that we need
to select from for our conversion project.
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The
AXI line of outrunner motors has several selections in this
400 watt class, the heavier 4120 motor for 4-5 LiPo cells,
and, the lighter 2826 for 3-5 LiPo cells. The AXI motors
are direct drive brushless designs that are virtually maintenance-free.
Since there are no brushes to wear out and no gears to lubricate
or strip, the motors need no maintenance other than perhaps
a yearly lubrication of the ball bearings supporting the
drive shaft.
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I selected
the AXI 2826/12 motor because it can be powered by readily available
10-cell NiCd/NiMH packs or 3-cell Lithium Polymer (LiPo) packs.
This also means that I can use an ESC with a built-in Battery Eliminator
Circuit (BEC) to eliminate the complexity of using a receiver battery
pack. Most ESCs have a built-in 5v regulator that can power the
receiver and servos as long as the flight pack doesn't exceed 10-cells
NiCd/NiMH or 3-cells LiPo. New ESC designs are no longer limited
to this 3-cell limit for using the built-in BEC. Read on!
Simply
take the Lithium cell voltage under load of 3.6v, times the #
of cells used, times the maximum burst current to get power class
in watts.
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For
the AXI 4120/14, it is 3.6v x 4 cells x 40amps = 576w.
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For
the AXI 2826/14, it is 3.6v x 3 cells x 40amps = 432w.
The
Jeti 40-3P ESC comes with a Battery Eliminator Circuit (BEC) which
eliminates the need for a receiver battery. The BEC provides voltage
to the receiver and the servos through the ESC control cable which
eliminates the need for a second battery for the receiver. This
convenience means that you'll never need to worry about the
charge on the receiver battery again! When using a folded 10-cell
NiCd or NiMH pack, the model will balance perfectly without adding
and lead weight to the nose. These 10-cell packs weight about
22oz.
When
you combine the AXI 2826/12 motor with either a 10-cell NiMH pack
or 3-cell Lithium pack, a 40-amp ESC, and, an APC 13x10 e-prop,
the resulting power system will provide about 385 watts at 37amps.
Note that this power level can be easily measured with a wattmeter
by plugging it in-line between the battery pack and ESC. This
gives our 6lb application 385w/6lb or about 64w/lb which fits
in our rule of thumb range.
My
Taxi Cup II was ready to fly at 100 ounces (6-1/4 pounds) with
a power system that will provide about 385 watts or about ?
h.p. The plane can perform most aerobatic maneuvers while remaining
incredibly stabile in flight. My conversion to electric power
was made easy by Dr. Keith Shaw's general rule of thumb.
Another
solution to convert my 6lb Graupner Taxi Cup II to electric
power is the E-flite Power
46 Brushless Outrunner Motor from Horizon Hobby. Not
only is the correct conversion size apparent in the name
of the motor, the description of the motor reads as follows:
Ideal
for 40- to 46-size airplanes 4- to 7-pounds (1.8-3.2 Kg),
25- to 40-size 3D airplanes up to 5-pounds (2.2 Kg), or
models requiring up to 800 watts of power
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This
technique for marketing the motor is very informative to the customer.
Note that the specifications on this motor also suggest a LiPo
cell count from 4 to 5 cells.
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While
using more than 3 Lithium cells may require the use of a
separate receiver battery on some ESCs, the new 60-Amp Pro
SB Brushless ESC with Switch-Mode BEC allows you to
operate up to 7 analog or 6 digital standard-sized servos
with the BEC on any recommended input voltage from 3s to
6s LiPo.
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Summary
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The
intent of this column was to arm the reader with a simple
set of rules and steps to follow for selecting the right
motor. When
using the watts per pound general rule of thumb, choosing
an electric motor can be as simple as knowing the all-up
weight of your model and its desired flying style. Variations
on the type of flying needed, scale prop size, and speed
can be handled though the number of motor winds and battery
cells used.
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There
is no mystery in selecting an electric power system; only confusion
by the overload of choices we currently see on the market and the
less than obvious specifications from various numbering schemes.
While the abundance of motor manufacturers helps keep the cost down
through competition, it can be intimidating to narrow the field
to a few choices. We can empower ourselves by first determining
the power class of the motor we need to make it an easier selection.
Don't fall prey to the least expensive product on the market as
quality and support can often overcome a choice based on cost alone.
Good
luck on your next glow to electric conversion project! Your plane
will last much longer by staying clean and flying noise-free without
all the nasty vibrations. When
you fly electric, fly clean, fly quiet, and fly safe!
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This
section of AMP'D covers some of the questions that our
readers have sent in and I thought would be interesting
for others.
Dan
asks: "I was confused by your last 'Ask AMP'D' issue
where you talk about using one or two packs for separate
motors. How do I wire two brushless motors on a plane?"
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Greg:
Unlike brushed motors, brushless motors need one
ESC per motor. You can connect both ESC's to a
single throttle channel using a Y-harness or you
can plug them into separate channels and use a
transmitter mix from the throttle channel.
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Tim
G. asks: "In your last issue, I noticed that you were
using a Jeti SPIN ESC but didn't give any details on
how to connect the SPIN BOX to program it. I can't seem
to figure out how to get it to work?"
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Greg:
My focus on the last issue of AMP'D was on arming
the ESC. I'll admit that I was rather disappointed
in the instruction sheet that came with my Jeti
SPIN BOX. Fortunately, Hobby Lobby has a better
description of the set up needed on their SPIN
Brushless Speed Controls page.
The
key things to know are that the longer cable with
the (+ - and signal label) symbols on it connects
to the ESC and if you are using an OPTO controller,
a separate
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power supply is required like a 4.8 - 6V standard receiver
battery. The shorter lead with the red connector goes
to the programming box and the longer lead goes to the
receiver. The short lead has a sticker on it that is
marked "RX" with a circle and line thru it, which stands
for "not to receiver". The battery plugs into the connector
that is only labeled (+ -) for use with OPTO speed controls
and the testing tool for measuring and exercising the
servos.
I
ended up attaching a 36" long JR extension cable to
my SPIN BOX so I could more easily connect it to the
short lead on the SPIN controller. I also put some
Velcro? on the back of the SPIN BOX so the Rx. battery
pack could be attached.
Neal
writes: In your last "Ask AMP'D", you wrote,
"As long as the noisy Motor to ESC wires are
short, the battery lines to the ESC can be long."
I believe that this is contrary to Castle's (Castle
Creations) thinking. I believe that Pat (Patrick del
Castillo) wants the ESC to motor leads long and the
battery to ESC short. Am I correct?
Greg:
Yes, you are correct. My statement appears
to be contrary, but it is common to misinterpret
a message, especially if it is taken out of
context.
The reality of which wires to make longer
can be different depending upon the type of
ESC used, the position of the receiver or
ESC, and your definition of "long".
In most cases, we only need the ESC to battery
wires to be a few inches longer and it is
easier to lengthen the two battery wires than
extending the three motors wires, especially
if the ESC must be moved out of the air flow.
Look
for a consensus of opinion and examples of
what works in certain situations. For example,
most opto-coupled ESCs do not have any issue
with longer battery wires up to 2' in length
or more. When you make the battery wires longer
than a foot on non-opto coupled ESCs, you
run the risk of hurting the controller unless
you add a decoupling capacitor across the
battery wires near the controller. More detailed
information about this can be read on the
Schulze
Tips page.
That
being said, if you must run longer wires between
the battery and the motor when using a non
opto-coupled ESC, extending the wires to the
motor is a better choice for the controller,
but not always the application.
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Tom
M. writes: You stated that, "Many of my ESCs
also sparked when I connect more than a 5s LiPo supply
to them." Shouldn't they? How do you explain
this
- The
Spark is your Friend!
Greg:
Castle's position on eliminating sparks from
high voltage ESCs is rather odd. The link
to their FAQ section seems to promote sparking
without consideration for the battery connectors
or the easy solution provided by the anti-spark
feature. I would continue to do what works
best for you.
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Ask
questions by e-mailing me at greg@rcuniverse.com
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This
section of AMP'D reveals some of the feedback or suggestions
that our readers have sent in about previous issues.
Greg G. writes:
GREAT ARTICLE!!!!! I just got done with the second AMP'D
issue and thought it was great. I get lots of questions
about wiring arming switches and such and you've hit
them all right on the head. I love the way the photos
and diagrams enlarge for better viewing in the article.
Whoever is formatting these articles is doing a great
job. I'm working on the March column and would love
to point a reference to this article if it's okay with
you.
Just
about the time I think electric columns have outlived
their usefulness you do this. These are the kinds of
practical tips that modelers can take to the shop or
field and make a better and safer airplane. This just
proves columns written by guys like you are still needed
and I'm betting you'll get lots of good feedback on
it.
When
I used to have my Bristol M1C, the machine gun was attached
to a sermos connector and was the arming switch. It
disguised it nicely and I had a static gun and a flying
gun that actually armed it.
Take
care,
Greg Gimlick
Model Aviation Magazine
Park Pilot Magazine
Electric Flight Columnist
Make
suggestions by e-mailing me at greg@rcuniverse.com
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It's
Truly Addicting! |
Affordable
Outrunner Brushless Motor - 450W
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