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REMOTE CONTROL
BY BOB YOUNG
Motors for electric flight models
This month, we will discuss the electric motors
used in model aircraft & look at controller
requirements. In coming months, this will lead
to a construction article for an electronic speed
controller.
Two prototypes of the electronic
speed controller are being developed
and these will be test flown in a twin
Partenavia P68 Victor. This aircraft is
shown a photo accompanying this article, along with my old mate and
builder of the model, Wes Fisher.
Before we can begin to design this
unit however, we must have a full
understanding of what is expected
from an electronic speed controller
for modern electric motors.
Electric propulsion is an old dream
of modellers and appeals especially
to those in the modelling fraternity
who abhor the noise and mess of internal combustion motors. Burnt castor oil is one of the most obnoxious
substances one will meet in the hobby
game. It is almost impossible to remove completely from your hands or
models and over a period of time will
gradually soak into a balsa airframe,
rendering it very difficult to repair.
Internal combustion technology has
greatly improved over the past few
years, with synthetic oils which allow much higher methanol/oil ratios
.and are very easy to clean off. Fourstroke motors are now available too.
These use much less oil, deposit less
oil on the model and are much quieter
in operation. We also have super glues
which work well for bonding oily surfaces and we now have fibreglass
which is impervious to the ingress of
all oils .
However, the appeal of electric
propulsion remains
undimmed by this impressive catalog of
technological wizardry. If anything, it is
enhanced (or some
would say even finally made possible) by
a similarly impressive
catalog of technological wizardry peculiar
to the field of converting electricity into
mechanical energy.
At the very heart of
this mini technological revolution are dramatic improvements
in battery and magnet
Shown here is a reversing speed controller made
technology. Thus, we
some years ago by Sunlux. It was a 12V device rated
15-20 amps.
now find ourselves in
an era in which model electric motors
are capable of delivering staggering
performance by the standards of not
too long ago. What's more, we also
now have batteries that are more than
capable of supporting this demanding performance.
And now the relentless demand for
technology has reached out and encompassed the electronic speed controller. Thus, we are faced with the
difficult task of supporting these electron gulping monsters with their seemingly unlimited source of current, via
a variable speed controller, capable of
delivering up to 70 amps at 30 volts.
This amounts to a total power dissipation of 2.1 kilowatts, all inside a
flimsy model aircraft.
Speed controller development
Broadly speaking, speed controllers fall into two categories: forward
speed only and reversible. The forward speed type is used mainly in.
aircraft, track cars and electric boats.
All of these applications place weight
and voltage delivered to the motor
above versatility.
For applications where reversing
the motor is a must, a reversing controller is used. These controllers feature a bridge output or a relay to effect
the polarity change. Thus, they are
heavier and usually more expensive
than the forward only controllers.
Typical applications are off-road buggies, model ships and most sports
models where versatility and realism
are th,e key points. The forward-only
controller is often fitted with dynamic
braking, a feature easily achieved in
electric propulsion by simply placing
a dead short across the motor terminals.
As with all things, speed controllers had a fairly primitive beginning
and a typical controller of the 1970s
DECEMBER 1991
53
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This is the circuit of one of the most popular speed controllers of the late 1970s,
made by Futaba. It used a relay for reversing, was rated at up to 10 amps &
fitted into a case about the size of a cigarette pack.
would deliver about 10 amps in a
reversing controller and about 20 amps
in a forward-only controller. Keep in
.mind here that the motors they were
required to drive were equally as
primitive and typical current consumptions were in the order of 4-10
amps at 12V.
The reversing controller usually
used a relay to effect the reversal and
these speed controllers were not small.
Shown in one of the photos with this
article is a finned unit made by
Sunlux. This is a reversing 15/20A
12V device.
One ofthe most popular speed controls of the late 1970s was the Futaba,
giving a reversible 10 amps at 12V
and again featuring a relay. About the
size of a cigarette pack and built without a heatsink, it was a little prone to
overheating when pushed near its limits but gave good service for many
years when used with the more conventional motors.
Here lies the key issue in speed
control design: the ON resistance of
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SILICON CHIP
the switching semiconductors. At high
currents, even a small resistance results in a significant voltage drop
across the output stage. This lost voltage is critical in a racing application
and is the key factor in deciding as to
which unit you choose for your application.
Luckily for us, the relentless quest
for improved battery and magnet performance has been matched by the
semiconductor manufacturers who
have given us that magical little device known as a FET.
However, even a very good FET such
as the IRFZ44 has an ON resistance of
around .025Q. Therefore, at 50 amps,
the voltage drop will be about 1.25V,
giving a power dissipation of 62.5W.
This power is wasted as heat inside
the model, heating the electronics and
stressing the output stages.
It would, of course, be better if it
were delivered to the motor where it
would provide extra propulsive force.
Thus, we can draw several very significant conclusions immediately
from this simple observation:
(1) Nothing is better than a stout
piece of wire for connecting the motor to the battery.
(2) A relay is the next best thing for
switching the motor ON and OFF.
(3) If you must use semiconductors
for switching, use those with the lowest ON resistance that you can buy
and use plenty of them in parallel.
This reduces the effective ON resistance and therefore reduces the total
dissipation. It is quite common to see
anything up to eight FETs in parallel
in some commercial controllers. These
units are very small and very expensive - as much as $350 each.
The figures quoted for these modern commercial FET speed controllers
are breathtaking, such as 250 amps
sustained and 1000 amps instantaneous for forward only controllers. Reversing units are quoted at 150 amps
sustained, with a peak instantaneous
rating of 450 amps, while dynamic
braking on the unidirectional models
is quoted at 60 amps. Just how accurate these figures are, I cannot attest
to, however I suspect they are fairly
close to the mark. What's more, some
It may be hard to believe but electric propulsion is being used for progressively
larger and heavier models. This twin Partenavia P68 Victor, shown here in the
construction stage, will be used as a test bed for an electronic speed controller
currently under development by the author.
of these units are quite small and most
do not feature a heatsink - another
technological marvel.
Therefore, we now have our first
design parameter for our proposed
speed controller. The output stages
will feature FETs - several of them in
parallel. How many, what type and in
what arrangement will not be resolved
until we examine exactly what the
modern electric motor demands of the
proposed controller.
High power cells and motors
For those who have been away from
R/C modelling, the world of electric
propulsion is a complete revelation.
Gone are the days of button cell nicads
which melt down if too much curr.ent
is pulled from them and which explode if too much current is pushed
back into them. In their place stands a
glittering array of batteries, some quite
capable of melting down battery chargers or welding the connecting wires
to the car chassis if one is not very
careful.
What we have now is a source of
electrons of almost zero internal impedance, in case sizes which even as
little as five years ago were just a
dream. Electric flight is the main beneficiary of these advances, although
most models benefit from weight reductions . .
However, it is the motors themselves
which stagger the imagination. Technology has invaded the world of electric motors in a big way and everything from new magnetic materials to
class 10 bearings have been brought
into play to squeeze out every last
drop of performance. The result is a
bewildering array of motors and accessories described in a language
which is as mysterious as any ancient
Middle Eastern dialect.
What does it all mean? Well you
might ask, and we will spend the remainder of this column and all of
next month's to unravel the mysteries
of electric motor language.
Motor construction
A typical model motor consists of
three major sections: the motor can,
which houses the front bearing and
magnets; the armature and commutator; and the endbell containing the
rear bearing and motor brushes.
The motor cans are arranged in a
hierarchical order, the classification
of which seems to defy all but the
most expert electric enthusiasts. On
the face of it, there appear to be two
popular classification systems, one
American and one European, the most
logical being the American system.
The American motors range in size
from 020 and 035 (which usually use
4-cell battery packs) to the 05, 075,
10, 15 , 25, 40 and, less usual, the 60.
These numbers approximate the output power of an internal combustion
motor expressed in cubic inches.
The first surprise for tyro electric
fan (if you will pardon the pun) is the
size of the prop these motors swing
and the speed at which they spin it. A
10 x 6-inch prop spinning at 13,000
RPM is really quite ordinary!
Thus a 15 size electric motor delivers about the same output power as a
0.15 cubic inch glow plug motor.
These figures are usually but not always based on 6-cell battery packs.
The Japanese call the 05 housing the
540 series and these are remarkably
similar in construction to the American motors. Armature winds range
from 6-27 turns .
The European system seems to be
based on the armature winding and
the can sizes seem to be confined to
3 7mm, 42mm and 45mm diameters.
The length of the motor varies considerably in various models and the brush
housings are quite different in construction from the American and Japanese systems.
The Europeans offer armatures of
various lengths and windings to accommodate the number of battery cells
used in any particular application.
Thus, armature winds range from 320 turns to accommodate 7-30 cells.
They also use very large commutator
areas for adequate brush cooling, a
very big item in high power, high
revving motors. This also contributes
to armature length. Figures of up to
50,000 RPM can be achieved from some
motors and brush/commutator heating becomes a serious problem.
The magnets are the heart of the
modern motor and the rare earth magnets have revolutionised this area of
motor design. Low-cost motors are still
available with the old ferrite magnets
but some of the more exotic rare earth
magnets such as samarium cobalt and
neodymium are used extensively in
the more expensive high-performance
motors. Neodymium magnets are,
however, prone to demagnetisation at
high temperatures and suffer badly in
some ii;istallations. A well-ventilated
model aircraft is their best environment. Motors of this type are expensive and run at about $370-400. Compare this to a good, fun motor (Speed
600,075, 7-8 cell) at $21.95.
And that's it for this month. Stand
by for r..ext month's exciting developments.
SC
0ECE/11BE R 1991
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