Specification of an Electric Power System

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Specification of an Electric Power System
Specification of an Electric Power System
Lawrie Henrickson
Rev 2, June 2008
Specification of an Electric Power System
Table of Contents
Table of Contents
1.0
2.0
2.1
2.2
3.0
3.1
4.0
4.1
Introduction .............................................................................................................1
Disclaimers..............................................................................................................1
Non-Affiliation ........................................................................................................1
Safety ....................................................................................................................1
Electrical Nomenclature .........................................................................................2
Key Equations .......................................................................................................2
Electric Power System Components.....................................................................4
Batteries ................................................................................................................4
4.1.1
4.1.2
4.1.3
Working voltage ......................................................................................................................... 4
C-Rating ..................................................................................................................................... 4
Rechargeable Battery Types...................................................................................................... 5
4.1.3.1.
4.1.3.2.
4.1.3.3.
4.1.4
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.3
4.4
4.4.1
4.4.2
Nickel Cadmium (NiCd / NiCad) .............................................................................................................................5
Nickel-metal-hydride (NiMH) cells...........................................................................................................................5
Lithium Cells...........................................................................................................................................................6
Battery Chargers ........................................................................................................................ 8
Motors ...................................................................................................................8
Commutation .............................................................................................................................. 8
Out-runner versus In-runner....................................................................................................... 9
Motor sizing ................................................................................................................................ 9
Motor Speed Constant ............................................................................................................. 10
Gearboxes................................................................................................................................ 11
Motor Specification................................................................................................................... 12
Electronic Speed Controllers (ESCs) ..................................................................12
Battery Eliminator Circuits (BECs).......................................................................14
Types of BECs ......................................................................................................................... 14
Disabling an ESC’s BEC .......................................................................................................... 15
5.0
Case Study – Specifying an Electrical Power System .......................................16
5.1
Step 1 – Power Requirement ..............................................................................16
5.2
Step 2 – Battery Selection ...................................................................................17
5.2.1
5.2.2
5.3
5.4
5.5
5.6
5.6.1
5.6.2
5.6.3
Step 2A – Voltage and Current ................................................................................................ 17
Step 2B – Charge Capacity...................................................................................................... 17
Step 3 – Speed Controller Specification ..............................................................18
Step 4 – Motor Selection .....................................................................................18
Step 5 – Propeller Selection ................................................................................18
Cooling ................................................................................................................19
Equipment Layout .................................................................................................................... 20
Cooling Air Inlets ...................................................................................................................... 20
Cooling Air Outlets ................................................................................................................... 20
5.7
Testing.................................................................................................................21
6.0
Record of Personal Set-ups .................................................................................24
7.0
About the Author ..................................................................................................25
Lawrie Henrickson
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Specification of an Electric Power System
Introduction
1.0 Introduction
Electric flight offers an alternative power source for our aircraft. Heavy and inefficient
components rendered this approach generally non-viable for many years, but advances in
motor and battery technology have now achieved high efficiencies and low component
masses, making this a very viable power source. Improvements in manufacturing and
mass-production have also lowered the cost enormously, so electric flight in now an
attractive option.
The following document serves to explain the terminology of electric flight systems, and
then demonstrates the specification of a system by a case study.
2.0 Disclaimers
2.1 Non-Affiliation
The author is not affiliated with any vendors and obtains no gain, monetary or
otherwise, from recommending any particular vendor.
Vendors have been
recommended purely based on good personal experience.
2.2 Safety
Electrical power systems are sources of large amounts of energy. Care must be taken
in their specification and use. The author makes no guarantees and assumes no
responsibility for any resultant incidents as a result of following the specification guide.
The user must take all due care to ensure their safety and the safety of those around
them.
Lawrie Henrickson
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Specification of an Electric Power System
Electrical Nomenclature
3.0 Electrical Nomenclature
With the sudden onset of electrical flight, electrical terms are being thrown about, often
incorrectly. The electrical industry has standards for the proper terms, and these should
be adhered to.
Voltage or EMF:
Volts (V)
Current:
Amperes (A), or milliamperes (mA)
Charge:
Ampere-hours (Ah), or milliampere-hours (mAh)
We do not measure current in mAh. Nor do we measure charge in mA. Modellers do not
like it when non-flyers misname our equipment, so let’s not insult the electrical community
– get the units right.
3.1 Key Equations
1. Charge = current x time
or Q = I.t
Should be measured in Coulombs, but for simplicity we measure in mAh.
2. Ohm’s Law: V = I.R
Where
V = applied voltage or voltage drop (V)
I = current, measured in Amperes (A, not mA)
R = resistance, measured in Ohms (Ω)
3. Power (P) = V.I
Where power is measured in Watts (W) or Joules / second (J/s).
Application of Ohm’s law shows power may also be calculated according
to the following equations. Use whichever you like – they are all correct.
= I2.R
= V2/R
4. Mechanical Power = τ x ω
Where
τ = torque, in Newton.meters (N.m)
ω = rotating speed in Radians per second (Rad/s)
1 radian = 57.3º
1 rpm = 0.105 Rad/s
5. Torque (τ) = r x F
Where
τ = the torque, in Newton.meters (N.m) about a pivot
r = the distance (at right-angles) of the force from the pivot, in
metres (m)
F = the force, in Newtons
Where
1 Newton
= 0.1kilogram.force (kg.f)
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Electrical Nomenclature
6. Force = m.a
Where
F = the force, in Newtons (N)
m = mass, in kilograms (kg)
A = acceleration in m/s2 (usually gravity, 9.81m/s2)
These equations make it possible to understand and define an electrical
power system. Equations 4 and 5 (mechanical power and torque) are
particularly important when understanding the relationship between a
motor’s torque and its rotating speed.
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Specification of an Electric Power System
Electrical Components
4.0 Electric Power System Components
It is important to understand the various components that make-up an electrical power
system. The following explanations serve to give an overview of each component, but do
not fully explain the inner workings.
A typical electric power system will comprise the following items:
o Battery
o Speed Controller
o Motor
4.1 Batteries
Batteries are chemical energy reservoirs. Unlike fuel tanks, they do not get lighter in
use. A chemical reaction inside the cells during use develops a potential difference
between the positive and negative terminal, enabling a supply of electrical power to a
load (usually a motor in our hobby).
4.1.1 Working voltage
Many cells have a stated voltage, but in practise deliver less than this amount. The
reduction is due to internal resistance. By Ohm’s law above, when an electrical
current is passed through resistance, there will be a loss in voltage. Since batteries
have some internal resistance, the output voltage will be the rated (chemical)
voltage less the loss to internal resistance. Higher current capacity cells have less
resistance to minimise this voltage loss. Note that any loss to internal resistance
results in heat – this is why batteries get warm (or even HOT!) while in use. This
waste heat can damage the cells.
4.1.2 C-Rating
Before describing the cells, it is important to understand the capacity rating of cells.
All rechargeable cells have a charge capacity rating, specified in mAh. This is the
product of operating current and duration. A 1000mAh battery should delivery
1000mA for 1 hour, or 2000mA for 30 minutes, etc. The higher the current, the
shorter the duration, but multiplying the two should always give the same result.
It is often more useful to know how much current a battery is delivering as a fraction
of its charge capacity – this is known as the “Capacity” or “C” rating. Operating the
above 1000mAh pack at 1000mA current is operating the cell at 1C. The 2000mA
current corresponds to 2C.
The packs C-rating is calculated by the following equation, which is simply a rearrangement of Equation 1:
7.
C-rating = Current (in mA) / Charge Capacity (in mAh)
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Most batteries (especially LiPos) have a maximum C-rating. For example, a typical
11.1V 2000mAh LiPo pack should be rated at 20C (i.e. 40A max current). Above
40A current, the loss in output voltage due to internal resistance takes the actual
voltage below 3.0V, causing electronic speed controllers to limit the current or shutdown the motor.
Many flyers seek batteries with higher and higher maximum C-ratings to maximise
the power delivery but minimise the battery weight. However, this also gives
reduced run-time, and this is irrespective of the charge capacity. Any battery
operated continuously at 20C will last a maximum of 3 minutes, as calculated by
Equation 8:
8.
Run time (minutes) = 60 / C-rating
For example, a pack operating at 20C will fully expire in 60 / 20 = 3 minutes!
4.1.3 Rechargeable Battery Types
There are many different batteries available to store and supply electrical power.
The following descriptions are limited to those used for model aircraft applications,
and are all rechargeable.
4.1.3.1.
Nickel Cadmium (NiCd / NiCad)
For many years the NiCad cell was the staple battery. Good quality cells could
be fast-charged (at 4C!), and they could be discharged at frightening rates
(100C is possible – that’s 100A for a 1000mAh pack!). Most NiCads had a
declared service life of 1000 cycles, but few lasted this long. NiCads also have
a very poor energy density, so they were very heavy for a given capacity.
NiCads have a fully-charged cell voltage of 1.2V, an operating voltage (under
load) of 1.0V, and a discharge voltage of 0.8 – 1.1V (not very precise!). NiCads
can be fully discharged to 0.0V without harm, and can be stored at this voltage
for extended periods. They do NOT hold their voltage very well – sources vary
but the generally accepted degradation is 3% per day. Experience suggests the
typical degradation is much higher.
4.1.3.2.
Nickel-metal-hydride (NiMH) cells
These cells have a higher energy density than NiCads, giving valuable on-board
capacity for less weight. However, NiMH cells often have far less current
capacity than NiCads. Even the “performance” cells have to be limited to 2C
charge rates, and they typically cannot achieve the mega discharge rates of
NiCads. Suppliers would often trickle-discharge NiMH cells to get the high
declared charge capacities – these were often not achieved at high discharge
rates. NiMHs have a service life similar to NiCads. They have a fully-charged
cell voltage of 1.2V, an operating voltage (under load) of 1.0V and a discharge
voltage of 0.8 – 1.1V, exactly the same as NiCads. NiMH cells are also not
damaged by being fully discharged, and may be stored fully discharged.
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Specification of an Electric Power System
4.1.3.3.
Electrical Components
Lithium Cells
These cells are quite different from NiCads or NiMHs. They have a bad
reputation for being dangerous, but they are no more dangerous than NiCads
when handled correctly. The author has personally only ever had one battery
pack catch fire, and that was a NiCad. If you feel NiCads are safe, short-circuit
a cell and watch what happens1!
Lithium cells have a much higher energy density than NiCads or NiMH cells, so
they are much lighter for a given capacity. They hold their charge VERY well,
and can be left fully charged for extended periods, but this is not recommended
due to risk of a permanent loss in charge capacity. The decision to use NiCads
or Lithium cells is usually based on cost.
Connectivity
Lithium cells are connected in series to obtain the desired voltage, as per
NiCads. However, they are also sometimes connected in parallel to boost
capacity or current capability. This is becoming less common as larger
lithium cells become available, and the cells improve in current delivery
capability. Any lithium pack will be referred to as #S#P, where the #S refers
to the number of cells in series, and the #P refers to the number in parallel.
If there are no parallel cells, then the P number is 1, even though 1 cell
cannot be in parallel to anything! For example, a typical small-craft LiPo
battery has 3 cells in series, and none in parallel, so it is referred to as 3S1P.
It will have a rated voltage of 11.1V (see below)
Charging Lithium Cells
NEVER, EVER charge a Lithium cell using a non-Lithium capable charger.
These cells must be charged very carefully. The typical charging routine is
1C charging (some cells offer 2C, but this may reduce battery life), until the
charge voltage is at 4.2V/cell. At this point, the voltage is held constant and
the charge current is ramped down until the current is zero. Do not try to do
this manually using a variable-voltage transformer.
Because Lithium cells are easily damaged by overcharging, it is important to
balance the cells in a pack during charging, using a proper balancer. These
are relatively low cost and make charging trouble-free. The balancer
typically connects between the charger and the battery, and uses a small
discharge current on the cells with higher voltages to equalise the cells.
Some modern chargers include a built-in balancer – excellent! All Lithium
packs come with balancing plugs to connect to the balancer – USE THEM.
1
Beware that this exact situation lead to the fire with the NiCad. For safety purposes this test is not recommended.
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Lithium Cell Types
There are currently 4 types of commonly available lithium cells:
o Lithium-ion (Li-ion),
o Lithium-ion-polymer (LiPo)
o Lithium-Manganese (LiMn)
o Lithium-ion-phosphate (LiPho, or “A123”)
Lithium-Ion (Li-ion)
This type are not capable of high discharge rates and are very
susceptible to explosions and fires, so they are typically not used for
model aircraft. Lithium-ion cells are commonly used in mobile (cell)
telephones – consider this when you worry about the safety issues
with LiPos. They have a “nominal voltage” of 3.6V/cell, a fully charged
cell voltage of 4.1V, and a minimum cell voltage of 3.0V. It is not
recommended that these cells should ever be used for model aircraft
applications.
Lithium Ion Polymer (LiPo)
This cell type has a polymer matrix to stabilise the chemistry, to
minimise the risk of fires and permit excellent charge and discharge
currents. These are the most common cells used for electric flight due
to their light weight and good discharge capacity.
LiPos have a
typical service life of 100 – 300 cycles, depending on duty. They have
a “nominal voltage” of 3.7V/cell, a fully charged voltage of 4.2V/cell,
an operating voltage of ~3.3V/cell, and a minimum cell voltage of 2.6V
– 3.0V/cell. They must never be allowed to discharge below the
minimum voltage, or the cell chemistry may be irreversibly damaged.
They should be stored at 40% - 60% of fully charged capacity. LiPos
should never be discharged beyond 90% of their rated capacity – do
not rely on your BEC to shut-down the motor on low battery voltage to
tell you it is time to land – time your flight instead.
Lithium Manganese (LiMn)
These cells were developed to ease the handling of LiPo batteries.
They have exactly the same voltage characteristics as a LiPo battery.
Some are claiming better minimum voltage tolerance. However, these
cells are claimed to be totally resistant to fire, by physical damage or
by overcharge, so they are very safe to use.
Lithium-ion Phosphate
These cells are packaged in a cylinder, and look very similar to
NiCads. They have better energy density than NiCads or NiMHs, but
not quite as good as a LiPo. However, they may be fully discharged
to 0.0V like a NiCad, are fire resistant, and typically achieve higher
discharge rates than LiPos (50C is typical!). They have a lower
working voltage than a LiPo (around 2.8V) so they are not a straight
one-for-one fit.
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Electrical Components
4.1.4 Battery Chargers
Many modellers have successfully used the standard trickle-charge wall-mount
chargers, commonly sold with radio control units, for many years to charge their
radio batteries. If you want to succeed in electric flight you will need to get a
modern fast charger, capable of around 5A charge current, with NiCad and Lithium
charging capability. These chargers often utilise electrical smarts to detect the
number of cells and the fully charged voltage, so there is no need for interaction
from the user apart from setting the charge current.
4.2 Motors
Electric motors typically comprise a rotor, which carries the windings of electrical wire.
There will also be a stator, that holds the permanent magnets that apply the magnetic
field. The current is transferred to the windings by a commutator. When a voltage is
applied across the windings, electrical current flows through the windings, and the
resultant current flow inside the magnetic field generates the force to rotate the motor
about the shaft. The higher the current and the stronger the magnetic field, the higher
the force. Equation 3 shows that increasing the current increases the electrical power.
Equation 5 shows that a force generated about a pivot creates rotating torque, where
the greater the force and the greater the rotor diameter, the higher the resultant torque.
According to Equation 4 this will increase the shaft mechanical power. As can be seen
from this, a higher electrical power to the motor resulted in a higher output power to the
shaft. The motor’s efficiency is determined by the following equation:
9.
Efficiency (ε)
= output power / input power
= τ x ω / V.I
4.2.1 Commutation
Until recently, the only readily affordable motors were brushed motors. These
motors utilise a DC electrical current passed from the speed controller via carbon
brushes into a split-ring commutator. Brushed motors are often very inefficient (the
brushes drag on the commutator, slowing the rotation motor) and heavy, so their
use has declined since brushless motors became more commonly available and
affordable.
In-runner brushless motors use a 3-phase AC current, supplied into the windings by
a slip-ring commutator, thereby negating the need for draggy brushes. Outrunner
brushless motors have stationary windings and instead spin the magnets, so there
is no need for a commutator – the electrical wires are joined directly to the windings.
Brushless motors are much lighter and up to 5 times more powerful than similarly
sized brush motors. Brushless motors are easily distinguished by their 3 supply
wires, whereas brushed motors only have 2.
Many modellers have successfully utilised brushed motors, and they are sometimes
a reasonable option due to their very low cost and simplicity. However, the recent
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Electrical Components
competitive pricing of modern brushless motors offers the modeller a cost-effective
high-performance alternative, which the author highly recommends.
4.2.2 Out-runner versus In-runner
Until recently, all motors were narrow “In-runner” motors. The stator was anchored
to the motor case, so the rotor turned the shaft. Due to their narrow rotors, these
motors could not develop high torque, so they had very high specific speeds (as per
Equation 4). However, when the designers opted to anchor the outer stator to the
shaft, rendering the rotor stationary and the stator free to rotate, this reversed the
torque situation. Motors began to have much lower specific speeds and much
higher torque. The high torque of these new rotating case “Out-runner” motors
often negated the need for a gearbox to swing large diameter propellers.
Note that Out-runner motors are not necessarily better than In-runner motors.
Some applications require a high rotating speed, such as in a ducted-fan jet.
However, an electric sailplane typically swings a large diameter propeller at
relatively slow speed, which is an ideal application for an Out-runner, provided the
sailplane’s nose can accommodate the larger diameter, and the motor speed
constant (see 4.2.4) is sufficiently low. Sometimes there is no option but to geardown.
4.2.3 Motor sizing
Unlike internal combustion (IC) motors, there is no convention for the size of electric
motors. However, suppliers have begun to use common measurements and terms,
greatly simplifying the task of specifying a motor and comparing different makes.
Brushed motors are typically sized by the total length of wire in the winding, e.g. a
Speed 400 has around 400mm of wire length in its winding. This does not tell you
much about the motor, apart from it having a longer winding than a “300” size
motor.
Brushless motors are typically sized as follows:
Brand-XXXX-XX
e.g. AXI-2808-16 or Himark 36-12-30.
The first two digits refer to the diameter of the rotor (the part that carries the
windings), in mm. The second pair of digits refer to the length of the rotor, in mm.
The last number refers to the number of turns per winding. In this case the rotor
has a 28mm diameter and 8mm length, with 16 turns per winding “pole”. Typically
as the first two pairs of numbers get bigger, so does the motor’s maximum power.
A bigger rotor carries more wire and generates more torque (remember Equation 4,
where power is proportional to torque). However, as the number of turns per
winding gets smaller, the motor will turn faster for a given voltage, draw more
current and typically develop more power (since power is proportional to current
from Equation 3).
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Electrical Components
End view of the STATOR of an Out-runner motor, showing 14 magnets and the central prop-shaft
attached to the case:
Casing (stator)
magnets
Prop shaft
End view of the ROTOR of an Out-runner motor, showing 12 poles with their associated copper
windings. The rotor fits inside the stator.
Power supply wires
Pole
Windings
Shaft bearing
4.2.4 Motor Speed Constant
All motors have a speed constant. For brushed motors, this is typically expressed
as the voltage required to spin the motor at 10,000rpm, e.g. “7.2V motor”. It does
not refer to the working voltage of the motor. A 6V brushed motor will spin faster
than an 8.4V brushed motor at the same applied voltage, and draw more power –
beware if you want to use a brushed motor.
Brushless motors are specified more sensibly by a “kV” speed constant, which is
the number of rpm the motor will achieve for each volt applied, e.g. 1200rpm/V.
Note that this is done without a load (propeller), so in practise it will spin somewhat
slower than this. However, the motor will attempt to achieve this rotary speed no
matter how big the propeller is, so if it is bogged down with a large prop it will draw
a very large current in an attempt to maintain its speed – beware if specifying a
large prop for a high revving motor.
A high speed constant is not necessarily good. When the author attempted his first
brushless application he connected a 10 cell NiMH pack to a 7,000kV In-runner
motor, thinking this would be more powerful. This set-up was turning a 10” x 6”
propeller without a gearbox – i.e. direct drive! In practise, the speed controller kept
shutting down the motor at anything above idle. This was because the motor was
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Electrical Components
attempting to spin-up to 70,000rpm (7,000kV x 10V operating voltage)! The power
required to spin a 10x6 prop at this speed is enormous, resulting in current overload
at the battery, causing the voltage to collapse (too much voltage drop across the
battery’s internal resistance, by Ohm’s Law). This set-up required a high-ratio
gearbox (around 7:1) to reduce the prop speed to something more reasonable.
The following table provides a general guide to the relationship between kV and the appropriate
number of LiPo cells in series for a direct-drive Out-runner motor:
Number of
Lipo cells
in series
2
3
4
5
6
7
8
10
12
Min kV
1500
900
700
600
450
350
250
200
150
Max kV
2000
1500
1100
900
750
600
450
300
250
4.2.5 Gearboxes
Propellers determine the load of an electrical power system. The bigger a prop is,
the more power it will draw to achieve a certain speed. Effectively, a gearbox
reduces a motor’s specific speed – it allows a motor with a high speed constant to
rev-out, while limiting the prop speed to something reasonable. Equation 4 says
Power = Torque x rotating speed, so a gearbox can enable a high-speed, lowtorque motor to turn a large prop at low speed but high-torque. Gearboxes are not
exclusive to in-runners – even some out-runner motor applications require a
gearbox.
The selection of a large propeller turning at low speed is pursued to maximise
efficiency, especially in sailplane applications. As the propeller speed is reduced,
there is less loss to drag (friction- and profile-drag) and more energy imparted to the
air as mechanical work. A target rotating speed of around 6000rpm is common for
good efficiency, and this often dictates the use of a gearbox. Large outrunners
have sufficiently low speed constants to achieve this, but will usually be too heavy
and too large to fit into a sailplane.
Note that some sailplanes have very narrow noses, so it may be essential to use a
narrow in-runner motor or small out-runner motor with resultant high kV. The high
kV must be offset by using a gearbox to enable the use of a large propeller
swinging at an efficient speed, as explained above.
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4.2.6 Motor Specification
The best indication of a motor’s capability is in its specification. Every brushless
motor will have a speed constant (kV), which is near-useless on its own. A good
supplier will provide the rotor dimensions (e.g. 42-48-20), but these are of limited
use. The best supplier will also provide a comment on the operating voltage
(sometimes as a number of LiPo or NiCd/NiMH cells in series) the maximum
current, and most importantly, the maximum power. Once the maximum power is
known, we can set-up the system accordingly. This will be shown in Section 5.
4.3 Electronic Speed Controllers (ESCs)
Modern speed controllers convert the DC current from the battery into a switching
current to the motor. Effectively, they switch the motor on and off rapidly to vary its
speed. The fractional duration of online current to the motor determines the motor
speed, e.g. at idle this will be low, typically 5%, whereas at full throttle there is no “off”
time and the current is continuously supplied. Speed controllers typically switch the
current at a frequency of 8,000 cycles per second (Hertz or “Hz”), but some offer higher
frequencies, up to 32kHz. A higher frequency will give a more stable motor speed, but
comes at a penalty of efficiency. Every time the current is pulsed, the resistance of the
speed controller causes a slight voltage drop (by Ohm’s Law), consequent power loss
and resultant heat build-up (referred to as a “transient loss”).
Effectively the speed controller is applying full power in pulses to the motor, at full
operating current. However, capacitors in the speed controller filter this out from the
connections to the battery, so the battery only “sees” (feels?) the average current.
Keep this in mind if operating a motor under reduced “throttle” to avoid exceeding its
current limit. A power meter will only show the average current, whereas the motor is
receiving full-current pulses at all throttle settings.
Just as there are brushed and brushless motors, there are also associated speed
controllers. Brushed speed controllers do not have brushes, but they can only be used
with brushed motors. A brushed controller has two wires to connect to the motor, while
a brushless speed controller has three wires. To reverse a brushed motor’s direction it
is a simple matter of swapping the connection of the two wires. For a brushless motor
it is equally simple – swap any two connections of the three wires.
Most suppliers offer programmable speed controllers, either via a PC interface or via a
special programming card. For maximum performance it is essential to program a
speed controller, which allows the user to vary the motor timing, the cut-off voltage, and
many other important factors.
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Specification of an Electric Power System
Electrical Components
The following plots graphically demonstrate the pulsed current sent by the ESC at three throttle settings,
in a system that draws 10A at full power.
Current at 5% Throttle Setting
10
9
8
Current (A)
7
6
5
4
3
2
1
0
0
10
20
30
40
50
60
70
80
90
100
70
80
90
100
Time (arbitrary)
Current at 50% Throttle Setting
10
9
8
Current (A)
7
6
5
4
3
2
1
0
0
10
20
30
40
50
60
Time (arbitrary)
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Current at 100% Throttle Setting
10
9
8
Current (A)
7
6
5
4
3
2
1
0
0
10
20
30
40
50
60
70
80
90
100
Time (arbitrary)
4.4 Battery Eliminator Circuits (BECs)
BECs enable the use of the motor battery to power the receiver and servos, greatly
simplifying an installation. Some flyers are not comfortable relying on the main battery
to also power the radio system, so they use a separate receiver battery. However,
BEC technology continues to improve, and offers major weight savings to the
performance conscious modeller. BECs also have an over-ride function to shut-down
the motor when the main battery voltage is getting low, giving the pilot time to land
before control of the radio system is lost.
4.4.1 Types of BECs
There are two types of BECs – linear and switching. Linear BECs are the most
common, and least reliable. A linear BEC uses resistors to drop the excess voltage
of the main battery to achieve the target 4.8 – 6.0V receiver voltage, as per Ohm’s
Law. Equation 3 shows that if there is a voltage drop, there will also be power lost
as heat, proportional to the voltage drop and the current. High voltage main
batteries require a large voltage drop across the BEC’s resistors, resulting in a lot of
heat build-up. This is more of a problem as more servos are used, drawing a larger
current for the radio system. Most linear BECs are limited to 2 – 3A, and no more
than 4S LiPos on the main battery. Note that larger main battery voltages require
fewer servos on the radio system to limit the radio system’s current draw and hence
limit the heat build-up in the BEC. If the BEC overheats in flight it will shut-down,
and the aircraft will be lost.
Switching BECs do not have the heat build-up problems of linear BECs. Switching
BECs operate just the same as speed controllers – they switch on and off to pulse
the current to the receiver. Large capacitors filter the full-voltage on-off pulses into
a smoother, lower voltage. Switching BECs can tolerate very large main battery
Lawrie Henrickson
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Specification of an Electric Power System
Electrical Components
voltages with no compromise to their supply current, and hence can also tolerate
high servo counts. Good speed controllers come with switching BECs, but more
commonly these BECs are purchased separately and are wired in parallel with the
ESC to power the radio.
4.4.2 Disabling an ESC’s BEC
If a separate receiver battery or a stand-alone BEC will be used, it is essential to
isolate the power supply to the radio from the ESC. This is easily done – remove
the pin on the red wire of the connecting lead from the ESC to the Receiver, and
cover this pin in heat-shrink tube or electrical tape.
Lawrie Henrickson
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Specification of an Electric Power System
System Specification
5.0 Case Study – Specifying an Electrical Power System
With all of the jargon and terms explained, the specification a power system for an aircraft
will be demonstrated, by example using a 700g AUW full-house thermal sailplane
(Passer-X from RVM Shop). The aircraft has 4 servos, including 2 digitals, so a separate
receiver battery or switching BEC is essential – a linear BEC would not cope. The aircraft
has a very narrow and small fuselage, so small, light equipment is essential. The
manufacturer suggested a Turborix 28mm diameter Out-runner motor of 110W maximum
power, with 11x6 folding propeller and 1000mAh 3S1P LiPo.
Note that the aircraft’s weight has to be estimated before starting work. As you become
more experienced, you will have a better feel for the required power system for any given
aircraft, and will have a better idea of what it will weigh. However, this process is
inherently iterative, so it is important to understand that your calculations must be
repeated. For this reason, spreadsheet programs are recommended rather than longhand calculations. The author has developed such a spreadsheet, where the user inputs
the estimated final weight of the aircraft, the desired flying type, and the target motor run
duration. The spreadsheet then calculates the required power system.
5.1 Step 1 – Power Requirement
The first step is to determine how much power the aircraft will need to fly in the desired
manner. The following table illustrates typical power loadings (power to weight ratios)
for various performance requirements. Note that these are input power loadings, so
the motor efficiency must be sufficient to ensure most of this reaches the propeller.
The following loadings are based on 70% efficiency.
Power Loading
W/lb
W/kg
50
112
75
167
100
223
125
279
150
335
200
446
250
558
300
670
Flying type
Slow-speed - will barely fly
Sport flyer, non aerobatic
Mild aerobatics, slow climbing
Good aerobatics, strong climbing
3D flight, unlimited vertical
Hot aerobatics, fast vertical
F3A pattern - speed control in vertical
Ducted fan jet
In the case study the intent is for the sailplane to climb out fast, so the required power
loading is 125W – 150W/lb. 150W/lb was selected as the basis (335W/kg). Given the
aircraft is expected to weigh 700g, this means it will require 335 W/kg x 0.7kg = 235W
of installed power. The power system will be able to supply just over 200W if the
system is correctly specified. Note that the aircraft manufacturer’s specification of a
110W motor is grossly inadequate and would have given a very sedate climb – this is
why it is so important to understand electrical power system specifications!
Lawrie Henrickson
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Specification of an Electric Power System
System Specification
5.2 Step 2 – Battery Selection
The battery will be the first item to specify. The motor and speed controller will be
defined from this step. A LiPo battery was selected to minimise weight.
5.2.1 Step 2A – Voltage and Current
Equation 3 says
Power = Voltage x Current
P = VI
At this point we need to make an assumption on the battery voltage, which should
be revisited when we finish our calculations. We want to keep the current as low as
possible, to minimise losses through any system resistances (e.g. battery internal
resistance). Voltage is an easy and guaranteed way to get power. It is worth
following the aircraft manufacturer’s suggestion and select a 3S LiPo battery, with
nominal voltage 11.1V as a starting point. It will have a working voltage of 3.3V per
cell, for an expected total voltage to the motor of 9.9V. The target peak current can
now be calculated:
Given
P = VI
Then
I = P/V
= 235 / 9.9
= 23.7A
That is not a bad mix: 11.1V LiPo supplying just over 20A. A 2S battery would
have required 36A to achieve the target power, which would develop a lot more
heat and require a heavy, high-capacity battery. A 4S battery would have drawn
18A – not a lot less, so 3S is a good balance between current and number of cells.
The next step is to specify sufficient charge capacity to provide that target current.
5.2.2 Step 2B – Charge Capacity
The user needs to decide the motor run duration of each flight. Assuming a climb
takes 30 seconds, and specifying a target of 6 climbs per charge, the total motor
run time is hence 3 minutes. The section on C-rating explained that a battery
discharging at 20C will be depleted in 3 minutes, so we want a battery with at least
20C current delivery – this is readily achievable with modern LiPos without any
need for parallel cells, so a 3S1P battery of 20C rating will be sufficient.
Utilising Equation 7:
7.
C-rating = Current (in mA) / Charge Capacity (in mAh)
Simplifies to: Charge Capacity = Current (in mA) / C-rating
= 23.7 x 1000 / 20
= 1184mAh
The calculation suggests a ~1200mAh 3S1P LiPo rated at 20+C is needed to
deliver the 23.7A for 3 minutes. Looking at the various vendors this is definitely an
achievable specification. However, a 1000mAh 3S1P 20C LiPo was selected,
simply because the author already had one. A Loong-Max LiPo from Hobby City
(www.hobby-city-com) capable of a sustained 20C discharge was selected. Given
the motor would be switched off during the glides, it was possible to take advantage
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Specification of an Electric Power System
System Specification
of this battery’s lower weight than a 1200mAh LiPo, and utilise the short-term
“burst” rating of 30C for the zooms to altitude.
5.3 Step 3 – Speed Controller Specification
With the current and voltage known, it is a simple task to specify the ESC. A brushless
speed controller rated to take at least a 3S LiPo, and at least 25A is required. Note
that it is best to over-specify the speed controller to ensure it is not maxed-out – an
over-utilised speed controller will overheat, shut-down and risk a crash. For this reason
a 40A brushless ESC was selected. While it was slightly heavier than a 25A ESC, this
particular ESC had an in-built switching BEC, which would easily handle the 4 servos,
and saved the weight and space of a separate receiver battery. Very few 25A ESCs
have switching BECs.
5.4 Step 4 – Motor Selection
This starts with a check of how much room there is inside the model. In this case, not
much – a 28mm diameter motor will fit, but no larger. To keep things simple an
Out-runner motor was selected. A gearbox would only be utilised if there was no
success finding a vendor who could supply a 235+W motor of 28mm diameter.
Looking at the vendors there are lots of 28mm motors, and a KD A20-22L motor from
Hobby City met the requirements at 250W max rated power (3S motor rated for 25A
max current). The motor has a kV of 1000 rpm/V – this is nice and low, so a gearbox
will probably not be required. This will make the system simpler to install, and save
valuable weight and space inside the glider. The motor will attempt to spin the
propeller at 10,000rpm (at the ~10V operating voltage), achieving around 7000rpm in
practise depending on propeller size.
The specification is almost complete! The specification is summarised as follows:
o 1000mAh 3S1P 20C LiPo
o 40A ESC with switching BEC
o 250W 1000kV motor.
The flight equipment all weighs 350g, and the plane’s airframe weight is just over 300g,
so the 700g target weight looks achievable. After final installation the plane weighed
680g – good result! It was lucky that this all came together first pass – often the
calculations must be iterated. Sometimes the current draw is too high and does not
match flight requirements, or the battery ends up unfeasibly large. However, it usually
only takes a few iterations to get it right. The only remaining task is to find the right
sized propeller that causes the motor to draw around 20A current when spun at ~7000
rpm.
5.5 Step 5 – Propeller Selection
Unfortunately there is no easy solution or magical calculation to determine the required
propeller size. There are dedicated software programs such as Motocalc available
online for a modest cost, but they can be a bit hit-and-miss (but continue to improve
with time). There is no substitute for bench experimenting, so it is time to call on
experience and select a likely prop size. In this case, the aircraft came with a supplied
11x6 folding prop – good for a first try.
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Specification of an Electric Power System
System Specification
To enable proper testing, a current meter is essential. There are many good-quality
combination watt-meters readily available from various hobby vendors – do not even
think about trying to use a multi-meter and attempting to hold everything together with
alligator clips! A Watts-up watt-meter was utilised, that cost $50. In practise the meter
is connected between the battery and the speed controller, and gives online
measurement of battery voltage, current, and power, amongst other things. The
battery voltage monitor is added security that allows the user to ensure the battery truly
can deliver its promised 20+C, especially as the battery would be pushed beyond its
rated sustained limit.
The final set-up requires low-loss connections. For the system to work efficiently, all
sources of voltage loss must be minimised. Low-loss Deans type connectors are used
to connect the battery to the ESC. The female connector is soldered to the battery
leads to minimise the risk of accidental short-circuit when not in use. 3mm bullet
connectors connect the speed controller leads to the motor, and are taped together for
security. The battery was cycled 4 times with 900mAh discharged each time at 1A to
condition it, per the manufacturer’s recommendations.
The following picture shows the equipment installation. The receiver is sitting behind
the battery, so the installation is tight – a bigger battery probably would not have fitted!
Note the battery’s connector is temporarily covered with a water balloon (pink) to
prevent accidental short-out – the balloon is removed just prior to connection, and the
system becomes “live”. There is no switch, as very few switches could cope with 20+A
current.
LiPo battery
Female Deans connector with
[temporary] balloon insulator
Motor
ESC
Heat-sink
Male Deans connector
Cooling Air Inlet
5.6 Cooling
There have been several references thus far to the waste-heat generated by electrical
power systems. This heat is unavoidable, and while every effort is made to minimise it,
there must also be allowance for it. For the test case it was assumed that the
installation would be 70% efficient, so of the ~230W input power, 30% of this would be
converted to waste heat, or 70W – a good sized home light bulb! Experimentation is
required with every installation, complete with follow-up thermal checks after the initial
flights to confirm the heat removal is sufficient. An infrared thermometer for accurate
checking is a worthwhile investment. The author has flown an 800W aerobatic aircraft
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Specification of an Electric Power System
System Specification
in the summer heat of Saudi Arabia, and that installation not only required generous
inlets and outlets for cooling air, but the ESC was also fitted with a large heat-sink and
dedicated cooling fan.
5.6.1 Equipment Layout
Good cooling starts with a good layout. There needs to be a clear flow-path for the
cooling air to reach all heat emitting components.
The motor’s position is somewhat fixed, so the only recommendation is to ensure
that there is adequate space for air to flow into it, and around it. The test-case did
not allow much freedom in this area, but the allowances have proved to be
adequate. In-runner motors should preferably have heat-sinks fitted.
The ESC deals with considerable heat loads, especially when it contains an integral
BEC, so it is important to ensure it also receives adequate cooling. Note that in the
test-case the ESC was fitted with an aluminium heat-sink to assist with heat
rejection, which is visible in the picture. Overheating of the ESC risks total loss of
the aircraft, and possibly a fire. The picture of the test-case suggests the ESC was
simply “shoe-horned in”, but it was actually placed free of other components, and
most importantly it was not stacked on the battery.
The battery will warm-up during flight, and should also stand alone with adequate
room for airflow around it. Some packs are supplied with internal spacers between
the individual cells to assist with heat removal.
The radio system should preferably be kept totally separate from the power train to
avoid exposing it to excessive heat. In the test-case this was not possible, and the
receiver was immediately behind the battery pack.
However, appropriate
specification of the system and free flow of cooling air has minimised the risk to the
radio of exposure to excessive heat.
5.6.2 Cooling Air Inlets
Good entry of cooling air requires adequately sized air inlets. The test-case has a
pair of inlets either side of the motor, which are simply openings to let the air in –
one of the inlets can be seen in the photo above. For higher power systems with
greater on-line time (e.g. an aerobatic aircraft) the inlets would need to be larger,
and preferably feature scoops to actively draw in cooling air. Optimal inlet sizing is
generally obtained by trial-and-error, but the modeller is encouraged to look for
similar successful aircraft and examine their inlets.
5.6.3 Cooling Air Outlets
It has been generally agreed that the cooling air outlets should be larger than the
inlets, to allow for thermal expansion of the cooling air as it passes through the
equipment bay. There have been attempts to accurately calculate the required
size, but these are highly subjective, as it is often not clear how much air enters the
equipment bay, and the heat transfer coefficients are not easily determined. The
exit air temperature should not be significantly higher than the inlet temperature,
otherwise this suggests that the system is working too hard, too inefficiently, or is
not receiving sufficient cooling air flow.
Lawrie Henrickson
Page 20
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Specification of an Electric Power System
System Specification
5.7 Testing
Test 1
Propeller: Hi-Mark 11x6 folder
Current
Voltage
Power
Weight
Power Loading
Power Loading
Results
A
V
W
kg
W/lb
W/kg
18.5
9.7
180
0.680
119
265
180W is not bad – much better than the paltry 110W the manufacturer had in mind, but
at 119W/lb it is less than the 125W/lb minimum target loading. The battery voltage at
9.7V is a concern – there is not a lot left given 9.0V is the absolute minimum for a 3S
LiPo, which is to be expected given it is operating near its rated 20C (20A) limit. An
11x8 prop was sourced and the set-up retested, with the following results:
Test 2
Propeller: Graupner 11x8 folder
Current
Voltage
Power
Weight
Power Loading
Power Loading
Results
A
V
W
kg
W/lb
W/kg
23.8
9.6
228
0.680
150
335
Success! The target 150W/lb power loading was achieved, and the battery was still
OK at 9.6V despite being pushed beyond its continuous rating of 20C. Initially the
throttle channel ATV was reduced until the motor was drawing no more than 20A for
the first flight, with the knowledge that more power was available if it proved to be
inadequate. Note that this only protects the battery – not the motor or ESC, as
explained in Section 4.3.
A tachometer was used to determine the propeller was turning at 6700rpm at full
power. The resultant “slip” of the motor was hence 32% (6700rpm vs 9600rpm no-load
speed), so the motor was heavily loaded, as expected given its maximum rated power
was 250W. The performance data was entered into the following free-to-use internet
worksheet at the following address:
http://www.badcock.net/cgibin/powertrain/propconst.cgi?C_prop=163&RPM=+6700&Volts=9.60&Current=23.80&Watts=220.80&Height=+++0&Temp=20.0
The efficiency estimate results were as follows:
Prop Speed
Torque
Prop Power
Efficiency
Linear Speed
Thrust
Lawrie Henrickson
rpm
N.m
W
%
km/h
g
6700
0.188
132
58%
82
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Specification of an Electric Power System
System Specification
The site estimated the shaft torque at 0.188N.m, resulting in a propeller power of
132W, as per Equation 4. With nearly 230W input power, the resultant power train
efficiency was 58% – not super inspiring! However, every effort had been made to
minimise inefficiencies. Note that this was the overall efficiency, not just the motor
efficiency, and included losses through the connectors, the wiring, the ESC and the
motor. It is not clear whether the quoted efficiency includes losses to drag by the
propeller. If not, then the 42% inefficiency was all waste heat, equating to nearly 100W
of heat emitted inside the aircraft!
The estimated static thrust of over 1kg and the static prop-wash speed of 82km/h
suggested the aircraft should climb vertically with authority, as intended. Note that as
these are static data – as the aircraft’s airspeed increases, the thrust will decline and
so will the prop-wash speed, until the aircraft reaches terminal climb velocity where the
propeller thrust equals the airframe drag force. For this reason, the ultimate top speed
would be somewhat less than the static prop-wash speed.
Test 3 – Flying!
Flight testing was done on a relatively warm 20ºC spring day in Toronto. With nearcalm conditions, the climb angle would be all-motor, with no wind assistance2. A handlaunch at 60% throttle showed a spirited climb, but subsequent push to full throttle gave
a fast vertical zoom to around 500 feet in barely 15 seconds! Near “Hot-liner”
performance had been achieved. The target input-power rating of nearly 150W/lb
proved to be worthwhile.
Subsequent climbs to altitude proved to be equally good. As the battery warmed from
use, its current delivery improved due to improved chemical kinetics. Upon landing 45
minutes later, the system was inspected for excessive heat build-up. All components
were barely warm. The system was a success. Any component operating in excess of
50ºC should be viewed with concern, and the system adjusted.
Final Cost
Brushless electric power trains have a reputation for being expensive. The following
table shows that this is no longer the case. The LiPo battery cost just $13.50, allowing
the easily justified purchase of a second pack. Admittedly the total cost is more
expensive than a hi-start, but the benefit is simpler field set-up, and on-demand climbs
to altitude. Given the plane could achieve target height in just 15 seconds, the installed
system enabled over 9 relaunches per charge. Now factor in that you can get back to
the field if you stray too far downwind, and the cost suddenly looks appealing!
Item
Battery Pack
ESC
Motor
Prop
Total
Specification
3S1P 1200mAh 20C
25+A
230+W brushless outrunner
11x8 folder (PP30, W10, S5, R2, C3)
Equipment Costs
Selection
Loong Max 1000mAh 3S 20C
Turnigy Plush 40A
KD A22-20L 250W
MP Jet Spinner + Graupner CAM
Vendor
www.hobbycity.com
www.hobbycity.com
www.hobbycity.com
www.aircraft-world.com
Cost
US$ 13.50
US$ 34.95
US$ 16.79
US$ 17.40
US$ 82.64
2
Unlike a tethered towline (hi-start or winch) launch, a motor launch does NOT benefit from wind. The wind simply
allows a shallower true climb angle for a perceived steeper climb.
Lawrie Henrickson
Page 22
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Specification of an Electric Power System
System Specification
The completed Passer-X e-glider, following the 45 minute test flight:
Sample of Spreadsheet approach to the above specification:
Electric Aircraft Power System Specification
Aircraft
Aircraft Flying Weight
Aircraft Type
Target Power Loading
Power required
Passer-X
0.700 kg
Hotliner
3D
334.5 W/kg
234 W
Battery and ESC Requirements
Target motor run time
3 mins
Peak C-rating
20 C
3
Cells in series
Pack working voltage
9.9 V
Peak Current
23.7 A
Required Cell Capacity
1183 mAh
Equipment Specified
Specification
Selection
3S1P 1000mAh 20C
Loong Max 1000mAh 3S 20C
25+A
Turnigy Plush 40A
200+W brushless outrunner
KD A22-20L 250W
11x8 folder (PP30, W10, S5, R2, C3) MP Jet Spinner + Graupner CAM
Item
Battery Pack
ESC
Motor
Prop
Total
Vendor
www.hobbycity.com
www.hobbycity.com
www.hobbycity.com
www.aircraft-world.com
Cost
US$ 13.50
US$ 34.95
US$ 16.79
US$ 17.40
US$ 82.64
Motor Speed Calc
Effective Kv
Slip
Operating Speed
Parameter
Current
Voltage
Power
Weight
Power Loading
Power Loading
Prop Speed
Torque
Prop Power
Efficiency
Linear Speed
Thrust
Lawrie Henrickson
1000
32%
6732 rpm
Results
Units
A
V
W
kg
W/lb
W/kg
rpm
N.m
W
%
km/h
g
11x6 Himark 11x8 Graupner
18.5
23.8
9.7
9.6
180
228
0.680
0.680
119
150
265
335
6700
0.188
132
58%
82
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Specification of an Electric Power System
Record of Personal Set-ups
6.0 Record of Personal Set-ups
The following table lists 20 different electric power set-ups the author has assembled, in chronological order, and their resultant
performance. Note the difficulties experienced with the early brushed set-ups, despite good power-loadings, demonstrating the
low efficiency of brushed systems. Also note the steady improvement towards systems that met expectations without rework. It
is important to understand that the performance cannot always be determined by the power loading. Some aircraft showed very
good performance (e.g. the Cermark New Timer Mk II) despite very low power loadings – this has been attributed to high
efficiency systems, which can vary from 60% to over 90%!
Aircraft
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Vendor
Wattage
GWS
Model
F86 Sabre
Formosa
Type
Ducted Fan
Aerobatic
Cermark
New Timer
Sailplane
Simprop
Align
E-Flite
Cap 232
Aerobatic
T-Rex 450XL Chopper
Brio
Aerobatic
Aeroflyte
Raven
Park Flyer
HET
Align
Extreme Flight
E-Flite
RVM Shop
F20
T-Rex 450 V2
Vanquish
Tensor 4D
Passer-X
Ducted Fan
Chopper
Aerobatic
3D Shockflyer
Sailplane
Lawrie Henrickson
Motor Details
Mk
1
1
2
3
4
1
2
3
4
1
1
1
2
1
2
1
1
1
1
1
Weight
g
500
400
400
450
600
693
800
800
700
600
800
1040
1350
230
250
1500
800
1595
340
680
Type
Generic
Generic
Multiplex 280
Axi 2208/34
Axi 2808/16
Generic
Himark C2808
Hobby City A20-20L
Hobby City A20-20L
Axi 2808/16
Align 430L
Hacker A30-16M
HXT 35-42D
GWS 2208-18T
GWS 2208-18T
HET Typhoon 2W-20
Scorpion HK2221-8
HXT 35-48B
KDA A20-28M
KDA A22-20L
kV
rpm/V
1650
1650
1650
1100
1800
1400
1200
1150
1150
1800
3500
1050
1000
1050
1050
3500
3600
900
1050
1000
Commutation
Brushed
Brushed
Brushed
Brushless
Brushless
Brushed
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Brushless
Battery Details
Casing
Inrunner
Inrunner
Inrunner
Outrunner
Outrunner
Inrunner
Outrunner
Outrunner
Outrunner
Outrunner
Outrunner
Outrunner
Outrunner
Outrunner
Outrunner
Inrunner
Outrunner
Outrunner
Outrunner
Outrunner
Gearbox
Ratio
1
3.3
2.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Page 24
Arrangement
Type Series Para
NiCd
10S
1P
NiMH
9S
1P
NiMH
9S
1P
Lipo
3S
1P
Lipo
3S
1P
NiCd
7S
1P
Lipo
3S
1P
Lipo
3S
1P
Lipo
3S
1P
Lipo
3S
1P
Lipo
3S
1P
Lipo
3S
1P
Lipo
4S
1P
Lipo
2S
1P
Lipo
3S
1P
Lipo
5S
1P
Lipo
3S
1P
Lipo
4S
1P
Lipo
3S
1P
Lipo
3S
1P
Performance Details
CMax Voltage Current Power Loading
Charge
V
A
W
W/lb Performance
Capacity Rating Current
600 mAh
50 C
30A
10.0
25
250
224 Sedate climb
800 mAh
20 C
16A
9.0
10
90
101 Weak aerobatics
800 mAh
20 C
16A
9.0
4
36
40 Could not take-off
1500 mAh
8C
12A
9.9
8
79
79 Gentle aerobatics
2000 mAh
15 C
30A
9.9
17
168
126 Good aerobatics
600 mAh
50 C
30A
7.0
25
175
113 Sedate climb
2000 mAh
20 C
40A
9.9
15
149
83 Good climb
2000 mAh
20 C
40A
9.9
18
178
100 Strong Climb
1000 mAh
20 C
20A
9.9
20
198
127 Unlimited vertical (slow)
2000 mAh
20 C
40A
9.9
17
168
126 Good aerobatics
2000 mAh
20 C
40A
9.9
33
327
183 Strong aerobatics
2000 mAh
20 C
40A
9.9
35
347
149 Good aerobatics
2500 mAh
20 C
50A
13.2
60
792
263 Unlimited vertical (fast)
800 mAh
5C
4A
6.6
3.5
23
45 Could not take-off
600 mAh
20 C
12A
9.9
8
79
142 Unlimited vertical (slow)
3700 mAh
20 C
74A
16.5
60
990
296 [not yet flown]
2200 mAh
20 C
44A
9.9
33
327
183 Strong aerobatics
3000 mAh
20 C
60A
14.4
50
720
202 Unlimited vertical (fast)
1000 mAh
20 C
20A
9.9
12
119
157 Unlimited vertical (slow)
1000 mAh
20 C
20A
9.6
23.7
228
150 Unlimited vertical (fast)
16/06/2008
Specification of an Electric Power System
About the Author
7.0 About the Author
Lawrie Henrickson is a Process Engineer, specialising in Alumina refining. He has a
degree in Chemical Engineering from Curtin University in Western Australia. His
engineering degree required the completion of some electrical engineering units, which
helped his understanding with electrical power systems – a key requirement of the job.
The engineering calculations required for slurry pumping systems are not dissimilar from
aircraft power systems, where a motor converts electrical energy into mechanical energy,
to drive a flow. The chemistry component of the degree assisted with the understanding of
electrolytic cells.
Lawrie started aeromodelling at the age of 4, flying control-line from his back lawn, with
considerable support from his experienced father. He has dabbled in free-flight, but
elected to switch to radio control after having one too many fly-aways! He started in radio
control at age 15, obtaining his solo certificate on a full-house IC power model. He has
contest victories in Sportsman Pattern and Sports Pylon racing, but no longer flies
competitively. Lawrie’s first glider was an Aeroflyte Albatross, a 100” span floater, built
when he was 17. The success with this plane encouraged him to ensure there was always
a glider in his hangar.
He branched out into electric power in 2003, with a ducted-fan F86 Sabre. He
subsequently converted a GWS Formosa from brushed power to brushless, chasing
additional performance. He has since specified 20 different electrical power systems for
his own aircraft, and equally as many for other people. He continues to develop
calculation tools to assist in the successful specification of these systems, to minimise the
guess-work and improve the chance of success.
Lawrie Henrickson
Page 25
16/06/2008
Specification of an Electric Power System
Revisions
Document Revision History
Rev #
Description
Status
Date
Prepared By
Rev 0
First draft
Issued for Internal
Review
April
2008
LMH
Rev 1
Revised
comments
brushed commutation
May 2008
LMH
Rev 2
Added equations 5 and 6 to Issued
for
Section
3.1.
Added Publication on the
Equations 7 and 8. Expanded SOGGI website
on the impact of these
equations. Added Section 5.6
– Cooling. Added efficiency
data to Section 5.7 – Testing.
Added Section 7 – About the
Author.
Intensive general
editing.
June 1,
2008
LMH
Rev 2.1
Updated Section 4.2.1 on Issued
for
commutation
to
explain Publication on the
outrunners do not have any SOGGI website
commutation.
June 15,
2008
LMH
Lawrie Henrickson
on Issued for Internal
Review
Page 26
16/06/2008

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