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SIMPLE
ELECTRICS
By
Stan Yeo
For
a number of modellers electrics is a black art that induces a mental
block when the word is mentioned. This is unfortunate as the basics
are simple and easy to understand if presented in a digestible form.
This is the challenge of this article but before starting an apology
to the purists as some of the terminology may not be academically
pure.
Definition
of Terms (Back
to top)
Volts
(V) This is a measurement of electrical pressure the equivalent
of air pressure in a car tyre.
Amps
(I or A) This is a measure of the amount of electricity (current)
flowing in a circuit. In non electrical terms gallons or litres
per minute.
Resistance
(R or Ohms) This is a measure of electrical resistance measured
in Ohms. This is equivalent to drag on a model or the force required
to push/pull an object on a flat surface.
Impedance
(Ohms) This is a measure of resistance in an alternating current(
AC) circuit.
Watts
(W) This is a measure of electrical power, i.e. the equivalent of
the horse power developed by an engine. 748 watts = 1 horsepower.
The metric equivalent of horsepower is PS or Pferdestarke. 1HP =
1.0139 PS. For the purposes of this article 1 PS = 1 HP = 750 watts.
Ohms
Law (Back
to top)
The
basics of Ohms law is that if you increase the voltage (pressure)
then more electricity (amps) will flow in the circuit. A practical
example of this is if you are watering the garden and turn the tap
on more i.e. increase the pressure more water flows out the end
of the hose. A practical modelling example is using a 5 cell (6v)
receiver battery pack instead of the usual 4 cell (4.8v) pack will
not only increase servo torque but increase current consumption
by 25%, which is why a larger capacity battery should always be
used.
Ohms
law is very easy to use. Just draw a triangle, divide it horizontally
in two then divide the bottom half vertically in two. In the top
half write 'V'. In the bottom half write 'I' and 'R'.
If
you know two of the three variables block out the third and what
is left is the formula for the unknown variable i.e. R = V/I, V
= I x R and I = V / R
Power
(watts) (Back
to top)
The
formula for power is even simpler it is just
Watts
= Volts x Amps.
Selecting
the Battery, Speed Controller and Motor
(Back to top)
Selecting
the right battery, speed controller (ESC) and motor combination
are very critical when compared to choosing the equivalent IC motor.
There are a number of factors to be taken into account which have
a big impact on performance i.e. propeller, motor windings, ESC
rating and battery type / capacity / current rating. Each will be
discussed in turn but before we start there are two basic rules
for the novice electric flyer. One, if you know the current required
i.e. current being drawn by the motor, multiply it by 1.5 when selecting
the battery / ESC. Secondly if you know the current rating of the
ESC / battery divide by 1.5 to select a safe operating current.
Brushless
Motors (Back
to top)
Currently
there is very little in common in the way brushless motors are classified
but most manufacturers supply three important pieces of information.
These are RPM per volt (kV), maximum output power in watts and maximum
current draw. All three should be used in selecting the optimum
propeller. The RPM per volt (kV) is useful in determining the NO
load motor RPM, the actual motor RPM acheived will vary with propellor
loading. Just multiply the kV by the battery voltage under load
to achieve the motor revs per minute (RPM). Unless the motor is
to power a ducted fan model or drive an electric helicopter avoid
high kV motors. A typical kV range for non-ducted fan, fixed wing
models is 800 to 1200. Unlike IC motors, the efficiency / power
output graph of an electric motor is very narrow. To small a propeller
= less thrust than the motor is capable of. Too large a propeller
= less thrust and increased current draw / shorter flight times
with the added danger of damaging the motor, ESC and battery. An
indication of how hard the 'electrics' are working is how hot they
get, assuming they are adequately ventilated. They should get warm
but not hot. This means choosing the right propeller is critical
for optimum performance and may require trying a selection of propellers
before finding the one most suitable for that particular model.
If
you increase the battery voltage i.e. cell count and the motor is
operating at maximum recommended power then the propeller must
be changed for one with less pitch and or reduced diameter. Remember
power (watts) is the multiple of Voltage x Amps so increasing the
voltage means that you must reduce the current (Amps)
to stay within the maximum permitted power (Watts) to avoid damaging
the motor. One
reason why modellers increase battery voltage is to reduce current
consumption to either increase flight times or overcome a battery's
inability to deliver the current required to produce maximum power.
Very occasionally motor information is available showing thrust
per watt for a range of battery / propeller combinations to assist
in choosing the right one.
To
assist in choosing a suitable propellor we have produced a propellor
loading chart for a wide range of pitches and diameters. Use
the propellor recommended for initial flights. Obtain the loading
for that propellor then select a suitable propellor to either increase
or reduce the load as required.
A final
point for consideration when selecting a motor is the that maximum
power rating (watts) is usually only attainable on the maximum battery
voltage i.e. max cell count due to the current considerations mentioned
above. This means that when selecting your motor you must multiply
the max. current rating of the motor by the voltage of the battery
you intend using to determine the power it will produce with the
correct propellor. As an example, if the max. power of a motor is
500w, max current is 40A and max voltage is 14v (4 cell LiPo) it
can only produce 500w at 14.8v. On a 3 cell LiPo pack (11.1v) it
will only produce 444w due to the current limit of 40A (11.1 x 40).
On a 2 cell pack (7.4v) it will on be 296w (7.4 x 40).
Electronic
Speed Controller (ESC) (Back
to top)
There
are two types of speed controller, one for brushless motors and
one for brushed motors. The brushed ESC has two wires to connect
to the motor whilst the brushless speed controller has three. The
reason for this is that for the motor to work electrical power to
the motor windings have to be switched on and off in sequence. In
the brushed motor this is done mechanically by the commutater and
the brushes. With a brushless motor the switching is done electronically
by the speed controller hence the need for three wires. As an aside
if a brushless motor does not work or runs in reverse when switched
on try swopping the position of the three wires before panicking!
Most ESCs have a BEC (Battery Eliminator Circuit) to power the receiver
and servos. Often there are restrictions on the use of the BEC i.e.
the number of servos it can drive and the battery voltage (number
of cells) it can be used with. If the BEC current is not enough
to drive the number of servos fitted in the model then we recommend
the use of a UBEC or using a separate receiver battery. The UBEC
reduces the flight pack battery voltage to 5v and is connected directly
to the battery.
If
using more than one speed controller i.e. in a multi motor setup
then the positive lead on all but one speed controller must be disconnected
to prevent them 'fighting' each other to supply the Rx with electrical
power. The same applies if using a UBEC or separate Rx battery.
Also, if possible, to minimise motor to motor interference on a
multi motor setup it is advisable not to use a 'Y' lead to link
two ESCs together but use a spare channel and link the channels
using a mixer in the transmitter.
As
well as controlling the speed of the motor ESCs are also responsible
for switching the motor off when the battery voltage is low. This
is to prevent the battery being discharged below a safe level (particularly
important when using LiPo batteries) and to reserve sufficient energy
in the battery to allow the model to be landed safely. To do this
there are two types of motor cutoff systems, one that works on a
percentage of the battery start voltage and one the that cuts the
motor off at a preset voltage. The latter is the better system as
with the percentage cutoff system you must always start with a fully
charged battery, particularly if using LiPos, otherwise there is
a danger of discharging the battery below safe limits and ruining
the battery / crashing the model due to radio failure. Most current
ESCs either allow you to preset the cutoff voltage or automatically
determine the number of cells present and set the cutoff voltage
accordingly. Read the instructions!
When
selecting the ESC use the 1.5 rule to determine the current rating
i.e. if the max current draw on the motor is 30 Amps use a 45 Amp
ESC. The harder the ESC works the less efficient it is.
Electric
Flight Packs (Back
to top)
There
are basically two types of electric flight packs on offer currently
although more are under development. These are the now dated nickel
cadmium / nickel metal hydride batteries and the class commonly
known as Lithium Polymer (LiPos). LiPos. These are approximately
40% lighter than Nicad / NimH batteries and require careful handling.
LiPo battery packs from reputable sources normally come with an
extensive list of does and don'ts and I strongly recommend that
these are read. The important points to remember are:
1.
Always charge the batteries on a concrete floor or large ceramic
tile. Avoid charging if possible inside domestic accommodation,
charge in the garage or outdoors. If something does go wrong during
the charge and the batteries self ignite they can easily cause
a major fire.
2.
Regularly check the batteries when on charge. A typical from flat
charge time is approximately 1.5 hours.
3.
Always charge through a dedicated LiPo charger and cell balancer.
Balance charging is where each cell is charged individually. Failure
to regularly balance the cells can result in overcharging some
cells and over discharging others. In either case the battery
will be irreparably damaged.
4.
Regularly check the battery for damage and puffing out. Any signs
of either discard the battery.
4.
DO NOT charge LiPo batteries from the car battery with the engine
running. Electrical spikes from the alternator, particularly when
starting the engine, can damage the charger or at the very least
befuddle the micro processor in the charger resulting in the charger
malfuctioning and damaging the battery.
As
with the speed controller select the battery capacity using the
1.5 rule i.e. if the max motor current is 30A then select a battery
capable of delivering a constant current of 45A. To determine the
max. constant current draw of a LiPo battery, multiply the battery
capacity by the battery's 'C' rating. If a recommended max. current
draw is printed on the battery label then use this. To determine
the battery capacity simply divide the battery max. current by the
max. motor current. Most
current LiPos have a 'C' rating of 20C. In the example above the
minimum battery capacity would be 45A (30Ax1.5) / 20C = 2250mAhr.
The reason for using the 1.5 rule with LiPos is that there is a
direct relationship between current draw and the number of battery
life cycles. The higher the current draw the lower the number of
battery life cycles. If the battery does not have a 'C' rating printed
on the label as is the case with some direct imports please make
an effort the find the information. If it is not available, for
safety reasons, assume a value of 50% of the current norm i.e. 10C.
It could be old stock!
One
final point, as a general rule the higher the 'C' rating of a battery
for a given capacity the longer the flight time due to the battery's
better performance under load. Remember the higher the load the
lower the battery voltage, the sooner the speed controller cuts
the motor. The battery could only be half discharged but if the
battery voltage under load drops below the cut-off voltage then
the ESC will cut the motor.
Nicad
and NiMh batteries should have a recommended maximum discharge current
on the label. Again select the battery using the 1.5 rule. The problem
with these batteries is that the higher the discharge current then
the greater the internal voltage drop due to the internal resistance
of the battery. In other words the harder the battery is driven
the less efficient it is.
Converting
IC Model to Electric (Back
to top)
The
key to converting an IC model to electric flight is knowing the
power the I.C. engine is developing in flight. This is best done
measuring the RPM of the motor at full throttle and reading the
power output off the RPM power curve for the recorded RPM. This
not easy as the RPM Power output curves are not always readily available.
If this is the case then an educated guess must be made based on
the published maximum power output of the engine. This, in reality,
is a hypothetical figure as the RPM at which it is produced is unrealistically
high necessitating the use of a very small propeller that would
make a lot of noise! In reality the power the engine is developing
is probably only 60% of its max. quoted power. If we take a 0.25cu
in engine with a max power output of 0.66HP then the equivalent
electrical power produced would be 0.66 x 0.6 = 0.396hp i.e. 296
watts (0.396 x 748). Using an 8 cell 9.6 volt sub C battery back
this would equate to a maximum current of 296 / 9.6 = 30.8 amps.
This of course is subject to an optimum propeller / motor combination
as previously discussed.
We
have recently converted one of our EPP Peppi Trainers to electric.
Originally it was fitted with and OS 25FP and climbed at about 60
- 70 degrees under full power. It is now fitted with a Twister 09,
which is equivalent to a 0.25cu in. IC motor, and a Tornado 40 amp
ESC. The climb under full power is similar. For initial flights
an 11 x 6in propeller was fitted but the speed controller got very
hot (it melted the heatshrink) so we fitted a 10 x 6 in propeller
instead. With the 10 x 6 propeller the ESC and battery just got
comfortably warm, flight times increased by more than 50% and there
was a marginal increase in performance, thus emphasising the need
for good propeller / motor selection. Dividing the battery capacity
by estimated full power duration time (the model was not flown on
full power all the time) would suggest a maximum current draw of
between 30 and 35 amps which is conveniently in line with our rough
calculations. They have also been tested on other models and with
similar results.
Useful
Tools (Back
to top)
The
two most useful readily available tools for electric flyers are
a Tachometer and a Watt Meter. The tachometer is used to measure
propeller RPM allowing it to be compared with the expected RPM achieved
by multiplying the motor kV (revs per volt) by battery voltage whilst
the watt meter measures the power being consumed. A third useful
piece of equipment is some means of measuring the thrust produced.
This could be a sophisticated rig on which the motor is mounted
with a high capacity battery used as a stable power supply or something
as simple as a tethered spring balance attached to rear of the model.
Once the thrust is known this can be used to calculate the thrust
per watt for a given propeller. The thrust is measured using a variety
of propellers to determine the one with the best thrust per watt
ratio for that model.
Noise
/ Interference Suppression (Back
to top)
All
electric motors and electronic switching devices generate electrical
noise, some more than others. In radio control systems this often
results 'glitching' i.e. un-commanded servo movement.
To
minimise this risk there are a number of precautions that can be
taken.
1.
Keep the leads from the speed controller (ESC) to the motor to
less than 100mm (4 inches).
2.
If using a brushed motor fit additional noise suppression capacitors.
There should be three, one from each terminal to earth (case)
and a third across the two terminals. The largest value capacitor
is the one fitted across the two terminals.
3.
The lead from the ESC to the receiver should be fitted with a
torrodial or noise suppression choke. The lead, with the plastic
end, removed is wound around the choke at least 4 times close
to the receiver end. The more turns the better the noise suppression.
4.
Is using a UBEC instead of the ESC onboard BEC (battery eliminator
circuit) fit noise suppression chokes as above and carry out a
range check. Some makes have been known to significantly reduce
range without a choke being fitted.
5.
Where possible install the receiver as far away as practical from
the motor / ESC.
6.
If you are experiencing 'glitching' in flight then we recommend
changing the receiver to one of a higher specification. If single
conversion try a dual conversion one. If dual conversion try either
a PCM Rx or an Rx with IPD. Both have built in signal verification
systems i.e. they check the information the Rx has received has
not been corrupted before it is passed on to the servos.
7.
If 6 fails then try a different make speed controller or try one
with an higher current rating.
8.
If 7 fails then try a separate Rx battery but remeber to disable
the BEC by to removing the positive lead from the ESC.
9.
Finally if after trying all the above you still have problems
try changing the battery, possibly for one of a higher capacity,
and by a process of elimination try and identify the noisy component.
Summary
(Back to top)
I
hope you have found this article useful and informative. I have
erred on the safe side, kept it as simple as possible and made
a number of assumptions but if you understand the basics then
it should be easier to put the information to practical use.
Stan
Yeo, June 2007
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