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SIMPLE ELECTRICS

By Stan Yeo

Definition of Terms
Ohms Law
Power (watts)
Selecting the Battery, Speed Controller and Motor
Brushless Motors
Electronic Speed Controller (ESC)
Electric Flight Packs
Converting IC Model to Electric
Useful Tools
Noise / Interference Suppression

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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