Electric R/C Flight
Part 2: Motors
Another month has past and now we start on the interesting stuff, the different types of electric motors. There are two main types of motors used in electric flight, brushed and brushless. In this months column I will discuss brushed motors and our first "rule of thumb" for choosing a power plant.
As the name implies these types of motors use "brushes" to feed the DC current into the armature windings via the commutator. Most brushes are made of a carbon compound and will eventually wear away due to friction with the commutator. The size and style of the commutator and brushes can determine the current handling ability of the motor. Cheaper motors with totally enclosed wiper style brushes cannot take very high currents as they will build up too much heat and have higher resistance losses due to their smaller size.
There are three main types of magnets used in electric motors, the most common being ferrite. Ferrite magnets are cheap to produce and can also withstand fairly high temperatures (~200C) before they loose their magnetism.
Next up the price and performance line are Neodym magnets. Neodym has a much stronger magnetic field than ferrite, which means more power can be got out of a motor before the magnets reach their saturation level. Neodym is more costly than ferrite and cannot withstand temperatures in excess of about 130C.
Samarium Cobalt is the last type of magnet commonly used. Cobalt has a similar strength to Neodym but usually commands a higher price. Its advantage is its ability to handle temperatures of over 300C (Of course the rest of your motor may be melting at this point)
This is the name given to a range of motors which are mass produced, usually by either Mabuchi or Johnson. These DC brushed motors are made for appliances, power tools and the auto industry to name a few. Several of these have proven useable for electric flight and have taken on new naming conventions started by the German company Graupner.
Nearly all of these motors have ferrite magnets, fully enclosed endbells, non replaceable brushes, fixed timing, and plain bronze bushes. A few models do have ball bearing supported shafts and are usually designated ‘BB’ . Some of these models are produced in the millions which is why we can buy them for as little as $20 each. The efficiency of these motors ranges from about 55% - 65%.
This describes a range of ferrite brushed motors that have been designed and made for the R/C car market. A few of these are suitable for electric flight use. These motors cost a bit more but have several design features that result in better performance and longer life.
The first feature is an exposed endbell with replaceable brushes. This allows better cooling. Another feature common to "Car" motors (as with this one pictured) is adjustable timing. This means the timing of the motor can be optimised for either forward or reverse rotation to suit the application.
Many of these Car motors are too "Hot" for our purposes, meaning they have very low numbers of turns, giving very high RPM’s with a corresponding high current draw. Some of these motors if used correctly can achieve efficiencies of around 70%.
The following motors are examples of car motors used in electric flight with excellent results.
Kyosho Mad Science series (Magnetic Mayhem, Atomic force, Endoplasma)
Trinity Speed Gems (Ruby, Sapphire, Quartz)
This describes all motors that are designed with electric flight at the outset. Most of these motors use either Neodym or Cobalt magnets, are ball raced and feature replaceable brushes.
These motors may achieve efficiencies of between 75 – 85%
Rules of Thumb #1
Our first rule comes from Bob Kopski & Keith Shaw
Note: Watts = Volts * Amps, All rules assume average motor efficiency of 70%. For a twin engine plane you can subtract about 10%.
Example: Great planes ElectriCub
All up Weight (with 7 cells) ~3lb
Power in: 8.4v * 20A = 168W
Watts per pound = 56
"Electric’s don’t have enough duration"
Most electric models have a duration of between 5 & 10 minutes. Although most glow powered planes can run for longer than this, the average flight time is about 7 – 8 minutes.