RC Electric Motors
Brushless rc electric motors have virtually superceded the previous brushed types so far as model plane propulsion systems are concerned. It is still possible to buy the brushed types but, because brushless motors are so much more efficient than brushed ones, they are few and far between.
For our purpose we will concentrate on brushless rc electric motors as our choice for powering your trainer plane.
There are two types of brushless motor available to us.
You need to understand the way in which a brushless electric motor works to appreciate which is most suitable for your plane. First of all you need to know that brushless motors use Three Phases of electrical alternating supply (AC) unlike brushed types which are single phase motors and use simple positive (+) and negative ( – ) DC supply.
Let us look first of all at the “Outrunner Configuration”.
The above diagram shows the way in which the three electrical phases are wired to the “Stator” or None Moving part of the motor. This is the central core on which our diagram shows 9 poles with the three phases wound around 3 sets of poles in turn.
Phase A is wound around poles 1, 4 & 7.
Phase B is wound around poles 2, 5 & 8.
Phase C is wound around poles 3, 6 & 9.
We use a special piece of electronics called an “Electronic Speed Controller” ( ESC ) to cause each phase to receive power in rotation.
Inside the outer casing are an array of magnets set with alternating North and South poles. This is our “Rotor” or moving part of the motor. The shaft to which we mount our propeller is fixed to this rotating case at one end whilst the other end runs through the centre of the fixed core carrying the windings in either a ball or roller bearing housed in the opposite end of this fixed Stator.
In this picture you can see the fixed shaft in the centre of the casing on the right and the three phases of wiring connected to the Iron stator on the left. The washers and split ring are used to hold the shaft in place once the motor is assembled.
The “Inrunner Motor” is arranged in the opposite way. The Outer casing is our Stator and remains still whilst the Inner core is our Rotor and spins around with the shaft. To enable this to happen our three phases must be wound around poles fixed to the outer casing and the magnets are fixed to the inner Rotor.
Generally speaking the Outrunner is most suitable for our purpose as it produces more Torque (rotational force) and, because there is more room in the outer casing for larger numbers of magnets, the speed of rotation can be varied more easily.. This enables us to use propellers with larger diameters. Inrunners are generally faster revving and are used for applications where higher speeds are required, e.g. Electric Ducted Fans (EDFs) for model jets.
Now we need to decide on the size and power rating of our electric motor. The trainer planes we are considering require approximately 500 to 600 Watts of maximum power for the appropriate flight characteristics.
Power (Watts-W) is calculated by multiplying the Voltage (Volts-V) of our supply battery by the amount of Current (Amps-A) consumed by the motor. (W = V x A).
For our size of plane the battery will normally be of the Lithium Polymer (Lipo) variety. Each cell of this type of battery carries 3.7 volts(V) of electrical energy. If we connect 3 of these cells in series (+ to -) as in the diagram below, the total voltage of the combined cells will be
3.7 + 3.7 + 3.7 = 11.1Volts (V)
Ignore the centre connector for the moment. The only connections we are interested in at this stage are the thick Red and Black ones.
If we want more voltage an extra cell can be added by connecting the Black “- Out” lead to the + terminal of the new cell and taking the “- Out” from the negative “-” lead of new cell. This new cell now becomes Cell#1 and the top cell becomes Cell#4.
By adding this new cell the overall voltage of our battery becomes
3.7 + 3.7 + 3.7 + 3.7 = 14.8Volts (V).
From our Power formula (W=VxA) if we know that we need 600Watts of power and we select a 3 cell battery giving 11.1Volts (V) then we can calculate the amount of current needed to produce the 600Watts.
Amps (A) = W/V (Watts divided by Volts) = 600/11.1 = 54 Amps (A)
Now, if we use a 4 cell battery instead our formula will give us:-
Amps (A) = W/V (Watts divided by Volts) = 600/14.8 = 40.5 Amps (A)
A larger battery does not need to deliver as much current to produce an equal amount of power. So how does this help us? The amount of current drawn depends on the load put on the motor and battery by the propeller used. Larger diameter propellers and coarser pitched propellers increase the amount of current needed to swing them.
Its interesting to note that a three cell battery will swing a larger propeller than a four cell one for the same current demand. This is because the speed of rotation of the motor is proportional to the voltage applied. More voltage means faster rotation requiring more power to overcome the increased load.
Fortunately most motor manufacturers will provide guidelines as to the best propeller selection for their motors along with maximum ratings for current and voltage. this means that providing we keep within these guidelines we should not risk overheating the motor or any other component in our power train. Whenever power is consumed heat is generated as a result of electrical resistance within all of the components in our system.
Electric motor power trains are not unique in this respect. Consider the heat generated in a glow motor as a result of both burning fuel and friction between the moving parts. All of this heat, whether it be electrically generated or fuel generated is power loss and effects the overall efficiency of the system.
Choosing a Motor
Having provided all this fairly technical information, I don’t want you to feel daunted by it. As with most components used by modellers, assumptions are made that most of us are not engineering whizz kids and that we require help and advise on choosing the most appropriate components for our needs. Brushless Motors are no exception. Manufacturers and stockists will have all the information needed to enable the purchaser to select the best option.
Let us look at a range of brushless motors suitable for our chosen plane. There are many manufacturers producing motors ranging from high end state of the art products to the cheaper products from Far Eastern sources. Again the choice is up to the individual and no doubt if you are looking to fit one of the upper price range motors, be aware that they will fail just as readily as a cheaper option if badly abused.
I have been using brushless outrunners now for over seven years and I have yet to have one fail through abuse. Crash damage…..YES! but abuse….NO! My philosophy is that if I choose a motor capable of providing a good margin of power over and above the base requirement of the plane, I will never have to drive it to its maximum and damage to its working parts through overload and associated heat will never be a problem. As a result, I have never been tempted to purchase top range expensive motors for my projects. Economy motors perform equally as well if treated with respect.
OK, enough of my preaching, lets look at some suitable motors. From personal experience I have found that the ideal motor size for this type of model has external measurements of 35mm case diameter and a length between front and back of the casing of 48mm. There are a number of motors in this size range.
When you look at brushless motor spec. you will see a figure quoted for the Kv rating. This is very simply the number of revolutions the motor will rotate at without load for every volt applied to it. For example, a motor rated at 900Kv will theoretically rotate at 900 x 11.1 = 9990RPM under no load conditions using a three cell (11.1Volt) lipo battery.
The reason I elected to use a 900Kv figure for this example is because this is an appropriate Kv value for our plane. You could use a motor with a slightly lower Kv value but for most trainers, 900Kv gives good acceleration and forward thrust under most circumstances. It also suits a 12 x 8 or 13 x 6 electric propeller.
Please don’t be tempted to fit a standard glow engine propeller to your brushless motor. They are to heavy and imbalance can cause excessive vibration. E-props have been specially designed to produce more thrust using a thinner airfoil section. Unlike glow motors, brushless motors do not cause vibration. There are no heavy metal components being forced up and down inside cylinders. Rotation is very smooth and the only cause of vibration will be an out of balance propeller or spinner.
Here then are just two suggestions for suitable rc electric motors for your trainer that I have used personally.