Long before RC electric motors appeared on the scene. RC kit plane building’ was often though of as a winter project. These affairs were often a heavier, sturdier and more robust construction that needed the extra torque of a gas powered model airplane engine to handle the larger propeller.
RC electric motors became more powerful, as we will discover. The other key element, often overlooked, in the equation was the humble battery. Up until then, rechargeable Nickle-Cadmium (NiCad) batteries sucked, big time!
It wasn’t long before batteries with more and more capacity were rolling off the production lines. Jam packed, with exotic metals such as Lithium, Lithium-ion (LiOn), Nickle metal hydride (NiMh) and finally Lithium-ion polymer (Lipo).
We have the makers of cell phones and laptops to thank for driving the demand for a reliable battery. Armed and empowered with greater and longer lasting power, the humble battery finally came into its own.
Now that the world had a rechargeable battery it could rely on, all it needed was an outlet (no pun intended). RC electric motors coupled with longer lasting batteries, paved the way for a newer lightweight class of electric RC planes.
Fuselage and wing components made from EPO & EPP foam were much lighter. Surprisingly strong, they are more crash resistant than wood. Small dents and dings could be smoothed out and easily repaired. More importantly, it meant that RC electric motors could easily keep these new designs flying.
Harnessed to mass productions techniques, electric RC planes became more affordable and made ideal Christmas and birthday presents and helped spawn a new generation of park flyers.
So much for the history lesson. So, how do these RC electric motors actually work?
RC electric motors – The basics
When we place the north pole of one magnet close to the south pole of another, they stick together (opposites attract). If you place the same poles of the magnets (north to north or south to south) together, they push apart (repel). This principle of magnetic attraction and repelling is what makes an electric motor work.
If we pass an electrical current through a coil of copper wire, it generates a magnetic field. The copper wire coil inside an electric motor is referred to as windings or the armature.
The position of the magnetic poles depends on the direction the current is flowing. The magnetic field alternates between positive and negative. As the rotating core, in the RC electric motors, align with the ‘actual magnets’ opposite polarity. The magnetic fields now repel each other and the core rotates to the next alignment position where the polarity again changes. The changes in magnetic polarity are caused by changing the direction of the current’s flow through the armature (copper wire windings).
Metal plates (brushes) inside the electric motor rub (or brush) against the rotating core. Between the shaft and the brushes is a ‘commutator’. The brushes provide current to the commutator which, causes the current flowing through the windings to reverse as the brushes contact it. How much current, determines how fast your propeller spins.
These RC electric motors have few moving parts. However, the brushes rubbing against the commutator cause friction. This is not energy efficient because it will lead to the motor losing power as the brushes wear down. This is why brushed motors are only 75 – 85% energy efficient.
Where the older ‘brushed’ motors changed the magnetic polarity (and the current flow) of the armature using a commutator. Brushless motors have a different approach. The motor’s rotation is still achieved by changing the polarity of the winding, as the magnet approaches.
Brushless motors are basically built inside out compared to a brushed motor. The permanent magnets are now positioned on the rotor. The electromagnets are attached to the stator. The electromagnets in the stator are electronically controlled as they rotate through 360 degrees. Brushless motors are found to between 85 – 90% efficient. They also tend to be much quieter in operation.
Brushless motor types
There are two types of brushless motors, ‘Inrunner’ and ‘Outrunner’.
They both have copper wire windings wrapped around a component called a stator. The windings generate the magnetic field. Inrunners and outrunners both have rotors which include the magnets that interact with the windings magnetic field.
Inrunner motors have their copper wire windings on the outside with a motor shaft containing the magnets running through the center of the motor. These RC electric motors have a high KV output (turns faster) but lower torque value.
The heat that is generated through running is easily dissipated through the outer casing because the windings are just underneath. The inrunner motor tends to be physically longer in length and narrow in diameter. Inrunner motors are more efficient than outrunner motors.
The outrunner motor has the stators and the windings in the center of the motor and the magnets are attached to a housing that spins on the outside. The shaft passes through the center of the motor allowing the housing and shaft to rotate together.
Outrunner has a lower KV output (turns slower) but a much higher torque value. Because the outrunner has more copper windings it can generate more heat. These RC electric motors are easily identified by the large air vents front and back that allow cooling air to pass through the motor. The outrunner motor is also physically larger in diameter.
It’s important that we correctly match the KV output to the application. Because we could end up burning out the speed controller, the battery or even the motor itself. When we are selecting RC electric motors for a particular application, the KV output value is usually what most people are interested in.
An outrunner motor is perfect for an RC training plane where we want low speed handling and a large propeller (that needs more torque). Outrunner motors are also used extensively in RC helicopters and drones. Again the correct application comes into play. Low KV, with high torque. Means those larger drone propellers and helicopter rotors can bite through big chunks of air to get the lift they need. RC jet models would benefit from a high speed inrunner motor. Sleek racing RC planes would be an ideal application for an inrunner RC electric motor.
An RC electric motors power output is referred to as its KV rating. The KV rating determined by the number of copper wire windings (or turns) it has.
More turns equates to lower KV values and therefore, fewer revolutions of the propeller. On the upside you get more torque. Fewer turns means a higher KV value and more propeller revolutions but much less torque.
Larger electric RC planes, helicopters and drones need electric motors with more windings because they are driving bigger, slower turning propellers that need more torque behind them. As mentioned, they need the torque to handle the larger air mass the prop is chomping into. If a motor with less windings (high KV) was used there is a risk of damage as the effort to turn the larger propeller would draw too many amps and damage the motor.
To ensure a constant torque rating the motor windings are placed in groups of three which are called triplets. This will allow the polarity to change causing smooth torque production. Brushless motors usually have several sets of triplets to ensure smooth, consistent operation.
Manufacturers labels – Method 1
There are two methods of how brushless motors are labelled by their manufacturer.
Method one, a four digit number is used. The first two digits are the diameter of the stator. The second two digits are the length of the stator. After which the manufacturer will add the KV rating.
If you know the stators width and length it will give you a general idea of the RC electric motors capacity. As a rule of thumb, the larger the numbers the higher the capacity.
Manufacturers labels – Method 2
The second method is becoming more common. In this system the label makes a comparison to an established motor or engine. You will find small brushless electric motors with a label displaying Power 480 or Park 450 or the model number that represents an equivalent size of a small brushed motor. On the larger motors you will find the equivalent glow engine displacement next to the manufacturer’s identifier, this allows you make a direct comparison.
Speed controller (ESC)
Inrunner and outrunner electric motors have an electronic speed controller (ESC). The ESC controls the motor’s movement and speed by using a metal-oxide-semiconductor field-effect transistor, better known as a MOSFET. This sets up the rotating magnetic field which enables the motor’s rotation. In layman’s terms, the ESC detects where the motor is in the rotation cycle and changes the polarity of windings to keep the propeller turning.
The electronic speed controller (ESC) usually has three connecting wires which attach to the motor to power the windings. If you are buying components separately, make sure they are compatible. Not every ESC will work with every electric motor. It’s a good idea to do some tests before you go flying, use a ‘current limiter’ to prevent the ESC and motor from overheating and getting fried.
In recent years, outrunner motors are becoming more popular than inrunners because of their range of sizes, power outputs and applications. Adopt the mantra, ‘Check the spec’ before buying.
Electric motors might look similar but will have different power outputs and weight. As well as limits on what it can handle. After reading this, you will have a clearer idea on what to look for and how to identify them.
The editorial team