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Powertrains 101: An Advanced Guide To RC Powertrains

Article by Kenny McCormick

RC vehicles are powered by a myriad of methods. Gravity, wind, updrafts, steam, internal combustion engines, electricity, you name it there's probably an RC that uses it. The most commonly used methods, however, are electricity and internal combustion. Therefore I feel it'd be a good idea to explain how these methods work in detail. This article will cover electric power, both brushed and brushless. Powertrains 102 will cover basic ICE theory as well as how the various ignition systems used in RC work, Powertrains 103 will cover 2-cycle glow engines, and Powertrains 104 will cover 4-cycles.

Let us begin by how we get our electric power in the first place. Batteries are very simple little devices. They use a chemical reaction to produce electricity, and in rechargeable batteries, this reaction is reversible. What we refer to when we say "NiCD" or "Li-PO" is the chemistry used. NiCD and NiMH use nickel as the primary reagent, whereas LiPOs use lithium. Different chemistires have different efficiencies and characteristics, which is why they're used in different applications. LiPO is probably the best available for powering electric RCs. Batteries ALWAYS produce DC, they cannot produce AC without an additional device put in between the battery's cells and the connector.

Ok, so we have electricity. But how do we get it to the motor in a controllable manner? The speed controller is tasked with metering out the power from the battery. Some of it is fed off to the receiver and steering servo, the rest is fed to the drive motor. Mechanical ones are just variable resistors, electric ones use quite a bit of complex circuitry to pulse current towards the electric rc motor. ESCs are just better in every way, they can handle more current and don't waste anywhere near as much of it as heat. They're also more reliable and less vulnerable to dust and dirt.

lipo battery

Now, onto the motor itself. In RC we use two types, brushed permanent magnet and brushless permanent magnet. Brushed motors are about as old as electricity itself, and are incredibly simple, so we'll go after them first.

Take any length of wire and run electricity through it. A magnetic field is formed around this wire. If this wire is near a magnet it will attempt to move around until the magnetic field of the wire and the one from the magnet have aligned. If this wire is wrapped around something that's allowed to spin it will rotate whatever it's attached to. Now, the wire will stop once it has aligned with the field of the magnet, so if we want to continue that rotation we cut power from that wire and apply power to the one behind it instead. Do this repeatedly and we have rotation! The commutator, located at the back of the armature, handles this task. Each wire, or winding as it's found inside motors, is attached to two points on the comm. These points send power through it only when the wire is in a suitable position to rotate the armature. Breaks between adjacent contacts ensure the current is shut off when appropriate.

This type of motor loses a lot of power through the brushes, both immediately and potentially. The brushes introduce a point of resistance that limits how much current the motor can draw. Additionally, if you exceed this limit anyway, you can burn the brushes up which will result in motor failure and necessitate a rebuild. The brushless motor aims to do away with these brushes. Advantages include: they cost next to nothing to make, speed controllers cost next to nothing to make, and they will accept any DC source within their voltage and amperage range. You can hook them straight to a battery and they will work fine. In RC they're being phased out, as even indoor stuff down to 1:36 scale can use brushless power, but you can still find brushed motors all around you. The starter on your car is one good example.

In a brushless motor the roles are a bit reversed. The armature, now called a rotor, houses the magnets. The can now houses the windings, called the stator. There is no commutator at all, instead the windings are run directly to the speed controller. This is why brushless motors have three wires. The speed controller takes up the job of the comm instead, pulsing power to the windings as necessary to create a rotating magnetic field inside the motor. The permanent magnets inside the rotor try to align to this rotating magnetic field, and in doing so, they rotate with it. This key difference removes a weak link in the motor and massively increases the efficiency. Brushless motors, therefore, last much longer than brushed ones, and can surpass the power output of most ICE powerplants as well.

A sensored brushless motor also has an additional bit of wiring inside it, which is able to read and transmit back to the speedo the position of the rotor, as well as what direction and how fast it's turning. A sensored speed controller can use this information to further improve how it operates the stator coils. This results in a much smoother motor at low speed, but is a bit more expensive and complex. Sensorless motors tend to be a bit jerky at very low speed, but they're a bit cheaper and work just fine for RC use. Most modern speedos support both types anyway, unless you get a really cheap one.

Electric rc power is anything but complex, and with the amount of raw horsepower available using LiPO batteries and brushless motors, it's no wonder so many people run them. But for some, like me, they lack that special... pizazz. They lack soul. Happily, there are fuel-burning alternatives, which I shall explain in the next article!

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