An electric motor converts electrical energy into a mechanical force.
It uses a fundamental principle of Physics, if you pass an electric current through a wire it creates a magnetic field around the wire. Wind the wire into a coil and you have more powerful magnet. In an electric motor we place a permanent magnet close to a coil of wire and pass a current through that coil. The magnetic force induced into the coil causes the coil to move either towards or away from a magnet, depending on the direction of current through the coil and the polarity of the magnet produced. Two North Poles will move apart, a North and a South Pole will attract each other.
There are two types of DC motor used in RC Racing, Brushed and Brushless Motors.
These are examples of Brushed motors:
These are the cheapest form of DC electric motor and the coil of wire is on the rotating part of the motor, known as the armature. To connect to this wire, we use small devices called brushes, since they ‘brush’ against the contacts on the end of the armature.
The armature in the simplest motors (and 99.9% of those used in Model Cars) has three coils and the can of the motor has two magnets, so when the armature is stationary there is always at least one coil sitting between the two magnets. This means the armature will start to rotate as soon as power is applied.
The brushes carry all the power being used by the motor and wear against the surface of the Armature, called the Commutator. This means these motors need frequent maintenance to maintain their performance.
The amount of power produced by an electric motor depends on several factors, the voltage applied (usually fixed by the battery pack allowed in the class), the current passed through the coils, the number of turns on the coils and the strength of the permanent magnets.
The strength of the magnets is largely set during manufacture. High temperatures during operation can cause this magnetism to fade. The hotter a motor gets, the faster the magnets will fade.
The current passed through the motor is limited by the resistance of the coil of wire. A long thin piece of copper wire has more resistance than a thick short piece. Motors are rated by the number of turns of wire wound onto each pole of the armature. If a single strand of copper wire is wound around each pole 27 times, we refer to that as a 27 turn single motor. If 2 strands are wound around each pole 12 times that would be known as a 12 turn double motor. The 12 turns will be shorter and can be made of thicker wire, so will have less resistance causing more current to flow, increasing the power of the motor. It will also increase the wear rate of the brushes and commutator and heat build-up in the motor.
These are examples of Brushless Motors.
Brushless Sensorred 540 size motor.
Sensorless Motor (not normally used for competition)
Sensorred Motor for 1/8th
Brushless Motors have the magnet on the rotating part, now called the rotor, and three coils inside the can.
There is no Commutator in this design of motor, so that weakness is eliminated. They require much less maintenance than a Brushed motor.
An RC Brushless motor has three connections to the coils. The Speed Controller (ESC) that powers it is more complex than for a brushed motor as the ESC has to know where the magnet pole is before it applies power to the coils. Inside the ‘rear’ end of the motor, away from the output shaft there is a circuit board similar to this:
The three small black rectangles on the small board are Solid State Hall effect switches. These components send a signal to the ESC when the pole of the magnet is close to the component. These signals are sent to the ESC through a sensor cable. For more information, check out the ESC section of these web pages.
Just as with brushed motors, Brushless motors also vary in power output depending on the number of turns. The turns are expressed as 5.5T, 13.5T, 17.5T etc. The coils connect to the three tabs you see at the rear of the motor, then connect to each other at a ‘star point’ at the front (output shaft end) of the motor where they connect to a collector ring. If the coil is wound with 13 turns of wire starting at the tab, it then has to make another ½ a turn to get to the collector ring at the front, so it is known as a 13.5 turn.
Controlled Class racing using motors with a fixed minimum number of turns are very popular, so you will commonly see classes using a minimum of 13.5T, 17.5T or 21.5T motors.
Brushless motors are just as sensitive to heat as Brushed motors. The Rotor’s magnetism is lost when they over-heat and the wire resistance can also increase with temperature. Many of today’s Brushless motors have extensive cooling slots in the can and drivers frequently use cooling fans on the motor.
Selecting a Motor
If you have joined a Club and have an idea what you want to race, ask around at the Club and most Racers will be only too happy to help. If you buy a motor that is popular at your local club, there will always be someone there who can help you get the best from it.
Your first choice will depend on the type of motor allowed in the class, Brushed or Brushless. If the class has a motor limit, then that will also decide what ‘turns’ you will have to select.
You do not need to buy the most expensive motor for your first model, look out for lower priced models or maybe even Second Hand. Regular Racers often have used but fully working motors they are happy to sell.
You can upgrade to the latest and greatest when your skill level and pocket allows. Save your money for Race Fees, Practise makes perfect!