All About RC Car Brushless Motors

This is really only relevant to people who are involved in radio controlled cars as a hobby, or who are looking at getting into it. I intend to clear up some of the misconceptions about the brushless motors used in RC cars with extreme prejudice, explain how they differ from standard brushed motors, and general cool stuff about them. But first:

What are they?

Brushless motors are essentially standard motors turned "inside out". That is to say, when normally a brushed motor would have a commutator and a rotor that contacts the commutator ("comm") to transfer current and feed its magnets, a brushless (BL) motor does not. It works by a principle called induction: Immersing a magnet in a changing magnetic field will cause it to move. Thus, this brings up the most interesting fact about BL versus brushed motors:

There is no physical contact between the electrical components at all.

This is important. Without this particular factoid, we would not be able to produce motors that are

• Efficient
• Fast
• Powerful

This is where the "inside-out" bit comes in. You see, brushed motors are essentially a magnetic can with a rotor inside that can change the polarity of its magnetic field in response to changing electrical contacts. In a BL motor, this does not happen. Rather, the permanent magnets are on the rotor ("rotating part"), while the changing magnetic field is supplid by elements in the can, or the stator ("static part"). This lack of direct contact is important. It means far less friction than a standard brushed motor, and less wear on the moving part (not "parts", as only the rotor moves in a BL motor).

The changing field in the stator is supplied by electronic control via the car's ESC (electronic speed control). That's why there are three poles (labelled A, B and C on the can) for providing power: so that the ESC can set the motor's speed by altering how quickly it "moves" the magnetic field. Faster moving = faster spinning rotor

These are two common terms that you will see.  A sensorless motor is one that uses internal timing in the ESC to set motor speed.  The advantage is that the overall system is simpler.  The bad news is that, at low speeds, the ESC has no way of "knowing" which way the motor is turning.  Thus, you may pull the throttle forward and have your car shoot off in reverse.  This has happened to me before, and it's why I tend to adhere to sensored motors.

Sensored motors have something called a Hall effect sensor in them that judges the position of the rotor based on the magnetic field that the sensor "feels" at a given time.  The information is then relayed to the ESC and it can, from there, figure out if the motor is going forward, backward and at what speed.  Quite handy.  Thus, sensored systems tend to be smoother at low speeds, although "cogging" (lurching) is still a common problem.

These are two of the most misunderstood terms/concepts in electric RC-car-dom. People often make the assumption that more turns = better, and they are stumped as to what Kv even refers to (a lot of publications even refer to it as kV, which is entirely wrong as I shall explain below).

Kv

This is a basic constant. In fact, the 'K' is German for Konstant. Guess what that means. The v refers to velocity. Thus, Kv is a velocity constant. The number associated with a motor simply means that, for every Volt of input, the motor will turn that many RPM (revolutions per minute). Thus, a 5000 Kv motor will turn 5000 times per minute faster as every Volt is added. At 8 volts, it would spin at 40000 RPM.

Turns

The number of turns that a motor has is roughly related to its power output.  A lower number of turns means, generally for a given manufacturer, a higher power output (measured in Watts).  But, you're probably wondering:

WHAAAATTT?

How does it make sense that fewer 'turns' means more power???  Well, consider: Turns are simply made of wire wrapped around the stator's phases.  Fewer turns means less wire.  Less wire means less resistance and, thus, a higher current can pass through the wire.  And, since power = current x voltage, more Amps = more POWERRR.

Cool.

 Brushless motors for RC cars are a relatively new technology.  They have truly prospered in the last few years, largely due to lower manufacturing costs and increased market competition.  Power levels have steadily ramped up over the years, too, and the most powerful 1/10 scale (540 size) motor that I know of is the one the I just recently purchased, the Team Orion Vortex 2.5.  It produces 897 Watts, which is 1.2 horsepower!

Add to that their smoothness, efficiency (usually over 85%) and cleanness compared to nitro engines, it's easy to see why the technology has blossomed as it has.