Motors

Brushed, brushless motors and ESC’s

You will probably be familiar with the traditional brushed motor which has been around for longer than most of us. A brushed motor uses both permanent and non-permanent electromagnets to create a rotational movement which is perfect for a propeller.

A brushed motor consists of an armature of copper windings, normally with three “arms” (the electromagnet) which spins inside two permanent magnets. The polarity of the arms is changed by the rotation of the commutator ring against two brushes, one negative and one positive. As polarity of the arms on the armature change the permanent magnets repel the armature and rotational movement is achieved. This technology is inexpensive and robust and can give many years of service, even when subject to minor abuse and if you want more power you simply fit a bigger motor, and/or feed it with a higher voltage.

A brushless motor may now be familiar to many of us but it’s operation is less well understood. In essence a brushless motor works using the same physics, but more efficiently. Inside the casing there are more (and more powerful) permanent magnets (called something I can neither pronounce or spell) and the armature will have more “arms”. There is no commutator ring or brushes. This motor must be controlled by a brushless ESC, electronic speed controller.

There are three wires, two providing power and another which sends a signal to the ESC telling it the position of the armature. The ESC sends power to the motor changing the polarity of the electromagnets which are both attracted and also repelled by the permanent magnets at the right time to achieve rotational movement.

A brushless motor of the same size and weight as a brushed motor will produce significantly more power and very powerful motors can be contained within a relatively small casing. Used in conjunction with new technology batteries a model speed boat can be propelled forward at frightening speeds!

A brushed motor does not need a speed controller; it can be controlled by a simple on/off switch or a variable resistor which reduces the voltage to the motor, slowing its rotation. A brushed speed controller is a more efficient way of controlling the rotation as it acts as a “pulse width modulator” which is effectively a sophisticated on/off switch. The controller controls the on and off pulses to reduce the power to the motor and so slow the rotation. The advantage of this compared to a variable resistor is that the voltage doesn’t change so the torque of the motor stays the same, even if the rotation slows which make it a better way of controlling the speed of a propeller in the water.

A brushless motor needs an ESC as it receives the signal from the motor indicating the position of the armature and sends the necessary power to cause the armature to rotate. It is important that the brushless ESC is rated in amps to at least the rating of the motor otherwise it will burn out. The brushless ESC will also perform some other duties, e.g. slowing the motor when the minimum voltage of a LiPo battery is achieved and some more sophisticated ESC’s can be programmed to fine tune the motor to its application.

Brushless motors can be “inrunners” or “outrunners”. In inrunner is configured the same as a brushed motor with the armature inside the permanent magnets and is better suited to marine use, especially when using an outer cooling jacket. An outrunner has the permanent magnets on the inside and the electromagnets rotate around them. This is better for air cooling and these are generally, but not exclusively found on aircraft.

Brushless motors are categorised by size and KV. A 4080 motor with a 800KV rating will have a casing 40mm in diameter and 80mm long. The bigger the casing the more powerful the motor. KV is the RPM of the motor per volt. An 800 KV motor will turn at 800RPM per volt supplied so a 3S LiPo (11 volts nominal) will produce an RPM of 8880. Note that a lower KV number indicates a motor that will produce greater torque.

So, brushed motors remain very applicable for use in scale models, and other models where outright speed is not the objective. They are simpler and cheaper and although “old” technology will do the job required of them exceptionally well. The brushless motor with an ESC especially when used with LiPo batteries can produce huge power outputs and are better suited to models where outright speed is the main objective.