What is the difference between a brushed and a brushless RC motor?

Answer 1

The main difference between a brushed motor and a brushless motor, (besides the differences implied by the names), is that the brushed motor uses an electromagnet system as the rotor surrounded by permanent magnets (and sometimes more electromagnets), and the brushless motor has a magnetic rotor surrounded by electromagnets.

There are many aspects about the two types of magnets and electromagnets of both types of motors, which could fill several pages if everything was covered - but I'll just describe the basic important bits here.

Names and descriptions of main parts:

• Electromagnet> a coil of wire, wound around a metal form, will magnetise the form when energised by electricity.

• Rotor> the entire rotating part of the motor that includes the output shaft and the "technology" that reacts with the surrounding "technology" to create motion. The rotor's "technology" will be either a series of coils, or one or more magnets.

• Stator>the magnetic field-producing "technology" that provides the opposing magnetic field needed to oppose the magnetic field of the rotor to create motion. A stator's "technology" will be either a series of two or more magnets, or a series of coils.

• Commutator> a series of two or more segments of copper, or other conductible metal, mounted around a form side-by-side, with each segment electrically connected to one or two ends of any electromagnet's coil. This arrangement of a conductor rubbing on a rotating conductor is similar to another system called "slip rings", but commutators have multiple segment whereas a slip ring is a solid ring.

• Brush> an electrically-conductive solid (metal or carbon) that is physically softer than the commutator, used for transferring electrical energy to power the electromagnets while rubbing on the metal segments as they rotate. To minimise the energy loss from its conversion into heat due to contact resistance, the brush is pressed firmly against the commutator using a spring or a strip of spring-metal to provide enough force. The "brush" can refer to the actual metal or carbon block on this component, or the entire component itself that includes the arm, or the spring in the case of a carbon brush. Brushes are usually used in sets of two, or less commonly, four.

Brushed MotorThe typical DC (direct current) motor has its coils mounted on the rotor to create the magnetic field that opposes the stationary stator's magnetic field produced by multiple permanent magnets. Some brushed motors use two or more coils as a stator for more rotational power, some of which may also run on AC. In either case, rotation is caused by the "like" poles of the stator and rotor, the polarity (North or South) changes as the rotor spins as the commutators switch positive and negative, to negative and positive at the correct angle of rotation to maximise the opposing magnetic forces.

* Note: The word "commutation" means "switching", which describes the result of the action of the commutator's segments alternating the power and polarity of the rotor's coil and thus the polarity of the magnetic field.

Brushless MotorThe brushless motor is basically the opposite of the brushed motor, where the rotor consists of one or more permanent magnets and the stator consists of two or more coils. In this case, the "commutation" (or switching) must be done outside the motor in an electronic circuit. To visualise the way this motor works, picture a magnet on the rotor having its North pole on one side and the South on the other. The coils surrounding the magnet are electronically switched in sequence, creating a rotating magnetic field that runs around in a circle - similar to the visual effect of "chasing" lightbulbs around an outdoor sign. This rotating magnetic field pushes or pulls the magnetic rotor around in a circle to create motion.

Pros and cons of BRUSHED versus BRUSHLESS motors:

• >Brushed motors are inefficient due to the power losses from imperfect power transfer through the commutator system.
• >Brushless motors are much more efficient without these losses.

• >Brushed motors have short lifetimes due to the wear of the brushes and commutators.
• >Brushless motors do not use commutation parts so they do not suffer from this.

• >Brushed motors require more complicated methods for speed control, lowering voltage reduces speed but also reduces torque (rotational power). Newer methods using PWM (pulse width modulation) is an improved method, but is still not ideal. Torque vs. speed is such that torque drops sharply at lower speeds.
• >Brushless motors are very simple to control speed, rotational power (torque) vs. speed characteristics are opposite those of brushed, where torque is highest at slow speeds and drops off with higher speed. Specialised types of brushless motors are designed for very high speeds, but they require unusually excessive current to do so.

• >Brushed motors run much too fast to be useful to most applications, requiring a gearing system to reduce this speed (torque increases as a side benefit) to make the motor match the application.
• >Brushless motors excel in this regard, as they are very frequently used directly without gearing, although some applications requiring high precision or additional torque will use a gearing system.

I'm sure I've missed a few things here and there, but hopefully I've covered enough to helpful and provide a basis for understanding the fascinating and important world of these types of motors.

Peace,

Dave