The difference between brushed and brushless servo motors is the mechanism used for commutation. Servo motors move or create torque based on magnetic forces being in opposition. In the simplest case there is a fixed magnet and a rotating magnet. We can cause the rotating magnet (rotor) to move by changing its polarity through a process of alternating the direction of the current through the magnet. The act of modifying the direction of the current in the windings is called commutation.
Figure 2 shows stators and rotors of brushed and brushless motors. On the left of the left-hand image is a stator and brushes of a brushed DC motor, while to the right is the stator of a brushless motor. The right image shows on the left a rotor of a brushed and on the right a rotor of a brushless motor. Both motor types are so called inrunners. The fixed part, the stator, is located outside and the rotating part, the rotor, is inside.
Brushless Servo Motors:
In a brushless motor, you lose the commutator and brushes while gaining an electronic controller. Now the permanent magnets act as the rotor and rotate around on the inside while the stator is made up of the fixed electromagnetic coils now on the outside. The controller powers each coil according to what charge it needs to attract the permanent magnet. In addition to moving the charge around electronically, the controller can also provide a same charge to oppose the permanent magnet. Since like charges oppose each other, this pushes the permanent magnet. Now the rotor is moving thanks to a pull and a push.
The permanent magnets are moving in this case, so now they are my running partner and me. We aren’t changing our minds about what we want anymore. Instead, we know that I want the Boston creme doughnut and my partner wants the smoothie. The electronic controller keeps our respective breakfast delights moving in front of us and we’re always chasing the same thing. The controller also puts what we don’t want right behind to offer a push.
Brushed Servo Motors:
Brushed motors use mechanical brushes to commutate the current in the motor windings. Because brushed motors commutate the incoming current, they can be powered by a direct current (DC) source. Brushed servo motors have two main parts:
A housing that contains the field magnets (stator)
A rotor made up of coils of wire that are wound in the slots of an iron core and connected to a commutator.
The brushes are in contact with the commutator and carry current to the coils. Over time these brushes can wear out and introduce friction into the system, this does not happen with brushless servo motors.
Brushed Motor Pros
Two wire control
Some are rebuildable for extended life
Low cost of construction
Simple and inexpensive control
No controller is required for fixed speeds
Operates in extreme environments due to lack of electronics
Brushed Motor Cons
Periodic maintenance is required
Speed/torque is moderately flat. At higher speeds, brush friction increases, thus reducing useful torque
Poor heat dissipation due to internal rotor contsruction
Higher rotor inertia limits the dynamic characteristics
Lower speed range due to mechanical limitations on the brushes
Brush arcing will generate noise causing electrical magnetic interference (EMI)
Brushless Motor Pros
Electronic commutation based on position sensors vs mechanical switch for brushed
Less maintenance due to absence of brushes
Speed/Torque- flat, enables operation at all speeds with rated load
High efficiency, no voltage drop across brushes
High output power to size ratio.
Reduced size due to superior thermal characteristics. Because the windings are connected to the case the heat disipation is better
Higher speed range – no mechanical limitation imposed by brushes/commutator
Low electric noise generation (EMI)
Brushless Motor Cons
Higher cost of construction
Control is complex and expensive
Electric controller(ESC) is required to keep the motor running which is sometimes more expensive than the motor.