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In a brush motor, the magnets are fixed to the case. The armature is made up of thin laminated sheets of steel and wound with copper windings. Current is supplied to the rotor windings through two brushes and a commutator. As the armature rotates, the brush and commutator assembly automatically directs current to the correct winding to keep the motor spinning. The spped controller for a brush motor is relatively simple, needing only a single variable switch. A brushless motor looks like a brush motor turned inside out. As shown in the figures below, the rotor has permanent magnets mounted to it and is supported by a pair of ball bearings. These are the only moving parts in the motor and are also the only parts to wear out. The stator or armature is also made up of a laminated stack and wound with copper windings. Magnetic sensors in the rear detect the angular position of the rotor with respect to the stator. These sensors tell the controller which phase to turn on and off allowing the motor to produce a continuous torque in one direction. Consequently, the controller is much more complicated and requires a minimum of 6 MOSFET’s to produce the required 3 phase AC current. Both types of motors are bound by the same limits, torque and RPM. In operation the brush motor generates heat on the armature. The only conduction path is either through the shaft or the commutator. This thermal insulation ultimately limits the torque a brush motor can safely produce because the windings can easily over-heat. Also the brush friction further concentrates the heat generated on the commutator. Unfortunately, the commutator can easily over-heat and lead to motor failure. The brush motor usually has an RPM limit that is either caused by the windings flying out of the armature or where the brushes "bounce" over the commutator not making good contact. In a brushless motor the windings are attached to the case which is relatively massive. Also, there is a direct thermal path to the ambient air. This allows the brush-less motor to have a much higher peak and continuous torque output. The only thing that limits the torque output of a brushless motor is the available cooling and magnetic saturation. The RPM is limited only by the retaining mechanism of the magnets.

A standard brushed motor.

A brushless motor.

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