Physical Size and Power Density for brush motor

  This is primarily a function of where the wound field is located in the motor. The brush motors have a higher thermal impedance because the windings are located on the inner rotating element and have a less efficient thermal path in which to dissipate the heat generated in them. The brushless DC motor has a lower thermal impedance because the windings are located on the outer member giving it a more efficient thermal path and a resultant higher heat dissipation rate. Thus, a brushless unit of the same size can generate more continuous torque than a brush motor. Another consequence of the winding location is that brush motors tend to have a higher inertia then a brushless motor of similar torque rating. In servo applications this can result in a significant motor size reduction when motor inertia is more

Typical Speeds
  Brush units work best continuously between 1000 RPM and 10,000 RPM. The word “continuously” is important here because we know that brush units can run slower than 1000 RPM occasionally. However, when trying to run at these slower speeds over extended periods of time, as the brushes wear away, brush particles begin to accumulate in the commutator slots between the bars and cause shorts in the coils connected to them. The higher speeds are limited to a practical limit of 10,000 RPM due to the mechanics of brush to commutator interface characteristics. That is, as the rotor speeds up, the brushes begin to float over the commutator making poor physical and, thus, electrical contact. On the other hand, brushless motors typically can run at higher speeds being limited only by the mechanical integrity of the
rotor, speed related losses and the stability of the bearings being used.

 By the use of application specific brush materials, electrical filters and suppression devices such as  capacitors, ferrite chokes and varistors placed across the commutator. In both types of units, noise can be reduced to acceptable limits by the proper selection of bearings and fits along with rotor balancing.