How to Select a Brushless DC Motor
If an application requires high speed, quiet operation, low EMI and longevity, then brushless DC technology (BLDC) might be what you are looking for. There are many advantages to brushless motor technology and speed is one of them. Higher speeds are achievable because there are no mechanical limitations being imposed by the brushes and commutator. Another advantage is the elimination of the current arcing/ electro-erosion problem commonly experienced with brushed motors. BLDC motors also possess higher efficiency, and generate lower EMI which is excellent when used in RF applications. They also possess superior thermal characteristics over brushed motors since the windings are on the stator.
The stator is connected to the case, thus the heat dissipation is much more efficient. As a result, the maintenance on a brushless motor is virtually nonexistent.
Unfortunately, the higher cost of construction puts BLDC technology out of reach for many applications. You can easily spend twice as much on a brushless system and lose the simplicity of a brushed motor. Don’t forget to save room for the control/drive electronics too. You’ll need to mount it somewhere it if it isn’t integrated in the motor. Keep in mind, the motor can’t be mounted too far away from the drive as long cable runs tend to introduce noise into the system. To compensate, the phase leads can be twisted and shielded from the sensitive feedback leads to reduce noise.
As with brushed motors, brushless must overcome starting friction as well. Again, this is the sum of torque losses not depending from speed. Dynamic friction is dependent upon speed. In fact, dynamic torque friction is the only thing defining torque losses proportional to speed for BLDC. A function of speed (for example in metric units of mNm/rpm), dynamic friction is due to the viscous friction of the ball bearings, as well as to the eddy currents in the stator originated by the rotating magnetic field of the magnet. Overall, you can expect the speed-torque curve to demonstrate excellent linearity for BLDC technology.