There are two types of DC motors: Brushed (DC) and Brushless (BLDC). Even with a brushed DC motor, the current through the windings must be reversed as the motor rotates; this is done automatically by the brushes and the commutator ring, so a steady DC current is all that is needed from the supply. BLDC motors do not have mechanical commutators to change the current, they are electrically commutated (EC) by using Hall sensors to signal the controller when to switch the currents. When operated this way, they still behave as DC motors where the drive voltage determines the speed. The speed of DC and BLDC motors operated this way are dependent on the drive voltage, torque load and temp, making speed regulation more difficult, especially without feedback.
BLDC motors can also be operated as a 3 phase synchronous AC motor where the speed is determined by the frequency of the drive signal, which is made up of 3 sinewaves 120° out of phase. The electronics to accomplish this correctly is more sophisticated than EC control, but it produces smoother operation, less cogging and more precise speed control. The SOTA Eclipse motors use a 3 phase AC controller. The speed can be controlled very precisely without feedback.
You can find more information at these links:
https://en.wikipedia.org/wiki/Brushless_DC_electric_motor
http://www.turntablepsu.com/technology.html#BLDC
BLDC motors can also be operated as a 3 phase synchronous AC motor where the speed is determined by the frequency of the drive signal, which is made up of 3 sinewaves 120° out of phase. The electronics to accomplish this correctly is more sophisticated than EC control, but it produces smoother operation, less cogging and more precise speed control. The SOTA Eclipse motors use a 3 phase AC controller. The speed can be controlled very precisely without feedback.
You can find more information at these links:
https://en.wikipedia.org/wiki/Brushless_DC_electric_motor
http://www.turntablepsu.com/technology.html#BLDC