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DC Servo Motor :
1. Brushes and the commutator, sequence the current through the coils.
2. DC servos are simpler in control
3. Independently control flux, which contributes to an independent control of torque when consistently maintained
4. Has brushes that will wear and need replaced.
5. In some cases the inertia is generally higher.
6. Control requires only the control of the field or armature current magnitudes, providing simplicity not possible with AC machine control
7. Uses very simple duty-cycle-controlled PWM to control the speed of the motor
AC Servo Motor :
1. A feedback system tells a controller where the rotor is so it can sequence the current through the coils
2. Most AC Servo motors have a specific feedback alignment to properly function with a controller/amplifier.
3. Lower Rotor Inertia
4. Require a coordinated control of stator current magnitudes, frequencies, and their phases, making it a complex control
5. No Brushes to wear and replace
6. Control scheme more complicated. It has a closed loop communication system with the controller/amplifier.
7. Uses sinusoidal PWM, where the speed control is in the frequency of the PWM
8. Less maintenance
9. Higher RPM, many servo motors are capable of 6000 RPM.
NOTE: AC sinusoidal and BLDC (Brushless DC) are constructed almost identically. BLDC require a commutation method. AC has the smoothness of a DC. AC sinusoidal all three windings are fed with 3ph sine wave, with BLDC, only two windings are powered at any given instance.
