Fanuc Servo Motor-Difference Between AC and DC


Many people often question the differences between an AC and a DC Servo Motor, so we put some facts in one place for you.

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.

If you require servo motor repair, please contact RepairZone at 989-922-0043, or click the link below:

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  1. Thank you very much for this wonderful information, I have benefited greatly from reading your article

  2. This was EXTREMELY helpful. I have been doing all the research I can on servo motors. I am not very well educated on what they are, but I keep coming across people that say they have had trouble because they bought the wrong one. I want to make sure I am as educated as possible before I make a purchase. From this article it sounds like the DC servo motor is more simple to operate, but in the long run may be more complicated because of the increased need for maintenance.

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