AC Servo Motors and DC Servo Motors



Indramat’s AC servo motors were a significant innovation in the industrial automation industry. But what really sets AC and DC motors apart?

We’ll start with the standard DC motor, which is powered through a two wire connection and continuously spins at an average speed of 5000 rpms. Now the actually speed of a DC motor is controlled by what is known as pulse width modulation, which in essence is just the concept of turning the motor’s power off and on at a specific rate. This technique is used to control the speed of the motor since by default the motor only runs at one specific speed. Certain motor drive circuit boards will utilize the “on/off” technique at preprogrammed increments, making it easier for the user to control the motor.

AC servo motors on the other hand are designed to be able to adapt easily to changes in speed and direction. They use a 3 wire connection (power, ground, and control) which must be constantly applied, unlike DC motors. In application, the motor will receive a specified output position control signal, which it will then use to apply an appropriate amount of power so that the servo shaft arrives at the commanded position. The position sensing device is what determines when the motor shaft has arrived at the specified position. Servos also utilize pulse width modulation, but instead the duration of positive pulses is what ultimately determines the ending position.

However, the use of the terms is not always that clear. Since Indramat created brushless servos and called them AC, people sometimes use “AC” for brushless and “DC” for brushed servos, even though there are now brushless DC motors. Since both brushless DC and AC motors use an electronic mechanism to control current, the AC/DC split is no longer a central difference. However, AC motors may still be less energy efficient than a DC motor — all things being equal.

It’s like the spherical chicken, though… How often are all things equal?

If you have Indramat or Indradrive servos to contend with, let us provide support.

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