DC vs AC Servo Motors: Which Type Is Right for Your Application?

Not all servo motors solve the same motion challenge. Brushed DC, brushless DC (BLDC) and AC permanent magnet servo motors each use a different commutation method, resulting in different levels of efficiency, maintenance, smoothness and overall performance. The best choice depends on your application's motion and performance requirements.

Servo motor technology has evolved significantly over the past century, expanding the range of applications that rely on precise motion control—from industrial automation and robotics to medical devices, CNC machines and more. As the technology has advanced, so have the options available to machine designers.

Let's look at where today’s brushed DC, BLDC and AC permanent magnet servo motors perform best and the tradeoffs to consider when making your selection.

What Are the Key Differences Between DC, BLDC and AC Servo Motors?

Brushed DC, BLDC and AC permanent magnet servo motors each use a distinct commutation method that determines their efficiency, maintenance requirements and suitability for specific motion profiles.

• Brushed DC motors use mechanical switching relative to rotor position via copper bars and graphite brushes.
• BLDC motor systems replace mechanical commutation with electronic switching relative to rotor position, that steps through two active phases at a time, at its highest-implemented design approximating a sine wave, where the third phase is always off.
• AC permanent magnet servo motor commutation relative to motor position, effectively applies all three phases simultaneously, resulting in the smoothest torque and highest efficiency of the three types.

The servo drive plays a central role in all three technologies. It interprets feedback signals from devices such as encoders or tachometers and applies pulse-width modulation to correct speed and position errors in real time. The quality of that correction—and the smoothness of the resulting motion—depends heavily on which commutation approach the motor and drive use together.

Understanding these differences is the foundation for any servo motor selection decision, but let’s examine each technology in depth before comparing them side-by-side.

How Do Brushed DC Servo Motors Work, and When Should You Use Them?

Brushed DC servo motors use a brush-and-commutator switch to mechanically commutate current through the rotor windings, while the servo drive uses real-time feedback to continually correct motor speed and position. This relatively simple design makes them the simplest and lowest-cost servo motor type, but also the highest-maintenance.

In a brushed DC permanent magnet motor, permanent magnets are mounted on the stator and copper coils are wound on the rotor laminations. A tachometer or similar feedback device reports velocity to the servo drive, which modulates the voltage supplied to the motor using pulse-width modulation (PWM). A rotating commutator made of copper bars and fixed conductive brushes routes current through the rotor coils in sequence, generating a spinning magnetic field that interacts with the permanent magnets to produce torque.

Based on data from the velocity feedback device, the drive continually adjusts its output to correct for errors and maintain consistent speed and positioning as loads change.

Advantages of brushed DC servo motors

• Lower initial purchase cost, although the limited lifespan can negate this advantage.
• High torque upon startup and at lower speeds.
• Relatively straightforward drive electronics and control system.

Disadvantages of brushed DC servo motors

• Worn brushes require routine replacement.
• Worn commutators require rework, new bearings or even motor replacement.
• Physical commutation limits maximum power transfer and can cause arcing if exceeded.
• Higher lifetime maintenance cost can offset the lower initial price.

Ideal use case: Brushed DC servo motors are well-suited for cost-sensitive applications with modest duty cycles, where simplicity and low-speed torque matter more than service life or maintenance overhead.

How Do Brushless DC Servo Motors Work, and What Are Their Limitations?

BLDC servo motors eliminate mechanical commutation by placing permanent magnets on the rotor and wound coils in the stator, with the servo drive handling commutation electronically. This design improves efficiency, increases service life and enables higher operating speeds compared with brushed DC motors, but torque ripple and cogging can limit smoothness at very low speeds.

The basic design of a BLDC motor inverts the brushed architecture. Armature coils are wound within the stator/frame on slotted laminations, and permanent magnets are affixed to the rotor. A feedback device such as an encoder continuously reports rotor position to the servo drive, which switches DC voltage directly to the stator coils to achieve the commanded direction, speed and position. This electronic switching steps through a pattern that mimics a sine wave, with the current at any given rotational point applied to two phases with the third phase always off.

That stepped waveform introduces two performance constraints. First, torque ripple results from the abrupt current transitions between steps. Second, a cogging effect arises from the magnetic attraction between the permanent-magnet rotor and the slotted stator's steel teeth. Both effects are most pronounced at low speeds — typically below 200 rpm — where they can compromise smooth motion and precise control. Slotless BLDC designs eliminate cogging by removing the stator teeth, but this significantly reduces torque output.

Advantages of BLDC servo motors

• Precise, responsive closed-loop control at medium-to-high velocities.
• No physical commutation means longer service life, higher efficiency and minimal maintenance.
• Capable of far higher speeds than brushed DC motors, with less torque drop-off.

Disadvantages of BLDC servo motors

• More complex drive electronics, programming and tuning requirements.
• Cogging and torque ripple can degrade smoothness and control at < 200 rpm.

Ideal use case: BLDC servo motors are well suited for high-speed industrial automation, semiconductor handling, and applications where maintenance intervals and service life are primary constraints and low-speed smoothness isn’t critical.

How Do AC Permanent Magnet Servo Motors Work, and Why Are They the Most Widely Used?

AC permanent magnet servo motors use true sine-wave commutation to deliver the smoothest torque, highest efficiency and broadest operating range of the three servo motor technologies. Because the technology pairs high performance with long life and low maintenance, they dominate demanding industrial automation, robotics, CNC and medical device applications.

The fundamental architecture resembles a BLDC servo motor. Permanent magnets are mounted on the rotor, while three-phase winding coils are inserted between laminated steel teeth in the stator. The key difference is commutation. Instead of stepping current between phases, the commutation current for an AC servo motor is delivered in a sine wave to all three phases as a function of the rotor position. The current is varied in both amplitude and frequency to provide more powerful torque and more precise control. The smooth sine wave also minimizes the effects of cogging and torque ripple.

Kollmorgen's AKM Series AC permanent magnet servo motors exemplify this approach, delivering industry-leading torque density in compact form factors with low rotor inertia for fast dynamic response to changing loads. Combining it with the AKD servo drive firmware can further compensate for any residual cogging in slotted designs, extending smooth operation across the full speed range.

Advantages of AC permanent magnet servo motors

• Exceptional torque density, power output and efficiency in a compact package.
• Versatile form factors, sizes and power ranges to match a wide variety of applications.
• Low rotor inertia enables fast response to dynamically changing loads and speeds.
• Long, reliable service life with minimal maintenance requirements.

Disadvantages of AC permanent magnet servo motors

• Slotted motors can exhibit cogging at low speeds, although sine-wave commutation and sophisticated drive firmware can compensate in many systems.

Ideal use case: AC permanent magnet servo motors are best suited for applications that require position accuracy, torque consistency, dynamic response, and long service life simultaneously.

How Do DC, BLDC and AC Servo Motors Compare Side by Side?

What’s The Right Motor Type for Your Application?

The choice between brushed DC, BLDC and AC permanent magnet servo motors comes down to three primary considerations: commutation method, application speed range and acceptable maintenance overhead.

Brushed DC motors deliver simplicity and low-speed torque at the cost of ongoing brush and commutator maintenance. BLDC motors remove that maintenance burden and excel at high speeds, but their stepped commutation can limit smooth operation at very low speeds. AC permanent magnet servo motors, with their sinusoidal commutation and high torque density, represent the most versatile option for demanding servo applications.

Matching motor type to application requirements from the outset is the most reliable path to a system that performs and scales, but it’s not the only important decision to make. If you’re curious about the right combination of motor, servo drive and feedback device for your specific application, the motion experts at Kollmorgen are here to help.