A Comprehensive Guide to Understanding the Differences Between DC Motors, Brushless DC Motors, and AC Servo Motors

Introduction: Overview of Servo Motor Development and Technology Selection
A century ago, the invention of the servo motor revolutionized electric motion control, enabling precise speed and position control in industrial automation, robotics, medical devices, flight control, CNC machines, and more. Over the years, servo motor applications have continued to expand, but this has also raised challenges in technology selection. This article focuses on brushed DC permanent-magnet motors, brushless dc motors, and AC permanent-magnet motors, analyzing the advantages and disadvantages of each type and their appropriate use cases.

Brushed DC Permanent-Magnet Servo Motor
Principle Explanation:
The servo drive interprets feedback signals and uses pulse-width modulation (PWM) to provide voltage to the motor via pulses of varying widths. These pulses are commutated by a rotating switch (commutator) made of multiple copper segments, which contact fixed brushes made of conductive materials like graphite, delivering pulses to multiple rotor coil phases. As the coils switch sequentially, a rotating magnetic field is generated, interacting with permanent magnets to rotate the rotor. The drive continuously corrects errors using PWM based on speed feedback. As a result, even with varying load, the rotor maintains stable speed and accurate positioning.

Advantages of Brushed DC Servo Motor

Initial cost is low, though shorter lifespan may offset this advantage.

High torque at startup and low speed.

Motor control system is relatively simple.

Disadvantages of Brushed DC Servo Motor

Worn brushes must be replaced regularly.

Worn commutators require servicing, bearing replacement, or even motor replacement.

Physical commutation limits power transmission, and overloading may cause arcing.

Brushless DC Motor
Principle Explanation:
Compared to brushed motors, brushless DC motors have significant improvements. Without physical brushes and commutators, efficiency and performance are enhanced. The armature coils are wound in stator slots laminated in the frame, while permanent magnets are fixed on the rotor. Feedback devices like encoders continuously report rotor position to the drive, which then electronically switches DC voltage to coils to correct rotational errors, achieving desired rotational direction, speed, and position. This electronic commutation occurs in steps approximating a sine wave. At any given rotation point, current is applied to two phases while the third phase is off.

This stepped waveform results in torque ripple, and the interaction between permanent magnet rotor and stator slots causes cogging, affecting low-speed precision and smoothness, though less problematic at high speed. Slotless motors, free from stator teeth, eliminate cogging but at the expense of significantly reduced torque.

Advantages of Brushless DC Servo Motor

Precise and responsive closed-loop control at medium to high speeds.

No physical commutation means longer lifespan, higher efficiency, and minimal maintenance.

Higher speed capability than brushed motors with minor drop in torque.

Disadvantages of Brushless DC Servo Motor

Drive electronics and programming/tuning are more complex.

Below ~200 rpm, cogging and torque ripple hinder smoothness and control.

AC Permanent-Magnet Servo Motor
Principle Analysis:
For most precision speed and position control applications, synchronous AC permanent-magnet servo motors are generally the best choice. These motors are similar in basic design to brushless DC servo motors. The external stator has three-phase windings inserted between laminated steel teeth, and an electronic commutation system sequentially energizes the coils to generate a rotating field interacting with the rotor’s permanent magnets, driving the motor. Unlike brushless DC motors, AC servo motors supply sinusoidal current to all three windings based on rotor position. By varying current amplitude and frequency, they deliver stronger torque and finer control, with smooth sine-wave commutation minimizing cogging and torque ripple.

Advantages of AC Permanent-Magnet Servo Motors

Excellent torque density, power, and efficiency in a relatively compact structure.

Available in various form factors, sizes, and power ranges.

Low inertia for rapid response to dynamic load and speed changes.

Long service life, reliable performance, and minimal maintenance.

Disadvantages of AC Permanent-Magnet Servo Motors

Slotted motors may experience cogging at low speed, but sine-wave commutation and precision firmware largely compensate for this.

Servo Motor Comparison Guide