Understand the technologies and how to choose
The invention of the servo motor around 100 years ago transformed the capabilities of electric motion, enabling precise speed and position control for applications ranging from industrial automation to robotics, medical devices, flight control, CNC machines and many others.
Servo motor technologies have evolved over the years, expanding the scope of application possibilities while also creating a dilemma: Which technology should you choose for your application? Let’s take a look at brushed DC permanent magnet motors, BLDC brushless DC motors and AC permanent magnet motors, evaluating the strengths, weaknesses and best uses of each category.
Brushed DC permanent magnet servo motors
Invented in 1926, the earliest servo motor technology uses a brushed DC motor, a concept that dates all the way back to 1832. The designs commonly used today incorporate permanent magnets on the stator and copper coils wound on the rotor laminations, with servo actuation accomplished using a real-time feedback device such as a tachometer.
The servo drive interprets feedback signals and modulates the voltage supplied to the motor in a series of pulses of varying width, a process known as pulse-width modulation. These pulses are commutated — that is, delivered to the multiple phases of the rotor coils — by means of a rotating switch consisting of a series of copper bars (the commutator) in contact with fixed brushes made of an electrically conductive material such as graphite.
As the coils are switched in sequence, they generate a spinning magnetic field which interacts with the permanent magnets to spin the rotor. Based on data from the velocity feedback device, the drive uses pulse-width modulation to continually correct for any errors, for example, to ensure consistent rotor speed and positioning even when the motor is driving varying loads.
Advantages of brushed DC servo motors
- Lower initial cost, although the limited lifespan can negate this advantage.
- High torque upon startup and at lower speeds.
- Relatively simple motor control system.
Disadvantages of brushed DC servo motors
- Worn brushes require routine replacement.
- A worn commutator requires rework, new bearings or even motor replacement.
- Physical commutation limits power transfer and, if exceeded, can result in arcing.
Brushless DC servo motors (BLDC)
Brushless DC motors were invented in the 1960s, made possible by advancements in semiconductor-based electronics. Early models were limited in power, but BLDC motors came into their own in the 1980s with the growing availability of powerful permanent magnet materials. Today, these motors are far more commonly used in industrial applications than brushed DC servo motors.
With no physical brushes contacting a spinning commutator, the basic design of a BLDC motor is reversed from a brushed motor to improve efficiency and performance. The armature coils are wound within the stator/frame on slotted laminations, and the permanent magnets are affixed to the rotor.
A feedback device such as an encoder continuously reports the rotor’s position to the drive, which in turn directly switches the DC voltage delivered to the coils to correct any rotation errors and achieve the desired direction, speed and position. This electronic switching occurs in steps that crudely mimic a sine wave, with the current at any given rotational point applied to two phases with the third phase always off.
This stepped waveform results in torque ripple, while a cogging effect is created by the attraction between the permanent-magnet rotor and the stator’s toothed steel laminations. These characteristics hinder precise control and smooth operation at low speeds, but this is rarely an issue at high velocities. Slotless motors with no stator teeth eliminate the cogging effect, but smooth, low-speed operation comes at the expense of significantly lower torque.
Advantages of brushless DC servo motors
- Precise and responsive closed-loop control at medium to high velocities.
- No physical commutation means longer life, greater efficiency and minimal maintenance.
- Capable of far higher speeds than brushed motors, with less torque drop-off.
Disadvantages of brushless DC servo motors
- More complex drive electronics and programming/tuning.
- Cogging and torque ripple can hinder smoothness and control at < 200 rpm.
AC permanent magnet servo motors
Synchronous AC permanent magnet servo motors are the preferred choice for most applications that require precise position and speed control. First developed in the 1980s, this technology has continuously improved due to advancements in drive electronics, permanent magnet materials, manufacturing tolerances and other innovations.
The basic design is similar to a BLDC servo motor. The outer stator consists of 3-phase winding coils inserted between a series of laminated steel teeth. An electronic commutation system energizes these coils in sequence to produce a rotating magnetic field, which interacts with the permanent magnets on the inner rotor to create rotation.
Unlike BLDC servo motors, 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.
Advantages of AC permanent magnet servo motors
- Exceptional torque density, power and efficiency in a relatively compact package.
- Versatile form factors, sizes and power ranges.
- Low inertia for fast response to dynamically changing loads and speeds.
- Long, reliable service life with minimal maintenance.
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 for this.
Servo motor comparison guide
| DC Servo Motor | BLDC | AC Servo Motor | |
|---|---|---|---|
| Power Source | Direct current | Direct current, switching | Alternating current |
| Efficiency | Lower efficiency | High efficiency | Highest efficiency |
| Velocity control | Excellent (rpm limited at high end) | Medium | Excellent |
| Position control | Excellent | Fair | Excellent |
| Maintenance and lifespan | Higher maintenance, shorter lifespan | Low maintenance, long lifespan | Low maintenance, long lifespan |
| Torque density | Average | High | Highest |
| Smoothness | Good (limited by feedback resolution, update rates, etc.) | Low (except advantaged at high rpm) | Best (capability is a function of feedback resolution, update rates, rpm, etc.) |
| Responsiveness | Horsepower limited | Good | Best |
Kollmorgen servo motors for precision and performance
Kollmorgen offers a broad selection of AC permanent magnet and BLDC servo motors to meet practically any motion requirement. No matter which servo technology you choose, our experts can help you make informed decisions that will optimize your system's performance, efficiency and speed to market. Contact us today to get started.
