A servo motor is a closed-loop motor that measures and corrects its own speed and position. The encoder at the end of the shaft continuously measures the current angle and rotational speed; the drive compares this information with the command and instantly adjusts the current if there is a difference. Thanks to this feedback, the motor can hold the desired position with precision down to roughly one-thousandth of a degree even when the load changes. This is why CNC machines, robot arms, packaging machines and all automation lines that require positioning use servo motors.
What Is a Servo Motor?
A servo motor is not a single component but a system that works as a whole. The system has three main components:
- Motor body: A rotor structure, typically with permanent magnets, designed for low inertia and fast response.
- Encoder (feedback unit): A sensor that measures the shaft's angular position and speed and can generate thousands of pulses per revolution.
- Servo drive: The control unit that receives the command, compares it with the data from the encoder and corrects the current sent to the motor within milliseconds.
When a conventional induction motor is switched on, it simply rotates; it does not know how far it has turned or whether it has stopped at the correct point. What distinguishes a servo motor is its ability to measure and correct its own motion. For this reason, a servo motor is defined less as a "motor that produces power" and more as a "system that controls motion".
How Does a Servo Motor Work?
The operating principle is based on closed-loop control . In open-loop systems, a command is issued and the result is assumed to be correct; in a closed-loop system, the result is measured and any error is corrected immediately.
The cycle works in the following steps:
- Command. The PLC or control unit sends the target position, speed or torque to the drive.
- Motion. The drive supplies current to the motor and starts rotating the shaft.
- Measurement. The encoder continuously reports the shaft's actual position back to the drive.
- Comparison. The drive calculates the difference between the target value and the actual value. This difference is called the error signal.
- Correction. The current is increased or decreased until the error reaches zero. The cycle repeats thousands of times per second.
In practice, this means the following: when the machine load increases—for example, when a transfer carriage picks up a heavy part—the motor begins to slow down. The encoder detects this slowdown within milliseconds, the drive increases the current and the speed remains constant. The same behavior applies when stopping; the motor stops at the target angle and continues to produce torque to hold that position if an external force tries to move the shaft. This is called holding torque.
Servo systems can run this loop in three different modes: position control (go to a specific point), speed control (rotate at a specific speed) and torque control (apply a specific force). Depending on the application, one of these modes is selected or several are used in sequence.
Types of Servo Motors
Servo motors are divided into two main groups according to the power supply type. Both operate with the same feedback principle; the difference lies in the motor's electrical construction.
| Comparison | AC Servo Motor | DC Servo Motor |
|---|---|---|
| Construction | Brushless; commutation is performed electronically | Mostly brushed; mechanical commutation |
| Maintenance requirement | Low; there are no brushes to wear out | Brushes and commutator are replaced periodically |
| Power range | From hundreds of watts to tens of kilowatts | Generally low and medium power |
| Control complexity | Requires an advanced drive | Simpler drive structure |
| Typical use | Industrial automation, CNC, robotics | Battery-powered and mobile systems, laboratory setups |
Today, brushless AC servo motors are used predominantly in industrial machinery; the fact that they require little maintenance on continuously operating lines is a decisive advantage. DC servo motors, on the other hand, are preferred in low-voltage mobile equipment; in this context, our article on the features of 24V DC motors may be useful for comparison. In fixed-speed applications powered from the mains, instead of a servo, geared AC motor solutions offer a more economical alternative.
Servo Motor Applications
A servo motor is used wherever it matters not only that motion occurs, but exactly where and how fast it occurs.
CNC machines. In machining, the path followed by the cutting tool is defined at micron level. In axis drives, the servo motor maintains the commanded coordinate even when the cutting force changes. For linear axis motion, it is commonly used together with a rack and pinion mechanism .
Robotics and manipulators. Each joint of a robot arm is a separate servo axis. Keeping the end point of the arm in the correct position depends on all joints operating synchronously.
Packaging machines. Film feeding, the cutting blade and the filling unit must operate in sync with one another. On lines producing hundreds of packages per minute, millisecond deviations turn into scrap. Paper and packaging is one of the sectors where servo drives are used most intensively.
Textile machines. Keeping yarn tension constant, pattern control and feed synchronization are achieved with servo axes. Textile applications make especially strong use of torque-control mode.
General automation lines. Labeling, stacking, cutting, positioning and transfer stations operate with servo drives. For system architecture and communications, you can review our automation page.
Relationship Between Servo Motors and Gearboxes
Servo motors operate at high speed and relatively low torque. Most machines in the field require the opposite: low speed and high torque. The component that bridges this gap is the gearbox, and in servo applications the gearbox is not an accessory but part of the system.
A gearbox adds four key benefits to a servo system:
| Benefit | Effect on the system |
|---|---|
| Torque increase | Torque is multiplied by the gear ratio, allowing a smaller motor that draws less current to be selected |
| Speed reduction | The high speed at which the motor operates efficiently is reduced to the operating speed required by the machine |
| Inertia matching | Load inertia is reflected to the motor reduced by the square of the gear ratio; the system operates more stably and with less vibration |
| Resolution gain | Each degree on the motor shaft corresponds to a smaller angle at the output shaft after being divided by the gear ratio |
Inertia matching is one of the most frequently overlooked issues in servo applications. When load inertia is much greater than motor inertia, the system oscillates, settling time increases and the machine vibrates while the encoder feedback continually tries to correct the error. Choosing the correct gear ratio resolves this issue at its source.
The preferred type for servo drives is a planetary gearbox. The reason is structural: the load is distributed across multiple planet gears instead of a single gear pair. This distribution provides high torque capacity, low backlash and high torsional rigidity in a compact housing. Since positioning accuracy depends directly on backlash, low-backlash planetary gearboxes are used in servo applications. For details of the operating principle, see our article on how planetary gearboxes work .
Two calculations are decisive during selection: the output torque required by the application and the necessary gear ratio. For these calculations, the formulas in our articles on torque calculation and gearbox speed calculation can be applied directly.
Let's Select the Right Gearbox for Your Servo Application Together
In a servo-driven machine, performance is determined not by motor power alone but by how well the motor–gearbox–load combination is matched. An incorrectly selected gear ratio or a gearbox with excessive backlash will cause positioning errors and vibration even with the best servo motor.
Since 1987, Remak Redüktör has been manufacturing reducers and gearboxes. Based on your application's torque, speed and inertia values, we can determine the appropriate planetary gearbox series together. You can review our product family on the planetary gearboxes page and contact us through our contact page to share the technical details of your project.
Frequently Asked Questions About Servo Motors
What is the difference between a servo motor and a stepper motor?
The main difference is feedback. A stepper motor operates open-loop; it rotates by the number of steps it is commanded to take but cannot verify whether it actually moved. If the load is heavier than expected, it may miss steps without detecting it. A servo motor, however, always knows its position from encoder feedback and corrects deviations immediately. Stepper motors are economical at low speeds and constant loads; servo motors are used for applications requiring high speed, variable loads and precise stopping.
Why is a gearbox added to a servo motor?
It is added for four reasons: to multiply motor torque by the gear ratio, reduce high motor speed to the machine's operating speed, reduce reflected load inertia to a level the motor can handle and prevent vibration, and improve positioning resolution. Without a gearbox, achieving the same torque requires a much larger and more expensive motor; high load inertia can also make the system unstable.
How accurately can a servo motor position?
Accuracy is determined by three factors: encoder resolution, gearbox backlash and the rigidity of the mechanical transmission components. Industrial encoders can produce hundreds of thousands of pulses per revolution, so theoretical resolution can reach roughly one-thousandth of a degree. In practice, however, the limiting factor is usually not the motor but the mechanical connection. If a high-backlash gearbox is used, deviation appears at the output shaft regardless of encoder accuracy.
When should a standard (induction) motor be preferred over a servo motor?
If it does not matter exactly where the motion stops and the speed can remain constant, an induction motor is sufficient. In fans, pumps, mixers, continuously running conveyors and similar applications, the servo system's positioning capability is not used, so the added cost brings no benefit. If variable speed alone is sufficient, an induction motor driven by a frequency inverter provides a much more economical solution. A servo is economically justified only when positioning or synchronization accuracy is genuinely required.
Can a servo motor operate without a servo drive?
No. The drive is the unit that supplies the servo motor windings in the correct sequence and at the correct current while reading and evaluating the encoder signal. Applying voltage directly to the motor does not create controlled motion. The motor and drive must also be compatible; if the drive's current capacity, encoder type and communication protocol do not match the motor, the system will not operate.
Does a servo motor hold its position when power is cut?
When power is removed, the motor cannot produce holding torque, so on vertical axes the load may move downward under its own weight. In applications with this risk, an electromagnetic brake that engages when de-energized is used. To retain position information, an absolute encoder is preferred; with backup-battery support, this type of encoder remembers the position after a power loss and eliminates the need to re-home the machine.
