What is a servomotor?
A servomotor is an electric motor with built-in position feedback, usually from an encoder or resolver, that works with a servo drive in a closed control loop to follow position, speed and torque commands precisely. The motor on its own is “dumb”: only the combination with the drive, which continuously compares the measured position with the commanded position, turns it into a servo system. Servomotors are the muscles of motion control in robots, CNC machines, packaging lines and pick-and-place systems. The word servo comes from the Latin servus (servant): the motor faithfully follows what the controller commands.
🧠 How does a servomotor work?
A servomotor always works closed loop. A motion controller or PLC calculates where the axis should be in each cycle and sends that setpoint to the servo drive. The drive controls the axis with a cascade of three nested loops:
- Position loop - compares the commanded position with the measured one; the difference is called the following error
- Velocity loop - turns the position correction into a required speed
- Current loop - controls the motor current, and therefore the torque directly, often within tens of microseconds
The encoder on the motor shaft continuously reports the actual position. If the axis deviates, for example because of a heavier load or an opposing force, the drive corrects it within milliseconds. If the following error exceeds a configured limit, the drive stops the axis with a fault. This is how a servo system detects a collision or a jam, something an open-loop drive cannot do.
🔧 What are the parts of a servomotor?
An industrial servomotor is compact and slim, with low rotor inertia so that it can accelerate and decelerate quickly.
| Part | Function | In practice |
|---|---|---|
| Stator with windings | Creates the rotating field | Three-phase, fed by the servo drive |
| Rotor with permanent magnets | Delivers torque without slip | Rare-earth magnets such as neodymium (NdFeB) |
| Feedback device | Measures position and speed | Resolver, incremental or absolute encoder |
| Holding brake (optional) | Holds the shaft at standstill | Spring-applied: closes when power is lost |
| Temperature sensor | Protects the winding against overload | Sensor in the winding, read by the drive |
| Connector(s) | Power and signals | Increasingly a single cable for both |
So the servomotor is not the whole servo system. The servo drive (or servo amplifier) converts the setpoint into motor current and closes the control loops; the motion controller plans the movement. Manufacturers often offer two variants: a low-inertia version for high dynamics and a medium-inertia version for stable control of medium to large moving masses. Siemens, for example, calls these High Dynamic and Compact in its SIMOTICS S-1FK2 range; at the same frame size, the Compact version has roughly 2.5 times the rotor inertia.
Feedback in detail. A resolver is a rugged rotary transformer without electronics, and it tolerates heat and vibration well. Encoders are more accurate. An absolute multi-turn encoder knows the position immediately after power-up, so the axis does not need to move to a reference point first. Modern motors offer resolutions of 20 bits or more per revolution: the 1FK2 has 22 bits (just over 4.1 million positions per revolution) plus 12 bits for counting revolutions. Digital encoder protocols such as EnDat, BiSS and HIPERFACE DSL replace the older analogue sine/cosine signals; with one-cable technology (for example HIPERFACE DSL or Siemens OCC), the encoder signals run over the same cable as the motor power. The encoder memory then also holds an electronic nameplate, so the drive reads the motor data automatically during commissioning.
🔄 What types of servomotor are there?
| Type | Principle | Typical application |
|---|---|---|
| AC synchronous servomotor (PMSM) | Permanent magnets, sinusoidal commutation | The standard in machine building and robotics |
| Brushless DC motor (BLDC) | Permanent magnets, block commutation via Hall sensors | Small axes, drones, compact devices |
| Brushed DC servomotor | Commutator with carbon brushes | Older machines, simple applications |
| Asynchronous servomotor | Squirrel-cage rotor with encoder and vector control | Main spindles and higher power ratings |
| Linear motor | ”Unrolled” motor, direct linear motion | Fast gantries, semiconductor industry |
| Torque motor (direct drive) | High torque at low speed, no gearbox | Rotary tables, indexing tables |
A hobby servo, as used in model building and simple robot kits, is a separate category: a small DC motor with a gearbox and a potentiometer for feedback. It is driven by a pulse signal of about 50 Hz in which a pulse width of roughly 1 to 2 ms sets the angle, with about 1.5 ms as the centre position.
⚖️ How does a servomotor differ from a stepper motor and an induction motor?
| Characteristic | Servomotor | Stepper motor | Induction motor with VFD |
|---|---|---|---|
| Control principle | Always closed loop | Usually open loop (counting steps) | Usually speed control, open or closed loop |
| Position accuracy | Very high, depends on the encoder | Good, but can lose steps unnoticed | Limited without an encoder |
| Torque at high speed | Stays high up to rated speed | Drops sharply | Constant up to rated speed |
| Overload capacity | Briefly several times rated torque | Low | Limited |
| Cost | High (motor, drive, cables) | Low | Low to medium |
| Typical use | Dynamic, synchronised axes | Small, slow positioning | Pumps, fans, conveyors |
Rule of thumb: choose a servomotor when the axis must move quickly, precisely and in sync with other axes. A conveyor at constant speed only needs an induction motor with a frequency converter; for a simple dosing unit, a stepper motor is often cheaper.
🛠️ How do you select and size a servomotor?
A servomotor is chosen on its motion profile, not on power alone. A common approach:
- Define the motion profile - distance, cycle time, acceleration, constant speed and dwell time per cycle.
- Determine the load inertia - inertia reflected through a gearbox is divided by the square of the gear ratio. A 10:1 planetary gearbox therefore reduces the inertia the motor sees by a factor of 100.
- Calculate the torque - peak torque during acceleration must fall within the peak range of both motor and drive; RMS torque over the whole cycle must stay below the continuous torque, or the motor will overheat.
- Check the inertia ratio - a rule of thumb is load inertia divided by motor inertia of no more than 10:1. A higher ratio causes overshoot and resonance; very dynamic axes benefit from a lower ratio.
- Consider braking energy - when decelerating hard, the motor acts as a generator. That energy goes to a braking resistor or, with a shared DC bus, to other axes.
- Choose options - encoder type (absolute or incremental), holding brake for vertical axes, degree of protection (usually IP64 or IP65) and shaft seal.
- Tune - after installation, the drive’s auto-tuning function sets the control parameters; fine-tuning uses PID-style gains and feedforward in the motion software.
An oversized motor is also a mistake: it costs more, uses more energy and can actually control worse. Vendors such as Siemens, Beckhoff and Rockwell Automation provide sizing software that runs through these steps. With virtual commissioning you can even test the profile before the machine exists.
🔐 Which safety and IT/OT aspects apply to servomotors?
Functional safety. A moving servo axis is a significant hazard. Servo drives provide safety functions from IEC 61800-5-2, such as Safe Torque Off (STO, stop category 0 under IEC 60204-1), Safe Stop 1 and Safe Brake Control. The holding brake needs attention: on a vertical axis, the load drops after STO if there is no brake, so the brake must be monitored and tested periodically. The achievable level (PL under ISO 13849 or SIL under IEC 62061) depends on the certified components and the wiring. Read more under functional safety.
Network and data. Servo drives communicate with the controller over real-time networks such as EtherCAT, ProfiNET IRT or SERCOS III, using standardised profiles such as CiA 402 and PROFIdrive. That makes every servo axis a data point as well: motor current, temperature, following error and bearing load feed condition monitoring and predictive maintenance. A rising current for the same movement, for example, can point to wear in the mechanics.
Cybersecurity. A servo drive contains firmware and parameters that can be changed remotely through engineering tools. Anyone who manipulates speed limits or control parameters can affect product quality or machine safety. Place drives in a shielded machine network (network segmentation) and control who may change parameters. The Machinery Regulation (EU) 2023/1230, which applies from 20 January 2027, also requires machinery and safety components to be protected against corruption through malicious interference.
Energy efficiency. Ecodesign Regulation (EU) 2019/1781 makes IE classes mandatory for asynchronous induction motors. Synchronous permanent-magnet servomotors fall outside that regulation, although they are inherently efficient because the rotor has no slip losses.
❓ Frequently asked questions
What is the difference between a servomotor and a servo drive?
A servomotor is the motor with feedback that delivers the motion. A servo drive (servo amplifier) is the power electronics that controls the motor current and closes the position, velocity and current loops. Together with the motion controller, they form the servo system.
Why does a servomotor need an encoder?
The encoder continuously reports the actual position and speed of the motor shaft to the servo drive. Without that feedback, the drive cannot correct deviations and there is no closed control loop. That is why every servomotor has an encoder or a resolver.
What is the difference between a servomotor and a stepper motor?
A servomotor always works closed loop and corrects deviations continuously, so it handles high speeds, high dynamics and brief overloads. A stepper motor turns in fixed steps, usually without feedback, and can lose steps unnoticed when overloaded. Stepper motors are cheaper and suit small, slower positioning tasks.
What does the inertia ratio mean for a servomotor?
The inertia ratio is the inertia of the load divided by the rotor inertia of the servomotor. A widely used rule of thumb is a maximum of 10:1; at a higher ratio, the axis becomes more prone to overshoot and resonance. A gearbox reduces the load inertia by the square of the gear ratio.
Why does a servomotor sometimes have a brake?
A servomotor with a holding brake can hold an axis while the motor delivers no torque, for example at standstill, during a power failure or after Safe Torque Off. This is especially important on vertical axes that would otherwise drop under gravity. The brake is meant for holding, and for stopping only in emergencies.
Do EU energy-efficiency rules apply to servomotors?
Ecodesign Regulation (EU) 2019/1781 sets efficiency classes for asynchronous induction motors, not for synchronous permanent-magnet servomotors. Servomotors are nevertheless inherently efficient thanks to their permanent magnets. Correct sizing and reusing braking energy through a shared DC bus help you save more.
📌 In summary
A servomotor is a motor with feedback that, together with a servo drive and motion controller, follows position, speed and torque in a closed control loop. Usually it is an AC synchronous motor with permanent magnets, low rotor inertia and a high-resolution encoder. You select it on the motion profile, paying attention to RMS and peak torque, inertia ratio, braking energy and a holding brake on vertical axes. Because servo drives are networked and contain safety functions, they also belong in your approach to functional safety and OT security.
A servomotor only becomes a servo together with its drive: the encoder closes the loop, enabling precise, synchronised motion and useful machine data.