Servo Motors and Servo Drives Explained: Precision Motion Control in Industrial Automation

Quick overview: This guide explains servo motors and servo drives and the practical checks that help you understand and plan an installation. Use the contents below to go directly to the relevant section.

Many industrial applications need more than simply starting a motor or adjusting its speed. Packaging machines, indexing systems, positioning tables, cutters, feeders and other automated equipment can require precise control of position, speed and torque. Servo systems are designed for this type of motion control.

A typical servo system combines a servo motor, servo drive and feedback device with a controller or motion-control system. These components operate together as a coordinated system, which means selecting a servo motor by kilowatt rating alone is rarely sufficient.

This guide explains the fundamentals of servo motors and servo drives, how they differ from conventional motor-control equipment, the specifications that matter during purchasing and the information that should be collected when replacing or selecting a servo system.

What Is a Servo System?

A servo system is a closed-loop motion-control system designed to control variables such as position, speed and torque with high accuracy and dynamic response.

A simplified servo architecture can look like:

PLC or motion controller → Servo drive → Servo motor → Machine load

A feedback device, commonly an encoder, provides information about the motor's actual movement back to the drive or controller.

The system compares commanded motion with actual motion and continuously adjusts motor output to reduce the difference.

What Is a Servo Motor?

A servo motor is a motor designed to operate as part of a controlled feedback system.

Industrial servo motors are commonly permanent-magnet synchronous motors, although servo technology is broader than one motor construction.

Servo motors are designed for applications requiring characteristics such as rapid acceleration, accurate positioning, controlled torque and repeatable motion.

What Is a Servo Drive?

The servo drive is the power-electronic controller that supplies and controls the servo motor.

It receives a motion command from a PLC, motion controller, CNC system or other control device and regulates the motor according to the required operating mode.

The drive also processes feedback from the motor encoder and can provide diagnostics, alarms and operating data to the automation system.

Servo Drive vs Servo Motor

The motor and drive are different components but normally form a matched system.

The servo motor produces mechanical motion.

The servo drive controls the electrical power supplied to that motor.

Compatibility between them can involve voltage, current, motor parameters, encoder interface, feedback protocol and manufacturer-specific commissioning data.

A servo motor should therefore not be assumed compatible with an arbitrary servo drive simply because the power ratings appear similar.

What Does Closed-Loop Control Mean?

Closed-loop control uses feedback to determine what the motor actually did after receiving a command.

For example, if a controller commands the shaft to rotate to a particular position, the encoder reports the actual position. The drive continually adjusts motor torque to reduce the error between commanded and measured position.

This feedback loop is one of the major differences between a typical servo system and basic open-loop motor control.

What Is an Encoder?

An encoder is a feedback device that measures motor movement.

Depending on the servo system, the encoder can provide information about:

• Position
• Speed
• Direction
• Rotor position for motor commutation

Servo encoders can use incremental, absolute or manufacturer-specific feedback technologies.

The encoder interface must be compatible with the servo drive.

Incremental vs Absolute Encoders

Incremental encoders generate position information relative to movement. The control system commonly establishes a known reference position through a homing procedure after power-up.

Absolute encoders can retain or communicate an absolute position reference according to their design, which can reduce or eliminate certain homing requirements.

Absolute encoder capabilities vary. Some systems use battery-backed multi-turn encoders, while others use batteryless technologies.

Encoder Resolution

Encoder resolution describes how finely the feedback system can represent shaft position.

Higher resolution provides more feedback counts per revolution, but overall machine positioning accuracy also depends on mechanical transmission, backlash, stiffness, load behaviour, tuning and control architecture.

A high-resolution encoder does not by itself guarantee equally high machine accuracy.

Position Control

In position-control mode, the servo system moves the load to commanded positions.

Applications can include:

• Indexing tables
• Cut-to-length systems
• Packaging machinery
• Pick-and-place mechanisms
• Feeders
• Labelling systems
• Assembly equipment

Position commands can be provided through pulse/direction signals, analogue references, industrial networks or manufacturer-specific motion interfaces depending on the system.

Speed Control

Servo systems can regulate motor speed with feedback.

Speed-control applications benefit from rapid response to changes in command or load.

Although VSDs also control motor speed, the required dynamic performance, feedback and motion functions determine which technology is appropriate.

Torque Control

Servo drives can also regulate motor torque.

Torque-control applications can include winding, tension control, pressing and other processes where controlled force is important.

Mechanical design and appropriate safety measures remain essential because servo systems can produce high torque very quickly.

Servo Motor vs Standard Induction Motor

A conventional industrial induction motor is well suited to many pumps, fans, conveyors and general machinery applications.

A servo motor is designed for high-performance controlled motion and commonly includes integrated feedback.

Typical differences can include:

• Rotor construction
• Feedback system
• Dynamic response
• Torque-to-inertia ratio
• Positioning capability
• Motor-drive matching requirements

The more expensive servo solution is not automatically the better choice. The application determines the required technology.

Servo Drive vs VSD

A variable speed drive is primarily used to control the speed and torque of AC motors in applications such as pumps, fans, conveyors and process machinery.

A servo drive is typically used where precise position, rapid acceleration and deceleration, synchronised motion or highly dynamic torque control are required.

Modern VSDs can provide sophisticated closed-loop and positioning functions, while servo systems can perform speed-control tasks. The technologies therefore overlap in some applications.

Selection should be based on the required motion performance rather than the product name alone.

Power Rating

Servo motors and drives are available across a wide range of power ratings.

Power is important, but it is not enough to size a servo axis.

The design also needs to consider torque, speed, inertia, acceleration, duty cycle and mechanical transmission.

Rated Torque

Rated torque is the torque a servo motor is designed to produce continuously under specified operating conditions.

The required continuous torque is influenced by the load, friction, gravity, mechanical transmission and operating cycle.

Running continuously above the permitted thermal capability can cause overheating or protective trips.

Peak Torque

Servo motors can often provide peak torque for short periods to accelerate or decelerate a load.

Peak torque capability is one reason servo systems can achieve rapid dynamic motion.

Peak torque should not be treated as a continuous operating rating. The permitted magnitude and duration depend on the motor and drive.

Speed

Servo motors have rated and maximum speed specifications.

The required motor speed depends on the machine's mechanical transmission and desired load speed.

Gearboxes, belts, ball screws and other mechanisms convert motor rotation into the required machine motion.

Acceleration and Deceleration

High acceleration is a common servo requirement.

Acceleration torque depends strongly on the combined inertia of the motor and reflected machine load.

A motor that has enough steady-state power can still perform poorly if it cannot accelerate the load at the required rate.

What Is Inertia?

Inertia describes resistance to a change in rotational speed.

In servo applications, the inertia of the motor rotor and the inertia of the machine load are important for dynamic performance and tuning.

Mechanical components such as couplings, pulleys, gears and screws contribute to the total reflected inertia seen by the motor.

Load-to-Motor Inertia Ratio

The relationship between load inertia and motor inertia affects how easily the servo system can control the mechanical load.

Permitted or recommended inertia ratios vary between servo families and applications.

Very high load inertia can make tuning more difficult and can reduce dynamic performance.

Manufacturer sizing tools and application calculations should be used rather than applying one universal ratio to every servo system.

Servo Motor Sizing Is More Than kW

For a meaningful servo selection, useful application data includes:

• Required motion distance or angle
• Maximum speed
• Acceleration time
• Deceleration time
• Load mass
• Load inertia
• Friction
• Duty cycle
• Mechanical transmission
• Required positioning accuracy

Without this information, selecting only by motor power can result in poor performance or unnecessary oversizing.

Rotary vs Linear Motion

Most servo motors produce rotary motion.

Machine mechanisms can convert that rotation into linear movement using ball screws, belts, rack-and-pinion systems or other transmissions.

Linear servo technologies also exist for applications requiring direct linear motion.

Gearboxes and Servo Motors

A precision gearbox can be used with a servo motor to change speed and torque and improve the relationship between motor and load inertia.

Servo gearboxes are commonly designed for low backlash and high dynamic performance.

Important gearbox specifications include:

• Reduction ratio
• Rated torque
• Peak torque
• Backlash
• Efficiency
• Input speed
• Output bearing loads

The motor, gearbox and machine load should be considered as one mechanical system.

What Is Backlash?

Backlash is mechanical free movement between mating transmission components before motion is transferred in the opposite direction.

Gearboxes, gears, screws and couplings can contribute backlash.

In positioning applications, excessive backlash can reduce repeatability and accuracy even when the servo motor feedback is highly precise.

Couplings

Servo motors are commonly connected to machine shafts using suitable couplings.

A coupling should accommodate the required torque and speed while considering alignment, torsional stiffness and permissible misalignment.

An inappropriate flexible coupling can reduce motion performance or fail mechanically.

Servo Motor Brakes

Some servo motors are available with holding brakes.

A holding brake is commonly used to hold a stationary load when motor power is removed, particularly on vertical axes.

It should not automatically be treated as a dynamic service brake for repeatedly stopping a moving load unless the manufacturer specifically permits that use.

Vertical Axes

Vertical motion requires particular attention because gravity acts continuously on the load.

The design can require:

• A motor holding brake
• Mechanical counterbalance
• Appropriate torque margin
• Controlled stopping
• Safety measures against uncontrolled descent

A servo brake alone should not be assumed to satisfy every machine-safety requirement.

Regenerative Energy

When a servo motor decelerates a load, mechanical energy can be returned to the drive's DC bus.

Vertical lowering and high-inertia deceleration can also generate significant regenerative energy.

The drive must manage this energy appropriately.

Braking Resistors

A braking resistor can dissipate regenerative energy as heat where supported by the servo drive.

Resistor selection depends on resistance, power, duty cycle and the drive manufacturer's requirements.

An arbitrary resistor should not be connected to a servo drive merely because its resistance value appears close.

Regenerative Units

Larger or highly regenerative systems can use regenerative power units that return energy to the electrical supply or share energy through a common DC system where supported.

The appropriate approach depends on the servo platform and machine duty cycle.

Servo Drive Supply Voltage

Servo drives are manufactured for specific electrical supply ranges.

Common systems can use single-phase or three-phase AC supplies depending on the drive size and product family.

Always verify the exact input voltage, phase, frequency and protective-device requirements before ordering.

Motor Voltage Is Not Necessarily the Mains Voltage

The motor is supplied by the servo drive, not directly from the mains in a normal servo system.

Motor electrical specifications are therefore matched to the drive output and servo family.

Do not connect a servo motor directly to an AC supply unless the manufacturer specifically designed the product for that arrangement.

Servo Motor Cables

Servo systems commonly use dedicated motor power and feedback cables.

Depending on the system, separate cables can be required for:

• Motor power
• Encoder feedback
• Motor brake

Some modern servo systems combine functions into hybrid or single-cable arrangements.

Connector type, cable length and shielding requirements are product-specific.

Encoder Cables Matter

The encoder cable carries sensitive feedback information.

Using an incorrect cable, connector or pinout can cause feedback faults or damage equipment.

When replacing a servo motor or drive, check whether existing motor and encoder cables remain compatible with the replacement generation.

Cable Length

Servo manufacturers specify permitted cable lengths and available cable assemblies.

Longer motor and encoder runs can affect signal integrity, voltage stress and electromagnetic compatibility.

Do not assume that a standard cable can simply be extended indefinitely.

Electromagnetic Compatibility

Servo drives switch power electronically and can generate electrical noise.

Correct cable type, shielding, earthing, separation and panel design are important for reliable operation and electromagnetic compatibility.

Manufacturer installation instructions should be followed, particularly for motor and encoder cables.

Servo Drive Control Methods

A servo drive can receive motion commands in several ways depending on the model.

These can include:

• Pulse and direction
• CW/CCW pulse commands
• Analogue speed or torque reference
• Industrial Ethernet motion networks
• Fieldbus communications
• Internal positioning tables

The control method must match the PLC or motion controller and required performance.

Pulse Train Control

Traditional servo systems often accept high-speed pulse commands from a PLC or motion controller.

The number of pulses can represent movement distance while pulse frequency relates to commanded speed, depending on the configuration.

The controller output must support the required pulse frequency and electrical interface.

Networked Servo Control

Modern servo systems increasingly use industrial communications networks for motion commands and diagnostics.

Depending on the manufacturer and system, technologies can include EtherCAT, PROFINET-based motion, EtherNet/IP-based motion and other real-time motion networks.

The PLC, motion controller, servo drive and engineering software must all support the selected architecture.

EtherCAT

EtherCAT is widely used for high-performance industrial motion control because it supports deterministic communications and synchronisation across multiple axes.

Having an Ethernet connector does not mean a servo drive supports EtherCAT. Protocol support must be confirmed from the exact model specification.

Multi-Axis Motion

Machines can use multiple servo axes that operate independently or in coordination.

Examples include packaging, printing, robotics, material handling and assembly equipment.

Coordinated motion can require a dedicated motion controller or PLC with appropriate motion functions and a deterministic communications network.

Electronic Gearing

Electronic gearing allows one servo axis to follow another axis or reference at a defined ratio without a physical gear train performing the synchronisation.

This can be used in coordinated machine processes where multiple axes need a controlled relationship.

Electronic Cam

Electronic cam functions allow a servo axis to follow a programmed motion profile relative to another axis or master position.

They can replace certain mechanical cam mechanisms in advanced machines.

Availability depends on the controller and servo platform.

Homing

Homing establishes a known machine reference position.

A homing routine can use:

• Home sensor
• Limit switch
• Encoder index
• Mechanical reference
• Absolute encoder position

The required method depends on machine design and encoder technology.

Limit Switches

Servo axes often use limit switches to identify travel boundaries or provide reference signals.

Software travel limits can also be configured in the control system.

Where hazardous movement is possible, ordinary software limits should not be treated as a substitute for required safety functions.

Servo Tuning

Servo tuning adjusts the control-loop response to the motor and mechanical load.

Poor tuning can cause:

• Oscillation
• Noise
• Slow response
• Position error
• Overshoot
• Mechanical vibration

Modern servo drives commonly provide auto-tuning functions, but the mechanical system still needs to be suitable for the required performance.

Auto-Tuning

Auto-tuning functions estimate machine characteristics and configure control parameters.

They can simplify commissioning, but they are not a substitute for correct motor sizing and sound mechanical design.

Highly flexible, resonant or unusual mechanisms can require additional tuning.

Notch Filters and Vibration Suppression

Advanced servo drives can include filters and vibration-suppression functions to reduce the effect of mechanical resonances.

These features can improve performance but should not be used to conceal serious mechanical problems such as loose couplings or excessive backlash.

Positioning Accuracy vs Repeatability

Accuracy describes how closely the actual machine position matches the commanded absolute position.

Repeatability describes how consistently the machine returns to the same position.

A machine can be highly repeatable but still have an offset from the true desired position.

Servo feedback, mechanics, calibration and measurement all influence these characteristics.

Servo Alarms and Diagnostics

Servo drives monitor operating conditions and can generate alarms for issues such as:

• Overcurrent
• Overvoltage
• Undervoltage
• Overtemperature
• Encoder fault
• Following error
• Overload
• Communication fault

Alarm codes should be interpreted using the manual for the exact drive model.

Following Error

Following error is the difference between commanded position and actual position.

Excessive following error can indicate that the motor cannot keep up with the commanded motion because of load, acceleration, tuning, mechanical restriction or another fault.

The permissible error is application and system specific.

Servo Overload

A servo motor can produce peak torque for short periods, but repeated or prolonged high torque can exceed its thermal capability.

Servo drives commonly calculate motor loading and provide overload protection based on motor data.

Frequent overload alarms can indicate incorrect sizing, excessive mechanical load or an overly aggressive motion profile.

Motor Temperature

Servo motor temperature depends on torque demand, speed, ambient temperature, mounting and duty cycle.

Some servo motors include temperature sensing integrated into the drive system.

Operating conditions should remain within manufacturer limits.

Servo Drive STO

Many modern servo drives include Safe Torque Off, or STO, inputs.

STO is a safety function designed to prevent the drive from generating motor torque when correctly integrated into a suitable safety system.

STO does not automatically provide mechanical braking, position holding or every required machine safety function.

Safety design must follow the drive documentation, risk assessment and applicable standards.

Emergency Stops and Servo Systems

Emergency stopping a servo-controlled machine requires an engineered safety strategy.

Depending on the risk and machine, the strategy can involve controlled stopping, STO, contactors, mechanical brakes or other safety functions.

An ordinary PLC command or HMI stop button should not be assumed to constitute an emergency-stop safety function.

Servo Drives and PLCs

A PLC can coordinate servo operation through digital I/O, pulse outputs or industrial communications.

More advanced applications use PLCs with integrated motion-control functions.

Compatibility should be checked at the controller, network, software and servo-drive level.

Servo Drives and HMIs

An HMI can display servo information made available through the PLC or supported direct communications.

Examples can include:

• Axis position
• Speed
• Torque
• Servo enabled status
• Alarm codes
• Motion mode

The HMI normally provides operator interaction while the motion controller or PLC executes the motion logic.

Servo Drives and Remote I/O

Remote I/O can provide distributed sensors, limits, pushbuttons and machine signals associated with servo axes.

Time-critical motion feedback is normally handled through the servo feedback and motion-control architecture rather than ordinary remote I/O.

The complete control system should be designed according to the timing requirements of the machine.

Replacing an Existing Servo Motor

A failed servo motor should ideally be replaced using the exact part number or a manufacturer-approved replacement.

Important information includes:

• Manufacturer
• Exact motor model
• Rated power
• Rated and maximum speed
• Rated and peak torque
• Encoder type
• Brake option
• Shaft dimensions
• Flange dimensions
• Connector orientation
• Servo drive model

A motor with similar kW and flange size can still be electrically or electronically incompatible.

Replacing an Existing Servo Drive

When replacing a servo drive, collect:

• Exact drive model
• Servo motor model
• Input supply
• Motor power
• Encoder type
• Control method
• PLC or motion controller model
• Network protocol
• Existing parameters
• Engineering software

A newer drive generation can require different cables, software, parameter conversion or PLC configuration.

Back Up Servo Parameters

Servo drives contain application-specific parameters including motor configuration, tuning, electronic gearing, limits and communications settings.

Where possible, maintain documented backups of servo parameters and engineering projects.

A physically identical replacement drive still needs the correct configuration before the machine can operate as intended.

Discontinued Servo Systems

Servo equipment can remain in machinery for many years, and older motor-drive families eventually become obsolete.

Migration can involve replacing:

• Servo drive
• Servo motor
• Motor cable
• Encoder cable
• PLC interface
• Network hardware
• Engineering software

The complete axis should therefore be assessed before assuming one obsolete component can be substituted independently.

Do Not Mix Servo Components by Power Rating Alone

Two 750 W servo motors from different manufacturers, or even different families from the same manufacturer, are not automatically interchangeable.

Differences can include:

• Voltage
• Current
• Torque curve
• Encoder protocol
• Connector
• Shaft size
• Flange
• Brake
• Motor parameters

The exact motor-drive combination should be verified.

Common Servo Selection Mistakes

• Selecting a servo by kW alone
• Ignoring peak torque
• Not calculating acceleration requirements
• Ignoring load inertia
• Assuming any servo motor works with any servo drive
• Ordering the wrong encoder type
• Forgetting the holding brake on a vertical axis
• Not checking motor and encoder cable compatibility
• Ignoring shaft and flange dimensions
• Assuming an Ethernet connector means the required motion protocol is supported
• Ignoring regenerative braking requirements
• Not backing up drive parameters
• Replacing a discontinued drive without checking motor compatibility
• Treating STO as a complete machine-safety solution
• Assuming high encoder resolution guarantees machine accuracy

A Practical Planning Checklist

For an accurate servo quotation, provide as much of the following information as possible:

• Application: ___
• New system or replacement: ___
• Required motion: rotary or linear
• Load mass: ___
• Load inertia if known: ___
• Required travel: ___
• Maximum speed: ___
• Acceleration time: ___
• Deceleration time: ___
• Cycle time: ___
• Required positioning accuracy: ___
• Mechanical transmission: direct, gearbox, belt, ball screw, rack and pinion or other
• Gear ratio if applicable: ___
• Vertical axis: yes or no
• Motor brake required: yes, no or unsure
• Supply voltage: ___
• Control method: pulse, analogue, network or unsure
• PLC/motion controller manufacturer: ___
• PLC/motion controller model: ___
• Required network protocol: ___
• STO required: yes, no or project dependent
• Motor cable length: ___
• Existing servo drive model if replacing: ___
• Existing servo motor model if replacing: ___
• Existing gearbox model if applicable: ___
• Parameter backup available: yes, no or unknown
• Quantity: ___

For replacement enquiries, clear photographs of the servo motor and drive nameplates, connectors and existing cables can help identify the correct equipment.

How Servo Systems Fit Into a Complete Automation System

Servo motors and servo drives form the motion-control layer of many automated machines. They can operate alongside PLCs, HMIs, remote I/O, industrial Ethernet switches, sensors, safety equipment and other automation components.

Understanding these relationships matters during purchasing because servo hardware must be compatible not only with the mechanical load but also with the controller, feedback system, communications architecture and existing machine design.

Softcore Group Industrial Solutions's Knowledge Centre covers these technologies individually to help customers, engineers and purchasing teams identify the specifications required before products are selected or quoted.

Products listed in our active online catalogue can be purchased or quoted through the Softcore Group Industrial Solutions store. Other brands, models and project-specific requirements can be quoted on request, with pricing, availability and lead time confirmed separately.

For servo enquiries, provide the exact existing motor and drive model numbers where applicable. For new applications, information about the mechanical load, motion profile, speed, torque, positioning requirements, controller and communications architecture helps narrow the appropriate servo system before quotation.

Technical Note

This article provides general industrial automation information. Servo sizing, motion profiles, mechanical design, drive configuration, regenerative energy, electrical installation, tuning, network architecture and machine safety must be assessed for the specific application. Manufacturer documentation, applicable standards, machine risk assessments and site requirements take precedence. Safety-related functions require appropriately designed and validated safety systems and should be implemented by competent personnel.

Related catalogue examples: Servo Drives & Motors | Variable Speed Drives. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.