Choosing the right servo motor for your industrial application is vital for achieving the right balance of functionality, reliability, and of course, cost.
Selecting the wrong motor can result in degraded performance (if it even performs at all), damage to ancillary hardware, system failure, and further downstream, quality issues in the goods or services that you are providing.
In this article, we will be exploring the types of servo motors available, and how to select the right one, so you can get the best bang for your buck in terms of functionality and reliability.
What is a Servo Motor?
Servo motors are highly precise electromechanical devices used in motion control for industrial machinery.
They operate on the same fundamental principles as regular electric motors, in the sense that they work by energizing stator coils to create a magnetic field that drives a rotor. The main difference is that servo motors include feedback systems, typically in the form of encoders or resolvers, that monitor speed, torque, and position.
This data is sent to a controller, which adjusts the motor’s operation using algorithms like PID control. The result is a closed-loop system that corrects errors in real time, ensuring precise movement. This level of control is why servo motors are essential in robotics, CNC machines, and high-accuracy automation.
How to Choose the Right Servo Motor?
From a technical perspective, your choice of servo motor will be determined largely by the requirements of the hardware that it is moving. These requirements include various torques, speeds, voltages, positioning accuracy, and other performance metrics, which we will delve into in the following section.
Determine Torque
The first step in selecting the right servo motor for your needs is to determine the required torque(s) needed to drive whatever it is that you need driving.
Torque is the rotational equivalent of force, and is, in layman’s terms, how much twisting force your motor will apply when turning about an axis (or turning the shaft, in this case).
When you unscrew a bottle of drinking water, it’s the torque that will remove the lid. If you apply too little torque, the lid won’t move. Too much torque, and the lid could break, or you could insure yourself.
It is vital to know the torque requirements, because as with the water bottle analogy, if you don’t apply enough torque, the thing that you are trying to rotate won’t go anywhere (it will stall). Conversely, if you apply too much torque, then this can result in damage to machinery.
The fundamental torque formula is as follows:
T = F x r
Where:
T= torque (measured in Nm, for SI units)
F = force (usually measured in N)
R = the radius, or lever arm (typically measured in meters)
There are a variety of different torques to consider during the servo motor selection process, but the main three of importance are continuous torque, peak torque, and acceleration torque. These describe how the servo motor operates under steady load, handles short bursts of high load, and responds to changes in speed, respectively.
Continuous torque
Continuous torque is the torque a servo motor must deliver consistently without overheating or experiencing performance degradation. It represents the steady-state torque required to keep the system running under normal conditions.
To calculate continuous torque (Tcont), sum the torques from all external forces, including gravity and friction:
Tcont = Texternal + Tgravity + Tfriction
where:
Texternal accounts for applied loads (e.g., conveyor belts, robotic arms).
Tgravity = Fg × r, where Fg is gravitational force and r is the lever arm.
Tfriction accounts for system resistance.
Peak torque
Peak torque is the maximum torque a servo motor can produce for short bursts without sustaining damage. It is necessary for overcoming sudden load changes, such as starting motion, handling impact forces, or dealing with temporary resistance spikes. Unlike continuous torque, which must be maintained indefinitely, peak torque is only required for brief moments.
To calculate peak torque (Tpeak):
Tpeak = Tcont + Tacceleration
where:
Tcont is the continuous torque required for steady operation.
Tacceleration is the accelerating torque and is explained below.
Accelerating Torque
Accelerating torque is the torque required to change the speed of a system, whether it’s starting from rest or increasing its velocity during operation. It is vital for applications where the motor needs to respond to rapid changes in speed or overcome inertia quickly.
This type of torque is highly dependent on the system’s moment of inertia (J), which represents the resistance of an object to changes in its rotational speed, and the angular acceleration (a), the rate at which the motor’s speed changes.
To calculate accelerating torque (Tacceleration):
Tacceleration = J × a
Selecting a motor with the appropriate accelerating torque ensures smooth and precise speed adjustments without causing strain or instability.
Friction Torque
Friction torque is the resistance encountered when two surfaces move against each other, which requires additional torque to overcome. It arises from the contact between moving parts within a mechanical system, such as bearings, gears, or shafts. The frictional resistance depends on factors such as surface material, lubrication, and the normal force between the surfaces. Friction torque can significantly impact the motor’s efficiency, causing energy loss and heat generation.
To calculate friction torque (Tfriction):
Tfriction = μ × Fnormal × r
Where μ is the coefficient of friction, Fnormal is the normal force, and r is the radius or distance from the center of rotation.
Minimizing friction through proper lubrication and material selection can help reduce friction torque, enhancing motor performance and lifespan.
External Forces
External forces refer to the external loads or resistances that a servo motor must overcome to perform its task. These forces can include mechanical loads (such as weight or friction from moving parts), external disturbances (like vibrations or shocks), and environmental factors (such as wind or thermal resistance in outdoor applications). The torque required to overcome these external forces depends on the magnitude and direction of the load, as well as the distance from the motor’s axis of rotation (lever arm).
To calculate the torque from external forces (Texternal):
Texternal = F × r
Where F is the external force applied and r is the distance from the pivot point or axis.
RMS Torque
Root mean square (RMS) torque is a measure of the effective or equivalent constant torque that a motor produces over time, taking into account the variations in torque that occur during operation.
RMS torque is especially useful for motors operating under variable loads or speeds, as it provides a more accurate representation of the motor’s continuous workload compared to average or peak torque. The RMS value helps assess motor performance and thermal management, ensuring that the motor operates within safe limits without overheating.
To calculate RMS torque (TRMS):
TRMS= √( (T₁² + T₂² + … + Tn²) / n )
Where T₁, T₂, …, Tn are the instantaneous torque values over a period and n is the total number of values.
The root-mean-squared computation should be used to determine the continuous torque demand, however this is laborious without a software tool, especially for larger values of n and longer periods.
Determine Speed
Choosing the right speed for your servo motor is essential for matching your application’s needs. Speed, measured in revolutions per minute (RPM), affects how quickly the motor operates. Speed and torque must be balanced, as higher speeds generally reduce torque.
To determine the right speed, consider the motion profile and task time. For example, if a robotic arm needs to complete a specific action in a set timeframe, the motor must reach the required speed within that limit. Keep in mind that motors with fewer poles spin faster but deliver less torque due to factors like Back EMF.
Also, consider mechanical components such as gears or belts that adjust speed through gear ratios. The load’s inertia and required acceleration are vital too, as they impact both speed and efficiency. Additionally, the motor’s temperature is important, as running too close to its limits can cause overheating, affecting performance.
Determine Servo Motor Type
Servo motors come in a range of types, with each type being suited to particular applications. The list below shows how the different servo motors can be subdivided according to various attributes. Most servo motors provide either linear or rotary motion, and can be further divided according to their specific features.
- Linear Servo Motors
Linear servo motors provide direct linear motion without additional hardware, offering high precision, efficiency, and minimal maintenance for dynamic applications. - Rotary Servo Motors
Rotary servo motors are versatile and efficient, offering precise control in both light and heavy-duty applications, often with additional hardware. They are the most common type of servo motor found in industry. - AC Servo Motors
AC servo motors are widely used in industrial settings for their ability to handle various power levels and voltages with high efficiency. They are the most common type of servo motor and can be categorized according to the following types:
- Low to Medium Voltage AC Servo Motors
These compact motors offer high torque density and precision, ideal for applications with limited space but requiring reliable, efficient performance. - High Voltage AC Servo Motors
High voltage AC servo motors provide robust power output, handling heavy-duty industrial tasks while minimizing size and maximizing torque efficiency. - Synchronous AC Servo Motors
Synchronous AC servo motors feature excellent precision, with the rotor and stator rotating at the same speed, offering superior control and reliability. - Asynchronous AC Servo Motors
Asynchronous AC servo motors use induction to generate torque and require complex vector control, making them suitable for cost-effective, precision-demanding tasks.
- DC Servo Motors
DC servo motors are efficient, bidirectional, and compact, making them ideal for low-power, variable-speed applications with high precision and low inertia. They can be divided into brush type, and brushless types.
- Brush DC Servo Motors
These motors use brushes to supply current to the rotating part (the armature). They are commonly used for low-cost, high-torque applications but require periodic maintenance because the brushes wear out over time due to friction. The key feature here is the use of brushes that transfer electrical power to the motor’s rotor. - Brushless DC Servo Motors
Brushless DC servo motors are efficient, low-maintenance, and compact, offering superior performance and longevity for high-precision tasks in industrial settings.
- Stepper Servo Motors
Stepper servo motors combine stepwise motion with feedback, offering superior accuracy and torque efficiency, ideal for closed-loop control in precise operations. They are mostly found in DC type, although AC variants are also available.
Determine Voltage
Selecting a servo based on voltage is fairly straightforward, as it will be dependent on the supply voltage, current type, and phase (in the case of AC servo motors).
For example, if you’re working with a small DC servo motor powered by a 12V DC supply, you’ll need a motor that’s rated for 12V DC. It’s a simple match. However, if your application is in an industrial setting with a three-phase power supply, such as 100VAC, 200VAC, or even 400VAC, you’ll need to select an AC servo motor that matches the voltage and phase requirements of the system.
It’s also important to consider how the motor will be integrated into your setup. Ensure that the motor can handle the input voltage, and remember that AC motors require more attention to both voltage and phase because they rely on the three-phase power cycle.
In short, selecting the right voltage ensures compatibility with your power source and helps avoid potential issues like underperformance, overheating, or even damage to the motor. Always match your motor’s voltage with the specifications of your system.
Understand Duty Cycle
Servo motors are classified based on their duty types as defined by IEC 60034-1 and similar standards.
Duty types describe the motor’s operating conditions, including load, time, and thermal characteristics.
There are 10 standard duty types for rotating machines according to IEC 60034-1, each specifying the motor’s operational cycle. Generally, S1, S2 and S3 are applied to most servo motors, although certain types of servo motors in specific conditions can use the other duty cycle descriptions.
- S1 – Continuous Duty
The motor operates continuously under a constant load until reaching a stable thermal state. Common in conveyors and pumps. - S2 – Short-Time Duty
The motor runs at a constant load for a limited time without reaching thermal equilibrium, followed by a rest period. Common in actuators and short-cycle applications. - S3 – Intermittent Periodic Duty
The motor operates in cycles with periods of running and rest. Thermal equilibrium is not achieved. Typical in cranes and presses.
Determine the Required Motion Profile
Defining the motion profile involves specifying key parameters like maximum speed, acceleration, and deceleration. These factors ensure smooth and efficient operation while preventing mechanical stress.
For example, a pick-and-place robot may need to accelerate at 5000 mm/s² and reach a speed of 1000 mm/s to meet cycle time requirements. Conversely, a CNC machine might require precise acceleration control to avoid overshooting, using values like 200 mm/s² for gradual speed changes.
Additionally, factors such as dwell time, jerk (rate of acceleration change), and system inertia impact motion efficiency. A well-optimized motion profile balances speed and control, reducing wear and energy consumption while ensuring accuracy in applications like robotics, conveyors, or precision machining.
Determine Inertia Ratio
The inertia ratio is a vital parameter in servo motor system design. It is defined as the ratio of the load’s inertia to the motor’s rotor inertia, divided by the square of the gear reduction ratio. Mathematically, it is expressed as:
Inertia Ratio = Jload / Jmotor × (Gear Ratio)2
Where:
Joad = Load Inertia
Jmotor = Motor Rotor Inertia
This ratio quantifies how much the load resists changes in motion relative to the motor’s ability to control that motion.
Understanding and calculating the inertia ratio is a critical step in understanding servo system performance. A properly matched ratio allows the motor to respond quickly without being overwhelmed by the load’s inertia. If the load’s inertia is too high, the motor may become sluggish, with poor accuracy and longer settling times. A low inertia ratio, however, can lead to instability or excessive responsiveness, causing oscillations or stress.
Many beginners overlook this factor when sizing a servo motor, potentially resulting in inefficient systems or failure. Ideally, an inertia ratio between 3:1 and 10:1 is recommended, though it may vary depending on the specific application.
Impact of Inertia Ratio on Servo Performance
Performance Improvement
Servo performance improves as the inertia ratio decreases. Lower ratios make the system more responsive, reduce overshoot, and improve accuracy and stability.
Control Loop Tuning
A lower inertia ratio also makes tuning the control loop easier. The servo responds more predictably, with less oscillation or delay, which helps achieve precise, stable motion.
Inertia Ratio Guidelines
An ideal inertia ratio is around 5:1 for most systems, offering a good balance of control and efficiency. 10:1 is often the maximum acceptable limit. Ratios below 1:1 typically don’t yield better performance and can lead to oversized, costlier motors without real benefit.
Cost vs. Performance
Lowering the inertia ratio too much can drive up cost without meaningful gains. After a certain point, going lower doesn’t translate to noticeably better performance.
Real-World Mechanisms
Real systems have imperfections: belts stretch, couplings flex, and gears have backlash. These effects get worse with high inertia ratios, causing a delay or “springiness” in how the motor moves the load.
Feedback and Control Issues
Servo motors use encoders and PID control to maintain accuracy. But at high inertia ratios, that spring effect can cause instability or oscillation. Reducing gain helps, but at the cost of responsiveness.
High-Performance Servo Drives
Modern servo drives with features like auto-tuning, resonance suppression, and disturbance rejection can handle inertia ratios up to 30:1. Simpler drives, though, often need ratios 3:1 or lower to work reliably.
How Do Environmental Factors Affect Servo Motor Selection?
When selecting a servo motor, it’s crucial to consider environmental factors that can impact its performance and longevity. Here are some key environmental considerations:
Ambient Temperature
Higher ambient temperatures can reduce the motor’s continuous torque capacity. In environments with high temperatures or demanding continuous torque needs, liquid cooling systems can help maintain performance. However, it’s essential to avoid direct contact with the motor’s mechanics and connections.
Contamination
Dust, dirt, and other contaminants can affect motor performance, especially in industrial environments. Sealing the motor or using protective covers can help keep contaminants out, ensuring smoother operation and longer motor life.
Vibration
Excessive vibration can impact the motor’s precision and reduce its lifespan. For environments prone to high vibration, selecting motors with vibration-resistant designs or using additional vibration dampening equipment can help protect the system.
Speed-Torque Curves: Understanding and Utilization
Speed-torque curves show how much torque a servo motor can deliver at different speeds. They help ensure the motor meets application demands without overheating or underperforming.
Reading the Curve
The curve has two main zones:
- Continuous region – the motor can run here indefinitely without overheating.
- Intermittent region – higher torque, but only for short bursts, based on RMS torque limits.
Voltage Matters
Motor performance depends on input voltage. At 480 VAC, a motor might deliver full torque up to 3000 rpm, but torque drops at higher speeds. At 380 VAC, the same motor may struggle to reach 2500 rpm or deliver required torque.
Field Weakening
At very high speeds, motors enter the field weakening range, where torque drops regardless of voltage. Choosing the right voltage is key to staying within safe operating zones.
Practical Tools
Software like KEB’s drive tools visualize these curves, making it easier to match motors (like the TA3S) to your application based on real performance data.
Select the Appropriate Gearing and Reducers
Gearing helps optimize servo motor performance for real-world loads. While some applications benefit from direct drive motors, most machines require gear reduction to make full use of a servo’s torque across a wide speed range.
Why Use Gear Reduction?
Gear reducers adapt the motor to the load in three main ways:
- Reduce speed – slows down motor output to match application needs.
- Increase torque – output torque increases in proportion to the gear ratio.
- Lower inertia ratio – the load inertia reflected to the motor is reduced by the square of the gear ratio, improving control and responsiveness.
Key Considerations When Selecting Gearing:
- Gear Ratio – Calculated from the ratio of teeth between gears. Choose a ratio that balances torque needs and inertia matching.
- Motor Speed – High-speed motors often need reduction to avoid overspeeding the load.
- Torque Requirements – Gear reduction can amplify torque without needing a larger motor.
- Inertia Matching – Proper gear ratio significantly improves servo stability and control loop tuning.
- Gearbox Inertia – Use data from gearbox manufacturers to factor this into total system inertia.
Though direct drive is ideal in some cases, geared servo systems remain essential for many high-torque, precision applications.
How Does the Control System Compatibility Influence Servo Motor Choice?
Ensuring control system compatibility is a critical step when selecting a servo motor. The motor must integrate smoothly with your existing drive and control architecture to guarantee reliable communication, accurate motion control, and efficient system performance.
Key Control System Considerations:
Control Interface Compatibility
Servo drives use various control methods. Choosing the right one depends on your existing system:
- Analog control – Uses voltage or current signals to regulate speed or torque. Simple but less precise.
- Pulse-and-direction control – Common in CNC and stepper replacement applications.
- Servo networks (e.g., EtherCAT, CANopen, PROFINET) – Provide high-speed, synchronized communication, improved diagnostics, and minimal wiring. Ideal for complex or multi-axis systems.
Drive-Motor Communication
Verify that the servo drive can communicate effectively with both the motor and the PLC or controller, using supported protocols. Incompatible systems may require converters or custom integration, adding complexity and cost.
Servo motors must match the communication standards, control methods, and feedback systems used by your existing control infrastructure. Selecting compatible components simplifies integration, reduces setup time, and ensures smooth, precise control.
Select the Right Servo Drive and Power Transmission Components
After selecting the servo motor, the next step is to choose a servo drive that fits your system, as well as the various hardware items needed to transmit the torque from your motor to your load. Here’s how to ensure it matches your needs:
Choose the Right Servo Drive
- Current Capacity: Make sure the servo drive has at least 25% more current capacity than what you expect your system to use. This extra headroom helps handle unexpected conditions like increased friction or machine wear.
- Voltage Headroom: Choose a power supply and servo drive with a 25% buffer above both the undervoltage and overvoltage limits. This is especially important in applications with voltage fluctuations or regenerative braking, like vertical loads.
- Isolation: Use an isolation transformer or optical isolation to protect your system from electrical hazards and component damage. This keeps the signal and power grounds separate, avoiding issues from floating grounds.
- Power Appropriation: Don’t overestimate your power needs. Too much power can lead to wasted space and cost. Match the power output to the specific needs of your application for efficiency.
Power Transmission Features:
When selecting power transmission components, consider essential features such as keyed motor shafts, shaft seals, holding brakes (especially for vertical loads), and external braking resistors.
These features are necessary for ensuring that the motor’s torque is reliably transferred to the load, while also enhancing system safety and performance. Keyed motor shafts prevent slippage, shaft seals protect against contamination, and holding brakes ensure stability for vertical load applications. External braking resistors manage excess energy from regenerative braking, preventing overheating and ensuring efficient energy use.
Efficiency
Servo motor efficiency is about how much current is needed to maintain constant torque (known as the torque constant, Kt). Motor windings come in different configurations:
- Windings with better current efficiency have lower speed capabilities.
- Windings with higher speed capabilities have lower current efficiency.
Choose the winding based on the required speed and the most efficient choice for your needs.
Life Expectancy and Maintenance
A servo motor’s lifespan is influenced by various factors, including its operating environment and application-specific demands. While most manufacturers estimate a service life of 20,000 to 30,000 hours, the actual longevity depends heavily on setup, maintenance, and usage conditions.
In ideal conditions, a servo motor could run for decades, but under extreme stress or poor maintenance, it might fail in under a year. Selecting the right servo motor for your required application can extend the lifespan of your servo motor significantly. Or to put it another way, picking the wrong one, can lead to early failure very quickly indeed.
Cost
And finally, we come to the cost element, which for many, might be the first thing to consider.
The cost of a servo motor system varies depending on factors like motor type, drive specifications, and the complexity of the application. While higher-end models like brushless DC or AC servo motors offer greater efficiency, precision, and longevity, they come at a premium price.
Additional components like servo drives, power transmission elements, and optional features (such as holding brakes or external resistors) can also add to the cost. However, investing in a quality servo system is often more cost-effective in the long run, as it reduces maintenance, improves performance, and extends the lifespan of the equipment.
As the old saying goes, buy nice…or buy twice!
Conclusion
After reading this article, you should hopefully be better equipped to make a decision on which servo motor you should select for its intended application.
As we have seen, selecting the right servo motor depends on a variety of technical requirements ranging from torques and speeds, to environmental considerations. Buying an underspec servo motor can result in either your system not moving at all (best case) to the destruction of equipment, and potentially even injury (absolute worst case).
And of course, there is the cost aspect. Buying the right servo motor (maybe with a little extra margin in terms of performance) will ensure that your machinery will perform optimally, for the best price, while going overkill on certain unwanted elements can jack the price right up.
If you follow our guide, and make an informed choice, your motor will have a long life, running reliably, doing what it does best – torquing about, turning things, and moving its loads in a highly precise manner.
