News Details

When selecting a servo motor, most engineers start by calculating torque.

The torque looks sufficient, the motor size seems appropriate, and everything appears correct on paper. But once the machine starts running, problems begin to appear: vibration during operation, slow response, excessive impact during acceleration and deceleration, and unstable positioning accuracy. The drive parameters get adjusted repeatedly, yet the performance still isn't satisfactory.

After hours—or even days—of troubleshooting, the real cause often turns out not to be insufficient torque, but an improperly matched inertia ratio.

If the torque is too low, the motor simply cannot drive the load. If the inertia ratio is incorrect, the machine may never achieve stable performance no matter how much tuning is done.

Many commissioning issues that consume days or even weeks can be traced back to this overlooked factor.

In this article, we'll skip the complicated formulas and focus on the practical servo motor selection principles that matter most in real-world engineering applications.

1. Torque Matters, But It's Not the Whole Story

Torque is often the first parameter engineers evaluate during servo motor selection, and for good reason. Torque determines whether the motor can move the load.

For example, a lifting platform must continuously overcome gravity, making rated torque a critical consideration. In contrast, high-speed pick-and-place machines, robotic arms, and transfer systems perform frequent acceleration and deceleration cycles, where peak torque becomes equally important.

In most industrial applications, engineers typically reserve a safety margin of approximately 1.5 times the calculated rated torque. For highly dynamic systems with frequent start-stop movements, a peak torque margin of 2 times or more is commonly recommended.

Typical Torque Reference by Application

However, many engineers encounter a frustrating situation:

The torque calculation is correct, sufficient safety margins are included, yet the machine still vibrates, overshoots, or struggles to achieve stable positioning.

In many cases, the answer lies in the inertia ratio.

2. Inertia Ratio: The Real Key to Servo System Performance

If torque determines whether a machine can move, inertia ratio determines whether it can move smoothly and accurately.

The inertia ratio is calculated as:

Inertia Ratio = Load Inertia ÷ Motor Rotor Inertia

At first glance, the concept may seem abstract. A simple analogy makes it easier to understand.

Imagine pushing an empty shopping cart versus pushing one fully loaded with heavy goods. Both can be moved, but the loaded cart is harder to accelerate, more difficult to stop, and less responsive to directional changes.

Servo motors behave in much the same way.

When the inertia ratio becomes too large, the motor must work harder to control the load, often leading to performance issues such as:

• Excessive motor vibration

• Residual oscillation after stopping

• Difficulty increasing servo gains

• Unstable positioning accuracy

• Higher motor temperatures

These symptoms are frequently mistaken for tuning problems or insufficient motor power, when the real issue is often poor inertia matching.

Recommended Inertia Ratios for Common Drive Mechanisms

For most automation equipment, maintaining an inertia ratio below 5 significantly simplifies commissioning and improves overall system stability.

3. Three Commonly Overlooked Details That Cause Problems Later

Many machines perform well during initial testing but begin experiencing reliability issues months later. Often, the root cause is not torque or inertia but several small details that were overlooked during the selection process.

Don't Focus Only on Rated Speed

Most servo motors are available with rated speeds of either 3000 rpm or 5000 rpm.

A common mistake is assuming that a higher speed rating automatically means better performance.

In reality, continuous torque typically decreases as motor speed increases. Therefore, engineers should always review the motor's torque-speed curve rather than relying solely on the rated speed listed in the catalog.

Encoder Resolution Directly Affects Accuracy

For general automation equipment, 17-bit or 20-bit encoders are usually sufficient.

For robotics, precision positioning systems, semiconductor equipment, and other high-end applications, 23-bit encoders provide significantly higher feedback resolution and improved positioning accuracy.

Engineers should also determine whether the machine requires:

• Position retention after power loss

• Automatic coordinate recovery after restart

If these functions are necessary, an absolute encoder is generally the preferred choice.

Vertical Axes Should Always Use a Brake

This is one of the most commonly overlooked and potentially costly mistakes.

Any axis subjected to gravity should be equipped with a holding brake.

Without a brake, a power failure or emergency stop can allow the load to fall freely, potentially damaging equipment, workpieces, or even creating safety hazards.

The additional cost of a brake is minimal compared to the potential consequences of not having one.

4. Why Are More AGVs and AMRs Adopting Low Voltage Servo Systems?

As AGVs, AMRs, autonomous forklifts, and other mobile robots continue to grow in popularity, traditional 220V AC servo systems are increasingly being replaced by 48V, 60V, and 72V low voltage servo solutions.

The primary reason is simple: low voltage servo motors can be powered directly by lithium battery systems.

This reduces power conversion losses, improves overall energy efficiency, simplifies electrical architecture, and enhances system safety.

For mobile robots, servo motors must deliver not only torque but also high efficiency, fast response, compact size, and seamless communication capabilities.

To address these requirements, Yikong Intelligent Equipment Co., Ltd. has developed the TEC Series Low Voltage Servo Motor and Drive System, designed specifically for intelligent logistics and mobile robotics applications.

Typical applications include:

• AGV drive systems

• AMR mobile robots

• Autonomous forklifts

• Lifting and towing robots

• Smart logistics equipment

• Automated material handling systems

The TEC Series supports 24V–72V low voltage operation and industrial communication protocols such as CANopen, enabling easy integration with AGV drive wheels, differential drive systems, and steering drive units.

For mobile robotic applications that require high efficiency, fast dynamic response, and reliable operation, low voltage servo systems are becoming the preferred choice.

Key Servo Motor Selection Rules at a Glance

Conclusion

Many engineers entering servo motor selection focus primarily on torque.

However, real-world project experience quickly reveals a more important truth:

Torque determines whether the load can move, while inertia matching determines whether the system can move smoothly and reliably.

At the same time, factors such as encoder selection, braking systems, and environmental protection ratings play a critical role in long-term reliability.

Addressing these considerations during the design stage is far more effective than spending days troubleshooting performance issues after installation.

For automation equipment, AGVs, AMRs, autonomous forklifts, and intelligent logistics systems, proper servo motor selection is not simply about choosing a power rating—it's about finding the optimal balance between performance, efficiency, reliability, and cost.

Keywords: Servo Motor Selection, Servo Motor Sizing, Inertia Ratio, Low Voltage Servo Motor, 48V Servo Motor, AGV Servo Motor, AMR Servo Motor, CANopen Servo Motor, Mobile Robot Motor, TEC Servo Motor, Industrial Automation Servo System.