In practice, a well-functioning drive is not achieved solely by selecting sufficient torque or the right speed. The extent to which a motor actually controls the connected mechanics is largely determined by the moment of inertia. Especially in applications involving positioning, high dynamics, or short cycle times, this becomes a decisive factor in achieving stable and repeatable performance.
The relationship between motor and mechanics
The motor’s moment of inertia must be selected so that the rotor is capable of both accelerating the connected mechanics in a controlled manner and decelerating them safely. It is therefore not just about initiating motion, but about maintaining control throughout the entire motion profile.
In practice, the ratio between the load inertia (JL) and the motor inertia (JM) is often used. A common guideline is to keep this ratio below 10. This JL/JM ratio is not an absolute limit, but a proven design condition for achieving stable control behavior.
When this ratio becomes too high, the influence of the load on the system increases, and the motor effectively loses “authority” over the motion.
Effect of the JL/JM ratio
An excessively high JL/JM ratio has direct consequences for servo control. The system may become unstable, resulting in less accurate and slower positioning. In practice, this means:
- the desired position is reached less accurately
- the system responds more slowly to control commands
- the likelihood of oscillations or overshoot increases
- achieving high dynamics becomes more difficult
The controller struggles to keep the mechanics tightly on track. This is especially limiting in applications with high demands on positioning accuracy or short cycle times.
Is a larger motor the solution?
Increasing motor size may seem like a straightforward way to improve the JL/JM ratio. In practice, however, this is not always the right choice. A larger motor almost always requires a larger drive controller. This can lead to higher investment costs and potentially higher operating costs, for example due to increased energy consumption.
As a result, there is a risk of unnecessary overdimensioning, both technically and economically. Moreover, the underlying mechanical situation remains unchanged. The load stays the same, meaning the solution is not necessarily optimal.
An alternative is to adjust the transmission ratio. By applying the right mechanical reduction, the effective inertia ratio “seen” by the motor can be reduced. In many cases, this makes it possible to use a smaller motor and controller without compromising functionality.
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Speed and acceleration
Requirements for speed and acceleration further increase the importance of a correct inertia ratio. The higher the desired dynamics, the more critical the alignment between motor and load. If the JL/JM ratio is not properly selected, the desired speed profile may simply not be achievable. The drive will then be unable to follow the required accelerations and decelerations. This may result in:
- failure to meet cycle times
- limited acceleration capabilities
- deviations in the motion profile
This becomes immediately visible in the performance of dynamic machines.
Always ensure the correct inertia
The moment of inertia is therefore not a parameter that is only relevant in exceptional cases. In fact, it plays a role in every drive design. From simple linear motion to complex multi-axis machines, the relationship between motor and load always influences system behavior.
A proper match of mass inertia ensures that the motor maintains sufficient control over the mechanics, both during acceleration and deceleration. The commonly used guideline of JL/JM < 10 helps achieve stable and accurate performance, but always requires evaluation within the context of the application.
When this balance is incorrect, instability arises along with limitations in accuracy and dynamics. Increasing motor size is not automatically the best solution. By also considering the mechanical transmission in the design, a better balanced and more efficient drive can be achieved.
LM Systems is ready to support you with in-depth technical expertise and tailored solutions that precisely match your challenge. Contact us to discover what customization can mean for your application.