Inertia’s role in miniature motor selection

When specifying a miniature motor, inertia is a key consideration. As a measure of a motor’s resistance to changes in rotational speed, its inertia value is based on a calculation involving the mass and radius of its rotor. High rotational inertia presents a greater challenge in accelerating the system, whereas a lower inertia indicates ease of acceleration. As less energy is required to accelerate or decelerate motors with lower inertia, they’re more energy-efficient, which can make a significant difference in applications with frequent start-stop cycles. Motors with lower rotational inertia also usually offer improved control; a positive attribute for applications requiring precise positioning.

While it may seem lower inertia is optimal, matching the motor’s inertia with the load’s inertia is essential. In the most basic terms, if the load’s inertia is significantly greater than the motor’s, the motor will struggle to control the outmatched mass or size. The closer the inertias match, the more accurately the motor and drive system can control the load, especially in applications requiring precise movements.

Tackling inertia mismatch

Although a 1:1 inertia ratio is theoretically perfect, it’s neither practical nor necessary. In real-world applications, striving for an inertia ratio close to 1:1 can result in oversized components, higher system costs, and increased energy consumption. Instead, each use case has an acceptable range, although for applications demanding dynamic control and positioning, a low load-to-motor inertia ratio is crucial.

The common challenge is an inertia mismatch caused by a high load-to-motor inertia ratio. The imbalance can introduce stability issues, causing increased response times and lower system bandwidth. It can also waste energy as the motor works harder to move the load. At its most serious level, it causes oscillations and resonance that could damage the motor as well as the load and connections.

Simplifying the transmission

Many motor manufacturers provide online tools and calculators to assist design engineers when selecting a miniature motor, including Portescap’s MotionCompass. However, a comprehensive awareness of the contributing factors to inertia is useful in motion design and integration. While a motor catalog and sizing tool can provide the inertia rating, holistic design tactics can improve overall application design and help close the inertial gap.

Gear reduction is a common step, and this technique reduces the load inertia in proportion to the square of the gear ratio. Additionally, modern control systems with advanced algorithms and high-resolution feedback devices can address inertia mismatch issues. However, the adverse effects of inertia mismatch, or a high load to motor inertia ratio, can become worse by a lack of stiffness in the system, also known as load compliance. To minimize these issues, look to wider components of the application or machine to optimize rigidity.

Load compliance challenges are more common in indirect drive systems. Here, the motor isn’t directly coupled with the load but is connected through one or more power transmission elements such as a gear mechanism, pulley belt systems, chain drives, or ballscrews. It’s useful to consider the effect of each connected component beyond the motor shaft when simplifying an indirect system to increase stiffness and reduce further potential causes of inertia.

Direct drive systems

A direct drive system is advisable for demanding applications that would significantly benefit from a close motor-to-load inertia ratio. This motion design approach will keep the power transmission elements to a minimum while optimizing compliance.

With a direct drive, the motor is directly coupled to its load. This connection removes the inertia resulting from power transmission components, and the less inertia the motor needs to overcome, the less torque required to meet the desired acceleration rate. A direct drive system also minimizes effects such as backlash – the play between mechanical components potentially impacting power transfer and system reliability.

A comprehensive review of all design aspects impacting the motion cycle can minimize the overall inertia within an application or machine, and optimize inertia balance between the motor and the load. Involve motion engineers early in the overall design process to effectively achieve this.

First, engineering input can help improve the wider machine or application design and advise on the best motion solution approach. Second, early involvement of motion designers means if customization is required for the motor or transmission, the most effective results can be reached early in the process with no impact on time to market.

Portescap provides tools and motor data to help guide original equipment manufacturer (OEM) engineers in their specifications, with company motion specialists available to discuss design and customization needs across a range of applications.