The Mass Inertia Ratio – Expert Tips for Designing Dynamic Motion Control Systems Motor design & inertia ratio Motor design & inertia ratio In today’s motion control systems, the right inertia ratio λ between motor and load often makes the difference between precise performance and instability. This article takes a practical look at the key factors in motor design—and how to effectively improve unfavorable inertia ratios.The mass inertia ratio λ describes the ratio of the inertia of a motor’s rotor mass compared to the inertia of a load. The more dynamic a task is, the smaller λ should be. This means that if a load is to be accelerated and positioned quickly and precisely, the inertia of the motor should be as close as possible to the inertia of the load. As an everyday example, imagine a shopping cart that you want to steer around a corner. If it is empty (low inertia), this can be done easily and quickly; if a few crates of drinks are already loaded in (high inertia), the cart takes on a ‘life of its own’ and it is more difficult to turn quickly and precisely as it pulls the shopper along. Options to improve an unfavourable inertia ratio 1. Use a gearbox – A gearbox significantly improves the inertia ratio, as the gear ratio i is squared in the calculation. Example: A robotics application with an inertia ratio of 150 is not acceptable. By using a gearbox with a transmission ratio of i=8, λ can be improved to 2.3 which is a good value for this type of application. An external transmission, such as a belt drive, also works in the same way. Replacing a stepper motor with a BLDC with gearbox can open space for new solution options. Positioning is also possible by using an encoder 2. Motor with greater rotor inertia – usually larger flange dimensions. With BLDC motors, external rotors also offer advantages. 3. Reduce acceleration ramps – braking ramps are particularly important. 4. For stepper motors: use an encoder – does not improve the inertia ratio, but position feedback allows position errors to be corrected after overshooting. This also opens more tuning options in the controllers and enables feed-forward control.If you have any further questions or need help with your design, our Technical Sales Team will be happy to help.
