Moment of Inertia
It is the measure of a rotating mass's resistance to acceleration and deceleration, given in kg·m². It grows with the square of the mass's distance from the axis of rotation. It determines how long the start-up will take and how long the high current will flow during that time.
Acceleration torque is the moment of inertia multiplied by the angular acceleration. If the load inertia is large, the motor draws high current for a long time; the start-up surge typically lasts 0.5–3 seconds and this duration depends directly on the inertia.
Its counterparts on the driver side:
- Thermal load: A short start-up fits inside the peak current window, whereas a long start-up consumes the continuous current budget with the same current.
- Ramp time: Setting the ramp shorter than the acceleration time the inertia requires does not prevent the current surge, it only shortens it on paper. A meaningful ramp must be longer than the measured start-up time.
- Braking: The same inertia comes back at you when stopping; stopping a high-inertia load in a short time pushes a significant amount of energy back into the supply line.
A gearbox reflects inertia to the motor side reduced by the square of the ratio. A large load behind a 1:50 box looks like a far smaller inertia from the motor's point of view; this is a frequently overlooked advantage of using a gearbox.
The context in which this term is used: DC Motor Driver Selection Guide