Mechanical Time Constant
The time it takes an unloaded motor to reach about 63 percent of its final speed after voltage is applied. It arises from the ratio of the rotor's inertia to the motor's own damping and is of the order of milliseconds. It shows how quickly the motor responds to speed commands.
In small brushed motors it is typically in the 10–100 millisecond band; when a load is connected the total inertia grows and this time gets many times longer. Do not confuse it with the electrical time constant: the electrical constant describes the time for the current to build up in the winding and is generally much shorter than the mechanical one.
Why it matters:
- Ramp setting: Reducing the ramp time below the mechanical time constant of the load does not produce real acceleration; the motor already cannot respond faster than that.
- Starting current: Until the motor reaches its final speed the back-EMF stays low and high current keeps flowing. The longer the time, the greater the thermal load.
- Control: In closed-loop tuning, if the controller's response time is brought below this constant the system starts to oscillate.
Practical measurement: run the motor together with its load and watch the current waveform. The time it takes the current to come down from the starting peak to its steady value shows the system's real mechanical response time well enough.
The context in which this term is used: DC Motor Driver Selection Guide