does ceiling fan dimmers (triac based speed controllers) reduce fan motor's lifetime?
The dimmers in fans are actually potentiometers. Every motor has a maximum currrent it can handle. If you limit the current, providing less and less current, the fan will go slower but the lifetime will go up. If you go over the current level, the lifetime shortens. It's just like an LED. On LED specs, you'll find something like 20,000 hours or 30,000 hours based on a specified current which is usually 20-30mA.