Fan Impedance Protection
Impedance protection is commonly used on small fan motors.
This relies on the inherent electrical properties of the coil which limit the temperature rise within a motor under locked rotor condition.
- Rotor is locked
- Current to windings increases
- Temperature in windings increase due to increase in current
- As temperature rises resistance of windings increases
- Temperature stabalises because resistance in winding reaches a level that reduces the current.
- No further temperature increase takes place.
- Maximum temperature is well within max temperature as dictated by motor insulation class.
From UL:
http://www.ul.com/motor/faqs.html
What is an Impedance Protected Motor?
A: A motor that relies solely upon the impedance of the windings to prevent overheating.
Tests for an Impedance protected motor include Winding Resistance, Input (no-load and Locked Rotor), 72 hr, Locked Rotor Temperature Test, Dielectric, 15 day Locked Rotor Endurance Test and dielectric. 3 samples of each motor are required for testing.