Bathroom Fan Fires

© 2005 Dr. Ray Franco, PhD., PE    r.a.franco@ieee.org

Bathroom fans usually fail because of wear to the motor bearing. Almost all bathroom fans use a shaded pole motor , which has the lowest starting torque of any induction motor. Hence, if there is any mechanical resistance in the barring, there is a good chance that the motor will not start. This is a locked rotor condition. Experimental results show that with the rotor locked, the temperature of a bathroom fan motor can reach in excess of 340 °F. Bathroom fan motors are typically NEMA Class B (NEMA Insulation Classes ), which corresponds to a maximum insulation temperature of 266 °F. Thus, the varnish insulation on the wire will start to fail, and depending on how the motor winding fails, this can cause a fire. This problem is compounded by lint build up and plastic propellers, which can ignite and/or add fuel to the fire. To prevent this, new bathroom fans have a thermal fuse, which opens at approximately 260 °F. The accompanying photographs depict actual cases and experimental results.


For Maximum Resolution, Click on the Body of the Picture.

Picture of Light Blub Pulled toward Fan

Light Bulb Pulled Toward Bathroom Fan (F05-18).

Better View of Distored Bulb

Better View of Distorted Bulb Pointing Toward the Heat Source (F05-18).   Click this link for an explanation.

Shaded Pole Motor

Shaded Pole Motor (F05-18).

Hot Spot on Top of Fan Housing

Hot Spot on Top of Fan Housing (F05-18).

Copper Bead on Motor Winding

Copper Bead on Motor Winding (F05-18).

Copper Beading Motor Winding

Copper Beading Motor Winding (F05-18).

Copper Bead on Fan Power Cord

Copper Bead on Fan Power Cord (F05-18).

Plastic Wire-Nuts Were Not Consumed by the Fire

Plastic Wire-Nuts Were Not Consumed by the Fire (F05-18).


Experimental Result:

For Maximum Resolution, Click on the Body of the Picture.

Broan Exemplar Bathroom Fan

Broan Exemplar Bathroom Fan (F04-21).

Shaded Pole Fan Motor

Shaded Pole Fan Motor (F04-21).

Disassembled Shaded Pole Fan Motor

Disasembled Shaded Pole Fan Motor (F05-20).

Intentionally Locked Rotor

Intentionally Locked Rotor (F04-21).

Temperature at the Steel Motor Core with the Rotor Locked

Temperature at the Steel Motor Core with the Rotor Locked (F04-21).

Damaged Wire Insulation

Damaged Wire Insulation (F04-21).

Heat Damage to Plastic Spool

Heat Damage to Plastic Spool (F04-21).

New Bathroom Fan Motors Have a Thermal Fuse

New Bathroom Fan Motors Have a Thermal Fuse (F04-21).


 

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