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Bathroom Fan Fires


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, 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.

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Picture of Light Blub Pulled toward Fan
Light Bulb Pulled Toward Bathroom Fan (F05-18).
Better View of Distored Bulb
Better View of Bulb Pointing Toward the Heat Source (F05-18).
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:

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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).